Chemiluminescent Indicator Titration of Lead with Potassium Chromate In lead-Tin Alloys and in Metallic Samples Containing lead, Tin, Antimony, and Arsenic FREDERIC K E N N Y and R. B. KURTZ Department
of Chemistry, Hunter College, New York, N. Y.
Lead in lead-tin alloys and mixtures was rapidly determined by titrating with potassium chromate. Siloxene indicator was used. Interference hy tin, antimony, and arsenic was avoided by elimination of these elements as bromides prior to titration.
L
EAD has been determined (6) with an accuracy of 0.7 part per 1000, and an average deviation of 1.3 parts per 1000,by titrating it with potassium chromate in a dark chamber titrator ( 4 ) and employing siloxene chemiluminescent indicator (3). The light emitted a t the end point was detected by a Photovolt multiplier photometer 520-A (6). The pH a t the beginning of the titration was approximately 2.6. To study the possibility of interference by other metals, the present authors ( 5 ) titrated solutions which were 0.1M with respect to an additional metal ion as well as 0.1M with respect to lead. The additional metal ions were manganese (11), nickel (11), iron (111), zinc (11), calcium (11), and magnesium (11). These ions do riot form a precipitate with chromate at the pH employed. The errors encountered n'ere for the most part between 1 and 2 parts per 1000. The largest error, which was encountered with the zinc solution, was 3 parts per 1000. To determine the effect of the presence of tin, lead was titrated in a solution of an alloy containing 47.83y0 tin and 52.17Y0lead. The results, which were totally unsatisfactory, were low to the extent of 96 parts per 1000 and had an average deviation of 13 parts per 1000. In the unsatisfactory method used the alloy was dissolved in 6iM nitric acid, after which the pH was adjusted to 2.6 with ammonia. Most of the tin was present as hydrated stannic oxide. If the effect of the stannic oxide was merely to absorb the light evolved by the indicator, high results would have been produced, because additional chromate would be required to attain a stronger light. The low results can be accounted for if it is assumed that stannic oxide adsorbs lead ions or that some stable complex ion of lead is formed; the end point, therefore, comes too soon. In order to eliminate the interference produced by tin, attempts were made to drive off the tin as stannic iodide by treatment with ammonium iodide. The results were completely unsatisfactory. The decision to use hydrobromic acid ( f , 2 )instead of ammonium iodide, however, led to the development of the method described in this paper. The lead-tin alloy, finely divided and containing about 0.5 gram of lead, is treated with 5 ml. of 48% hydrobromic acid in a covered 100-ml. beaker. Then 5 ml. of bromine reagent, previously prepared from 2 volumes of bromine and 7 volumes of 487, hydrobromic acid, is added dropwise to the covered sample in a good hood. The watch glass and sides of the beaker are washed with 48Y0hydrobromic acid from a small dropping bottle, and the watch glass is discarded. The solution is then evaporated to dryness in the hood with the use of an overhead radiant heater to avoid spattering; 45 minutes to 1 hour is required for the operation. The heating is continued for 15 minutes after dryness is reached. This operation eliminates tin as stannic bromide. The beaker is cooled and the sides of the beaker are washed down with about 8 ml. of 6 M nitric acid. The solid residue is broken up with a stirring rod and the beaker containing the stirring rod is covered and placed on a hot plate in the hood until the color of bromine is removed completely. The sides of the beaker are then washed with water. The volume.is kept below 25 ml. The pH is adjusted to 2.6 to 2.8 by the addition of ammonia. A glass electrode pH meter is employed. The stirring rod is then washed off and discarded. The solution is now ready for titration.
Eight samples can be treated by this procedure in less than 2 hours. In titrating the samples in the dark chamber titrator, approximately 75 mg. of siloxene indicator was used and the solution was stirred mechanically. The titrations reported in Tables I and I1 were continued until the pointer of the photometer, when operating on the No. 3 scale, moved swiftly from zero to one unit, which corresponds to 0.002 microlumen. The samples of Table I11 were titrated to a predetermined end point. The equation for the reaction involved in the titration is: Pb++
+ CrOd--
--+
PbCrOl
Two lead-tin alloys were prepared from chemically pure lead and chemically pure tin. The temperature was very carefully controlled, so as to be only slightly above the melting point, and the melt in a borosilicate glass container, stirred with a glass rod, was held in the furnace only long enough to ensure complete mixing. No smoke or oxide film was observable and the melt had a mirrorlike surface. Each alloy was disintegrated by filing and was carefully sampled. Because of the great care exercised, it was felt that the errors in preparing these alloys were exceedingly small. The compositions were 52.17y0lead and 47.83y0tin, and 70.83% lead and 29.17y0tin, respectively.
Table I.
Sample,
G.
1.1912 1.1913 1.1920 1.1923 1.1921
Titration of Lead in 52.1770 Lead-47.8370 Tin Alloy with 0.1000-WPotassium Chromate (75 mg. of siloxene indicator) Potassium Lead Found DeviaLead in Chromate, by Analysis, tion, Parts/ Sample, C. M1. G. Lead, % 1000 0.6216 52.18 0.76 0.6214 30.00 52.21 0.19 0.6215 30.02 0.6220 0.6226 52.23 0 19 0.6219 30.05 0.95 0.6220 52.17 0.6220 30.02 52.32 1.91 30.10 0,6237 0.6'219 ,Mean 5 2 . 2 2 0.80
Table I gives the results of hhe titration of the 52.177, lead47.83y0tin alloy. The precision obtained, in terms of the standard deviation, is 1.1 and in terms of average deviation is 0.8. The mean error involved is 1.0 part per 1OOO. Table I1 gives the results of the titration of the 70.83% lead29.17% tin alloy. The precision obtained, in terms of the standard deviation, was 2.5 parts per 1000 and in terms of the average deviation wm 1.8. The error was 3.3parts per 1000. The small average error of 1.0 part per 1000 presented in Table
Table 11. Titration of Lead in 70.83% Lead-29.17% Tin Alloy with 0.1000M Potassium Chromate Sample,
G.
0.8765 0,8770 0,8782 0,8769
1206
(75 mg. of siloxene indicator) Potassium Lead Found DeviaLead in Chromate, by Analysis, tion, Parts/ Sample, G . MI. G. Lead, % 1000 0.53 0,6185 70.56 29.85 0.6208 70.41 2.50 29.80 0.6175 0,6212 0.79 0.6195 70.54 29.90 0.6220 3.29 0,6216 70.85 30.00 0,6211 Mean70.60 1.78
1207
V O L U M E 28, NO. 7, J U L Y 1 9 5 6 Table 111. Titration of Lead in Sanlples Containing Antimony, .Arsenic, and Tin with 0.1000.M Potassium Chromate (75 mg. of siloxeiie indicator)
Sample Xo.
Lead in Sample, G.
Potassium Chromate, Ml
by Analysis,
G.
Error, Parts/1000
1
0.6229 0.6230 0.6216 0.6238 0.6226
30.02 29.97 30.05 30.10 30.10
0.6220 0.6210 0.6226 0.6237 0.6237
1.44 3.21 1.60 0.16 1.77
2 3 4 5
.
Lead Found
Mean error 1.64
Table LV. Standardization of 75 Mg. of Siloxene Indicator in Titration of 30.00 M1. of O.1OOOM Lead Nitrate with 30.00 M1. of 0 . 1 O O O M Potassium Chromate Titration
Photometer Reading Scale divisions Microlumen
1 2 3 4
1.0 1.5 2.0 2.0
0.0020 0,0030 0,0040 0.0040
Mean
1.6
0,0032
bromides ( 1 , 2 ) ,it appeared probable that the treatment employed for the elimination of tin would also eliminate both antimony and arsenic. Consequently, five mixtures of chemically pure portions of lead, antimony, arsenic, and tin were prepared. These mixtures had the compositions: Sample
1
2
3
4
5
Lead, % Antimony, Gh hrsenic, C;; Tin, %
69.19 2.42 0.69 27.70
69.08 2.27 0.79 27.86
69.11 2.20 0.53 28.16
69.17 2.27 0.60 27 96
69.00 2.43 0.72 27.85
These samples were dissolved and prepared for titration in exactly the same manner as the two lead-tin alloys previously considered. Table I11 gives the results of the titrations. For very accurate work standardization of the indicator is desirable. Variations in the method of preparing the siloxene indicator and its age, which can conceivably affect the end point, can be taken into account by standardization. Thus for the titrations presented in Table I11 the indicator had been previouslj standardized by titrating 30.00 ml. of O.1OOOM lead nitrate solution with 30.00 ml. of 0.1OOOM potassium chromate (Table IV). As a consequence, the titration of the samples shown in Table I11 nas continued until the photometer reading was 1.6 on the No. 3 scale. LITERATURE CITED
I as compared with the corresponding value of 3.3 parts per 1000 obtained from Table I1 probably results from a fortuitous balancing of errors, as the errors involved in the physical measurements alone would lead one to expect an accuracy within 2 parts per 1000. Errors of method could, of course, increase this t o an even larger value. Eight samples of alloy were subjected to the entire analysis, including p H adjustment and titration, in 2.5 hours. Based on the properties of antimony and arsenic and their
(1) (2)
Blumenthal, H., Metal2 u. Erz 37, 233 (1940). Hillebrand, W. F., Lundell, G. E. F., Bright, 13. A., Hoffman, J. I., “Applied Inorganic Analysis,” p. 290, Wiley, New York,
(3) (4) (5) (6)
Kenny, F., Elurta, R. B., ASAL. CHEY.22, 693
1953. (1950).
Ibid., 23, 383 (1951). Ibid., 25, 1550 (1953). Photovolt Corp., New York, N. Y , “Operating Instructions for Multiplier Photometer 520-A.”
RECEIVED for review November 1, 1955.
.4ccepted April 6, 1956.
Dielectric Values for the System Water-Ethyl Acetate P. H. BYRNE’ and C. Department
P. BROCKETT
o f Chemical Engineering, University of Toronto, Toronto, Canada
A study has been made of the relationship between a varying moisture content of ethyl acetate and the corresponding dielectric values, water being regarded as the solute. Up to nearly 2% of water, by weight, this correlation is linear. Conspicuous departure from linearity occurs only when sufficient time has elapsed for hydrolysis to intervene, a matter of hours or days at room temperature. Data are presented for linearity, and analytical applications are suggested.
T
HE quantitative determination of small amounts of dis-
solved water in an organic liquid by chemical means, whether in industry or in the laboratory, is usually very time-consuming and instantaneous results cannot be obtained in continuous-flow systems. The need for a speedier method has been reflected of late in the increasing resort to dielectric measurements for inferring moisture content in various liquid-liquid solutions. An important industrial product, ethyl acetate, was suggested for study of moisture content by this means (1). No previous 1 Preaent address, Ferranti Electric, Ltd., Research Division, Mount Denis. Toronto, Canada.
data have been published on dielectric value us. dissolved moisture for this compound. The call for such data is thus easily surmised. EXPERIMENTAL
The heterodyne-beat method, employing a C-R oscilloscope to show an unequivocal Lissajous figure a t the end point, was used for determining the test-cell capacitance-Le., dielectric v a l u e s a n d Karl Fischer reagent with dead-stop end point was utilized for corresponding moisture determinations. The arrangement of the apparatus is shown in Figure 1. With so sensitive a null-point arrangement, much care was taken to assure stability in the oscillators. The Clapp circuit (3) was used in the variable oscillator, and the crystal oscillator was based on the Colpitts circuit (7) modified to be additionally stable. Test Cells. In neither of two cells which were made was there any departure in principle from the conventional form of having two capacitor plates, as seen in Figure 1. One cell, waterjacketed for constant temperature and allowing total immersion of the plates (6), was made t o serve as a measure of the purity of the starting-point, or standard, benzene and to check for dielectric value against that published by the National Bureau of Standards ( 6 ) . No further use for this cell was called for.
