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.
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).
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 1–2 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.
- 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.