Question

Write out equations for the dissociation of the aci of both H2PO4^- and tris buffer systems.

What are the normal source of error in buffer/dilutions lab. Please list 4 common errors and how to avoid them.

Answer 1

The acid dissociation reactions for H₂PO₄^- and Tris acid (Tris-H⁺) can be shown as below.

H₂PO₄^- (aq) ---------> H⁺ (aq) + HPO₄^2- (aq)

(HOCH₂)₃CNH₃⁺ (aq) ----------> H⁺ (aq) + (HOCH₂)₃CNH₂ (aq)

Tris Acid                                                      Tris Base

The question asks for errors associated with buffer preparation. Since dilution is mentioned, we will assume that the buffers are prepared by dilution of a stock solution.

There are a number of errors associated with dilutions, some of them being:

i) A number of glassware like conical flasks, burets, pipettes, etc are used in a dilution lab. Each glassware has an uncertainty of measurement associated with it and therefore, the buffer preparation will entail an uncertainty in the exact concentrations of the weak acid/conjugate base used to prepare the buffer.

ii) Another common method of preparing a buffer is to titrate a weak acid with a strong base of known concentration so as to obtain the conjugate base of the weak acid. The equivalence and end points of titration experiments are slightly off hence, the exact concentrations can be skewed.

iii) It is important to prepare a buffer at the exact pH value desired. Measurement of the pH has an uncertainty associated with it since the electrode used for measuring the pH has an intrinsic uncertainty. Moreover, the electrode needs to be properly calibrated and errors can creep in during proper calibration of the pH-meter.

iv) Finally, volumes of solutions are susceptible to temperature changes and hence, the concentrations of the buffers will depend on the temperature of the experiment. A slight change in temperature can alter the concentrations of the weak acid/conjugate base and hence affect the pH.