The Budget Minded ιTitration System from RADIOMETER
Automatic End Point Titration with DIGITAL READ-OUT This Radiometer system offers outstanding features to gether with savings in both time and equipment costs. It's:
Automatic — Allowing "Hands Off" operation for spe cific or routine End Point Titrations. Digital — Providing direct n u m e r i c a l r e a d - o u t of titrant consumed, accurate to 0 . 0 4 % buret volume. Flexible — Offering a base for a more sophisticated system including recording capabilities as well as allowing use of the pH meter by itself. Economical — Since it's easy on the equipment bud get and results in considerable time savings in the laboratory.
These benefits are inherent in Radiometer modular analyt ical systems. In this case the new A B U 1 1 Auto'- Buret adds the important digital read-out to the automatic oper ation of the versatile pH m e t e r / t i t r a t i o n control unit combination. Complete information on this and other Radiometer analyt ical systems is available on request.
Lοndοn In Canada: B a c h - S i m p s o n Limited
Circle No. 127 on Readers' Service Card
114 A • ANALYTICAL CHEMISTRY
THE LONDON COMPANY 811 Sharon Drive Cleveland, Ohio 4 4 1 4 5 Telephone: (216) 8 7 1 - 8 9 0 0
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INSTRUMENTATION factory. In practically all cases, no data are given and it is difficult to ana lyze the emf vs. time curves in any pre cise fashion. Dr. Rechnitz has made repeated pleas for more information (data), to which we would add, "always give the final value of the equilibrium potential, even if you have to wait hours or days for the answer." There is the temptation to quote the half-time (fin)—»•£·> the time required for the emf to attain half the final equilibrium value. In the usual analytical tech niques, this is an illusory constant, be cause the measurement, in the early stages, is a fruity mélange of mixing time and electrode equilibration. It is not unusual to encounter in the literature a statement that "the electrode had fast response with a half time of 1 second" and then followed by "the potentials were recorded after an interval of 2.5 minutes." This still gives no indication of the true potential equilibrium or what fraction thereof is attained in 2.5 minutes. There is an empirical approach which affords an accurate prediction of the equilibrium value, obtainable from a few values taken at moderately short times. We speak of this briefly because of its utility, particularly in kinetic studies. Basically, the object is not to delineate the entire emf vs. time curve, but to get an accurate prediction of the equilibrium value. But in simpler terms, "one is interested in where he is going rather than where he has been." For some time, here at Baton Rouge, we have looked into this matter in collaboration with Dr. Doris Miiller and Dr. Philip W. West. Measurements of emf to ±0.1 mV as a function of elapsed time were made with an Orion Ag2S membrane electrode for "jump" increments of Ag+ ion. The data were accurately represented by the equation: Ε = t/(a + bt) where Ε is the increase in potential caused by the sud den increment of Ag+ concentration. The curve is a hyperbola and a plot of t/E vs. t yields an excellent straight line, the slope of which is ο and the in tercept is a. The maximum value of Ε (for t = °° ) is equal to 1/5, the recip rocal slope. As expected, there is a small but progressive deviation from linearity for the lower values of ί in which range the experimental condi tions are ill-defined. The prediction of the equilibrium potential Ee does not involve any dubious extrapolation pro cedure because it is given by the recip rocal slope of a straight line and the de gree of linearity of this line is a reliable criterion of the confidence which can be placed in the calculation of Ee.
