Table II. Results of Coulometric Titrations of 4-Aminopyridine Leading to a Value for the Faraday Work done at Iowa State University (ISU) and repeated at the National Bureau of Standards (NBS)
EXAMINE... Spectral
Data
F in 1972 NBS coulombs per g-equiv-wt (Uncertainty 3 in ppm) ISU NBS
Anodic oxidation
at hydrazine-platinum anode
Cathodic reduction0
96 486.40 (6.2) 96 486.78 (2.3)
96 486.58 (2.3) 96 486.53 (2.7)
a Standard deviation of the mean combining the random errors with the systematic errors estimated at the 70 % confidence level (one standard deviation). ° Direct titration with hydrogen ion generated at the anode- c Back titration with hydroxy ion generated at the cathode after addition of excess perchloric acid.
are given here, in Table II, that is the values obtained for the faraday and an estimate of the combined random and systematic uncertainties. As you see, the results obtained at ISU and NBS were identical, the average being the value reported in Table I, 96 486.57 1972 NBS coulombs per gram-equiva lent-weight. T h e agreement between the ISU-4-aminopyridine value and the earlier NBS-Craig-silver dissolu tion method is striking. The discrep ancy, however, between the direct, ex perimental, electrochemical value and the calculated value now becomes a matter of real concern. It would, perhaps, have been wise to have terminated this work at this stage. T h e new value is after all two orders of magnitude better than the accuracy and precision characteristic of essentially all chemical work, indus trial, clinical, and academic. But while the policy of quitting while you are ahead is sound in playing a market, scientific research has not only many of the elements but much of the irre sistible lure of gambling. T h e physi cists report that a backlog of experi mental and theoretical work has accu mulated because a high-precision value for the faraday is lacking. More over, they are serious enough about the business to undertake an exhaus tive re-examination of the various ex perimental measurements used in ar riving at their value. T h e chemists can hardly do less. Then too, it may well be that the discrepancy does lie in the chemical work. If so, however, the fault is something common to the elec trolysis of aqueous solutions involving such diverse reactions as the anodic dissolution of silver, the anodic gener ation of hydrogen ion, and the cathodic generation of hydroxyl ion. Because the calculated value is smaller than the direct, electrochemical value, it appears as if a very small portion of the electricity passing through the electrochemical cell does so without effecting a chemical change at the
electrodes. Sufficient unexplained phenomena, having to do principally with the change in pH through the equivalence-point region, had been observed during our own work to keep us intrigued. And in addition, some astonishing and quite unorthodox re sults of simple electrolysis experi ments have been reported during the immediate past few years by the Indi an chemist Palit (14), work which has been unconscionably ignored by the electrochemists. The Palit observa tions may indeed strike directly at our assumption of 100.0000% current effi ciency in our faraday experiments. The pressure, though, to do better comes from the physicists. Almost without making a deliberate decision, I slipped into an effort to reduce the uncertainty in the electrochemical value to less than 1 ppm. A necessary prelude to such an ef fort is a re-examination of the uncer tainties in the various measurements involved in the determination just completed (ref. 3, page 511) and steps to reduce the uncertainty in each to less than 1 ppm. The uncertainties in the standards of emf and resistance are estimated to be 0.2 ppm. By a modification of the electrical measur ing circuit, it has been possible to eliminate the potentiometer and re place it with a simple null point detec tor good to 0.2 ppm. We have acquired a new constant current source, based on the Kroeger-Rhinehart circuit (15), stable to 0.5 ppm and have devoted considerable effort to eliminating from the circuits electrical pick-up noise, transients arising during switch ing, and grounding effects. As with the electrical measurements, the measure ment of time, previous uncertainty 0.2 ppm, is above reproach, although we have acquired a new timing device and are arranging to key it to the color TV broadcast timing which is reported (16) to be good to a few parts per bil lion. With this apparatus we hope to confirm the 4-aminopyridine value by
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ANALYTICAL CHEMISTRY, VOL. 5 1 , NO. 3, MARCH 1979 · 325 A
