Exp 1 Key Entered values are sample values
1
Part A. Buffer Preparation
1. pH = pKa + log (γ A x Vol A x [A] / γ HA x Vol HA x [HA]) and 0.10 M = [A]
... [Show More] = [HA]
7.4 = 7.22 + log (0.445 x Vol A / 0.744 x 19 ml); 1.51 = 0.445 x Vol A / 0.744 x 19 ml
Vol A = 48.0 ml ~ 48.1 ml (not what is required from Table A-1).
Phosphate buffers are far from ideal solutions, hence even the activity coefficients do not fully account for
the difference in required volumes. Other contributions include the effects of ionic strength (of all ions
present: not shown are the counter ions) and temperature. Hence, the phosphate buffer mixing table, Table
A-1, was derived empirically, not theoretically. An important point here is that when making a buffer, the
buffer should be titrated to the desired pH and the actual pH recorded, not merely “hoping” the pH is
correct from theoretical calculations, which is abundantly clear in this example using phosphate.
Part B. Buffer Capacity
1. As the buffer is diluted, the capacity of the buffer to resist changes in pH diminishes (the concentrations
of both the conjugate acid and conjugate base forms of the buffer are being diluted). The trend, if NOT
using Davis water*, would be for the pH to go towards 7.0 (neutral) for greater dilutions with water.
What is typically observed in Table B-1 data is the pH does not change much, or decreases down to 8.6 or
8.7 or 8.4. Note: if the acetate buffer, pH 5.7, is diluted, which direction would the pH trend towards?
*FYI: Davis well water is some of the “hardest” water known. Hard water has a high mineral content (in
contrast with soft water). Hard water primarily consists of calcium (Ca2+) and magnesium (Mg2+) metal
cations, and other dissolved compounds such as bicarbonates and sulfates. Calcium usually enters the
water as either calcium carbonate (CaCO3), in the form of limestone and chalk, or calcium sulfate
(CaSO4), in the form of other mineral deposits. The predominant source of magnesium is dolomite
(CaMg(CO3)2). Hard water is generally not harmful. The pH of hard water is alkaline.
2 & 3. Expected pH: find the concentrations of the A and HA forms (before titration with the strong acid
or strong base), then calculate the change of the concentrations of A and HA when the strong acid titrates
the A form of the buffer, or when the strong base titrates the HA form of the buffer. [Show Less]