ANALYTICAL CHEMISTRY
1476 entia1 formation of chlorine (4, It was found, however, that a t least under the conditions described, the presence of chloride had no effect on the stoichiometry of the coulometric process. Even in solutions that were 251 with respect to chloride ion, small amounts of base could be titrated. It was obvious iu such titrations that some free chlorine was being evolved a t the generating anode, but the electrolytic stoichiometry of the process is probably preserved by the reaction Clz
+ 20H-
-C
OC1-
+ C1- + H20
T o check this hypothesis, macrovolumes of sodium hi drovide were titrated in the presence of chloride ion using a glass-calomel system for the detection of the end point. No significant errors were involved, and the presence of hypochlorite ion was indicated by testing the titrated solution with iodide ion in the presence of starch. The error in the base titration was lowered by sweeping the solution with nitrogen. The above examples of typical titrations indicate that coulometric methods can be applied to the titration of small volumes of dilute solutions. The volume of simple was usually 10 pl.)
and the concentration of the arsenite solution ranged from 0.01 t o 0.0002M while in the acidimetric titrations the range of coneentration was 0.01 to 0.001M. The accuracy attained is comparable to that of conventional methods. ACKNOWLEDGMENT
The authors gratefully scknowledge the support of this research by the United States Air Force, through the Office of Scientific Research of the .4ir Research and Development Command. LITERATURE CITED
(1) Cooke, W. D., and Furman, S . H., ANAL.CHEM.,22, 896 (1950). ( 2 ) Cooke, W. D., Reilley, C. N., and Furman, N.H., Ibid., 23, 1662
(1951).
(3) Kirk, P. L., “Quantitative Ultramicroanalysis,” Wiley, S e w
York, 1950. (4) llitchell, J., Jr., Kolthoff, I. M., Proskauer, E. S., and Weissberger, A,, “Organic Analysis,” vol. 11, p. 186, Interscience, New York, 1954. (5) Pitts, J. N., DeFord, D. D., Martin, T. W., and Schmall, E. A,, -4N.4~.CHEM., 26, 628 (1954). RECEIVED for review October 25, 1954. Accepted hlarch 17, 1953.
Colorimetric Determination of Platinum with Stannous Chloride 0.I. MILNER
and G. F. SHIPMAN
Research and Development Department, Socony
M o b i l Laboratories, Socony Mobil Oil Co., Inc., Paulrboro,
In 0.3N acid solution, sometimes recommended in the literature for the reaction between stannous chloride and chloroplatinic acid, platinum probably forms a “hybrid” complex or mixture of complexes containing both stannous and stannic tin. The reaction product is sensitive to slight changes in acidity, time of standing, and concentration of reacting ions, so that rigorous control of conditions is necessary to ensure reproducible results. A t higher acidities (1.5 to 2.5N), a true stannous-platinum chloride complex is formed. This reaction is unaffected by the presence‘of stannic tin, and is generally more suitable as a basis for quantitative photometry.
0
NE of the most suitable methods for routine determination of platinum is the spectrophotometric determination based on the reaction of platinum with stannous chloride in hydrochloric acid solution. In the course’of adapting this method to the anrtlysis of platinum-bearing materials, some interesting observations were made regarding the nature of the reaction. REACTION OF PLATINUM AND ST4NNOUS CHLORIDE IN 0.3N ACID
Effect of Acidity and Time. Sandell ( 4 ) suggests that the reaction between chloroplatinate and stannous chloride involves a. simple reduction to chloroplatinous acid, and he reports that the intensity of the color varies with the acidity. Although the authors subsequently found the reaction to be different, a study of the effect of the acid concentration on the intensity and stability of the color confirmed Sandell’s statement in regard to acidity. This effect is shown in Table I. I n general, the intensity of the color increases with time a t acidities below 0.3.V, and decreases with time a t acidities above 0 . 3 N . At an acidity of 0.3N the color develops immediately and is stable for a t least 2 hours. Consequently, careful control of the acid concentration is necessary to ensure reproducible results. It was also found that a 400 to 1 mole ratio of stannous chloride to platinum was necessary to yield complete color de-
N. J.
velopment. These effects cast some doubt on the validity of the theory of simple reduction to chloroplatinous ion. Nonetheless, by careful control of conditions, the authors were able to obtain reliable results by the method recommended by Sandell. Variation between Different Grades of Stannous Chloride. Some time after the adoption of the method, it was observed that the intensity of the color depended on the grade of stannous chloride used. Technical grade stannous chloride, as used in the original adaptation of the method, yielded consistently reproducible results; however, with reagent grade material, the colors were not reproducible and were less intense. The color reaction was therefore re-examined to determine the cause of this anomaly. The difference between the two grades of stannous chloride was first investigated by reacting various amounts of each with 5 p,p.m. of platinum, the acidity of the final solution being kept constant a t 0.3N. The technical grade reagent gave constant absorbance with different quantities, but the reagent grade stannous chloride did not. Although the absorbance increased with increasing amounts of the reagent solution, results were lower than those with the technical grade material (Table 11).
Table I.
Variation of -4bsorbance with Acidity and Time (Pt
= 3 p.p.rn.)
Absorbance a t 350 mp .Acidity, .Y 0 20 0 2G 0 30 0 35 0 40
Irnmediate 0 279 0 270 0 298 0 241 0 212
10 min. 0 297 0 274 0 290 0 229 0 211
30 min. 0 299 0 28G 0 293 0 221 0 211
6.0 min. 0 300 0 288 0 290 0 220 0 210
120 nnn. 0 290 0 294 0 293 0 218 0 201
Table 11. Comparison of Different Grades of Stannous Chloride Absorbance Quantity of SnClr, ( A l l . of 10% Soln.)
Technical grade
4.0 5.0
0.479 0.471
8.0
Reagent grade 0.357 0,390 0.431
V O L U M E 2 7 , NO, 9, S E P T E M B E R 1 9 5 5 Table 111.
1477
Effect of Addition of Stannic Chloride Absorbance, SnCh Reagent Technical grade grade 0,461 0.340 0 467 0.395 0.467 0.467
0.0 1.0
10.0
Table 1V. Effect of Stannic Chloride on Reaction of Stannous Chloride with Bivalent Platinum Renyent Added, 311.~SnCL SnClr (10% soln.)
dbsorbance Observed Theory
(176 so1u.:
0.000 0 403 0.348
0.0
4 0 1 0 0 0
4.0
4.0
alii8 0.418
Effect of Stannic Tin. It was believed that the most likely contaminant of the technical grade material was stannic. tm To test the effect of stannic tin, 5-p.p.m. amounts of plat~num were made to react with both grades of stannous chloride, and various amounts of a 1%solution of stannic chloride pentahydrate were added. The reagent grade material initially gave a much loa er color intensity than the technical grade, but the intensitv increased with the amount of stannic tin present. With 10 ml. of added stannic tin solution, the intensity was identical to that yielded hy the technical grade material, which showed no change wit11 the addition of stannic chloride (Table 111). The iollowing experiment confirms that stannic tin was involved in the reaction: -4 solution of reagent grade stannous chloride was prepared fresh and divided into two portions. Air was bubbled through one portion for 2 hours, after which both were made t o react with 5 p.p.m of platinum. The platinum solution that contained the nonoxidized reagent gave an absoibance of 0.391, whereas the solution that contained the midieed portion gave an absorbance of 0.461. 111 view of the above findings, i t was thought possible that the stailnous chloride acted only as a reductant, and that the color
72
68
64
60
measured was actually a platinous-stannic reaction product, with the technical grade material containing sufficient stannic tin to carry the reaction to completion. (If 1% of the tin in the stannous chloride were present as stannic tin, the amount added would correspond to a tin-platinum mole ratio of about i to 1.) To test this possibility, solutions containing 4.4p.p.m. of bivalent platinum as potassium chloroplatinite, prepared according to the method of Fernelius (a),Irere made to react with various combinations of reagent grade stannous arid st'annic chloride (Table IV). Stannic chloride alone did not react s t all; a, spectrogram of the solution showed no absoi.!);rnce between 350 and 4.50 niN. \\'hen hoth stannous and stannic chlorides were present,, the color intensity was reasonably close to the calculated value for 4.4 p.p.m. of platinum. With only stannous chloride present, the color developed, but showed a 17% deficiency, indicating that both stannic and stannous ions were necwary for full color development a t 0.3N acidity. Spectrograms of the two colored solution? were similar (Figure l), but the presence of an absorption p ~ a k(at about 400 mp) in the curve of the solution containing both stannic and stannous tin, indicates that a second complex is iiivolved in the reaction. Since stannic tin itself gives no reaction, the inference is that this complex probably contains both staniious and stannic tin. From the above, it is apparent that. platinum cannot be determined by reaction with stannous chloride in 0.3.Y acid, unless the stannic tin content of the reagent is carefully controlled. As this is extremely difficult, in viex of thr ease with which tin is oxidized by air, a variation of the method which might ovcrcome this problem was sought. REACTION OF PLATJNU\I A S U STAY7iOUS CHLORIDE IK 2.Y ACID
11eyer and A4yres( 3 ) showed that if thr reaction is cari.iet1 out in 2-V acid, the absorbing species is a complex of the form (PtSn,Cl,)++++. It \vas also found that the final color is independent of acid concentration, provided the concentration exceeds 0.i.xr, and that stannous chloride concentration litis no effect ( 1 ) . The authors' data confirm many of the findings of these investigators, and show the advanhges of the higher ' acidity. Spectral Characteristics. A solut,ion of chloroplatiniv acid 2 5 in hydrochloric acid, was made to reart with reagent grade stannous chloride. A spectrogram of the reaction product showed it t,o have a true minimum transmittancy at 405 mp, in agrwment with the work of Ayres. Alt,hough the color intensit,?-of the complex a t this wave length was only approximately half that of the complex formed in 0.3:Y acid, the same sensitivity was maintained in practice by developing the color in half the volume used a t the lower acidity.
56
Tahle \-. Variation of Absorbance w-ith Acidity and Time at 2.V Acid Level 52
_______ .ibsorbanre at 10 Immediate min. 1.5 0,204 0.202 2.0 0.207 0.204 2 5 0,208 0.205 N o change during additional 3 days
Acidity.
s
48
405
ing ~~
20
iiiin. 0.205 0.207 0.208
30" niin. 0.204 0.205 0.208
44
40
36
0
I
I
I
I
I
I
340
360
380
400
420
440
WAVE LENGTH, M I L L I M I C R O N S
Transmittance of platinum-tin chloride complexes in 0.3N hydrochloric acid
Figure 1.
Variation of Absorbance with Acidity. The effect of varying the acidity a t the 2N level was checked on a solution containing 5 p.p.m. of platinum. I n c-ontrast to the results a t the lower acidities, the absorbance of the complex a t the 2N level shows no significant change with time or acid concentration (Table V). For convenience, a concentration of 2N was selected. Although the values refer to the final acidity of the solution, the stannous chloride must not be added until the acidity has been adjusted to the approximate required strength. If the reagent is added
1478
ANALYTICAL CHEMISTRY
Table VI.
Variation of Absorbance with Stannous Chloride Concentration
Table VII. Effect of Stannic Chloride on Reaction in 2.V Hydrochloric Acid
( P t = 5 p.p.m.)
SnClt, 311. of 20% S o h 1.0 2.0 5.0 10.0 15.0 20.0
( P t = 5 p,p.rn.)
SnClr, Ml. of 1 % Soln. 0.0
dbsorbance 0.200 0.202 0.202 0.203
0.5 1.0 5.0 10.0
::%
Table VIII.
to a solution that is only weakly acidic, the reaction appears to proceed irreversibly along the lines already discussed; subsequent addition of acid does not change the absorbance. Variation of Absorbance with Stannous Chloride Concentration. I n 2Y acid, 1.0 ml. of 207, stannous chloride, or a stannous chloride-platinum mole ratio of 300 to 1, \vas sufficient to yield essentially full color development; the absorbance of the solution showed a negligibly slight change with increasing amounts of stannous chloride (Table VI). A 5-ml. portion of the reagent per 100 ml. of solution Kas selected to ensure an adequate excess. Effect of Stannic Tin. Since stannic tin had a pronounced effect on the reaction with stannous chloride in 0 . 3 s acid, its effect on the caomplex a t the higher acidity was studied by adding stannir rhloride in varying amount. to the reaction mixture. I n contrast to the effect a t the lower acidity. there was no change in the intensity of the color (Table 1‘11). Linearity. The color intensity is directly proportional to the platinum concentration (Table VIII). The average absorption coeffirient (absorbance units per p.p m I is 0.0398 with a standard deviation of f0.00044. Application of Method. The method has proved accurate and precise when applied to a variety of platinum-bearing materials and in the presence of numerous other elements. Details of this work cannot be revealed a t present. However, aside from gold and some of the other platinum metals, or high concentrations of ions which are themselves colored, few interferences have been encountered. Rhodium, which is a particularly troublesome interference * in the determination of platinum, interferes less in 2‘V acid than
-4bsorbance 0.201 0,201 0,201 0,200 0,200
Variation of ..ibsorbance with Platinum Concentration
(Beckman Model B spectrophotometer; 1-cm. light path: P t , P. P.M. 2.0
4,0 6.0
8.0 10.0 12.0 16.0 20.0
-4bsorbance 0.078 0.162 0,239 0.319 0,400 0.473 0.639 0,800
h = 405 in@)
Sbsorption Coefficient (Absorbance/P.P.bI.) 0,0390 0,0406 0.0398 0,0399 0.0400 0,0394 0.0399
0,0400
in 0 . 3 s acid. At the lower acidity, the rhodium absorbance increases rapidly with time until it is about double that of an equal concentration of platinum; in 2 5 acid, the absorbance gradually decreases to about one third that of platinum. By allon.ing the solution to stand for approximately 3 hours before measuring the absorbance, the rhodium interference can be held to a minimum. LITERATURE CITED
( 1 ) Ayres, G. H., and Neyer, A.
S.,ANAL.CHEM.,23, 299 (1951). (2) Fernelius, W. C., “Inorganic Syntheses,” vol. 11, p. 247, 1IcGraw. Hill, Ken. York, 1946. (3) lleyer, b. S., and Ayres, G. H., J . A m . Chem. SOC.,77, 2672 (4)
(1955). Sandell, E. B., “Colorimetric Determination of Traces of lietals,” p. 494, Interscience, New York (1950).
RECEIVED for review April 1 4 , 1955. Accepted M a y 27, 1955. Presented a t the Pittsbiireh Conferenre on hnalytical Chemistry and Applied Spectroscopy, Februa,ry 28 t o March -1. 1955.
Sensitive Determination of l o w Boiling Organic Sulfw r Compounds JOHN A.
R. COOPE’ andG. 1. MAINGOT2
Department o f Chemistry, University o f British Columbia, Vancouver, Canada
Improvements in the reduction of disulfide in organic solution permit determinations of 99% accuracy. Direct determination of hydrogen sulfide in the presence of thiol is effected by separating precipitated sulfide from mercaptides by adjustment of acidity. Procedures, suitable especially where homologs of low molecular weight may be present, form a convenient scheme for sensitive determination of hydrogen sulfide and low boiling thiols and disulfides in gaseous organic mixtures.
T
HE analysis of sulfur compounds in organic mixtures has been widely studied, particularly by the petroleum industry ( 1 , 5 ) , and a number of methods of determining the various compound types have been developed. However, methods of improved accuracy or sensitivity are often desired, as well as Present address, T h e Mathematical Institute, Oxford, England. Present address, Trinidad Leaseholds Ltd., Forest Reserve, Trinidad. British West Indies. 1
2
methods applicable to anomalous cases, and a group of procedures may be of interest which have been in use in this laboratory in kinetic studies of gas phase reactions of organic sulfur (6). Significant improvements in the reduction of disulfide in organic solutions, and a method of determining hydrogen sulfide directly in the presence of thiol(mercaptan) are described. Procedures designed for the case where homologs of low molecular weight may be present are presented as a convenient scheme for the sensitive determination of hydrogen sulfide and low boiling thiols and disulfides in gaseous samples which may contain double bonds. I n kirietic studies by static methods gaseous samples are normally obtained at low pressure in pipets of several hundred milliliters; procedures are in a form suitable for such samples. Under typical conditions, 0.5 mm. of gas in a 250-ml. reaction vessel a t 600” K. will correspond to about 0.000003 mole of sample. or when dissolved in 100 grams of solvent to the order of O . O O O l ~ o sulfur. The procedures have a sensitivity of this order.
