Sulfuric Acid and Magnesium pH
Date: Fall 2013
I recently had students (and myself) do an experiment looking to measure changes in solutions with a chemical reaction taking place. We measured pH and temperature.
0.1 0.2 and 0.3g Mg(s)
reaction was H2SO4 + Mg(s) ----> MgSO4 + H2g
as predicted, the temp rises with a nice curve. HOWEVER, as the acid is consumed, we predicted the pH to rise, which was not the case. In multiple tries, we got plummitting pH curves proportionate to the quantity of Mg and rise in temp. In fact, as we added more Mg, the curve was steeper. I have no idea why that happened and have not seen any litterature explaining the matter. So I tried the experiment a dozen times with the same results....
I would also expect the pH to rise. However, the temperature effect on the measurement of pH ( using pH electrodes) can be significant.
As a result, at temperatures as high as 45 degrees Celsius, a positive error of 0.3 to 0.5 units of pH can be observed. The shift in pH electrode behavior is usually corrected by an automatic temperature correction arrangement that comes in pH meters. It may be worth cooling the reaction liquid and then measuring the pH again to verify if there is an error of this type.
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Update: November 2011