Select Page
NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 3

NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 3

Polyethylene terephthalate (PET): A brief introduction

PET is a very common plastic, mostly encountered in our lives as PET bottles and as a food packaging material. In this article you will learn how NIR spectroscopy can improve the efficiency of your PET analysis at different steps along the production cycle. Before getting into this, let’s introduce some background information about PET.

Polyethylene terephthalate (PET)

Polyethylene terephthalate (PET) is a general-purpose thermoplastic polymer which belongs to the polyester family. Polyester resins are known for their excellent combination of properties such as mechanical, thermal, and chemical resistance as well as dimensional stability.

PET is one of the most recycled thermoplastics and has the number 1 as its recycling symbol. Recycled PET can be converted into fibers, fabrics, sheets for packaging and for manufacturing automotive parts. PET is a highly flexible, colorless, and semi-crystalline resin in its natural state. Depending upon how it is processed, it can be semi-rigid to rigid. It exhibits good resistance to impact, moisture, alcohols, and solvents.

The chemical formula of PET is (C10H8O4)n and its molecular structure is shown in Figure 1.

Figure 1. Molecular structure of linear PET.

In addition to linear PET, there is also a branched version of the polymer. Branched PET is typically mixed with a small percent of isophthalic acid (C8H₆O4), because purified isophthalic acid (PIA, Figure 2) reduces the crystallinity of PET, serving to improve its clarity and increase the productivity of bottle manufacturing processes.

Diethylene glycol (DEG) as an additive also reduces the rate of crystallization of PET when crystallizing from the melt, isothermally and dynamically.

Figure 2. Molecular structure of isophthalic acid.
The key properties and advantages of PET resin are numerous:
  • very strong and lightweight, and therefore easy and efficient to transport
  • has good gas (oxygen, carbon dioxide) and moisture barrier properties, meaning low gas permeability (particularly against CO2)
  • exhibits excellent electrical insulating properties
  • broad range of use temperature (-60 to 130 °C)
  • high heat distortion temperature (HDT)
  • suitable for transparent application purposes
  • practically shatter-resistant – PET does not break or fracture and is used to replace glass in some applications
  • recyclable material
  • transparent to microwave radiation
  • very resistant to alcohols, aliphatic hydrocarbons, oils, greases, and diluted acids
  • moderately resistant to diluted alkalis, aromatic and halogenated hydrocarbons
  • PET is approved as safe for contact with foods and beverages by the FDA, Health Canada, EFSA, and other health agencies

What is polyethylene terephthalate (PET) used to make?

Polyethylene terephthalate is used in several types of packaging applications as shown in Figure 3. Due to its strength, light weight, and many other attractive properties, PET excels as a food packaging material.

Figure 3. PET is an ideal choice for many food packaging applications due to its strength to weight ratio.

Polyester makes up nearly two-thirds of synthetic fibers produced. There are many different types of polyester, but the type most often produced for use in textiles is PET. When used in a fabric, it is most often referred to as «polyester» or «poly» (Figure 4). This material costs very little to produce, which is the primary driver for its use in the textile industry.

Approximately 60% of the global PET production is used to make fibers for textiles while about 30% is used to make bottles for various purposes. Its ability to be recycled is especially attractive for manufacturers looking to save costs and operate in a greener manner.

Figure 4. PET makes up a significant portion of produced polyester fabric.

In the electronics industry, PET is chosen to replace less ideal materials due to its excellent electrical insulating properties and resistance to distortion even at high temperatures. PET is also used to manufacture many parts in the automotive industry (Figure 5).

Figure 5. PET is often used in the manufacturing of various automotive parts.

NIRS as a tool to assess the quality of PET

For over 30 years, near-infrared spectroscopy (NIRS) has been an established method for fast and reliable quality control within the PET industry. Despite this, many producers still do not consistently consider the implementation of NIRS in their QA/QC labs. Limited experience regarding application possibilities or a general hesitation about implementing new methods are some of the reasons behind this.

The advantages of using NIR spectroscopy for QA/QC are numerous. One major advantage of NIRS is the determination of multiple parameters in just 30 seconds with no sample preparation! The non-invasive light-matter interaction used by NIRS, influenced by physical as well as chemical sample properties, makes NIRS a suitable method for the determination of several critical quality parameters in these polymers and many more.

In the remainder of this article, a short overview of PET applications is presented, followed by available turnkey solutions for PET, developed according the NIRS implementation guidelines of ASTM E1655-17.

Did you miss the first parts in this series? Find them here!

For more detailed information about NIRS as a secondary technique, read our previous blog posts on this subject.

Applications and parameters for PET with NIRS

During production of PET it is important to check certain parameters to guarantee the quality. These parameters include the diethylene glycol content, isophthalic acid content, intrinsic viscosity (ASTM D4603), and the acid number (AN). Determination of these parameters is a lengthy and challenging process due to the limited solubility of the sample and the need to use different analytical methods.

The most relevant applications for NIRS analysis of PET are listed in Table 1.

Table 1Available application notes for use of NIR for PET
Polymer Parameter Related NIRS Application Notes
Polyethylene terephthalate (PET)

Diethylene glycol, Intrinsic Viscosity, Acid number, Isophthalic acid

AN-NIR-023

Where can NIRS be used in the production process of PET?

Figure 6 shows the individual production steps from the plastic producer via plastic compounder and plastic converter to the plastic parts producer. The first step in which near-infrared lab instruments can be used is when the pure polymers like PET are produced, and their purity needs to be confirmed. NIRS is also a very useful technique during the next step where polymers are compounded into intermediate products to be used for further processing.

Figure 6. Illustration of the polyethylene terephthalate production chain.

Easy implementation of NIR spectroscopy for plastic producers

Metrohm has extensive expertise with analysis of PET and offers a turnkey solution in the form of the DS2500 Polymer Analyzer. This instrument is a ready-to-use solution to determine multiple quality parameters in PET.

Figure 7. Turnkey solution for PET analysis with the Metrohm DS2500 Polymer Analyzer.

Application example:

Pre-calibrations available for the PET industry on the DS2500 Polymer Analyzer

Due to the limited solubility of polyethylene terephthalate and the need to use several different analytical methods, the determination of the parameters listed in Table 2 is a lengthy and challenging process with conventional laboratory techniques.

Table 2. Primary method vs. NIRS for the determination of various quality parameters in PET samples.
Parameter Primary method Time to result (primary method) NIRS benefits
Diethylene Glycol content

Extraction + HPLC-MS

45 min. preparation + 40 min. HPLC-MS All four parameters are measured simultaneously within a minute, without sample preparation or the need of any chemical reagents
Isophthalic acid content Dissolve + HPLC 45 min. preparation + 40 min. HPLC
Intrinsic viscosity Dissolve + Viscometer 90 min. preparation + 1 min. Viscometer
Acid number Dissolve + Titration 90 min. preparation + 10 min. Titrator

The NIRS prediction models created for PET are based on a large collection of real product spectra and is developed in accordance with ASTM E1655-17 Standard practices for Infrared Multivariate Quantitative Analysis. For more detailed information on this topic, download the free white paper.

To learn more about pre-calibrations for PET, download our brochure and visit our website.

The result of this turnkey solution for rapid non-destructive determination of the key quality parameters for PET listed in Table 2  is shown in Figure 8.

Figure 8. Turnkey solution for diethylene glycol, isophthalic acid, intrinsic viscosity and acid number in PET using the Metrohm DS2500 Polymer Analyzer. A: Sampling and analysis of PET granulate. B: Results of the four analyses from NIRS compared to a primary laboratory method along with the Figures of Merit (FOM) for each analysis.

This solution demonstrates the feasibility of NIR spectroscopy for the analysis of multiple parameters in PET in less than one minute without sample preparation or using any chemical reagents.  Learn more about the procedure in our free Application Note!

Future installments in this series

This article is a detailed overview of the use of NIR spectroscopy as the ideal QC tool for the analysis of polyethylene terephthalate (PET). Future installments in this series will be dedicated to:

 

  • Polyamide (PA)
  • Polyols and Isocyanates to produce Polyurethane (PU)

For more information

About spectroscopy solutions provided by Metrohm, visit our website!

We offer NIRS for lab, NIRS for process, as well as Raman solutions

Post written by Wim Guns, International Sales Support Spectroscopy at Metrohm International Headquarters, Herisau, Switzerland.

Validation of titration methods

Validation of titration methods

Manufacturing products of the highest quality is a must, especially in the pharmaceutical and food industries. This requires accurate, reproducible, and simple analysis methods that eliminate human errors as much as possible. Automated titration is one such solution that offers additional time and cost savings to laboratories.

After applying automation to a titration method, how can you ensure that the chosen method also delivers a reliable result? And how do you know that it is suitable for the analysis of your analyte(s)? This requires method validation of a titration, which includes standardization of the titrant as well as determination of accuracy and precision, linearity, and specificity.

USP General Chapter <1225> Validation of Compendial Procedures and ICH Guidance Q2(R1) Validation of Analytical Procedures: Text and Methodology define the validation elements – some of the most important ones are described in the following article.

These include (click to go to each section):

Standardization

Dilution and weighing errors as well as the constant aging of all titrants lead to changes in the concentration of the titrant. To obtain results that are as reliable as possible, the most accurate titrant concentration is a prerequisite. Standardization of the titrant is therefore an integral part of a titration method validation. The standardization procedures for various titrants are described in the Volumetric Solution section of USP – NF as well as in the Metrohm Application Bulletin AB-206 regarding the titer determination in potentiometry.

The titrant to be used in the validation must first be standardized against a primary standard or a pre-standardized titrant. It is important that the standardization step and the sample titration are carried out at the same temperature.

Primary standards are characterized by the following properties:

  • high purity and stability
  • low hygroscopicity (to minimize weight changes)
  • high molecular weight (to minimize weighing errors)

The use of a standard substance (primary standard) allows accuracy to be assessed.

For more information about titrant standardization, check out our blog posts «What to consider when standardizing titrant» (for potentiometric titration) and «Titer determination in Karl Fischer titration».

Accuracy and precision

Accuracy is defined as the proximity of the result to the true value. Therefore, it provides information about the bias of a method under validation. Accuracy should be determined over the entire concentration range.

Precision is usually expressed as the standard deviation (SD) or relative standard deviation (RSD). It expresses how well the individual results agree within an analysis of a homogeneous sample. Here, it is important that not only the analysis itself but also all sample preparation steps are performed independently for each analysis.

Precision is evaluated in three levels:

  1. Repeatability: the precision achieved by a single analyst for the same sample in a short period of time using the same equipment for all determinations.
  2. Intermediate precision: analysis of the same sample on different days, by different analysts and with different equipment, if possible, within the same laboratory.
  3. Reproducibility: precision obtained by analyzing the same sample in different laboratories.

Determination of both accuracy and precision is necessary, as only the combination of both factors ensures correct results (Figure 1).

Figure 1. Only when both precision and accuracy are high can correct results be obtained, as high precision does not necessarily mean good accuracy, and vice versa.

For titration, accuracy and repeatability are usually determined together. At least two to three determinations at three different concentration levels (in total six to nine determinations) are recommended. For assays, the recommendation is to use a concentration range of 80% to 120% of the intended sample weight.

Linearity

Linearity expresses whether a particular method gives the correct results over the concentration range of interest. Since titration is an absolute method, linearity can usually be determined directly by varying the sample size and thus the analyte concentration.

To determine the linearity of a titration method in the range of interest, titrate at least five different sample sizes and plot a linear regression of the sample volume against the titration volume consumed (Figure 2). The coefficient of determination (R2) is used to assess linearity. The recommendation is to use a concentration range of 80% to 120% of the intended sample weight.

Figure 2. Linear regression curve for the assay of potassium bicarbonate.

Specificity

Impurities, excipients, or degradation products are among the many components that may be present in a sample. Specificity is the ability to evaluate the analyte without interference from these other components. Therefore, it is necessary to demonstrate that the analytical procedure is not affected by such compounds. This is the case when either the equivalence point (EP) found is not shifted by the added impurities or excipients, or in the event it is shifted that a second EP corresponding to these added components can be observed when a potentiometric sensor is used for indication.

Specificity may be achieved by using suitable solvents (e.g., non-aqueous titration instead of aqueous titration for acid-base titration) or titration at a suitable pH value (e.g., complexometric titration of calcium at pH 12, where magnesium precipitates as magnesium hydroxide).

How can this be implemented in practice? The titrimetric determination of potassium bicarbonate with hydrochloric acid will serve as an example here.

In this case, potassium carbonate is expected as an impurity with pkb values of 8.3 and 3.89. This makes it possible to separate the two species during the acid-base titration. Figure 3 shows the comparison of a curve overlay of the titration curves of potassium bicarbonate with and without added potassium carbonate.

Figure 3. Curve overlay of the specificity test using 1 g KHCO3 with and without 0.5 g K2CO3 (green and orange = no K2CO3 added; blue and yellow = K2CO3 added). Click to enlarge.

The lower titration curve corresponds to the solution containing both potassium bicarbonate and potassium carbonate. Two EPs are found here: the first EP can be assigned to the added potassium carbonate, while the second corresponds to the sum of potassium bicarbonate and potassium carbonate. The curve at the top of the figure clearly shows only one EP for the potassium bicarbonate solution without impurities.

Find out more about the proper recognition of endpoints (EP) in our previous blog post.

Conclusion

If you follow the recommendations above, you will be ready for titration method validation – and now it`s time to get started!

Using potentiometric autotitration instead of manual titration increases the accuracy and reliability of your results. In addition, the use of an autotitrator ensures that critical regulatory compliance requirements, such as data integrity are met.

Right from the start, Metrohm products provide peace of mind and confidence in the quality of the data you produce with proper IQ/OQ.

If you would like to learn more about Metrohm Analytical Instrument Qualification, have a look at our two blog posts dedicated to this important topic.

Additional security is also provided, e.g., by Metrohm Buret Calibration which ensures that the accuracy and precision of your dosing device are within the required tolerances. Traceable monitoring of the performance and function of the instrument through regular re-qualifications and tests is therefore a given.

Watch our free webinar

available on demand!

How to convert from manual to automated titration procedures

Post written by Doris Hoffmann, Product Manager Titration at Metrohm International Headquarters, Herisau, Switzerland.

NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 2

NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 2

Polypropylene and polyethylene: A brief introduction

Did you know that polypropylene (PP) and polyethylene (PE) are the most produced plastics in the world? Products made out of PP and PE are so ubiquitous that every single one of us encounters them several times per day. In this article you will learn how NIR spectroscopy can improve the efficiency of your PP and PE analysis along different steps along the production cycle. But first, let’s get a little bit of background information about PP and PE.

Polypropylene (PP)

Polypropylene (also known as polypropene or PP) has a chemical formula of (C3H6)n. It is a thermoplastic polymer mainly produced from propylene monomers. PP is a versatile plastic commodity that also functions as a fiber. In 1954, it was first polymerized simultaneously by the Italian chemist, professor, and Nobel laureate Giulio Natta and Karl Rehn, a German chemist.

Polypropylene has the unique ability that it can be manufactured via several different methods and be utilized in many applications like packaging, injection molding, and fibers. This plastic commodity is the second most popular in the world, only preceded by polythene.

Polyethylene (PE)

Polyethylene (or polythene, PE) is also a polymer, but it is made from ethylene monomers and has the chemical formula (C2H4)n. The first synthesis of PE in 1898 by the German scientist Hans von Pechmann was accidental. Similar to PP, PE is also a thermoplastic.

PE is the most used plastic worldwide. Polythene is very stable and is a good electrical insulator. It has a very low melting point and is used in large amounts for the automotive and food packaging industries. Approximately 70% of PE is utilized in food packages, food containers, pallets, and even in crates and bottles.

Polyethylene is available in many different types:

  • Ultra-High-Molecular-Weight Polyethylene (UHMWPE)
  • Ultra-Low-Molecular-Weight Polyethylene (ULMWPE or PE-WAX)
  • High-Molecular-Weight Polyethylene (HMWPE)
  • High-Density Polyethylene (HDPE)
  • High-Density Cross-Linked Polyethylene (HDXLPE)
  • Cross-Linked Polyethylene (PEX or XLPE)
  • Medium-Density Polyethylene (MDPE)
  • Linear Low-Density Polyethylene (LLDPE)
  • Low-Density Polyethylene (LDPE)
  • Very-Low-Density Polyethylene (VLDPE)
  • Chlorinated Polyethylene (CPE)
Figure 1. Molecular structures of PE and PP.

Differences between polypropylene and polyethylene

Which is better, polypropylene or polyethylene? It all depends on the application! For what purpose are they being used? Both polymers are considered «commodity plastics». These are plastics that are used in high volumes for a wide range of applications.

Let’s compare some of the properties of each.

Table 1. Comparison chart of polypropylene vs. polyethylene.
Polypropylene (PP) Polyethylene (PE)
Chemical Properties

Semi-crystalline

Polypropylene bag

Inert, translucent

Polyethylene bag

Electrical Properties

High static charge

Poor insulator

Low static charge

Good insulator

Melting Point
130–171 °C 115–135 °C
Chemical Formula
(C3H6)n (C2H4)n
Uses
Fibers, films, caps, hinges, synthetic paper Plastic bags, bottles, food containers, pallets, geomembranes, films made of plastic, crates, etc.
Density

0.855 g/cm3 amorphous

0.946 g/cm3 crystalline

0.88–0.96 g/cm3
Relative Cost
Low Medium

NIRS as a tool to assess the quality of PP and PE

For over 30 years, near-infrared spectroscopy (NIRS) has been an established method for fast and reliable quality control within the PP/PE industry. Despite this, many producers still do not consistently consider the implementation of NIRS in their QA/QC labs. Limited experience regarding application possibilities or a general hesitation about implementing new methods are some of the reasons behind this.

The advantages of using NIR spectroscopy for QA/QC are numerous. One major advantage of NIRS is the determination of multiple parameters in just 30 seconds with no sample preparation! The non-invasive light-matter interaction used by NIRS, influenced by physical as well as chemical sample properties, makes NIRS a suitable method for the determination of several critical quality parameters in these polymers and many more.

In the remainder of this article, a short overview on PE and PP applications are presented, followed by available turnkey solutions for PE and PP, developed according the NIRS implementations guidelines of ASTM E1655-17.

Did you miss the first part in this series about NIRS as the ideal QC tool for the polymer industry? Find it here!

For more detailed information about NIRS as a secondary technique, read our previous blog posts on this subject.

Applications and parameters for PE and PP with NIRS

During production of PE and PP it is important to check certain parameters to guarantee the quality. These parameters include the density to classify the PE type, copolymer level to enhance certain properties like strength and solvent resistance, and melt flow rate to make sure PP can be formed to the intended shape.

The most relevant applications for NIRS analysis of PE and PP are listed in Table 2.

Table 2Available application notes for use of NIRS for PE and PP
Polymer Parameter Related NIRS Application Notes
Polyethylene (HDPE/LDPE)

Identification, Density, Melt Flow Index, Copolymer level

AN-NIR-083

AN-NIR-081

AN-NIR-034

AN-NIR-003

Polypropylene (PP)

Identification, Melt Flow Index, Additives

AN-NIR-083

AN-NIR-082

AN-NIR-034

AN-NIR-004

Where can NIRS be used in the production process of PE and PP?

Figure 2 shows the individual production steps from the plastic producer via plastic compounder and plastic converter to the plastic parts producer. The first step in which near-infrared lab instruments can be used is when the pure polymers like PE and PP are produced, and their purity needs to be confirmed. NIRS is also a very useful technique during the next step where polymers are compounded into intermediate products to be used for further processing.

Figure 2. Illustration of the polyethylene/polypropylene production chain.

Easy implementation of NIR spectroscopy for plastic producers

Metrohm has extensive expertise with analysis of PE and PP and offers a turnkey solution in the form of the DS2500 Polymer Analyzer. This instrument is a ready-to-use solution to determine multiple quality parameters in PE and PP.

Figure 3. Turnkey solutions for PE and PP analysis with the Metrohm DS2500 Polymer Analyzer.

Application example:

Turnkey solution for the determination of Melt Flow Rate (MFR) of PP

The Melt Flow Rate of polypropylene pellets is an important parameter to measure so that PP can be formed in the intended shape. The model created with the chemometric software is based on a large collection of real product spectra and is developed in accordance with ASTM E1655-17 Standard practices for Infrared Multivariate Quantitative Analysis. For more detailed information on this topic, download the free white paper.

To learn more about pre-calibrations for PP, download our brochure and visit our website.

The result of this turnkey solution for the non-destructive determination of Melt Flow Rate of PP without rheological tests is shown in Figure 4.

Figure 4. Turnkey solution for Melt Flow Rate of PP using the Metrohm DS2500 Polymer Analyzer. A: Sampling and analysis of PP pellets. B: Results of MFR from NIRS compared to a primary laboratory method along with the Figures of Merit (FOM) for this analysis.

This solution demonstrates the feasibility of NIR spectroscopy for the analysis of MFR in polypropylene samples. The standard procedure (ASTM D1238) requires a significant amount of work including packing the sample, preheating, and cleaning. With no sample preparation or chemicals needed, Vis-NIR spectroscopy allows the analysis of MFR in less than a minute.

Learn more about the procedure in our free Application Note!

Future installments in this series

This article is a detailed overview of the use of NIR spectroscopy as the ideal QC tool for the analysis of polypropylene and polyethylene. Future installments in this series will be dedicated to:

  • Polyamide (PA)
  • Polyols and Isocyanates to produce Polyurethane (PU)

For more information

About spectroscopy solutions provided by Metrohm, visit our website!

We offer NIRS for lab, NIRS for process, as well as Raman solutions

Post written by Wim Guns, International Sales Support Spectroscopy at Metrohm International Headquarters, Herisau, Switzerland.

The importance of titrations in pharmaceutical analysis

The importance of titrations in pharmaceutical analysis

If you are in the pharmaceutical industry and wonder if a conversion from a manual titration to an automated titration is suitable for your work, this blog post should give you all the answers you need.

I will cover the following topics in this article (click to go directly to the topic):

Applicability of modern titration methods in pharmaceutical analysis

Perhaps you have already heard or read about automated titration and its benefits in comparison to manual titration, but are now wondering whether those guidelines are also applicable to pharmaceutical analysis.

Getting straight to the point: Yes, it is true that many USP monographs as well as USP General Chapter <541> Titrimetry still refer to the manual visual endpoint titration. But there’s good news! USP-NF General Notices and Requirements Section 6.30 states:

As long as the alternative method is fully validated and you can prove that both methods are equivalent, you are allowed to use alternative methods.

Since titration still plays an important role in pharmaceutical analytical procedures and processes, Metrohm offers a variety of applications for innumerous API monographs of the United States Pharmacopeia as well as pharmacopeia-compliant analytical instruments.

Automated titration procedure

Have you wondered about how to perform the procedure of an automated titration—how does it differ from a manual titration? Working with a pharmacopeia compliant analytical instrument from Metrohm is not so different:

 

  1. Titrant is added with an automated piston buret that safely controls the delivery of titrant to a precise level.
  2. The sample is homogenized with a stirrer.
  3. The electrode detects the titration endpoint, removing subjectivity of color changes.
  4. Results are automatically calculated and displayed allowing no room for human error.
Figure 1. Anatomy of an automatic titrator.

As shown in Figure 1, an automated titration procedure mainly consists of four steps. These steps are repeated until the end of the titration (Figure 2).

In addition, all Metrohm devices that run with proprietary tiamo® or OMNIS® software are 21 CFR Part 11 compliant meeting all ALCOA+ requirements. Thanks to improvements in productivity, accuracy, and precision, the human influence on analysis is reduced to a minimum.

Figure 2. The titration cycle illustrating the different steps in an automated titration procedure.

If you are wondering how to transfer a manual titration to automated titration, then check out our earlier blog posts on this topic. Also, download our free white paper comparing manual and automated titration.

Choice of electrodes for pharmaceutical titrations

For autotitration, either an electrode or a photometric sensor is used to detect the point of a sample analyte neutralization. Metrohm offers a wide range of different electrodes for titrations that are extremely suitable for various pharmaceutical applications. The electrode choice depends on the type of reaction, the sample, and the titrant used.

Download our free brochure to learn more.

If you want to know more about how endpoints are recognized using electrodes or photometric sensors, read our previous blog post to find out how the endpoint is determined during an autotitration.

Maybe you are not quite sure which is the best electrode for your application. Therefore, Table 1 shows an interactive electrode guide for different pharmaceutical titrations.

Type of titration Electrode Close-up view Pharma Application / API

Aqueous acid/base titrations

e.g. titrant is NaOH or HCl

phenolphthalein indicator

Combined pH electrode with reference electrolyte c(KCl) = 3 mol/L

e.g. Ecotrode Plus, Unitrode

Water-soluble acidic and basic active pharmaceutical ingredients (API) and excipients

API: Benzbromaron, Potassium carbonate, Potassium bicarbonate

Non-aqueous acid/base titrations

e.g. solvent is organic or glacial acetic acid

crystal violet indicator

Combined pH electrode with alcoholic reference electrolyte LiCl in EtOH

e.g. Solvotrode easyClean

Water-insoluble weak acids and bases

Assay of API

Acid value (free fatty acids)

API: Caffeine, Ketoconazole

Redox titrations

e.g. titrant is sodium thiosulfate

starch indicator

Pt metal electrode

e.g. combined Pt ring electrode, Pt Titrode

 

Antibiotic assays

Peroxide value in fats and oils

API: Captropril, Paracetamol, Sulfonamide

Precipitation titrations

e.g. titrant is silver nitrate

ferric ammonium sulfate indicator

Ag metal electrode

e.g. combined Ag ring electrode, Ag Titrode

Chloride content in pharmaceutical products

Iodide in oral solutions

API: Dimenhydrinate

Complexometric titrations

e.g. titrant is EDTA

hydroxy naphthol blue indicator

Ion-selective electrode

e.g. combined calcium-selective electrode with polymer membrane

Calcium content in pharmaceutical products

API: Calcium succinate

Photometric titration

e.g. titrant is EDTA

Eriochrome black T indicator

Photometric sensor

e.g. Optrode

Assay of various metal salts in APIs

API: Chondroitin sulfate, Bismuth nitrate, Zinc sulfate

Table 1. Electrode guide for pharmaceutical titrations.

To help you select the best electrode for your titrations, we have prepared a poster for you to easily find the perfect electrode for USP monographs. Additionally, you will find information about proper sensor maintenance and storage.

If you prefer, the Metrohm Electrode Finder is even easier to use. Select the reaction type and application area of your titration and we will present you with the best solution.

As documentation and traceability are critical for the pharmaceutical industry, Metrohm has developed fully digital electrodes, called «dTrodes». These dTrodes automatically store important sensor data, such as article number and serial number, calibration data and history, working life, and the calibration validity period on an integrated memory chip.

Conclusion

Metrohm is your qualified partner for all chemical and pharmaceutical analysis concerns and for analytical method validation.

In addition to full compliance with official directives, Metrohm instruments and applications comply with many of the quality control and product approval test methods cited in pharmacopoeias. Discover the solutions Metrohm offers the pharmaceutical industry (and you in particular!) for ensuring the quality and safety of your products.

Learn even more about the practical aspects of modern titration in our monograph and visit our Webinar Center for informative videos.

Need a reason to switch

from manual to automated titration?

How about FIVE?

Post written by Doris Hoffmann, Product Manager Titration at Metrohm International Headquarters, Herisau, Switzerland.

NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 1

NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 1

Undoubtedly, there is a trend nowadays towards stricter quality assurance and quality control in production processes, such as in the polymer industry. At the same time, this trend is accompanied by a stronger focus on cost-saving and time-efficient methods so that performing more testing will not automatically result in higher costs. 

Major driving factors for companies to voluntarily implement more testing and quality practices include: 

  • Cost pressure. Testing can reveal out-of-specification products, allowing production to be stopped in plenty of time, eliminating excess manufacturing costs.
  • Increased competition. Quality practices provide a competitive edge and can be used as a marketing tool to raise brand value.
  • Scarcity of resources. Qualified staff are difficult to find; therefore, checks that can be carried out by non-specialists are invaluable.

Near-infrared (NIR) spectroscopy is an analytical method that addresses the above drivers and is particularly suited for making quality control more efficient and cost-effective as shown in this article. A short overview of NIRS is presented, followed by application examples for the quality control of polymers, concluding with indications and examples regarding how polymer producing and processing companies can benefit from the utilization of NIRS.

NIR technology overview

The interaction between light and matter is a well-known process—just recall the last time your skin was sunburned. Depending on the applied light intensity and energy, the interaction can be destructive (as with a sunburn) or harmless (like radio waves). Light used in spectroscopic methods is typically not described by the applied energy, but in many cases by the wavelength or wavenumbers.

A NIR spectrometer such as the Metrohm DS2500 Polymer Analyzer measures this interaction between light and matter to generate spectra as displayed in Figure 1. NIRS is especially sensitive to the presence of certain functional groups including -CH, -NH, -OH, and -SH. Therefore, NIR spectroscopy is an ideal method to quantify chemical parameters like water content (moisture), hydroxyl value, acid number, and amine content, just to name a few. Furthermore, the interaction is also dependent upon the matrix of the sample itself, which allows the detection of physical and rheological parameters like density, intrinsic viscosity, and melt flow rate.

Figure 1. Nylon and polyethylene spectra resulting from the interaction of NIR light with the respective samples.

All this information is contained in just one spectrum, making this method suitable for quick multiparameter analysis. Solid samples, such as powders, are secured within an appropriate container or vial (Figure 2a) then placed as-is on the analyzer. Heterogeneous samples, such as polymer pellets, can be analyzed using larger measurement cups (Figure 2b).

Figure 2. Solid sample placement for NIR spectra measurement. A) Direct measurement of powders in a vial. B) Large heterogeneous sample such as pellets can be analyzed using large sample cups.

Learn more about the DS2500 Polymer Analyzer on our website!

The measuring mode is referred to as «diffuse reflection», generally an appropriate procedure for analyzing granules, fibers, flakes, and both coarse and fine powders. For diffuse reflection (Figure 3), the NIR light comes from below the sample, penetrating and interacting with it, while being partially absorbed. Unabsorbed NIR light reflects to the detector. In less than 1 minute, the measurement is completed, and the results are displayed.

Figure 3. Schematic display of the light path interacting with a sample during diffuse reflection.

The procedure to obtain NIR spectra already highlights two major advantages of NIR spectroscopy: simplicity regarding sample measurement and speed.

  • Fast technique with results in less than a minute.
  • No sample preparation required – measure sample as-is.
  • Low cost per sample – no chemicals or solvents needed.
  • Environmentally friendly technique – no waste generated.
  • Non-destructive – precious samples can be reused after analysis.
  • Easy to operate – inexperienced users are immediately successful.
Read our previous blog posts to learn more about NIRS as a secondary technique.

What kinds of polymer manufacturers in the production chain might benefit from using NIR spectroscopy?

Figure 4 illustrates the individual production steps from the plastic producer, via plastic compounder and plastic converter to the plastic parts producer. The first step in which near-infrared lab instruments can be used is when pure polymers are produced, and their purity requires confirmation. NIRS is also a very useful technique for the next step, when polymers are compounded into products to be used for further processing. 

Figure 4. Simplified illustration of the polymer production chain.

A plastic part producer, typically an injection molding or extrusion company, assesses the quality of the received polymer batches. In many cases, the certificate from the supplier is trusted without any further verification. However, a rapidly growing number of companies that create products for the medical industry or that produce parts of high value or in high quantities have started to assess the important rheological quality parameters of each polymer batch before using it for injection molding or extrusion. Feeding an out-of-specification polymer to the production process leads to costly standstill of the equipment and its time-consuming cleaning.

Here, a quick pre-check of the starting polymer material used in the process would be ideal to avoid such risks and potential downtime. For this purpose, NIRS is the ideal solution because it is fast, has low running costs, and can be operated by personnel without any extensive chemical education.

When the final part is created at the end of the production process, it can also be subjected to NIR spectroscopic investigations for quality control. This is useful for assessing the homogeneity or thickness of bottles or sheets of material, for example.

What kinds of polymer applications and parameters are possible with NIRS in general?

In principle, NIRS analysis is more suitable for measuring bulk materials and not for trace analysis. Furthermore, polymer samples should contain no more than 3% carbon black and a reference method should be available. When complying with these prerequisites, NIR spectroscopy can be used as a fast and cost-saving alternative measurement technology.

Metrohm Application Bulletin 414 describes several applications for the polymer industry that can be carried out with the aid of NIRS instruments. This document contains analyses of a wide range of parameters in a very large array of samples.

Examples for use of NIR spectroscopy for selected polymers are indicated in Table 1.

Table 1. Examples for use of NIRS for selected polymers.
Polymer type Parameter Conventional analysis method Advantage of using NIRS Related NIRS Application Note
Polyethylene (HDPE/LDPE) Density Densimeter Results within 30 seconds AN-NIR-003
AN-NIR-081
Melt Flow Index MFI apparatus

Time-saving

No cleaning of equipment

AN-NIR-083
Polypropylene (PP) Melt Flow Index MFI apparatus

Time-saving

No cleaning of equipment

AN-NIR-004
AN-NIR-082
AN-NIR-083
Polyamide (PA) Intrinsic Viscosity Ubbelohde viscosimeter

No time-consuming dissolution in hazardous chemicals

No waste

Cost savings

AN-NIR-005
AN-NIR-060
COOH, NH2, Moisture Titration

Time & cost savings

No chemicals needed

No chemically educated operator needed

AN-NIR-077
Polyethylene terephthalate (PET) Intrinsic Viscosity Ubbelohde viscosimeter

No time-consuming dissolution in hazardous chemicals

No waste

Cost savings

AN-NIR-023
Acid number Titration

Time & cost savings

No chemicals needed

No chemically educated operator needed

Isophtalic acid HPLC

No eluent solvents needed

Time & cost savings

No chemically educated operator needed

Polyurethane (PU) OH of polyols Titration

Time & cost savings

No chemicals needed

No chemically educated operator needed

AN-NIR-006
AN-NIR-007
Isocyanate content Titration AN-NIR-035
AN-NIR-065
AN-NIR-068
Polyvinyl Alcohol (PVA) Degree of alcoholysis Titration

Time & cost savings

No chemicals needed

No chemically educated operator needed

AN-NIR-076
Silicone Rubber Vinyl content Gas Chromatography

Time & cost savings

No chemicals needed

No chemically educated operator needed

AN-NIR-084
Polyvinylidene Chloride (PVDC) Sheet thickness Weighing

Time-saving

Reduced user error risk

AN-NIR-092

Save up to 90% on running costs with NIRS

Underestimation of quality control processes is one of the major factors leading to internal and external product failure, which have been reported to cause a loss of turnover between 10–30%. As a result, many different norms are put in place to support manufacturers with their QC process. However, time to result and the associated costs for chemicals can be quite excessive, leading many companies to implement near-infrared spectroscopy in their QC process.

Our free white paper illustrates the potential of NIRS and displays cost saving potentials up to 90%.

Future installments in this series

This article is only a general overview of the use of NIR spectroscopy as the ideal QC tool for the polymer industry. Future installments will be dedicated to the most commonly produced and commercially important polymers and will include much more detailed information. These polymers are:

  • Polyamide (PA)
  • Polyols and Isocyanates to produce Polyurethane (PU)

For more information

About spectroscopy solutions provided by Metrohm, visit our website!

We offer NIRS for lab, NIRS for process, as well as Raman solutions

Post written by Wim Guns, International Sales Support Spectroscopy at Metrohm International Headquarters, Herisau, Switzerland.

Recognition of endpoints (EP)

Recognition of endpoints (EP)

Like many of you, I gained my first practical titration experience during my chemistry studies in school. At this time, I learned how to perform a manual visual endpoint titration – and I can still remember exactly how I felt about it.

Using a manual buret filled with titrant, I added each drop individually to an Erlenmeyer flask that contained the sample solution (including the analyte to be measured) and the indicator which was added prior to the titration. With each drop and even slight color change of my sample solution, minutes passed with increasing uncertainty. I asked myself, «Have I already reached the true endpoint, should I add another drop, or have I even over-titrated?» You have probably been in the same situation yourself!

Sound familiar to you? Don’t forget to check out our other blog post about the main error sources in manual titration!

Several years have passed since then, and I am glad that I no longer have to face the challenges of performing a manual titration because Metrohm offers the possibility of automated titrations.

If you want to know how to determine the endpoint in an automated titration, I will give you all the answers you need. In the following article I will cover these topics (click to go directly to each):

    Different detection principles – an overview

    At this point you may ask yourself—if not visually, how the endpoint (EP) can be detected in an automated titration? Well, aside from the visual endpoint recognition (e.g., by a color change, the appearance of turbidity, or appearance of a precipitate), a titration EP can also be detected by the automated monitoring of a change in a chemical or physical property which occurs when the reaction is complete.

    As shown in the table below, there are many different detection principles:

    Now, let’s discuss the potentiometric and photometric EP determination in comparison to a visually recognized EP detection as they are the most commonly used determination principles for automated titrations. If you’d like to learn more about the principles of thermometric titration, read our blog post about the basics!

    Potentiometric principle

    As shown in the table above, in the potentiometric principle the concentration dependent potential (mV) of a solution is measured against a reference potential. Therefore, a silver-silver chloride (Ag/AgCl) reference electrode is used in combination with a measuring electrode (pH sensitive glass membrane or metal ring). In general, a combined sensor (electrode) including both measuring and reference electrode is used.

    Figure 1 illustrates with a simple example how a manual titration with a color change looks when being converted to an automatic system.

    Figure 1. Illustration of the same titration performed manually (left) and automatically (right).

    Step 1: Beginning of the titration before titrant is added.

    Step 2: Addition of titrant – as the titration approaches the endpoint you begin to see signs of the color change. At this point in an automatic titration the sensor will detect a change in mV signal and the titrator begins dosing the titrant in smaller volumes and at a slower rate.

    Step 3: Finally, the EP is reached with a faint pink color which corresponds with the inflection point in the titration curve.

    Step 4: Titrating beyond the endpoint leads to over titration, and here the mV signal is fairly constant.

    This is how you achieve the characteristic S-shaped titration curve you see when performing an automated titration.

    Not only acid-base titrations can be converted. Figure 2 shows how a simple chloride titration can be converted. The titrant, titrant concentration, sample size, and sample preparation remain the same.

    Figure 2. Illustration of a chloride titration – conversion from manual to automatic analysis.

    Only the indicator is replaced by the Ag Titrode, a silver ring electrode, and we get a titration curve (Figure 2, right side) with a clearly defined endpoint.

    For more examples of possible potentiometric titrations, download our free monograph «Practical Titration» or check out our Application Finder where you can find several examples for all endpoint recognition principles.

    Photometric Principle

    Titrations using color indicators are still widely used e.g. in pharmacopeias. When performed manually, the results depend, quite literally, on the eye of the beholder. Photometric titration using the Optrode makes it possible to replace this subjective determination of the equivalence point with an objective process that is completely independent of the human eye.

    The advantage here is that the chemistry does not change – that is, the standard operating procedure (SOP) generally does not have to be adapted. 

    The basis of photometric indication is the change in intensity at a particular wavelength of a light beam passing through a solution. The transmission is the primary measured variable in photometry, and is given by the light transmission (mV or % transmission) of a colored or turbid solution that is measured with a photometric sensor such as the Optrode from Metrohm.

    There are eight possible wavelengths to choose from that span nearly all color indicators used for titrations (see table below). The shaft is solvent resistant and there is no maintenance required. It connects directly to the titrator and improves accuracy and repeatability of color indicated titrations.

    I’ve also picked an example to show you how to convert an EDTA titration of manganese sulfate from manual titration to automated titration. Like in the example above, the procedure remains the same.

    Are you ready to take the leap and switch to using an automated titration system? Read our other blog post to learn more about how to transfer manual titration to autotitration.

    One advantage of automated titration is that a lower volume of chemicals is needed, resulting in less waste. With the same indicator Eriochrome Black TS, the Optrode is used at a wavelength of 610 nm. The titration curve (Figure 3, right side) shows a large potential change of the mV signal indicating a clearly defined titration endpoint.

    Figure 3. Illustration of the photometric EDTA titration of manganese sulfate according to USP.

    If you are not sure what the optimal wavelength for your titration is, then have a look at our blog post about photometric complexometric titration to learn more!

    Comparison: Optrode vs. potentiometric electrodes

    When you decide to make the switch to automated titration, there are some points to consider when comparing the Optrode with other Metrohm potentiometric electrodes. The following table lists the main criteria.

    1Optrode has a working life of tens of thousands of hours.

    You see, an autotitration is quite simple to perform and has the great advantage that a clearly defined endpoint is given.

    Believe me, whenever I`m working with such a device including a suitable electrode for an automatic titration, I have a big smile on my face thinking back to my university days: Bye bye subjectivity, time-consuming procedure, economic inefficiency and non-traceability!

    Maybe you are now also convinced to make the change in your laboratory.

    Save more money

    with automated titration

    Read our blog post to find out more.

    Post written by Doris Hoffmann, Product Manager Titration at Metrohm International Headquarters, Herisau, Switzerland.