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CIRCLE 21 ON READER SERVICE CARD 322 A · ANALYTICAL CHEMISTRY, VOL. 5 1 , NO. 3, MARCH
1979
ratio of five hydrogen ions generated for each four electrons passed. We ti trated the 4-aminopyridine with hy drogen ion generated at this anode. We also titrated the 4-aminopyridine by adding an excess of standard per chloric acid and back titrating the ex cess with hydroxide generated at the cathode. The perchloric acid was stan dardized coulometrically in the same way. Thus, the 4-aminopyridine was titrated coulometrically two ways, at the anode and at the cathode. Masses of about 3 g of 4-aminopyri dine were titrated. The main current, running for some 8 h, was in the neighborhood of 64 mA. The smaller current, used through the end-point regions at the beginning ("pretitration") and at the end of a titration, was approximately 6.4 mA. These cur rents were measured, of course, with high precision, actually by measuring the potential drop over a 20-ohm re sistor with a Leeds and Northrup Type K5 potentiometer. The position of the end-point in the titration curves was determined by fitting a cubic equation to the data and setting the second derivative equal to zero (73). To calculate a value for the faraday from our data, it is necessary to know the molecular weight of 4-aminopyri dine, and of course, with the same ac curacy as in the remainder of the work. For the atomic weights of car bon and hydrogen, we took the values, H = 1.00797 ± 0.00001 and C = 12.01115 ± 0.00005, from the "1961 Table of Atomic Weights" rather than from later tables in which the num bers have been rounded off for general use and the ± figure represents, not the uncertainty in the value, but the range which encompasses all values of the isotope mixture ever reported. For nitrogen, we calculated a value, Ν = 14.00672 ± 0.00001, from certain works of the 1960's on the isotope ratio of nitrogen and rejected other work not applicable to our 4-aminopy ridine. The molecular weight of 4-ami nopyridine used was 94.11702 with an uncertainty of 0.00003. When this work on the faraday was initiated, it was intended to determine the isotope ratio of the three elements in the spe cific 4-aminopyridine used; unfortu nately this part of the program was never carried out. It is barely possible, of course, for an analytical chemist of the Richards, Willard, Lundell, Wichers stripe to get through a lecture without presenting an extensive table of results. However, I will forego my contribution to defer ence to tradition. The published pa pers (2, 3) give the data in detail so that the interested reader, perhaps some future individual with a new the oretical approach which needs check ing, can repeat the calculations. The bottom lines of the respective tables
