According to the henderson hasselbalch equation, 50mM potassium phosphate buffer at pH 7.5 can be prepared by dissolving 41.1mMol of K2HPO4-2 and 8.9mMol of KH2PO4- in 1 L of water.
However, the pH of this solution measured by a well calibrated probe is 7.75 (not 7.5). I used a pKa value of 6.85 which is the one adjusted to 25C and 0.01M ionic strength.
When using the unadjusted pKa (7.2), and changing the amounts accordingly, the pH of the solution is 7.35
What is the source of the discrepancy between thermodynamic calculations and measured pH values. Should the calculations or the pH probe be trusted?
The temperature is an important factor when measuring pH.
Make sure you calculate the amounts of the salts based on the molecular weights written on the container. The number of waters of hydration can vary depending on which product you buy.
Also, did you do calibration of your pH meter before your measurement?
I believe it is common practice to adjust the pH of the buffer by adding small amount of diluent NaOH or HCl (or base/acid used for the buffer).
Very commonly, the solid crystals of potassium phosphate have more contaminating water than the label declares. In this case, assuming you do have the temperature at room temperature and that you have calibrated your pH meter correctly, I suspect that your KH2PO4 has about 10% more of the mass that is water than you think. When you are weighing out the mass, you are assuming that the molar mass of KH2PO4 can be used to determine the number of moles of the acid component. Since the water is actually part of the mass, you therefore have a higher molar mass of the material in the crystal and a lower proportion of moles of the acid to mass than you think. Hence, the pH skews upwards.
Calibration was done with 4 buffers at 25.0C with a slope of 98% and an offset of 2mV. All materials were weighed on dry basis (dried in vacuum oven at 110C for 3 hours). Temperature of the measured sample was also 25C
Other contaminations that will have the same effect, which are frequently encountered: dust in the KH2PO4, sodium in the K2HPO4, K2HPO4 in the KH2PO4. These are not uncommon things to deal with.
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