V O L U M E 2 7 , NO. 11, NOVEMBER 1 9 5 5 t h a t a standard error of 3.5 y in t h e weight of t h e samples would cause this deviation. As every sample weight is t h e difference of two weighings i t appears t h a t not even the best tested balances could be expected t o give much better results (11). The Ainsworth balance Model F.D.J. on which the weighings were carried out has been in daily use in the laboratory since 1948. So, if it were assumed that the errors in t h e determinations were caused by the 11 eighing errors alone-which of course is impossible-the results would still be a good proof of the excellent performance of this balance. T h e method is so fast and convenient t h a t one operator easily and rvithout having to hurry can carry out 10 complete analyses, including weighing, running t h e analysis, and calculating the results, in a 7-hour working day. Experiments t o apply the method to the determination of submicrograni amounts of nitrogen b y the use of a n ultramicroba1anc.e and finer nitrometers are planned. ACKNOW LEDG-M EYT
T h e authors are very indebted t o E. Stenhagen and G .tgren for their interest in the work, to A. Edenstrom for n-ktance
1809 with constructional designs, and to E. Sepp for drawing the figures. T h e work was made possible b y grantsJrom the Swedish Medical Research Council to E. Stenhagen, G. Agren, and IT-.Kirsten. LITERATURE CITED
(1) D u d , Clement, Anal. Chim.Acta, 2, 438 (1948). (2) Kirsten, W. J., Mikrochemie, 40, 121 (1952). (3) Koch, C. W., Simonson, T. K., and Tashinian, V. H., A s ~ L . CHEM.,21, 1133 (1949). (4) Lindner, Josef, “hlikromassnalytische Bestimmung des Kohlenstoffes und Wasserstoffes mit grundlegender Behandlung der Fehlerquellen in der Elementaranalyse,” Verlag Chemie, Berlin, 1935. (5) Pagel, H. .1.,and Oita, I. J.. d s i ~CHEM., . 24, 756 (1952). 16) Parks, T. D., Bastin, E. L., ..lgaeei, E. J., and Brooks, F. R., I b i d . , 26, 229 (1954). ( T ) Richards, Th. IT., 2. anorg. Chem., 1, 187 (1892). (8) Royer, G. L., Sorton, A. R., and Foster, F.J., IND. ENG.CHEM., ANAL.ED.. 14. 79 (1942). (9) Schonberg, N.E., Soderfors Bruk, private communication. (10) Trautz, 0. P., Mikrochemie. 9, 300 (1931). (11) Waber, J. T., and Sturdy, G. E., .%SAL. C H m r . , 26, 1177 (1954) I
_
,
R E C E I V E for D review October 20, 1954.
Accepted April 23, 1955
Complexometric Determination of Sulfide PEKKA KlVALO Department o f Chemistry, lnstitute o f Technology, Helsinki, Finland
A complexometric method for the determination of sulfide ion is described. The method is hased on the observation that an alkaline sulfide solution added to a neutral or slightly acid solution containing an excess of metal perchlorate gives a stoichiometric precipitate with copper hut not with zinc or cadmium.
T
HE modern volumetric methods of analysis employing ethylenediaminetetraacetic acid [( ethy1enedinitro)tetraacetic acid, EDTA] and related compounds are characterized by a high degree of accuracy and also by the fact’ t h a t the t’iter of the standard solutions is extraordinarily stable. I n seeking a complexometric method for the determination of sulfide ion in which the principle of difference could be used, the following results were discovered. K h e n the alkaline sulfide solution to be analyzed was added to a neutral or slightly acid solution containing an excess of a metal perchlorate, zinc and cadmium gave colloidal precipitates and erroneous results. The cupric ion, on the other hand, gave a stoichiometric precipitat.e of copper sulfide which is knon-n to he extremely slightly soluble ( 2 ) and was easily filterable. The sulfide solution to be analyzed ivas prepared from analytical grade sodiuni sulfide nonahydrate using oxygen-free water, and n-as kept under hydrogen atmosphere. Samples were taken by means of a buret connected to the sulfide flask and the standardization was done iodometrically ( 3 , 4). The titration of the standard solution and the excess of copper was per-
formed according to Flaschka ( 1 ) . The reagent solutions were of the order of 0.05M. Table I lists a few results obtained with the iodometric and the new methods. From the results obtained one can judge t h a t the new method IS as accurate as the classical iodometric one. Hoivever, in addition to the advantages mentioned above, the new method is also characterized by the fact that, in the medium used, the sulfide only will be precipitated as copper sulfide, leaving the sulfite and thiosulfite ions in the filtrate. -4s is knovm, iodine oxidizes all of these ions. -4possible disproportionation of the cupric sulfide into cuprous sulfide and sulfur obviously does not affect the final result. REAGENTS
Disodium salt of EDTA (Compleuon 111),0.05X. Cupric perchlorate, 0.05Alf. .Ammonia, 1-11, hcetate buffei (0.67M acetic acid 0.33.11 sodium acetate), 1.11. Saturated water solution of rniiinide.
+
PROCEDURE
Pipet 25 ml. of the copper solution into a 150-ml. Erlenmeyer flask, rinse it down with little water, and add 15 ml. of the acetate Iiuffer. Add slo~vlythe sodium sulfide solution (10 to 40 ml. of :in approximately 0.02121 solution or equivalent), constantly shaking the flask. Filter off the copper sulfide using a fineporosity (G4) fritted-glass funnel and a 250-ml. suction flask. Wash with 20 to 30 ml. of hot water. h d d a few milliliters of ammonia to the filtrate until the deep blue copper solution is vlear, and dilute to 100 to 120 ml. Add 3 to 6 drops of murexide iiidicator and titrate to a reddish violet color. ACKNOWLEDGMENT
Table I. Standardization of Sodium Sulfide Solution Using Iodometric and Complexometrie 3Iethods (All reagents approximately 0 . 0 5 3 4 )
MI.
Molarity of NarS Found
Cu, 111.
Sa&,
hI1.
MI.
Molarity of Na?S Found
25
10 10 15 20 40
0.01710 0.01710 0.01711 0 . 0 1696
25 25 23 25 25
10 15 20 30 40
0.01700 0.01696 0.01709 O.Olfi95 0.01700
12,
25
25 25 50
Na&,
O.OlB9G A r . 0.01704
.Ir.0 . 0 1 7 0 0
The autbor is indebted to Professor Anders Ringboni, Abo ;Ikadenii, Bbo, Finland, for the use of laboratory facilities. LITERATURE CITED
(1) Flaschka, H., Mikrochemie, 39, 38 (1952). (2) Goates, J. R., Gordon, 31. B., and Faux, S.D., J . A m . Chem. Soc., 74, 835 (1952). (3) Kolthoff, I. 31., 2. anal. Chern., 60, 451 (1921). (4) Scott, W. W., ”Standard Methods of Chemical Ana1yi;is.’’ 5th ed., p. 2183, Van Kostrand, Sew York, 1947. R E C E I V Efor D reyiew S o v e m b e r 18, 1954.
Accepted M a y 3 1 , 1955,
