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FAQ pH measurement

In this blogarticle we answer to possible questions, that may occur to you during installation and operation of our pH instruments. We also respected common questions. In the blog you will find more exciting topics about pH measurement.

Frequently Asked Questions about pH/Redox/ORP

1.1. Application

  • Is a pH measurement in cold brine possible?
    Yes, the possible electrodes can be found in the electrode list in the catalog (pg. 51)
  • Which electrode is suitable for which application?
    For the initial orientation, there is a selection table included in the electrode catalog. We will gladly advise you in person.

1.2. Life span

  • How long will the electrode last?
    A general statement is difficult here. Import factors include the measuring conditions, the condition of the measured samples and the care of the electrodes.

1.3. Maintenance

  • The KCl solution in the electrode has been partially crystalized: does the entire reference electrolyte need to be replaced or can 3 mol KCl solution be added? What happens if the concentration of the KCl solution in the electrode is no longer exactly 3 mol/l?
    Procedure: Completely remove the electrolyte, add water and dissolve the KCl crystals. After that, remove the solution and add some 3 M KCl (in order to remove possibly adhering solution inside the electrode) and then remove. Then, fill up with 3 M KCl. Differences in the concentration of the electrolyte solution lead to differences in the measured potential and therefore influence the measured ph value.
  • What do I do if an electrode is dried out?
    Dissolve possible KCl crystals in water, fill in new electrolyte solution and hydrate for 24 hours in 3 M KCl. After this treatment, a calibration must be performed to decide whether the electrode is still usable.
  • How do I keep or store the electrode properly?
    With liquid gel electrodes (one-rod measuring chains and reference electrodes), the respective electrolyte solution should be filled into the hydration cap. For gel electrodes, a 3 mol/l KCl solution is sufficient. For individual glass electrodes, fill de-ionized water.
  • What is the best way to clean the electrode?
    Contamination on membrane and diaphragm result in measurement deviations. Calcareous depositions can be removed with thinned mineral acids (e.g. diluted hydrochloric acid), organic contamination can be dissolved with suitable solvents, fats can be removed with surfactant solutions, and protein can be dissolved with pepsin solution (e.g. cleaning solution L510). General tips – Rinse the electrode with distilled water after the measurement and the cleaning, do not rub dray and do not use as a stirrer. – To refill and store the electrode, see question ""How do I keep or store the electrode?"". – Extreme pH values and temperatures shorten the life time and limit the accuracy of the electrode. – Hydrofluoric acid, hot phosphoric acid and strong alkalines destroy the ph glass membrane.
  • How long and where can I store electrodes?
    An electrode ages, even if it is not used, therefore it should not be stored for extended periods of time.

1.4. Measuring pH

  • Should you stir during the ph measurement or not? What are the pros and cons?
    This depends on the electrode you use. Electrodes with platinum diaphragms do not show any or only a very minor stirring effect; therefore, they influence the measured potential. Electrodes with ceramic diaphragms show a higher stirring effect. The advantage (pro) is the better mixing of the measuring medium. In order to keep this increased influence on the diaphragm as low as possible, shortly stir to mix the solution and then perform the measurement without stirring.
  • Why should the refill opening of the electrode be open during ph measurement?
    The open refill opening causes a pressure balancing. The KCl solution can flow out through the diaphragm into the measuring solution. At the same time, it prevents the submersion of the measuring solution through the diaphragm. This keeps the potential of the inner electrolyte unchanged.
  • Do I have to calibrate at the same temperature that I am measuring at?
    The calibration and measurement should be performed at the same temperature for the most precise result.
  • Which buffers should be used for calibration?
    It is impossible to achieve a higher accuracy than the accuracy of the buffer solutions. Therefore, the user must determine which accuracy is required for his measurement and then adapt the buffer solutions for the calibration. STATEMENT?
  • What do I have to watch out for with an electrode, so that it measures properly (opening the air supply, discolorations, diaphragm, replacing the inner electrolyte)?
    Regular calibration to verify whether the electrode is still measuring correctly. Visual inspection of the diaphragm for discoloration, the refill opening must be open for the measurement, replace the inner electrolyte if it has crystalized or discolored.
  • Which buffers are the best ones to use for the calibration?
    Buffer solutions in the acidic or neutral range, as alkaline buffer solutions quickly change their buffer value due to picking up CO2 contained in the air and can thus lead to miscalibrations.
  • Why temperature indication with ph in spite of temperature compensation? Why is T-Cal = T Meas?
    The temperature compensation of the measuring devices can only be calculated with the temperature dependency of the "Nernst factor", as the temperature dependency of the sample is usually unknown.

You can find suitable buffer solutions for your pH measurement here!

1.5. pH-Meters

  • Can SI Analytics electrodes be connected to any ph meter?
    SI offers different possibilities of use of their electrodes to ph meters made by other manufacturers. Fixed cable electrodes are available with compatible connectors. Plug head electrodes can be adapted to the matching connectors on measuring devices by used adequate cables.

1.6. General pH-Tips

  • How long can opened buffer solutions be used?
    Buffer solutions in glass vials are intended for single use. All other buffer solutions are marked with expiration dates.You can find suitable buffer solutions for your pH measurement here!
  • Are there differences between DIN and technical buffers?
    DIN buffer solutions are fixed in their composition (DIN 19266) and offer high accuracy. Technical buffer solutions can vary between manufacturers. Which buffer solution is used is therefore specific to the application. You can find suitable buffer solutions for your pH measurement here!
  • How do the electrode glasses differ? Which glass is suitable for which use?
    The different electrode glasses differ in their composition and are optimized for different uses. H glass: is well suited for high temperatures in the acidic and alkaline range as well as for high concentrations of sodium ions S glass: in hot alkaline media with good reproducibility and a short response time L glass: for low temperatures and general applications A glass: with a short response time in drinking, domestic and waste water; for general applications and in low-ion media.
  • When does it make sense to use ISE electrodes (measuring range, sample prerequisites)?
    The measuring range, sample preparation, and possible interferences are listed in the operating instructions for the ion selective electrodes.
  • What do I watch for when using ISE electrodes?
    Ion selective electrodes by SI Analytic are fitted with a plastic electrode shaft. This shaft should to be tested for resistance to the measuring medium. ISE are suited only for use in laboratories and not for continuous process measurements. The sample preparation should be adhered to in detail.
  • What does the incline/zero point of a ph electrode have to be in order to be able to measure sensibly? When should the electrode be replaced?
    The zero point should be between -25 and +25 mV, the incline between -56 and -61 mV/pH. If the values are outside the ranges, the electrode must be cleaned and the calibration must be performed once more. If the electrode cannot be improved by cleaning it, it must be replaced.
  • Why are there air bubbles when attaching the hydration cap?
    When you attach the hydration cap, the air that remains inside of the cap is pressed into the electrode via the diaphragm due to the increased pressure, which is the reason for the rising bubbles. This effect is a good sign, as this proves the permeability of the diaphragm.
  • Does an air bubble cause trouble in the glass electrode?
    As long as the conduction wire of the electrode is still protruding into the solution of the inner buffer, no interference must be feared.
  • Why does the electrode lose KCl solution?
    When the refill opening is open, KCl solution will flow through the diaphragm into the measuring medium.
  • Why can the one-rod measuring chain not be stored in distilled water?
    If the diaphragm is stored in distilled water, the electrolyte inside the diaphragm is diluted and the potential of the reference electrode is shifted. This will cause problems during ph measuring.
  • Why does the zero point migrate over time?
    The position of the zero point depends on the ph value of the inner buffer. This can change because of loosened components of the glass. Also, a change of the KCl concentration shifts the potential of the reference electrode and therefore the zero point.

For helpful and practical tips within your application area, you can read in our blog articles:

pH Electrodes for laboratory and field

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