PH Meters - Calibration and Anomalous Values

Good question... ph instruments do seem susceptible to electrical noise, bad cabling and other factors.
Are the samples being read whilst in a system or are they being tested 'on the bench'?
Del

Hi Del.

my samples are tested on the bench. I prevail samples of Demin Water for which I am reading different values on my PH meters and worse I cannot say which one is the right one. I have some suspicions about the conductivity of the pure water, which is so close to zero that it may cause the ions migration between the electrodes of the PH Instrument more difficult and which therefore renders the instrument unreliable.

This is probably unfounded theory, or simply can someone enlighten me on that.

Thanks for helping out.

Regards

Bamboche

I'm not that au fait with ph meters, but I suspect that the various factors like ph and conductivity do interact.
I guess the answer is to 'manage' the situation by standardizing on one meter, and maybe relegate the other meters for less critical duties.

Dunno if it's of interest but we were using conductivity to measure concentration of a vegetable washing solution which was mostly ascorbic and citric acid. The conductivity went up nicely with concentration.... And then started coming down again as concentration increased.
Del

Ah! The pH of demin water is not particularly relevant. It's the same as asking, "what is the concentration of the hydrogen ion concentration in a solution that has so little of anything else in it that the only substances present are hydrogen ions, hydroxide ions and water molecules?"

If there were some acid-making substances present, then the water would not be particularly demineralised. So use the conductivity instrument instead!

This seems crazy, but this occured with me as well. Temperature and product density gave two different results, but the buffer solution was exactly the same reading. By sampling several samples the electronic pH tester self calibrated and produced the same result. Hope this helps!

You need to perform a minimum of two point calibration with two buffers, usually 7 and 4 or 7 and 10. Try to bracket your expected sample value with the calibration buffers. The pH meter should then report an efficiency, usually as a percentage slope. Anywhere from 94 to 101 % is acceptable. Also use the pH 7 buffer first to set the zero potential and then the 4 or 10 to set the electrode slope. If both meters are calibrated the same way and both have acceptable slopes they will measure the unknown pH very closely.

The ions in solutions produce a potential across the electrodes. Buffer solutions are usually simple small ions but samples sometimes have ions, or other molecules, dispersed or dissolved in them which interfere with ion transport at the electrodes, and as a result change the electrical signal from the electrodes. The meter is actually measuring a tiny electrical potential across the electrode membranes. Since different electrodes will make slightly different electrical potentials in various sample solutions, it is possible to get variations in "measured" pH. Even electrodes which are supposed to be the same and made of the same materials can cause this if the electrodes have a different history. Various solutions can plug electrode tips or contaminate them with something that does not wash off easily. Soaking electrodes for a day or two in water or dilute HCl can sometimes help. The differences in measured pH should however not be very large (more than a tenth or two of a pH unit), and if they are, then something is hinkey (scientific term for messed up).

Hi Tad,

I thank you very much for the enlighting information that you have given. There is therefore a closely related link between Conductivity and PH-Measurements, since it involves the use of a sensitive voltmeter in a media that contains almost only H+ and OH- ions together with traces of contaminants that will alter the PH values. So far, so good; If I take your point, it will mean that when the conductivity of the Pure water is very low, that is, with low ions migration activity, the PH-Meter will be less effective. This is characterised by a longer stabilisation time to read PH. And I suppose that every PH meter from different suppliers will give different results (as it is actually the case at my place).

My questions:

1) Can you suggest alternative methods for measuring PH of Pure Water, such as your titration method. If the method can provide a means to determine the error values that are related to the drift of the PH-Meters.

2) Is there any type of Resistance that can be coupled with the probe to increase the internal Resistance?

3) To conclude, I still have doubts about the End result, I am not sure what will be measured?

Unfortunately, I do not have a good answer for you. I would think there is a colorimetric method that might work, but I could not find any specific references for ultrapure water.

I noticed that some of the pH probe manufacturers market pH probes designed specifically for this application. Try Googling pH and deionized water or ultrapure water.

It appears that ASTM, USP and other organizations have methods and procedures for making these measurements. This paper goes into some of the problems, and provides references for some of the standard methods.

I would get copies of the USP and ASTM methods, and use one of these procedures. If your results are ever questioned, it is hard to argue about results when using approved standardized procedures.

I personally feel that you are being asked to make a measurement which really doesn't make much sense, or provide any useful information. Please note that ISO has no requirement to measure pH of ultrapure water for this reason.

However, I know specs are specs, and it is seldom worth arguing over their validity. The best solution in this case is to use the ASTM or USP method. At least your butt is covered by using a "validated" standard procedure.

Good luck, and let us know how you make out.

Tad

Different electrodes will give different responses. Good quality liquid KCl electrodes will be more accurate than gel electrodes. Temperature compensation algorithms and temperature sensor quality will also impact on the measurement of solutions, whereas buffers are normally measured at 25 deg C.

There is no alternative measurement technique for pH. Your best bet is a good quality instrument with the best electrode and buffers you can afford.

Radiometer of Copenhagen make the best instruments and their IUPAC buffers are the closest you'll come to a NIST standard.

It is impossible to measure the pH of deionised water using a bench application. The DI water will immediately start to absorb CO2, which will depress the pH. Furthermore, any contaminants in the sample container will tend to equilibriate with the solution, thus influencing the pH. Adding KCl is NOT recommended, unless you are analysing the pH of soils or other normally solid substrates. The only way to accurately determine the pH of DI waters is to use a good quality on line analyser. The best analysers are made by companies such as Swan and Polymetron. The best alternative method uses a dual conductivity ( specific and cation conductivity ) analyser and calculates the pH using an algorithm specific to the solution. This is not normally a recommended technique for DI waters, because of the relatively small difference between SC & CC, but they can still be of use.

There are no titrimetric or colourimetric techniques to accurately determine pH, so don't even waste your time with them.