samplc aliquot and the volume of barium acetate solution to back titrate an ewess of about 4 ml. of 0.05-V barium acetate in a total volume of approximately 60 nil. Under optimum conditions, the precision and accuracy are comparable to that obtained by the potentiometric method. Effects of Foreign Ions. The effects of various anions and cations in the potentiometric determination of sulfate have been evaluated [ANAL. CHEM.33, 266 (196l)l. The same anions will interfere regardless of the method used to detect the end point. Certain cations t h a t form basic acetates also interfere in the titration of barium acetate. Attempts were made to determine conductometrically the sulfate in potassium sulfate, nickel sulfate, and ferric sulfate to learn whether the corresponding acetates, which are produced by the reaction of barium acetate with the sulfate salt, can be distinguished from barium
y - v r - - - -
4 0 Y O
i
7-
---
--
I l l 2 3 4 5 6 7 VOLUME OF 0 0 3 N HCI04 , m .
8
Figure 3. Conductometric titration curves of various sulfates Conditions Total volume of solution titrated, 50 ml. Ba(CHaC00)Z present, 0.20 meq. Sulfate present, 0.05 meq.
acetate by conductometric titration. The titration curves obtained are shown in Figure 3. I n each case, the theoretical end point is 5 ml. A correct
and well-defined end point was obtained for ferric sulfate. I n the case of potassium sulfate, a sharp break in the curve occurred only after both the potassium acetate and the excess barium acetate had been titrated. For nickel sulfate, there was no well-defined break at any point in the titration curve. It is apparent that in the conductometric titration barium acetate can be differentiated from the very weakly basic ferric acetate but cannot be distinguished from either potassium acetate, which is a somewhat stronger base, or nickel acetate, which is a somewhat weaker base. GERALDGOLDSTEIN D. L. MANNINQ H.E. ZI’ITEL Anal tical Chemistry Division Oak k d g e National Laboratory Oak Ridge, Tenn. The Oak Ridge National Laboratory is o erated by Union Carbide Corp. for the S. Atomic Energy Commission.
8
Determina tio n of Ethy le ned ia mine in 2-Met hy Ipiperazi ne SIR: Near infrared spectrometry has been very useful in organic quantitative analysis. Kaye (1, 2) in his reviews of near infrared spectroscopy points out that this region of the spectrum is concerned primarily with hydrogenic stretching vibrations of CH, NH, and OH, their overtones and combinations. In a general study conducted in this laboratory on methods of analysis of amine mixtures, a striking difference was observed in the near infrared spectra of ethylenediamine and 2-methylpiperazine. This difference was then used to develop a method for the determination of ethylenediamine in 2-methylpiperazine. At a wavelength of 2.02 microns, ethylenediamine exhibits a strong absorption band which is not present in 2methylpiperazine. A further investigation showed that although 2-methylpiperazine does not exhibit a strong
Table 1.
Differential Analysis of Ethylenediamine
Ethylenediamine, Moles/Liter Added Found 0,019 0.028 0.039 0.051 0.059 0.070
0,019 0.028 0.038 0.050 0.058 0.071
1 170
0
Deviation, Moles/ Liter
Deviation, Mole %
0.000
0.0
0.000
0.0
0.001 0.001 0.001
2.6
0.001
ANALYTICAL CHEMISTRY
2.0 1.7 1.4
band at 2.02 microns, increased concentrations of 2-methylpiperazine caused an increase in the background transmission. It was possible to eliminate this interference by differential analysis. This technique has been discussed by Robinson (4) and McDonald (3) in their publications. Washburn and Scheske (6) have applied it to the determination of traces of ketone in a carbinol. Differential analysis is particularly suited for use with double beam instruments. The effect of the interfering material in the sample solution is compensated by placing a solution of the solvent and interfering material in the reference beam. The resulting absorbance is due to the desired constituent only. EXPERIMENTAL
Apparatus. A Perkin-Elmer Spectracord 4000 double-beam spectrophotometer equipped for use in the near infrared with 10-mm. silica cells. Reagents. Although reagent grade carbon disulfide and carbon tetrachloride are excellent solvents for use in the near infrared, neither can be used with amines because amines react rapidly with carbon disulfide and slowly with carbon tetrachloride. Many solvents were found for ethylenediamine and 2-methylpipera~ine~ but most of them absorbed too strongly t o be of use. Pyridine, reagent grade, although exhibiting strong absorbance in the near infrared, was found to transmit sufficient energy at 2.02 microns to permit its use as solvent for this system. Both ethyl-
enediamine and Zmethylpiperazine are readily soluble in pyridine. Beer’s law. Solutions of varied concentrations of ethylenediamine in pyridine were prepared and the transmittance was measured at 2.02 microns. Ethylenediamine i n pyridine obeys Beer’s law at 2.02 microns as demonstrated by the constant absorptivity. For 12 solutions examined, the average deviation of the absorptivity was +0.77%. Procedure. Solutions of 2-methylpiperazine in pyridine were prepared with varied amounts of ethylenediamine from 0.019 to 0.071 mole per liter of pyridine plus 2-methylpiperazine. A reference solution was prepared containing approximately the same amount of Zmethylpiperazine in pyridine ea the sample. The differential absorbance between the reference solution and the sample solution was measured at 2.02 microns and the results were calculated as ethylenediamine. The data obtained are presented in Table I. The average deviation of ethylenediamine in moles per liter is 0.0007 or 0.7 mole %. The data presented in Table I correspond to a concentration range of approximately 3% to 18% ethylenediamine in 2methylpiperazine. DISCUSSION
Effect of Water. Water exhibits a strong absorbance at 1.94 microns. Its effect on the absorbance of ethylenediamine at 2.02 microns was investigated by preparing solutions of 2-methylpiperazine and ethylenediamine in pyridine with varied
amounts of water. The absorbance curves obtained from solutions containing approximately 17yowater and higher show that water causes an increase in the intensity and a decrease in the sharpness of the absorbance a t 2.02 microns. As the concentration of water increases, the amine band a t 2.02 microns becomes a shoulder of the water band until it is completely obscured by the broad water band. Several different types of desiccants and adsorbants were used in attempts
to remove the water without affecting the amines present. None Of the materials used produced satisfactory results. The differential analysis technique was extended to water* For samples containing reasonably large amounts of water, a reference solution must hold the same amount of water as the sample solution before it is placed in the reference beam. I n this manner, compensation can be made for the water present in the sample.
LITERATURE CITED
( 1 ) Kaye, W., Spectrochim. Acta 6, 257 (1954). (2) Kaye, W., Ibid., 7, 181 (1955). (3) McDonald, 1. R. C., Nature 1749 703 (1954). (4) Robinson, D. Z., ANAL.CHEM.24,619 (1952). (5) Washburn, W. H., Scheske, F. A., Ibid*,299 346 (1957)*
RODNEY J. QUENTIN Research Division
Wyandotte Chemic& carp. Wyandotte, Mich.
Reverse-Current and Current-Cessation Chronopotentiometry with a Two-Component System SIR: This note examines the chronopotentiometric behavior of solutions containing two separate reactants A and C which can be consecutively reduced and then reoxidized a t the electrode. The cathodic electrode reactions are A
+ nle-
+.
B
(1)
+
C n2e-+. D (2) The two-step reduction of a single starting species according to A B
+ rile+ n2e-
-+
B
(3)
4
D
(4)
is mathematically equivalent to the first case so that the equations presented will apply to both cases. Rouse (2) and Testa and Reinmuth (3) have given general theoretical treatments of current-reversal and currentcessation chronopotentiometry for multicomponent systems as well as some experimental results. However, there is no record of previous experimental study of systems in which both the oxidized and reduced forms of all species are soluble in the solution. We have studied the behavior of solutions of Fe(II1) and Cu(I1) in 1F HC1 and Ce(1V) and Fe(II1) in 3F HzSO4. I n both cases four chronopotentiometric waves are obtained, two preceding and two following current reversal. As pointed out by Testa and Reinmuth (S), the occurrence of the reaction nzA
e.g., Fe(II1)
tion times following current reversal, should also not be influenced by the occurrence of reacton 5 if all M u s i o n constants are assumed equal. This becomes apparent if a new concentration variable, C*, is used:
+ nlD = nlC + nnB Fe(I1)
+ Cu(I1)
+ nZC$
(7)
CA and CD are the time-varying concentrations of A and B and CCO is the initial concentration of C. C* obeys the same equations and boundary conditions that describe the chronopotentiometric behavior of a simple one-component system. Namely
ac* - -D-a"* bt
C*-+C*OasX-+
m;
C* = C*O at t = 0
where D is the common diffusion coefficient of all species, and io is the current density. Thus C* will obey the well known chronopotentiometric equations derived for a one-component system (I)-i.e., a t the electrode surface
C* =
c*o [l -
(5)
(-& t'
+ 1)1/' + = t + >0 (71
72)
(11) (6)
in the solution near the electrode does not affect the observed values of 71 or r2except for differences in diffusion coefficients of the various species. This is true regardless of the rate of this reaction. Analogous arguments can be used to shon- that r3 and r4, the transi-
The four transition times correspond to the following values of C*: 71:
c* = ?&@
72:c*
Rouse (8) expressed the quality in Equation 12 in the form
-
0
For the case where all concentrations and n-values are equaI, Equation 12 gives 7 ) = 0.0717 (71 72) which, taken with the ratio (71 r2)/(n 4r4) = 3, corresponds to 7 3 = 0.215 (r3 r4). Because, for this special case, r3 corresponds to the point a t which the concentration of the reduced species a t the electrode surface is just one half of its maximum value, the value of r3 could also be calculated from Delahay's equation 8-35 (1) with the same result. (The relation t' = 0.222 7' given by Delahay contains an arithmetic error; the correct relation is
+ +
+
t'
c* = nlCAo + mcco
:
= 0.215 T'.)
To test Equation 12 experimental data were obtained for Fe(II1)-Cu(I1) solutions in 1F HC1 and for Ce(1V)Fe(II1) solutions in 3F H804. A series of experiments with a solution approximately 0.01M in Fe(II1) and Cu(I1) gives the following results: 7,
= 2.88 sec.;
=
5.56 sec.;
78:C* = n2CP
74
+
+
3x2
2(*2)1/2],
+ Cu(1) =
- nzCD
C* = nlCn
The values to be expected for the successive transition times can be calculated from Equations 10 and 11. The results for r1and rn are well known (1). The fact that (78 r4) is one T ~ in ) the absence of third of (rl complications is also familiar (1). The value of 73which results from Equations 10 and 11 is
T~
TI
= 0.92 sec.
calculated from Equation 10 = 0.96 sec.
VOL. 34, NO. 9, AUGUST 1962
1171
