Dear Islam,

For pH meters to be accurate, they have to be properly calibrated (the meter is accurately translating voltage measurements into pH measurements), so they usually need testing and adjusting before you start to use them. You calibrate a pH meter by dipping it into buffers (test solutions of known pH) and adjust the meter accordingly. Another important consideration is that pH measurements made this way depend on temperature. Some meters have built-in thermometers and automatically correct their own pH measurements as the temperature changes; those are best if fluctuations in temperature are likely to occur while you're making a number of different measurements. Alternatively, you can correct the pH measurement yourself, or allow for it by calibrating your instrument and making pH measurements at broadly the same temperature.

Dear Yadav,

Thank you so much for your nice answer. I completely support your comments, calibration of pH meter is essential. I mentioned that we calibrated our pH meters in buffer solution of 4, 7 and 10 values. All of 3 pH meters gave similar result in buffer solutions but they gave different readings (values) in same soil solution. It is notable that all of our pH meters have temperature adjustment capability through thermometer. 

@ Dr. Islam - I am not sure about the quality of your pH meters you are using. Generally a pH meter of the reputed company (say Beckman) does not give such trouble.

Such differing measurements is unacceptable. There should be something wrong with your instruments/or electrodes. The pH elecrode is probably gone off, especially 6.4 measuring one. A special care shold be taken for the elecctrode when it is out of usage. It shoud be inserted either in distilled water or its conservation cup with KCl solution. Otherwise the fluid in the electrode become decrease and its accuracy decrease. I suggest replace them all if you have spare ones, calibrate and read the sample again.

Good points mentioned by colleagues on pH measurement in soil.As I understand one has to measure pH of soil water suspension,not sediment  or water. One has to read pH immediately after stirring.When one is reading pH of soil in alkaline range he  can adjust pH with  7.0 and 10  buffers..If the problem persists one has to change the electrode or even instrument.

Dear Islam 

I understand the problem seems with electrodes with respect to age, time lapsed you renewed the KCl solution in electrodes and un-recognized human error. Check these.

Dear Ghafoor sir,

Thank you so much for your nice answer. KCl solution might be a problem. I will renew it. However, I am confused because all pH meters gave same value in buffer solutions. 

Dear Rao sir,

Thank you so much for your nice point. So, I think capability of different pH meters is variable in soil suspension and capability in water or buffer is similar. As a result they give same value in buffer and different values in soil suspension, I think.

The calibration range of the pH meter is also significant fact. You should use a calibration range 4 and 7 or 7and 10 depending on the expected pH range of the samples. The other case is ionic streght effect. Each soil has different amounts of soluble salts thus the ionic strenght is to change depending on the polivalent cations/anions and amount of salts. This infact influence the activity of H ion in solution. As a result a pH meters performs differently depending on the soluble salt content of soils. Because it measures -log of H ion activity. 

Dear M N Islam

Please take care of the electrodes of your pH meters and adjust the pH according to the suggestions given by Veli Uygur.

Dear Islam

DR. Veli Uygur has given good point to consider. 

I will suggest if the soils under test are from areas where generally pHs is alkaline, then calibrate the meter with buffers of 7 and 9. If soils belong an area where generally soils are acidic, then calibrate meters with buffers of 4 and 9 pHs. I expect this will decrease the range of variation in pHs.

I agree as was suggested by Dr. Abdul Ghafoor. He is the great scientist in the field of soil science and expert in soil chemistry particularly.

However, I want to confirm that if soils belong an area where soils are generally acidic. then calibration would be with 4 and 9 OR 4 and 7????? Dear Dr. Abdul Ghafoor, can you please clear me. Thanks.

Dear Syed Shahid Hussain Shah

Thank you for appreciation. For acid region soils, calibration with buffers of 4 and 7, while for alkaline  area soils calibration with buffers 7 and 9 will provide better results, i.e. variation in pH measurements by different pH meters will be decreased. 

This phenomenon is very similar to spectroscopy technique. When we calibrate instrument with std solutions of 0, 2, 4, 6, 8 and 10 ppm solutions, many time readings fall below zero for samples having very low concentration of any chemical. But if we calibrate instrument with std solutions of 0, .02, 0.04, 0.06, 0.08 and 0.10 ppm solutions, the instrument will provide readings as 0.1, 0.2, 0.3 etc for the same samples.

Many factors contribute erroneous results, most important is maintenance, physical state and calibration of the instruments. 

Salam, nice discussion, Now I m trying to make research regarding PHmeter..

Several standard methods of measuring soil pH make use of a dilute sol. of CaCl2, added to the soil sample and shaken with it under specified conditions, before the pH of the resulting slurry is measured with a pH electrode. That being the case, and if there are indeed no reasons that may point to faulty calibration or poor condition of the used electrode, it seems advisable to revise the validity of the whole experimental procedure/protocol followed. Besides contact time ― between soil and solution ― and temperature; enough attention should be paid to the pH, exact concentration, and purity of the actual CaCl2 solution used. This salt is highly hygroscopic and deliquescent, what may possibly affect the weighting accuracy. Moreover, it is also prone to carbonation by the atmospheric CO2, what may have a rather detrimental effect on the salt purity (*) and consequently, on the pH of a prepared solution from contaminated salt.

(*) You may want to check also my answers elsewhere at this forum: https://www.researchgate.net/post/Why_is_my_anhydrous_calcium_chloride_not_going_into_solution https://www.researchgate.net/post/whats_the_best_choice_to_modify_the_PH_of_calcium_chloride_hexahydrate