pH: Frequently Asked Questions

pH Measurement in Samples

QUESTION: What do I need to test pH for reporting purposes?

ANSWER: A pH electrode and meter such as an HQd or sensION+ meter must be used if your results will to be reported to an agency such as the USEPA. pH meters will give the best accuracy and sensitivity, measure across the full pH range, and can be used with most water samples, however they require regular calibration with pH buffer solutions. Hach meters come with buffers and complete instructions for simplified calibration.

QUESTION: What methods other than an electrode are available for measuring pH and when can I use them?

ANSWER: If regulatory reporting is not required, pH measurements can be made using pocket pH testers, test kits, colorimeters, or test strips. Choose the method that best matches your accuracy requirements and ease of use.
A pocket tester should be used when the sample has color or is visibly turbid. Pocket testers can measure across the full pH range and give results to the nearest 0.1 pH unit, however they must be calibrated using pH buffers regularly.
Test kits and colorimeters give an accuracy of 0.1 to 0.5 pH units in a limited pH range, and test strips give semi-quantitative results. Oxidants such as chlorine can interfere with these tests. Add one drop of 0.1 N sodium thiosulfate before adding the pH indicator to remove chlorine interference.

QUESTION: What factors need to be taken into account when measuring low ionic strength samples and deionised water?

ANSWER: As a general rule, standard pH probes contain a 3 or 3.5 molar KCl electrolyte solution or a gel. As low ionic strength samples and deionised water contain little or no salt, they try to get ions (salt) out of the electrolyte solution. When conventional pH probes are used (e.g. with gel filling), this phenomenon may lead to poor response and unstable readings. For applications such as these, Hach offers pH probes specifically designed for low ionic strength samples. These probes have high reference electrolyte flow rates to insure a free flowing reference junction.
When KCl crystals are added, the ionic strength can be increased and the response time improved. It is important to use high purity KCl to avoid influencing the pH. Closed sample containers (flasks) present the advantage of avoiding CO 2 contamination. Use of scrupulously clean equipment and glassware and thorough rinsing before measurement are essential to avoid contamination from previous samples. Prior to sample measurement, the probe must be calibrated by using high-precision pH buffer solutions.

QUESTION: What is the best way to measure pH in soil?

ANSWER: It is advisable to use a specific pH probe with a strong glass tip and high electrolyte outflow. Mix a 5 g soil sample with 25 g deionised water while stirring carefully. Then let the mixture stand without stirring for 10 minutes to separate the particles from the liquid. Insert the pH probe in such a way that the glass bulb is totally covered by particles, but the diaphragm is not. Wait for a stable reading.

QUESTION: What is the best way to measure pH in high temperature samples?

ANSWER: Before measuring samples over 80C, check whether the probe is designed to stand such high temperatures. Gel-filled pH probes can be used up to 80C but no higher. Hach pH probes have an Ag/AgCl reference system which is ideal for higher temperatures. We recommend pH probes with saturated KCl filling solution. As KCl crystals are soluble at higher temperatures, it is a good idea to ensure excess crystals so that the solution remains saturated. If at all possible lower the temperature of the sample to the temperature of the buffers at calibration.

QUESTION: What is the best way to measure pH in high alkaline samples and samples with high salt content?

ANSWER: We recommend pH probes with saturated KCl filling solution. As KCl crystals are soluble at higher temperatures, it is a good idea to ensure excess crystals so that the solution remains saturated. Samples with high salt content and pH>12 may be subject to "sodium error". Hach offers high alkalinity electrodes that minimize "sodium error".

QUESTION: What is the best way to measure pH in emulsions or fatty solutions?

ANSWER: When measuring fats and emulsions, it is important to choose an electrode with the right diaphragm and reference electrolyte and a junction that is easy to clean. We therefore recommend open liquid junctions and sleeve junctions. After the measurement, the probe should be cleaned thoroughly, i.e. any remaining fat or oil must be removed with soap or surface-active agent.

Maintenance and Storage of the pH Electrode

QUESTION: How should pH probes be stored best?

ANSWER: All Hach pH probes are delivered with a plastic protection cap that can also be used for storage. Pour a few drops of saturated KCl solution into the cap to ensure that the glass membrane is kept hydrated and ready to use. For short-term storage of a few hours, the pH probe can be placed in a solution of 3.5 molar KCl or pH 4.0 or pH 7.0 pH buffer. Always rinse before use. As a general rule, pH probes (especially reference probes) should never be stored in deionised water. Overnight, the probe should be stored in the corresponding electrolyte storage solution, usually saturated KCl. For long-term storage (2 weeks or more), the pH probe should be stored with its protection cap filled with storage solution and sealed with Parafilm.

QUESTION: What is the best way to remove air bubbles from inside a pH probe?

ANSWER: Air bubbles can get trapped in the electrolyte solution of the reference system. This results in unreliable and unstable readings and may make it impossible to achieve a reading. In order to remove air bubbles, shake the probe vigorously using a downward motion so that the air bubbles move to the upper end of the probe. If air bubbles are trapped inside the solid KCl crystals, heat the electrode tip in warm water (max. 60C). This will dissolve the crystals and release the air. Afterwards, shake the probe down once more as described above and leave it to cool down. Air bubbles inside the pH glass membrane are normal and cannot be removed. They will not cause problems if they are at the upper end of the probe.

QUESTION: Can a pH probe be used straight out of the box?

ANSWER: Although Hach pH probes come with a protective cap which is wet inside, the glass membrane can dry out.
For best results, we recommend rinsing with deionised water then conditioning the probe in pH 4.0 buffer for at least 2 hours. After further rinsing, it is ready to be calibrated. The normal (quick) response time will be achieved after 24 hours hydration. If measurements are needed before this time, calibrations should be repeated often due to drifting potentials.

QUESTION: Do dirty and wet cables have an influence on the pH reading?

ANSWER: Due to the extremely small currents which pass through the pH electrode, the cable, plug and connector must be kept clean and dry if reliable measurements are to be obtained.

QUESTION: How much electrolyte should the pH probe contain?

ANSWER: The level of electrolyte solution should be 1 cm below the filling hole. This is the only way to ensure that the hydrostatic pressure applied to the diaphragm will be high enough to prevent sample passing through the diaphragm into the probe.

QUESTION: What causes a diaphragm to get blocked and how can this be remedied?

ANSWER: Liquid junctions with fibre or ceramic diaphragms can occasionally get blocked due to KCl crystallisation. Try soaking the electrode in warm tap water to dissolve the KCl crystals that cover the diaphragm. Other types of blockage can occur in the form of a precipitate, for example, silver chloride or silver sulphide. Gentle polishing with abrasive paper and soaking in saturated KCl can help.

pH Measurement Technique

QUESTION: What is the maximum cable length between the pH meter and pH probe?

ANSWER: All pH electrodes have a high impedance and the mV signal is amplified by the electronics in the pH meter or, with Hach digital IntelliCAL™ electrodes, in the probe head. With analogue electrodes, cable lengths of 3 m can normally be used. Digital probes work with cable lengths of up to 30 m or more, because the digital signal is not impaired by the magnetic fields of motors, e.g. pumps.

QUESTION: How does the temperature influence the impedance of the pH glass membrane?

ANSWER: The lower the temperature, the higher the impedance of pH glass. For every 10C decrease in temperature, the glass impedance will increase about 2.5 times, resulting in slow response. For example, if the pH glass impedance is 100 MOhm at 25C it will increase to 250 MOhm at 15C.

QUESTION: What is the lifetime of a pH probe?

ANSWER: The lifetime of a pH electrode depends on several factors including storage conditions, correct maintenance and the type of sample measured. Under normal laboratory conditions, for aqueous samples, the average lifetime is between 12 and 18 months, supposing of course that the electrode is kept clean and kept hydrated during storage. If the probe is used with dirty samples (e.g. stirred solutions with particles), is subjected to mechanical abrasion or used at high temperature or high pressure, the lifetime may be only a few weeks. In hot alkaline solutions, pH probes can be damaged after only a few hours. Regular maintenance helps pH probes keep working efficiently for several years.

QUESTION: What is the difference between pH probes with glass body and an epoxy body?

ANSWER: Both electrodes have a glass membrane, a diaphragm, a reference system and offer the same measuring quality. However, epoxy electrodes have a limited temperature range of maximum +80C, while glass bodies can withstand temperatures of 100-110C (with an Ag/AgCl reference system). If you are working in the field or in tough conditions, epoxy bodies have the advantage of being more robust and less liable to crack. Electrodes with glass bodies tend to be used in the lab, because they can be cleaned more easily and, unlike epoxy bodies, can withstand organic solvents. pH probes with epoxy bodies are usually less expensive and therefore represent a cost-effective alternative.

QUESTION: What is the point of automatic temperature compensation (ATC)?

ANSWER: Automatic temperature compensation (ATC) has to do with correcting the pH calibration slope to account for the actual temperature of the sample so the pH electrode gives an accurate reading of the pH of the sample. The pH calibration equation is linear and has a slope value at 25 o C. Any deviation of the actual temperature from 25 o C is compensated in the slope according to the Nernst equation. The pH of the internal solution of the pH bulb is called the isopotential pH. If the pH of the sample is the same as the isopotential pH there is essentially zero potential across the pH membrane. The isopotential pH of most pH electrodes is around pH 7. Temperature has little effect on sample measurement around the isopotential pH and becomes more important as the sample becomes more acidic or basic. ATC becomes important the further the sample pH is away from pH 7.

QUESTION: Is it always necessary to perform a 2-point calibration or is a 1-point calibration sufficient?

ANSWER: If the last calibration was performed on the same day, a control calibration with one buffer is sufficient. Then only the zero potential is adjusted, the old slope will remain as it was. Otherwise, a 2-point calibration is recommended, because only then an actual probe status can be determined and taken into account for the measurements to come.

QUESTION: What is the recipe of IUPAC pH buffers?

ANSWER: HCl (pH1.094): 0.1 M HCl, Oxalate (pH 1.679): 0.05 mol/kg KH 3 C 4 O 8 , Phthalate (pH 4.005): 0.05 mol/kg KHC 8 H 4 O 4 , Acetate (pH 4.650): 0.1/0.1 mol/kg C 2 H 4 O 2 /C 2 H 3 0 2 Na, Phosphate (pH 6.865): 0.025/0.025 mol/kg KH 2 PO 4 /Na 2 HPO 4 , Phosphate (pH 7.000): approx. 0.020/0.0275 mol/kg KH 2 PO 4 /Na 2 HPO 4 , Phosphate (pH 7.413): 0.008695/0.03043 m KH 2 PO 4 /Na 2 HPO 4 , Borate (pH 9.180): 0.01 m Na 2 B 4 O 7 , Carbonate (10.012): 0.025/0.025 mol/kg NaHCO 3 /Na 2 CO 3 , Ca(OH) 2 (pH 12.45): saturated (at 25C) and filtered.

pH Measurement Troubleshooting

PROBLEM: The pH probe response is slow, tends to drift and results are not reproducible

SOLUTION: This may be caused by one of the following: - the glass membrane or the diaphragm is dirty, e.g. oil, fat, paint, dirt. - the pH probe is reaching the end of its lifetime. - a sample of low ionic strength (< 100 S/cm) is being measured with a conventional rather than a specially designed pH probe.

PROBLEM: The pH measurement system can no longer be calibrated by auto-buffer- recognition

SOLUTION: Check all parts of the system. First eliminate possible mechanical defects such as loose plugs, damaged cable or probe, too low a level of electrolyte etc. Ensure that fresh pH buffers were used. Buffers are only detected automatically if the mV signal is within a certain range. Old buffers or buffers not meeting DIN/IUPAC specifications cannot be detected by the meter software (AUTOCAL). For instance, pH 6.86 and pH 7.00 buffers are too close together and cannot be distinguished. In order for auto buffer recognition to work properly, the correct type of buffers has first to be selected. Typical combinations of pH buffers are pH 4, 7, 10 or pH 4.01, 6.86, 9.18. Finally, the pH electrode must be cleaned using special cleaning and conditioning solutions which can be found in the GK Annex Electrode Maintenance Kit from Hach. If the pH probe still does not react normally and a slope of 95% to 102% cannot be achieved during calibration, it should be replaced.

PROBLEM: The response time of the pH probe keeps increasing.

SOLUTION: If the response time is gradually becoming longer, the diaphragm may be blocked or dirty, or the surface of the pH glass membrane may have fat, oil or paint deposits. In order to clean the diaphragm, use the special RENOVO.N and RENOVO.X cleaning solutions found in the GK Annex Electrode Maintenance Kit. Fat deposits can be cleaned off glass membranes by using surfactant solutions. If the glass membrane does still not react properly, it can gently be etched. Procedure for etching glass membranes The glass bulb is lowered for 1 minute into a 20% ammonium fluoride solution and then for 15 seconds into a 6 molar HCl. It should then be rinsed thoroughly with deionised water and stored for 24 hours in a slightly acidic buffer such as pH 4.01.

PROBLEM: pH buffer not reading correctly on colorimeter.

ANSWER: If you are using a pH 7 buffer solution to check pH using your DR/820, 850, 890, or Pocket Colorimeter and the reading is not close to what it should be, then make sure the buffer you are using is colorless (Cat. No. 12222-49 or 12222- 56) and not yellow. Pour 10 mL of the buffer into the sample cell, and add the phenol red. Zero the instrument on the buffer with no phenol red and read the buffer with the phenol red added to it. If the reading is slightly off, you can adjust the calibration using the standard adjust feature.

QUESTION: What is the usual stabilisation time for a pH probe in pH buffers?

ANSWER: At room temperature (20-25C) and with fresh pH buffer solutions, the stabilisation time should not exceed 1 minute; normally a stable reading should be achieved after 30 seconds. Otherwise the pH electrode should be cleaned and conditioned again.

QUESTION: Is it possible to use a pH probe in alcoholic solutions?

ANSWER: Yes, but short-term only. Longer exposure to high percentage alcoholic solutions leads to dehydration of the glass membrane which then has to be conditioned again.

www.HACH.com