Dear Aykut,

Firstly, it may be a measurement error caused by the fact that you calibrated the pH-electrode/meter e.g. with buffers 7.00 and 4.00. However, for this set of calibration buffers the linearity of the mV response of your pH-meter stops around 3. Any pH below that will give a non-linear mV-to-pH relation which can lead to the fact that you get erroneous, negative, pH-values.

Negative pH-values are not uncommon btw. pH is defined as the - log value of the H+ concentration. So at 1 M of a strong acid, pH = - log (1) = - 0 or simply pH = 0.

At 85% phosphoric acid one can calculate that the concentration is:

(0.85*1685)/98= 14.6 moles/litre, assuming all the first protons from phosphoric acid are released, this means [H+] = 14.6 moles/litre, so the pH to be expected is pH = - log(14.6) = -1.16. At 5% phosphoric acid, you simply calculate the phosphoric acid concentration like this:

(5/85)* 14.6 = 0.86 moles/litre, so the pH = - log (0.86) = 0.07 so a positive value.

Please recalibrate your pH-meter and electrode with buffers pH= 7.00, 4.00 and 1.00. This should cure the calibration error. If this does not work, please keep in mind not all meters, pH-electrodes can be used for such low pH-values. Check the manual for the electrode on its working range and recommended calibration procedure.

Hope this helps, kind regards, Leo

Thank you very much. I'm going to try again.