Exp_1_Key_+W12 - Exp 1 Key Winter 2012 Part A Buffer...

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Exp 1 Key Winter 2012 1 Part A. Buffer Preparation 1. pH = pKa + log (γ A x Vol A / γ HA x Vol 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 (still not what is required from Table A-1, but closer). Other contributions include the effects of ionic strength (of all ions present) and temperature; and variance introduced in weighing solids and measuring liquids. 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. Part B. Buffer Capacity 1. Dilution of a buffer with water will not change the [A] / [HA] ratio. 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 just water with greater dilutions as the buffer approaches the concentrations of H + and OH - for the ionization of water: [H + ] and [OH - ] are both 10 -7 M. What students typically observed was the pH did not change much, or decreased 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 water is some of the “hardest” water known. Hard water is a type of water that has high mineral content (in contrast with soft water ). Hard water primarily consists of calcium (Ca 2+ ) and magnesium (Mg 2+ ) metal cations, and sometimes other dissolved compounds such as bicarbonates and sulfates. Calcium usually enters the water as either calcium carbonate (CaCO 3 ), in the form of limestone
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