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Making a better beer with chemistry

Making a better beer with chemistry

Lager or ale? Pale ale or stout? Specialty beer, or basic draft? This week, to celebrate the International Beer Day on Friday, August 7th, I have chosen to write about a subject near and dear to me: how to make a better beer! Like many others, at the beginning of my adult life, I enjoyed the beverage without giving much thought to the vast array of styles and how they differed, beyond the obvious visual and gustatory senses. However, as a chemist with many chemist friends, I was introduced at several points to the world of homebrewing. Eventually, I succumbed.

Back in 2014, my husband and I bought all of the accessories to brew 25 liters (~6.5 gallons) of our own beer at a time. The entire process is controlled by us, from designing a recipe and milling the grains to sanitizing and bottling the finished product. We enjoy being able to develop the exact bitterness, sweetness, mouthfeel, and alcohol content for each batch we brew.

Over the years we have become more serious about this hobby by optimizing the procedure and making various improvements to the setup – including building our own temperature-controlled fermentation fridge managed by software. However, without an automated system, we occasionally run into issues with reproducibility between batches when using the same recipe. This is an issue that every brewer can relate to, no matter the size of their operation.

Working for Metrohm since 2013 has allowed me to have access to different analytical instrumentation in order to check certain quality attributes (e.g., strike water composition, mash pH, bitterness). However, Metrohm can provide much more to those working in the brewing industry. Keep reading to discover how we have improved analysis at the largest brewery in Switzerland.

Are you looking for applications in alcoholic beverages? Check out this selection of FREE Application Notes from Metrohm:

Lagers vs. Ales

There are two primary classes of beer: lagers and ales. The major contrast between the two is the type of yeast used for the fermentation process. Lagers must be fermented at colder temperatures, which lends crisp flavors and low ester formation. However, colder processes take longer, and so fermentation steps can last for some months. Ales have a much more sweet and fruity palate of flavors and are much easier to create than lagers, as the fermentation takes place at warmer temperatures and happens at a much faster rate.

Comparison between the fermentation of lagers and ales.

Diving a bit deeper, there are several styles of beer, from light pilsners and pale ales to porters and black imperial stouts. The variety of colors and flavors depend mostly on the grains used during the mash, which is the initial process of soaking the milled grains at a specific temperature (or range) to modify the starches and sugars for the yeast to be able to digest. The strain of yeast also contributes to the final flavor, whether it is dry, fruity, or even sour. Taking good care of the yeast is one of the most important parts of creating a great tasting beer.

Brewing terminology

  • Malting: process of germinating and kilning barley to produce usable sugars in the grain
  • Milling: act of grinding the grains to increase surface area and optimize extraction of sugars
  • Mashing: releasing malt sugars by soaking the milled grains in (hot) water, providing wort
  • Wort: the solution of extracted grain sugars
  • Lautering: process of clarifying wort after mashing
  • Sparging: rinsing the used grains to extract the last amount of malt sugars
  • Boiling: clarified wort is boiled, accomplishing sterilization (hops are added in this step)
  • Cooling: wort must be cooled well below body temperature (37 °C) as quickly as possible to avoid infection
  • Pitching: prepared yeast (dry or slurry) is added to the cooled brewed wort, oxygen is introduced
  • Fermenting: the process whereby yeast consumes simple sugars and excretes ethanol and CO2 as major products

Ingredients for a proper beer

These days, beer can contain several different ingredients and still adhere to a style. Barley, oats, wheat, rye, fruit, honey, spices, hops, yeast, water, and more are all components of our contemporary beer culture. However, in Bavaria during the 1500’s, the rules were much more strict. A purity law known as the Reinheitsgebot (1516) stated that beer must only be produced with water, barley, and hops. Any other adjuncts were not allowed, which meant that other grains such as rye and wheat were forbidden to be used in the brewing process. We all know how seriously the Germans take their beer – you only need to visit the Oktoberfest once to understand!

Determination of the bitterness compounds in hops, known as «alpha acids», can be easily determined with Metrohm instrumentation. Check out our brochure for more information:

You may have noticed that yeast was not one of the few ingredients mentioned in the purity law, however it was still essential for the brewing process. The yeast was just harvested at the end of each batch and added into the next, and its propagation from the fermentation process always ensured there was enough at the end each time. Ensuring the health of the yeast is integral to fermentation and the quality of the final product. With proper nutrients, oxygen levels, stable temperatures, and a supply of simple digestible sugars, alcohol contents up to 25% (and even beyond) can be achieved with some yeast strains without distillation (through heating or freezing, as for eisbocks).

Improved quality with analytical testing

Good beers do not make themselves. For larger brewing operations, which rely on consistency in quality and flavor between large batch volumes as well as across different countries, comprehensive analytical testing is the key to success.

Metrohm is well-equipped for this task, offering many solutions for breweries large and small.

Don’t take it from me – listen to one of our customers, Jules Wyss, manager of the Quality Assurance laboratory at Feldschlösschen brewery, the largest brewery in Switzerland.

«I have decided to go with Metrohm, because they are the only ones who are up to such a job at all. They share with us their huge know-how.

I can’t think of any other supplier who would have been able to help me in the same way

Jules Wyss

Manager Quality Assurance Laboratory, Feldschlösschen Getränke AG

Previous solutions failed

For a long time, Jules determined the quality parameters in his beer samples using separate analysis systems: a titrator, HPLC system, alcohol measuring device, and a density meter. These separate measurements involved a huge amount of work: not only the analyses themselves, but also the documentation and archiving of the results all had to be handled separately. Furthermore, Jules often had to contend with unreliable results – depending on the measurement procedure, he had to analyze one sample up to three times in order to obtain an accurate result.

A tailor-made system for Feldschlösschen

Jules’ close collaboration with Metrohm has produced a system that takes care of the majority of the necessary measurements. According to Jules, the system can determine around 90% of the parameters he needs to measure. Jules’ new analysis system combines various analysis techniques: ion chromatography and titration from Metrohm as well as alcohol, density, and color measurement from another manufacturer. They are all controlled by the tiamo titration software. This means that bitterness, citric acid, pH value, alcohol content, density, and color can all be determined by executing a single method in tiamo.

Measurement of the overall water quality as well as downstream analysis of the sanitization process on the bottling line is also possible with Metrohm’s line of Process Analysis instrumentation.

Integrated analytical systems with automated capabilities allow for a «plug and play» determination of a variety of quality parameters for QA/QC analysts in the brewing industry. Sample analysis is streamlined and simplified, and throughput is increased via the automation of time-consuming preparative and data collection steps, which also reduces the chance of human error.

Something to celebrate: The Metrohm 6-pack (2018)

In 2018, Metrohm celebrated its 75 year Jubilee. At this time, I decided to combine my experience as a laboratory analyst as well as a marketing manager to brew a series of six different styles of beer for the company, as a giveaway for customers of our Metrohm Process Analytics brand, for whom I worked at the time. Each batch was brewed to contain precisely 7.5% ABV (alcohol by volume), to resonate with the 75 year anniversary. The array of ales was designed to appeal to a broad audience, featuring a stout, porter, brown ale, red ale, hefeweizen, and an India pale ale (IPA). Each style requires different actions especially during the mashing process, based on the type of grains used and the desired outcome (e.g., flavor balance, mouthfeel, alcohol content).

Bespoke bottle caps featuring the Metrohm logo.
The 6 styles of beers brewed as a special customer giveaway to celebrate the Metrohm 75 year Jubilee.

Using a Metrohm Ion Chromatograph, I analyzed my home tap water for concentrations of major cations and anions to ensure no extra salts were needed to adjust it prior to mashing. After some of the beers were prepared, I tested my colleagues at Metrohm International Headquarters in the IC department, to see if they could determine the difference between two bottles with different ingredients:

Overlaid chromatograms from IC organic acid analysis highlighting the differences between 2 styles of the Metrohm 75 year Jubilee beers.

The IC analysis of organic acids and anions showed a clear difference between the beers, allowing them to determine which sample corresponded to which style, since I did not label them prior to shipping the bottles for analysis. As the milk stout contained added lactose, this peak was very pronounced and a perfect indicator to use.

Metrohm ion chromatography, along with titration, NIRS, and other techniques, allows for reliable, comprehensive beer analysis for all.

In conclusion, I wish you a very happy International Beer Day this Friday. Hopefully this article has illuminated the various ways that beer and other alcoholic beverages can be analytically tested for quality control parameters and more  fast, easy, and reliably with Metrohm instrumentation.

For more information about the beer quality parameters measured at Feldschlösschen brewery, take a look at our article: «In the kingdom of beer The largest brewery in Switzerland gets a made-to-measure system». Cheers!

Read the full article:

«In the kingdom of beer – The largest brewery in Switzerland gets a made-to-measure system»

Post written by Dr. Alyson Lanciki, Scientific Editor (and «chief brewing officer») at Metrohm International Headquarters, Herisau, Switzerland.

Best practice for electrodes in titration

Best practice for electrodes in titration

After my earlier blog post on the topic of «Avoiding the most common mistakes in pH measurement», I will now cover the subject of electrodes that are relevant for titration. Here you will not only find out how to select the right electrode for your application – but also how to clean and to maintain it, and most importantly, how you can assure that your electrode is still ok to be used.

The following topics will be covered (click to go directly to the topic):

 

How to select the right sensor

You might wonder what you need to consider when selecting a suitable sensor for your titration, as a huge variety of different sensors exist. The right sensor needs to be selected based on the type of titration that you want to carry out. For a redox titration, you will need a different sensor than for a complexometric titration.

Furthermore, the sensor selection is highly dependent on matrix, the sample volume, or possible interferences. If you are working in non-aqueous media, you must especially consider any electrostatic effects that might arise. Therefore, I recommend working with an electrode that offers an internal electrical shielding.

The sensor has to show a fast response time, and needs to be robust enough for the application, meaning it needs to be resistant to the chemicals used and the applied cleaning procedure.

Table 1. Overview of suggested sensors for various types of titration (click to enlarge).

For further guidelines regarding how to select the right electrode, either consult our online electrode finder or check out our flyer about «Electrodes in titration» which includes practical tips on care and maintenance.

Maintenance and cleaning

Proper cleaning between your titrations is a key factor for obtaining reliable results. The rinsing step has to assure that neither sample nor titrant contaminates the electrode, leading to carry-over and false results. Therefore, between titrations the electrode (as well as buret tip) has to be rinsed with a suitable solvent, such as deionized water, detergent solution, or any other solvent that removes remaining residues. For non-aqueous titrations, it is furthermore important to condition the glass membrane of the electrode in deionized water after each titration.

Furthermore, both the reference and measuring electrode require regular maintenance. For the reference electrode, it is very important that it is filled up to the opening with the correct (and uncontaminated) electrolyte. A daily check of the electrolyte level should be performed, and if necessary, the reference electrolyte should be topped up. Always refill the reference electrolyte level up to the filler opening. This assures a proper electrolyte outflow and reduced contamination of the electrolyte.

Figure 1. Always refill your electrolyte up to the filler opening for the best performance!

In addition to the regular refilling, the electrolyte should be replaced at least on a monthly basis to guarantee a clean electrolyte with the correct concentration (e.g., evaporation of water can increase the concentration of the electrolyte). Usage of old or contaminated electrolyte leads to an undesired change in the measured potential.

Also ensure that the diaphragm is clean, otherwise you might experience a blockage, leading to an unstable potential caused by the missing contact between electrolyte and sample. Figure 2 shows an example of a contaminated diaphragm. Table 2 suggests some possible cleaning agents to remove sticky substances from the diaphragm. After cleaning the diaphragm, always replace the electrolyte.

Figure 2. Close-up view of a dirty diaphragm.
Table 2. Common electrode contaminants and suggested cleaning agents for each situation. Contact your local Metrohm representative for further questions.

The measuring electrode needs a thorough cleaning at least weekly. Uncoated metal ring or ISE electrodes require regular polishing to maintain a quick response. Glass membranes or polymer membranes must not be polished or cleaned with abrasives. If the electrode is used in oily or sticky samples, degreasing or removing proteins might be necessary by using a suitable solvent.

Correct storage of your electrode

Another important point to consider is the right storage for your electrode. Incorrect storage reduces the lifetime of an electrode, and therefore it needs replacement more frequently. Unfortunately, there is not one single storage solution which covers all electrode types. The right storage solution highly depends upon the electrode type.

If it is a separate indicator or only a reference electrode, then it is much easier to determine the correct storage solution, as the perfect solution for only one part must be found. For combined electrodes, the situation is a bit more complicated. Combined electrodes contain a reference electrode and a measuring electrode that each have different preferences. Therefore, sometimes a compromise is necessary. The reference electrode prefers to be stored in reference electrolyte to remain ready to use, whereas an indicator glass membrane prefers deionized water. On the other hand, a metal indicator electrode prefers to be stored dry.

For combined pH electrodes with c(KCl) = 3 mol/L as reference electrolyte, a special storage solution was developed by Metrohm, which maintains the glass membrane as quickly as possible without impairing the performance of the reference system. All other pH electrodes are stored in their respective reference electrolyte (normally indicated on the head of the electrode, see Figure 3).

Figure 3. Different reference electrolytes for different electrode types.

Metal electrodes are also stored differently, depending on the type. Combined metal ring electrodes are stored in reference electrolyte to maintain the diaphragm properly, whereas Titrodes are stored in deionized water, as these electrodes contain a pH glass membrane that needs to be kept hydrated. Always fill the storage vessel of your electrode with approximately 12 mL of storage solution and exchange it regularly as it might be contaminated by sample or cleaning solution.

The table below shows typical storage conditions depending on the type of the electrode. 

Table 3. Storage conditions for various electrode types.

If you are not sure how to store your electrode correctly, check the information in the electrode flyer or on the Metrohm website.

Check your electrode

The easiest way to check the performance of your electrode is to monitor it during a standardized titration (e.g., titer determination) which is performed regularly (e.g., weekly) and where prerequisites such as sample size, concentration of titrant, and volume of added water are always very similar. Otherwise, you can also follow a procedure recommended by Metrohm. To check metal electrodes, you can find a test procedure in application bulletin AB-048, for surfactant electrodes in application bulletin AB-305 and for ion selective electrodes, a check procedure is given in the ISE manual.

As an example, I will explain the test procedure of a silver electrode a bit more in detail. Silver electrodes can easily be checked by a standardized titration using hydrochloric acid (c(HCl) = 0.1 mol/L) as sample, and silver nitrate (c(AgNO3) = 0.1 mol/L) as titrant. Perform a threefold determination with the recommended titration parameters and sample size.

The following parameters are evaluated and compared to optimal values:

  • added volume of titrant at equivalence point (EP)
  • time until equivalence point is reached
  • potential jump (potential difference) between the potential measured at 90% and 110% of the EP volume
Figure 4. Example testing procedure for evaluation of electrode performance.

If the evaluated data cannot meet the specified values, clean the electrode thoroughly and repeat the test. If no improvement is observed, the sensor must be replaced.

Further symptoms can indicate a necessary replacement: sluggish response, unstable or drifting signal, longer duration of titration, smaller potential jumps, and worse shape of the titration curve. In Figure 5 below,  two different titration curves for calcium and magnesium in water are shown using a combined Calcium ISE. In Figure 5, the upper curve is obtained with a new Ca-ISE; the titration is fast and you will obtain 2 equivalence points: one each for calcium and magnesium. In the lower curve, an old electrode was used. The titration takes much longer and the second equivalence point for magnesium cannot be recognized anymore due to the lack of sensitivity of the electrode.

Figure 5. Comparison of the response of a new ISE vs. an aged ISE.

To summarize

  • Select the right indication for your titration type.
  • The quality of the electrode highly influences the quality of your titration results.
  • Proper maintenance and storage can increase the lifetime of the electrode.
  • Check the electrode performance regularly or monitor the titration performance (duration, potential jump) over time to reduce downtime of your instrumentation.

 

If you would like to get some more information and practical tips on electrodes in titration, please check out our white paper on «Basics of potentiometry» or our webinar «Avoid titration mistakes through best practice sensor handling». 

On-demand Metrohm webinar

«Avoid titration mistakes through best practice sensor handling»

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

FAQ: All about pH calibration

FAQ: All about pH calibration

In a recent blog post, we discussed how to avoid the most common mistakes in pH measurement:

Here, I want to discuss in a bit more detail how you can correctly calibrate your pH electrode and what you have to consider to obtain the best measurement results afterwards by answering some of your most frequently asked questions.

Let’s get right into it! If you want to jump directly to a question, click on one of these links:

When do I have to calibrate my pH electrode?

Performing regular calibration of your pH electrode is important to get accurate results. The pH electrode can change its properties (e.g., by contamination of the reference electrolyte) which then leads to deviating calibration results. If you do not freshly calibrate your electrode, you obtain precise but inaccurate results of your pH measurement. Therefore, the more accurate the results need to be, the more often you have to calibrate.

Depending on the number of measurements and the sample matrix, I recommend calibrating at least weekly. If the sensor is used often, or if the sample matrix contaminates the sensor, then you should calibrate daily or even more frequently. If the pH electrode is not used often, then always calibrate it prior to a new set of measurements. Also make sure that you always calibrate your sensor if you have received a new one, or after maintenance.

How do I select the correct buffers?

Any time you perform a calibration, it is essential that appropriate buffers are used.

First, you have to select the pH values that you would like to use for calibration. Use at least two different buffers, though it is even better to perform a multi-point calibration. Furthermore, make sure that the pH of your sample is of course within the calibration range! For example, if you want to measure a sample at pH 9, your calibration should not be within pH 4 and 7, but at least up to pH 10. In the graph, you can see that errors become large especially outside of the calibrated range.

 

In addition, the quality of your buffer solutions is essential, as your calibration can only be as good as the buffers used. Never use expired calibration buffers! If the buffer solutions are meant for single use only, do not reuse them. Microbial growth in the buffer can alter its properties quickly. Always mark your buffer solution bottle with the opening date, and especially ensure that alkaline buffers above pH 9 are not used for too long (< 1 month), as CO2 will enter and change the pH value slowly. Moreover, never pour the standards back into the bottle, as they might have been contaminated!

How should I set up my instrument?

Not only is the right choice of calibration buffers essential, it is also very important that you set up your instrument correctly. It’s not only the pH measurement that is sensitive to temperature, pH buffers are as well, and the measured pH value can change with the temperature. This temperature dependency of the pH buffers is usually depicted with buffer tables.

Most instruments already include buffer table templates from various buffer manufacturers. Several tables are available that contain the information about the exact pH value at various temperatures for a certain buffer. These tables are unique for each manufacturer.

The instrument will then select the correct pH value according to the measured temperature. If your buffer is not available with a table, make sure you enter the correct pH value or use a custom buffer table to store the information. As seen here, a temperature change of only 5 °C can have an influence of > 0.04 pH units.

Therefore selecting the manufacturer of your buffer solutions within the calibration parameters is important to obtain an accurate calibration.

Why do I have to measure the temperature?

You might wonder why you should always measure the temperature when you perform pH measurements. Most pH electrodes used for pH measurement have a temperature sensor directly included. This is because the pH value is temperature-dependent. Let me digress for a moment:

In 1889 the Nernst equation was established, describing the potential of an electrochemical cell as a function of concentrations of the ions taking part in the reaction. The relationship between potential and pH [-log(H+)] is given by the formula:

Where U is the measured potential, U0 the standard electrode potential, R the universal gas constant, T the absolute temperature, n the charge (here, +1), and F the Faraday constant. The central term

is called the Nernst slope and gives the mV change per pH unit. As you can see, this term includes the absolute temperature, meaning the slope of your calibration is temperature-dependent. The higher the temperature, the steeper the slope.

Modern pH meters will correct the slope for this temperature variation when the calibration and measurement are not performed at the same temperature.

However, there is an effect that cannot be corrected by the instrument: samples do not have the same pH value at different temperatures! This can already be seen when looking at the example buffer table above. This temperature dependence is different for each sample. Therefore: Always measure your samples at the same temperature if you want to compare their pH values. Also be sure to carry out the pH calibration at the same temperature at which you are measuring your samples. This will greatly reduce the error of your pH measurement.

How do I perform my calibration?

First, prepare your electrode for calibration: open the refilling plug to ensure proper electrolyte outflow, rinse the electrode well with deionized water, and place the sensor into the buffer solution. An important note: both glass membrane and diaphragm must be covered with the buffer solution.

Additionally, assure that you position the electrode in the beaker for maximum reproducibility, especially when stirring. Never place the sensor haphazardly into the beaker where the glass membrane is touching the glass of the beaker; this can cause scratches on the glass membrane, leading to erroneous results.

Do you even have to stir at all? No, you do not! However, as there can be effects on the measured potential depending on the stirring speed, make sure that you always choose the same stirring speed among all buffer solutions, but also for calibration and subsequent measurements. Also, make sure that you do not stir so strongly that a vortex is formed, and avoid any splashing of the solution.

Now, you can start your calibration. Most instruments decide autonomously when the reading is stable by monitoring the drift (mV change per minute). Sometimes it is also possible to stop the buffer measurement after a fixed time interval. However, this requires enough time for the electrode to reach a stable potential as otherwise the calibration will be biased.

Between the buffer solutions, the electrode is rinsed with deionized water. Never dry the electrode afterwards with a tissue, paper towel, or a cloth! This can lead to electrostatic charges on the electrode or even scratches on the glass membrane. Both will lead to longer response times, and in the latter case – to irreversible damage.

What do «slope» and «offset» mean?

Once you’ve finished the calibration, the instrument will display the calibration result. The calibration results usually consists of a slope and offset value. In this section, I want to explain their meaning.

The slope is normally given in % and is calculated from

the measured slope of the calibration divided by the theoretical slope (Nernst Slope) which is equal to 59.16 mV per pH unit at 25 °C. This is done in order to be able to correct the slope for temperature differences between calibration and measurement.

The second parameter that is evaluated is the pH(0), which is the pH value measured at 0 mV. In an ideal case, 0 mV corresponds to a pH value of 7. However, reality usually does not stick to the ideal case. Sometimes, the offset potential (Uoff) is also given, which corresponds to the potential at pH 7.

After calibration, always check the slope and the pH(0). The slope should fall between 95 and 103% and the pH(0) should lie between pH 6.8 and 7.2 (Uoff within ± 15 mV).

If you would like to get more information about your pH electrode, you can either perform a pH electrode test, which is implemented in some instruments from Metrohm, or a test according to Metrohm application bulletin AB-188.

If the pH(0) is outside the recommended range, this can be caused by a contaminated electrolyte or your probe may require a general cleaning.

If the slope is lower than 95%, this can be related to expired or contaminated buffer solutions. However, old and slow electrodes can also exhibit slopes outside of the limits. Therefore, always use fresh buffers.

If the slope is still too low even with fresh buffer, or the pH(0) is outside the recommended range after cleaning and subsequent reconditioning, it is time to replace the electrode.

To summarize

  • Select the calibration frequency and buffer types according to your samples.
  • Make sure that you always use fresh, high quality, and certified buffers as your calibration can only be as good as your buffers.
  • Set up your instrument correctly and use a fixed electrode position for the best reproducibility.
  • Measure the temperature for calibration and subsequent measurement. Moreover, only compare pH values of samples measured at the same temperature.
  • After calibration, check that your data for slope and pH(0) are within the optimal limits.

Would you like to learn even more about pH measurement? Come to our website and check out our informative webinars!

Metrohm webinars

Available below for pH measurement

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

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.