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Avoiding the most common mistakes in pH measurement

Avoiding the most common mistakes in pH measurement

If you’re reading this, then I’m sure you have already performed at least one pH measurement in your lifetime, since it is one of the most important parameters in analytical chemistry. I remember my first contact with a potentiometric pH meter and a pH electrode – and I can still remember how I felt back then.

I was young and completely unsure how I should handle the instrument and the electrode. Was I doing everything correctly? Consequently I had many questions about the best practices.

Today, I am much more confident! Therefore, I would love to share with you some of the most common uncertainties and mistakes I see during my daily work when potentiometric pH measurements are performed. By the end of this article, I am certain that you will agree with me: pH measurement can be just as easy as it looks. I will cover the following topics (click to go directly to each topic):

Is this the correct electrode for your application?

Troubleshooting already starts before you put the sensor into your sample solution. A wide variety of electrodes are available on the market, and it can be quite difficult to determine which electrode is the best for your application. Many different diaphragm types as well as glass membrane materials exist:

We’ve prepared a flyer for you to help find the perfect electrode for your application. Additionally, we have provided valuable information about maintenance and storage. You can download the flyer in several languages: English, German, French, or Spanish.

What’s most important when preparing the electrode for calibration or measurement?

Before starting your measurement, check the electrode for cracks or contaminations. Open the plug to ensure that the electrolyte can flow out (otherwise you may observe unstable results), and check the level of the electrolyte.

The electrolyte should always be filled up to the opening in order to ensure an outflow from the hydrostatic pressure. If the level of the sample is higher than the level of electrolyte within the sensor, then sample will enter the reference system of your electrode. This causes the reference potential to shift, and results are no longer reproducible.

Make sure that you insert your sensor deep enough into the sample. At least the glass membrane and the diaphragm need to be covered, as shown in this example.

Calibration: When is it necessary, and what must I consider?

Calibrations must be performed on a regular basis. Depending on the number of measurements and the sample matrix, I recommend calibrating at least weekly. If used often, or if the sample matrix is contaminating the sensor, then you should calibrate daily or even more frequently. Of course you should always calibrate your sensor if you have received a new one, after maintenance, or after a longer storage period.

For calibration, consider the following points:

  • Always use fresh (not expired) buffers – the calibration can only be as good as the buffers used!
  • Perform at least a 2-point calibration.
  • Your sample pH should be within the calibration buffer pH value.
  • Always measure the temperature, as the pH value is temperature-dependent.
  • Most manufacturers already include buffer table templates with their instruments. Make sure that you select the correct one.

How should you store the pH electrode?

The correct storage of the pH electrode can increase its lifetime significantly. Never store the pH electrode dry! The glass membrane builds up a hydration layer, which is necessary for proper pH measurement. If you store the electrode dry, this hydration layer will be destroyed. Even though the layer can be recovered by conditioning the sensor in deionized water, the sensor will become slower.

For electrodes filled with potassium chloride (c(KCl) = 3 mol/L) as reference electrolyte, we have developed a dedicated storage solution which keeps the glass membrane in top quality without impairing the performance of the diaphragm.

The figure above shows how quickly the sensor responds when placed in a sample after a storage period. You can clearly see that storing the sensor in the dedicated solution leads to a much faster response time in comparison to storage in c(KCl) = 3 mol/L. This means even more productivity and less waiting.

All electrodes which are filled with another reference electrolyte than c(KCl) = 3 mol/L are stored in their reference electrolyte.

How should the pH electrode be cleaned?

Between the measurements, the electrode must be rinsed well with deionized water. If the sample is sticky or contains proteins, use a suitable solvent to remove the contamination. From time to time, it is important to give the electrode a «special treat» and clean it with the pHit Kit, shown below. This set includes everything that is necessary to gently and efficiently clean the electrode.

Very important: Never wipe the sensor off with a tissue! Similar to rubbing the surface of a balloon, you will charge the surface of the glass membrane. The built-up electrostatic energy will influence your measurement, which will get significantly longer. Additionally, you can scratch the sensitive glass membrane surface, thus destroying it.

To stir or not to stir?

Depending on the electrode type you are using, it is recommended to always stir constantly, at the same speed, during analysis. The following graph illustrates why:

The upper curve shows the measurement with an electrode having a fixed ground-joint diaphragm, and the lower curve utilized a very common electrode with a ceramic pin diaphragm. 

Not only does the top electrode show less signal noise, the signal remains nearly unchanged once the stirrer is switched off. However, there is a significant signal drop for the ceramic pin diaphragm (bottom). Therefore, the stirring speed should be identical for all buffers and samples to minimize such effects. 

Is my electrode still ok to use?

To get an idea about whether your electrode is still ok to use or not, it is generally enough to check the slope and the pH(0) after calibration. The slope should be between 95–103%, whereas the pH(0) should lie between pH 6.8–7.2. Further information can be gained if a pH electrode test is performed, which is implemented in some of Metrohm’s instruments, or a test according to application bulletin AB-188.

If the electrode does not meet the specifications, clean it according to the instructions and perform the test again. If the sensor still does not pass, a replacement is inevitable.

Check out our webinars:

«Basic of pH measurements» or «Troubleshooting of pH measurement

You can also download our whitepaper WP-003 «pH measurement: Six technical tips»  for free: 

Post written by Dr. Sabrina Gschwind, Jr. PM Titration (Sensors) at Metrohm International Headquarters, Herisau, Switzerland.

Benefits of NIR spectroscopy: Part 1

Benefits of NIR spectroscopy: Part 1

This blog post is part of the series “NIR spectroscopy: helping you save time and money”. 

People who are unfamiliar with near-infrared (NIR) spectroscopy frequently ask the question: “Why should I need to know more about this technique, and how can I benefit from it?”.

In this first installment of in this series of posts, we focus on the main advantages of NIRS over conventional wet chemical analysis methods and will provide examples of the types of parameters that can be measured with NIR spectroscopy.

Solid vs. liquid samples

In order to understand the benefits of NIRS, a good starting point is to understand how the NIR spectrum is measured. NIR spectroscopy can be used to analyze different types of samples. However, different instrumentation is required depending on the sample type. Several measurement methods are available for samples ranging from clear liquids to opaque pastes and powders. Choosing the right measurement method, sampling module, and accessories is the most important step to developing robust NIR methods. Below, the different methods are shown for various sample types (diffuse reflection, diffuse transmission, transflection, and transmission).

Diffuse reflection: Cream, paste, granulates, coarse & fine powders

NIR light penetrates into and interacts with the sample, and the unabsorbed NIR energy reflects back to the detector. This method is most suitable to measure solid samples without sample preparation.

Diffuse transmission: Tablets and capsules

As with diffuse reflection, the NIR light penetrates into and interacts with the sample. This light is scattered throughout the sample, due to interaction with the particles. The unabsorbed NIR light is transmitted through the sample prior to reaching the detector. This method is most suitable to measure solid dosage forms without sample preparation.

Transflection: Liquids and gels

This measurement method is a combination between transmission and reflection. A reflector is placed behind the sample, used to reflect the unabsorbed NIR light back to the detector. This method is most suitable to measure liquid samples.

Transmission: Liquids

In this situation, the sample is placed between the NIR light source and the detector. NIR light is transmitted through the sample, and any unabsorbed NIR energy continues to the detector. This method is most suitable to measure clear liquid solutions or suspensions.

Solid sample measurement

Solid samples (such as powders) must be placed on the window as shown here, secured within an appropriate container or vial. The instrument lid needs to be closed prior to starting the analysis so external light does not affect the results.

The NIR radiation comes from below, and is partially reflected by the sample to the detector, which is also located below the sample vessel plane. After 45 seconds, the measurement is completed and a result is displayed. As this reflected light contains all the relevant sample information, this measurement technique is called diffuse reflection.

Liquid sample measurement

As the image illustrates, for liquid analyses via NIRS, a vial or cuvette must be placed in the drawer of the instrument. After pressing start, the drawer closes automatically and a result is obtained after 45 seconds.

In this case, the NIR radiation travels through the solution before reaching the detector. This measurement technique is known as transmission.

Advantages of NIRS

The procedure for obtaining the NIR spectrum already indicates two main advantages of NIRS: simplicity regarding sample measurement and speed. These and other advantages of NIR are listed here:

  • Fast technique with results in less than 1 minute.

  • No sample preparation required – solids and liquids can be used in pure form.

  • 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.

How to quantify with NIRS

NIRS is a secondary technique, which means a prediction model will need to be created first. You can compare this, for example, to HPLC. If you want to identify or quantify a substance with that technique, you would need to prepare standard solutions of the substance and measure them to create a calibration curve.

This is similar with NIRS: first you need to measure a number of spectra with known concentrations or known parameter values gathered from a primary method such as titration. A prediction model is then created out of these spectra using chemometric software, e.g. the Metrohm Vision software. We will explain in more detail how prediction models are created in another installment of this series.

    Application versatility in all industries

    NIRS is a versatile technique and can be used for various applications, both for chemical and physical parameters. You can find many different application examples for NIR in the Metrohm Application Finder. Here, we have listed representative examples for some industry segments.

    • Polymers: Density of Polyethylene (PE); Melt Flow Rate; Intrinsic Viscosity

    • Chemical: Hydroxyl number of polyols

    • Petrochemical: Research Octane Number (RON) of gasoline; cetane index for diesel

    • Oils and Lubricants: Total Acid Number (TAN)

    • Pharma: Water content of lyophilized products; content uniformity in tablets

    • Personal care: Moisture content and active ingredients in creams

    Overall, near-infrared spectroscopy is a robust alternative technique for the determination of both chemical and physical parameters in solids and liquids. It is a fast method which can also be successfully implemented for routine analysis by staff without any laboratory education. 

    In the next installment we will answer another frequently asked question: “Is near-infrared the same as infrared spectroscopy?”.

    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 Dr. Dave van Staveren, Head of Competence Center Spectroscopy at Metrohm International Headquarters, Herisau, Switzerland.

    Why your titration results aren’t reproducible: The main error sources in manual titration

    Why your titration results aren’t reproducible: The main error sources in manual titration

    In the practical course in Analytical Chemistry during my first semester at university, I had to titrate a lot. Thinking back on it, I remember carefully dosing titrant with the glass buret, the cumbersome process of refilling the buret, and the constant suspicion that I hadn’t correctly chosen the endpoint.

    Everyone in class kept getting different results—but we were never quite sure why. At the time, I wasn’t as experienced as I am now. Today, after 10 years of experience in titration, I’ve learned that the results of manual titration depend quite a lot on the person carrying it out. Here are the top error sources in manual titration and how you can avoid them.  

    Choosing the right indicator I’m sure you’ve learned at some point that the pH value of the titration endpoint depends on the acid dissociation constant (Ka) of the acid and base that are used. If a strong base is titrated by a strong acid, the pH value at the endpoint is around 7. The titration of a strong base with a weak acid shifts the endpoint towards the alkaline range. The titration of a strong acid with a weak base will result in an endpoint in the acidic range. This explains why several different indicators are used in acid-base titrations. But which is the right one to choose?
    The chart above shows some of the most frequently used pH indicators. You can probably imagine that you won’t get correct results when the pH of your endpoint is around 7, but you use crystal violet or methyl orange as the indicator. Luckily, most standards and SOPs specify an indicator. Follow the instructions, and you’re on the safe side!

    Endpoint recognition is subjective

    The problems really start when you try to recognize the endpoint. Have you ever thought about the nuances of the color change?

    Above, you see five stages of an acid-base titration of c(HCl) = 1 mol/L with c(NaOH) = 1 mol/L. The only difference between each image and its predecessor is one additional drop of titrant. Where would you choose the endpoint in this case?

    Is the endpoint reached in picture 1, where only a faint pink is visible? Or is it reached in picture 3 where the color becomes more intense? Or even in picture 5, at which point the pink color is most vibrant? Between picture 1 and picture 5, just four drops of titrant were added. With the pharmaceutical definition of a drop as a volume of 50 µL, this corresponds to 200 µL of titrant or about 7.3 mg of hydrochloric acid—an enormous error.

    Reading the buret volume

    Do you remember how to correctly read the buret? You have to stand on a footstool and make sure that you read the meniscus value horizontally. Do you know why?

    The volume reading depends upon the angle from which you view the buret. In the case shown here, the readings vary up to 0.2 mL (200 µL) from the actual value, depending on the reading angle. The more your line of sight deviates from the horizontal, the more inaccurate the reading—and the result. You can assume an average error of 200 µL. This is a lot for a titration, as I showed in the previous example!

    Improving objectivity and accuracy

    How can you eliminate these errors? The easiest one to overcome is the reading error. The solution for this is to use an electronic buret. When using an electronic buret, all you need to do is fill it with the titrant and then you press a button. The device automatically measures the volume and gives you a digital readout. Using an electronic buret ensures already a high level of objectivity for your results.

    It also improves the accuracy of your results. I don’t have to tell you how important accuracy is in analytical chemistry, but I’ll give an example. Imagine you determined the purity of gold at 90%, but in reality, it’s 99% pure. You would lose a lot of money when selling your gold under this pretense!

    Earlier, I showed that visual endpoint recognition using a color indicator can result in errors of up to 200 µL. An inaccurate buret reading can lead to an additional 200 µL error. While using an electronic buret doesn’t help you achieve a more objective endpoint recognition, it does reduce the minimum volume addition per drop: it’s no longer 50 µL, but can be as small as 0.25 µL depending on the cylinder volume you use. This substantially lowers the error resulting from endpoint recognition. The following minimum volume additions are common:

    The next step: Automated titration

    If you want to overcome all sources of error described in this post, you’ll have to switch to automated titration, or autotitration. In this case, you will use a sensor to measure pH change in the sample and a mathematical algorithm to detect the endpoint—an indicator isn’t required anymore. Additionally you have the same precision as with the electronic buret.

    Want to learn more?

    Download our free White Paper:

    Manual vs. Automated Titration: Benefits and Advantages to Switching

    Post written by Iris Kalkman, Product Specialist Titration at Metrohm International Headquarters, Herisau, Switzerland.