and 1 low-carbon copper ring (Leco product). Ignite the sample in the Leco Conductometric Carbon hnalyzer and measure the resulting COz by recording the change in conductance betneen the weak 13a(OH)nsolutions with the conventional oscilloscope attachment. Correct for a blank which has been taken through the entire procedure, along with the sample portion. Standardize by running an appropriate Bureau of Standards steel sample.
is consistent with the metallographic results on the sample and may be due to traces of soot deposited on the sample surface during arc melting.
RESULTS
LITERATURE CITED
Duplicate analyses by this procedure gave 129 and 145 1i.p.m. of free carbon or an average of 137 p.p.m. This value
(1) Hansfn, 31 "Constitution of Binary Alloys, 2nd ed McGraw-Hill, Xen York. 1958
( 2 ) Kriege, 0. H., "The Analysis of Re-
fractory Carbides, Borides, Kitrides and Silicides,'' Los Alamos Scientific Laboratory, Report LA 2306 (1959). (3) Pauson, P. L., McLean, J., Clelland, m.J., S a t u r e 197, 1200 (1963). GEORGE FEICK
ACKNOWLEDGMENT
The authors are grateful to Gerald Clay of .\rthur D. Little, Inc., and Robert Liu of Ledoux and Co. for much of the experimental work.
Arthur D. Little, Inc. 15 Acorn Park Cambridge, Mass. \ \ r I ~ ~ I GIUSTETTI ~ V
Ledoux and Co. Teaneck, N. J.
~
WORKsupported by the L S Air Force Aeronautical Sj'stems IXviaion under Contract ASD-TDR-62-201
Effect of Input Resistance on Potentiometric Titrations in Nonaqueous Solvents SIR: T o obtain optimum results in following the course of acid-base titrations in low conducting systems by the usual potentiometric techniques, it is essential that an e.m.f. measuring instrument with a sufficiently large input resistance (impedance) be used. Most of the comniercial pH meters available today are designed 1 ure the e.m.f. of aqu the glass electrode. sistence encountered in the neighborhood of 10; ohms, which is approximately the resistance of the glass electrode itself. The aqueous solutions have relatively low re h.'-t i > ance and therefore contribute very little to the overall resistance of the cell. However, in the case of the extremely low conducting systems frequently encountered in nonaqueous titrations, the overall resistance of these cells, including the glass electrode, is very large. Hence the question arises as to what input resistance is required to measure the actual e.m.f. of these systems. The purpose of this communication is to call attention to the nature and the magnitude of the effect that input resistance can have on titrations in low conducting nonaqueous systems. Data are reported which show the actual increase in the e.m.f. range obtained for the titration of benzoic acid in acetone when the input resistance was increased from 10' to >10l4 ohms. Similar data were obtained when the titration was carried out in several other solvents inciluding dichloromethane and benzene. EXPERIMENTAL
Potentiometer. h Keithley Model 6lO;i electromet,er (Keithley Instru ments, Inc., Cleveland, Ohio) was used to measure e.m.f. The input, resistance of this instrument can be varied in decade steps from 1 ohm to 10" ohms by means of shunt resistors built into the instrument. The maxi-
mum input resistance is greater than l o i 4ohms Electrodes. 2l, inch glasz (Beckman, general purpose)-KCl, methanol modified calomel electrodes n ere uqed. These were connected to the electrometer \\ith 4 feet of coaxial cable (Type R G ,5S A L-) and a n Amphenol plug. T h e g1a.s electrode a a s a t tached to the center conductor and the calomel electrode u a s attached to the cable zhield and instrument ground.
900r----800
Input Resistance I
7001 2
E
Potential Range(mV)
>1014a 109n
540 465
lO8n
185
107n
25
+
500
0
5
3001
I
A
100
m l 0.1N(n-Bu14 NOH TITRANT
Figure 1 Effect of input resistance on potentiometric titration of benzoic acid in acetone I
Sample: 0.38 millimoles of benzoic acid in 4 0 ml. acetone Electrodes: glass-KCI, methanol modifled calomel Potentiometer: Keithley Electrometer M o d e l 61 OA 0.1N (n-Bu)cNOH i n 9:l benzeneTitrant: methanol
Titrant. Tetra-n-butylammonium hydroxide, O.l.Y, wa.5 prepared by diluting lJlI methanolic solution (Southwestern Analytical Chemicals, Auqtin, Texas) with reagent grade benzene. RESULTS A N D DISCUSSION
Titration curves for benzoic acid in acetone a t various input resistances are shown in Figure 1. The curve3 a t input resistances of 10'0 ohms and 10l1 ohms were determined also and were essentially identical to the curve for >loi4 ohms. I t is apparent from the data that the input resistance of the e.m.f. reading instrument used to follow the course of a titration in a low conducting can seriously affect the magnitude of the e.m.f. break a t the end point. Furthermore, the e.m.f. range of the solvent, from the viewpoint' of performing differentiating titrations, is also directly affected. The effect observed is undoubtedly caused by the amount of current drawn from the cell (loading) by the electrical circuit used to measure the e.m.f. I n other words, the greater the input resistance the smaller is the load, allowing therefore a more accurate measurement of the actual cell e.m.f. The data in Figure 1 show that to obtain the maximum possible e.m.f. range for the titration and differentiation of acids like benzoic acid in acetone, an instrument with an input resistance of at least 10'0 ohms is necessary. The actual minimum level of resistance for optimizing other titrations will, of course, depend upon the solvent and particular titration reaction involved, including the type of acid or base. MAXWELL L. CLIJETT Industrial and Biochemicals Department E. I. du Pont de Semours & Co. Wilmington, Del. VOL. 36, NO. 1 1 , OCTOBER 1964
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