V O L U M E 28, N O . 1, J A N U A R Y 1 9 5 6
‘ d / \
c’‘
/ \
CH
I
4ChNOWLEDGhlENT
CHB CHI
CH3 CHa
0
129
0
11
l
CHz l
The authors wish t o express their appreciation to 0. R. Trimble for performing many of the analyses in the course of this investigation. LITER4TURE CITED
I
I1
whereas 3,5-dimethyl-2-hexene-5-olide (I1j s h o w no tendency to brominate after 2 hours. Aliphatic aldehydic carbonyl groups have little effect when conjugated with the carbon-carbon double bond except, perhaps, to speed addition of bromine. Double bonds adjacent to ether linkages are not consistent in their behavior ton-ard bromine-bromide reagent. Vinyl alkyl ethers cannot be determined because of erratic results, probably hecause of hydrolysis, whereas 2-formyl-3,4dihydro-2H-pyran reacts quantitatively.
(1) Braae, B., ANAL.CHEM. 21, 1461 (1949). ( 2 ) Hanus, J., Chew&.Zerbtj.2, 1217 (1901). (3) Kaufman, H. P., “Studien auf dein Fettgebiet,” Verlag Chemie, Berlin, 1935.
(4) Nulliken, S., and Kakeman, R . IND.ESG. CHEV.,AXAL. ED. 7, 59 (1935). (5) Sutherland, IT. W.,and Quinn, J. H., Carbide and Carbon
Chem. Co., South Charleston, W. Va., unpublished data, July 12, 1942. (6) Trappe, W , Biochem. Z. 296, 174 (1938). ( 7 ) Uhrig, K., and Levin, H., IKD.EKG.CHEY.,ANXL.ED. 13, 90 (1941). (8) Wijs, J. J. A, Ber. deut. c h e m . Ges. 31, 750 (1898) RECEIVED for review July 23, 1955. -4ccepted September 29, 19%.
Vol umetric Dete rminatio n of Selenium SlLVlO BARABAS and W. CHARLES COOPER Canadian Copper Refiners, L t d , Montreal East, Quebec, Canada
The volumetric permanganate procedure of Schrenlc and Browning has been extended to the determination of selenium in refined selenium, sodiuni selenite, sodium selenate, and iron selenide.
T
HE conventional gravimetric procedure of determining
selenium by precipitation of elemental selenium with sulfur dioxide is subject’ to errors in washing and ignition which render the procedure unsuitable for the accurate est,iniation of selenium in refined selenium and selenium compounds. Of the various volumetric procedures for the determination of selenium, the iodomet,ric methods of Evans (a),Deshmukh and Sant (Z), and Schulek arid Koros ( 7 ) all require carefully controlled conditions, some of which are difficult to maintain. The reproducibility of these procedures, as n-ell as Stanim and Goehring’s permanganate method (8), left much to be desired. -1 conimendable permanganate method which has received scant at,tent,ionis that of Gooch and Clemons ( 3 ) as extended by Schrenk and Browning (6). In t,his method selenium and tellurium are oxidized i n sulfuric acid medium from the quadrivalent to the sexivalent st,ate by an excess of permanganate, the excess being determined by back-titration with ferrous amnionium sulfate. Disodium phosphate is added to prevent the precipitation of manganese dioside. This paper extends Schrenk and Browning’s procedure to the estimation of seleniuni in refined selenium, sodium selenite, sodium selenate, and iron selenide. PROCEDURE
Selenium, Sodium Selenite, and Sodium Selenate. Weigh accurately a 1-gram sample into a 300-ml. Berzelius beaker. Dissolve sodium selenite in 50 ml. of warm distilled water, and dilute to the mark in a 500-ml. volumetric flask. Treat refined selenium with 20 ml. of mixed acid water, nitric acid, sulfuric acid (1: 1:l),and allow to simmer under the boiling point. When all the nitrogen oxides have been driven off and the solution has turned colorless, cool the solution and dilute to the mark in a 500nil. volumetric flask. Run from a buret a 25-ml. aliquot of the sample solution (50 nil. for sodium selenite) into a 250-ml. Erlenmeyer flask. .%cidify
with 20 ml. of 1 8 s sulfuric acid, dilute Ivith 100 nil. of distilled water, and add 12 grams of disodium phosphate (XalHPO4.7H20). Stir the solution to dissolve the phosphate and add from a buret 20 ml. of 0 . 1 s potaspiuni permanganate solution. Let the solution stand for 30 minutes to complete the oridation. Back-titrate the excess of permanganate with 0.1s ferrous ammonium sulfate. Add one drop of o-phenanthroline ferrous sulfate indicator near the end point, which is detected by the color change from pale blue to bright red. Dissolve sodium selenate in 15 ml. of 1 8 s sulfuric acid a t moderate heat. Cool the solution and make up to the mark in a 250-ml. volumetric flask. Take a 25-m1. aliquot in a 250-ml. Erlenmeyer flask and add 1 gram of hydroxylamine hydrochloride. Boil gently for 10 minutes until all the selenium has precipitated. Cool and dissolve the selenium in 20 ml. of the mixed acid added cautiously to avoid violent reaction. Heat just under the boiling point to expel nitrogen oxide fumes. N o w proceed with the cool solution in the same manner as for aliquots for selenium and sodium selenite samples. Iron Selenide. Dissolve a 1-gram sample by adding gradually 30 ml. of hot mixed acid. Boil gently to dissolve t,he last particles of iron selenide and keep boiling until the first crystals of ferric sulfate have precipitated. Dilute the cooled solution with 50 ml. of distilled water, mix to dissolve the salts, transfer to a 250-ml. volumetric flask, and make up to the mark. Titrate a 25-ml. aliquot in the manner previously described.
It is recommended that the titer of the pernianganate and ferrous ammonium sulfate solutions be checked carefully with each set of determinations by duplicate analyses of high-purity selenium. DISCUSSION
Refined Selenium. Commercial selenium poxvder generally analyzrs around 99.570 selenium, wit’h tellurium as the principal impurity. A correction fur the tellurium content of the sample may be realized by Schrenk and Broivning’s dichromate method ( 5 ) or by direct permanganate titration follon.ing volatilization of the selenium by sulfuric acid fuming at elevated temperature (4). Following the procedure det’ailed, the selenium values reported in Table I were obtained. The impurity analTses are given to indicate clearly the composition of the samples. The superiority of the volumetric procedure is realized when it is noted that a careful gravimetric determination of samples 1, 2, 3,
A N A L Y T I C A L CHEMISTRY
130 Table I. Analysis of Refined Selenium Powder Se,
%
Te,
Sios,
Cu,
Pb,
Fe,
Hg,
Sb.
Sample
%
%
1
99.65 99.64 99.65 99.49
0.30
0.077 0.029 0.014 0.024
0.0014 0.0010 0.0017 0.0039
0.012
0.021 0.017
0.010 0,0009
0.0046 0.0033 0,0033 0,0039
Xi1 Nil h-il Nil
2
3 4
%
0.26
0.23 0.23
70
%
%
0.017 0.013 0.017
%
0.024
0.018
%
0.007
0.0005
S.
of the sample in hot mixed nitric-sulfuric acid. In order to check the selenium value indirectly, the iron content of a number of samples was det'ermined by hydrolytic precipitation from a separate aliquot of the sample solution following the removal of selenium and tellurium. A typical analysis gave selenium 58.49%, iron M.49%, and tellurium 0.47%. The theoretical selenium figure is 58.57%.
and 4 gave the values 98.73, 99.15, 98.81, and 98.73% selenium, respectively. Sodium Selenite. The permanganate procedure applied to sodium selenite was found to give selenium values in good agreement with the theoretical value, 45.65y0. Typical samples analyzed 45.45 and 45.67y0 selenium. Sodium Selenate. As no convenient means is available for reducing sexivalent selenium to the quadrivalent state, the most satisfactory procedure of analyzing sodium selenate by the permanganate method was found to be reduction of the selenate to elemental selenium by hydroxylamine hydrochloride, followed by oxidation to the quadrivalent state by nitric acid. Typical analyses gave 41.84, 41.77, and 41.84%,,values agreeing well with the theoretical value, 41.79%. Selenide, The selenium in iron may be determined readily by the permanganate met,hod following dissolution
xron
ACKNOWLEDGMENT
The authors are indebted to Herbert >farshall for the determination of the impurities in the refined selenium samples. LITERATURE CITED
Deshmukh, G. s., and Sant, B. R., Analust 77, 272 (1952). (2) E ~B. s,, ~Ibid.,~67, S46 ~ (1g42). , (3) Gooch, F. A . , and Clemons, C. F., Am. J . Sei. 50, 5 1 (1895). (4) Marshall, H., Canadian Copper Refiners, Ltd., unpublished work. ( 5 ) Schrenk, W. T., and Browning, B. L., J . Am. Chem. SOC. 48, (1)
139 (1926).
(6) Ibid.,p. 2550. (7) Schulek, E., and Koros, E., 2. anal. Chem. 139, 20 (1953). (8) Stammy H.3 and Goehring, LI.,I b d . , 120, 230 (1940).
RECEIVED for review Aplil
4, 1
9
Accepted ~ ~ June 28, 1955.
Coulometric Determination of Organic Bases in Acetonitrile CARL A. STREULI Research Division, American Cyanamid Co., Stamford, Conn.
The coulometric method of analysis is extended to the determination of amines in essentially nonaqueous solution. Nonaqueous solvents have been shown to be most useful in the determination of these weaker bases. The solvent employed is acetonitrile containing 0.O.W lithium perchlorate trihydrate. The water content of this solution is approximately 0.3%. Hydrogen ion is produced by anodic oxidation of the water and the ion detected by the conventional glass-calomel electrode combination. Pyridine, diphenylguanidine, triethylamine, and benzylamine have been titrated in milligram quantities; the usual error is less than 2%. Aromatic amines tested by this method cannot be titrated. They apparently undergo partial or complete oxidation by the oxygen in solution which is produced concurrently with hydrogen ion at the anode.
not oxidized to yield hydrogen ion anodically-the reaction water so readily undergoes. The small amount of water required in the solvent as the source of hydrogen ion was introduced in the form of a hydrate of lithium perchlorate. A 0.05,V solution of lithium perchlorate trihydrate in acetonitrile served as the solvent for the determinations. The water content of this solution is 0.3%. A potentiometric titration curve for pyridine dissolved in ace-
THE
value of coulometry as a method for the determination of small amounts of materials has been amply demonstrated in recent years ( 2 ) . DeFord ( 4 ) has applied this method for the titration of acids and bases in aqueous systems. Carson and KO ( 1 ) have titrated acids coulometrically in a 70/30 mixture of isopropyl alcohol and water. Extensive work (6, 7 )has shown, however, that weak acids and bases are most easily titrated in nonaqueou3 solutions. An extension of coulometry to essentially nonaqueous systems should then be useful in the determination of small amounts of these compounds. Coulometry has a further advantage in t h a t it is readily adapted t o automatic procedures. The present paper deals with a coulometric method for determining weak bases in acetonitrile. The usual source of hydrogen ion in the aqueous coulometric titrations for base is the solvent itself. Acetonitrile, however, is
ml. 0.IN HCQ in C Y N
Figure 1. Titration of pyridine in acetonitrile solution
