Spectrophotometric Study of the Platinum(lV)-Tin(ll) Chloride System GILBERT H. AYRES AND ALBERT S. RIEYEK, JR. The University of Texas, Austin, Tex. anal5sis error is less than l q ~ .B j nieasuring solutions containing 50 t o 60 p.p.m. of platinum against a .50 p.p.m. standard, the analysis error may be reduced to 0.1%. The color is almost quantitatively extracted into amyl acetate, and has essentially identical spectral characteristics as in the aqueous solution. The color fades rapidly, but may be stabilized for a t least a n hour b y addition of 1% resorcinol. The extracted colors are reproducible only to about +0.5% absolute transmittancy. A study was made of possible interference by many cations and common reagent anions; in general, the spectrophotometric method shows greater tolerance t h a n gravimetric methods. The method, without loss of accuracy, saves time in determination of platin u m and permits use of less rigorous preliminary separations; it is applicable to low concentrations.
This paper is part of a general study of the analj tical reactions of the platinum metals. Although titrimetric and colorimetric methods have been reported for the determination of platinum, t h e only generally accepted methods now in use are gravimetric. In addition to their usual limitations, the existing methods have little selectivity and require extensive separations prior to their application. The yellow color developed by the reaction of tin(I1) chloride and platinum(1V) chloride in hydrochloric acid solution was studied both in the original aqueous solution and in extractions by organic solvents. The aqueous solutions were found to be stable, reproducible to *0.10/0, and, over a reasonable range, independent of reagent concentration. For the method used, t h e optimum concentration is from 3 to 25 p.p.m. of platinum. Over this range the minimum relative
0
For preparation and gravimetric analysis of the standard platinum solution, weighings were made on an assay balance having a sensitivity of 0.002 mg., using weights that had been calibrated directly against National Bureau of Standards certified weights. Calibrated volumetric ware was used throughout.
S L Y two colorimetric methods for platinum have found
much application; the tin(I1) chloride method is superior to the potassium iodide method because of more rapid color development and greater selectivity ( 7 , 2 2 , 23). The reaction of tin(I1) chloride with platinuni(1V) was first reported by Wohler (25),who found that. platinum(1V) in hydrochloric acid solution, when treated with tin(I1) chloride gave a. hlood-red color that was extractable with ether. He considered the color to be due to colloidal platinum, analogous t.0 the “purple of Cassius” formed when gold chloride is similarly treated ( 1 6 ) . 1,ater authors have attributed the color to platinum(I1) ( 6 ) ,or to’ chloroplatinous acid ( 2 2 , 24). The method has been applied to the estimation of small amounts of platinum filtered from air in the vicinity of platinum works ( 6 ) , and to the determination of small amounts of platinum in nitric acid ( 2 ) . Poluektov and Spivak ( I O ) det,ermined platinum in ores containing 0.03 to 0.1 gram of platinum per ton; the developed color was extracted into r,t,hyl acetate. Sandell ( 2 2 ) studied the method photoelectrically, using a blue fiker, and shoFed that the system conformed to Beer’s law in the concentration range investigated (up to 2 p.p.m.); he found strong interference from palladium, and lesser interference from ruthenium, gold, and iron. Wolbling ( 2 7 ) Found that if the solution was first made ammoniacal, and then acidified to about 1 molar before addition of tin(I1) chloride, the palladium color was not extracted, while the platinum color was not appreciably affected. Hopkins ( 6 ) states that the tin(I1) chloride test for platinum must be carried out in the absence of organic matter; the original reference (8) indicates that interference is due to the yellow color formed when the sample is t,rclated with aqua regia, rather than to the organic matter per se.
REAGENTS
Grade 1 platinum therniocouple wire, specified as 99.99Oj, pure, was used for preparation of t,he standard platinum solution. Tin(I1) chloride solution, 1.0 molar in tin(I1) chloride and 3.5 molar in hydrochloric acid, was prepared from AMERICAN CHEMICAL SOCIETY reagent grade tin(I1) chloride dihydrate. -4fter the salt was dissolved in hydrochloric acid and diluted to volume, the clear solution was separated by decantation from the small amount of residue, and stored under a layer of xylene to protect against atmospheric oxidation. Stock solutions used for the study of int,erfering metals contained 1 mg. of metal per nil. All chemicals were A.C.S. reagent grade, except compounds of the platinum metals, which were Eimer and Amend C.P. materials. Solutions of copper(II), cobalt(II), nickel(II), chromium(III), palladium( 11), rhodium(111), iridium(IV), and ruthenium(II1) were prepared from their chlorides. Iron(I1) solution was prepared from ferrous ammonium sulfate hexahydrate; tellurium(1V) was prepared by dissolving the dioxide in sulfuric acid. Gold(II1) solution was made from chloroauric acid monohydrate. Osmium(IV), as chloroosmate, was prepared from the pure tetroxide. For testing anion interference, chloride, bromide, and sulfate were used in the form of their alkali salts; because of the high concentrations necessary to show any interference, these were added in the form of weighed amounts of the solid salts. Perchlorate was added in the form of accurately measured volumes of perchloric acid. Baker and Adamson purified isoamyl acetate was used for thfh extractions; 1% resorcinol was added t80this solvent, to stabilize the extracted color.
It was the purpose of the present investigation to make a detailed spectrophotometric study of the platinum( 11‘)-tin(I1) chloride color system, both in aqueous solution and in organic solvent extract,s; to establish optimum conditions for color formation; to evaluate the photometric accuracy; to determine the nature and extent of interferences, and methods for their elimination; and to attempt to elucidate the chemistry of the color-forming reaction.
EXPERIMENTAL
Preparation of Standard Platinum Solution. Exactly 1 g~1113 of grade 1 platinum thermocouple wire (99.9970 pure) was (lissolved in aqua regia, evaporated almost to dryness, taken up with 20 ml. of 1 to 1 hydrochloric acid, and again evaporated t o sirupy consistency. The hydrochloric acid treatment was rvpeated three times to remove all nitric acid and to destroy any nitrosoplatinic acid. After final evaporation, the material was transferred to a 1-liter volumetric flask, 10 ml. of concentrated hydrochloric acid were added, and the solution was diluted to volume, giving a concentration of 1 mg. of platinum per ml.
APPARATUS
Transmittancy measurements were made with a Beckman Model DU spectrophotometer, using matched 1.000-cm. cells. The instrument was operated at constant sensitivity, using slit widt,hs of the order of 0.02 to 0.1 mm., corresponding t,o nominal band widths of about 1 to 4 millimicrons.
299
300
ANALYTICAL CHEMISTRY
The concentration of the solution was checked by precipitating the platinum from 25ml. aliquots with formic acid, finally igniting and weighing as platinum (3,p. 290). The results of triplicate analyses were as follows: Sample No. Platinum found, mg. Deviation, % ’
1
2
3
llean
25.07 0.28
25.16 0.08
25.20 0.24
25.14 0.20
The slightly high results are believed to be due to silica, because the flask in which the platinum was dissolved was slightly etched. Color Development and Measurement. The desired volurno of standard platinum solution was transferred to a 100-ml. volumetric flask, 10 ml. of concentrated hydrochloric acid, 25 ml. of 20y0 ammonium chloride solution, and 20 ml. of 1.0 molar tin(I1) chloride solut,ionwere added, and the mixture was diluted to volume. A blank was prepared from identical amounts of reagents. A portion of the developed solution was used for transmittancy measurements. Another portion was extracted wit,h an equal volume of amyl acetate; an amyl acetate blank was prepared by ext,racting the aqueous blank. The aqueous solutions had a transmittancy of approximately 100% in the range of 1000 to 650 mp; a minimum transmittancy was found a t 403 mp, and a sharp maximum a t 355 mp. A second minimum was located a t about 310 mp, hut below 325 mp the absorption of t.he blank was so great as to render this minimum unsuitable for use. The spectral characteristics of the amyl acetate extrack were almost identical with t,hose of the aqueous solution, except that the minimum was displaced to 398 mp. Curves of transmittancy versus wave length for the aqueous solutions of various concent,rationsare shown in Figure 1.
Various inorganic and organic reducing agents were added, both before and after extraction, to test for stabilizing effect. These included sulfurous acid, crystals of tin( 11) chloride, hydroxylamine hydrochloride, formic acid, oxalic acid, and resorcinol. Sulfurous acid formed a pale yellow precipitate in the organic layer. All the other reagents gave some increase in color stability, but only resorcinol gave a stability suitable for quantitative measurement of transmittancy. When amyl acetate containing 1% resorcinol was used as the extractant, the solutions were found to give constant transmittancy readings for :tt least an hour, the color stability increasing as the platinum ronrrntr:ttion decreased.
>
y 60
20 IO
0
Figure 2
t
325
360
400
440
480
510
550
WAVELENGTH, m y
Figure 1
Stability of Color. The color developed so rapidly in the aqueous solution that it w&s impossible to obtain readings before complete development was attained. To test the stability of the color, solutions containing 1, 10, and 30 p.p.m. (mg. per liter) of platinum were developed by the above procedure; these concentrations more than covered the optimum concentration range. Transmittancy meawrements at 403 mp were taken immediately and a t intervals over a period of 5 days. The aqueous solutions showed no change in transmittancy over this period. Upon storage for several weeks, considerable fading occurred; apparently the fading was caused by atmospheric oxidation, for the addition of more tin(I1) chloride restored the color. Although the color in amyl acetate appeared to be stable while in contact uith the aqueous solution, after separation of the phases was made it faded too rapidly for measurement. The color in ethyl acetate was somewhat more stable than in amyl acetate, but even this showed rapid fading. This was found to he due to atmospheric oxidation of the tin(11) chloride, and was more rapid in amyl acetate than in ethyl acetate because of the lower solubility of tin(I1) chloride in the former.
Optimum Reagent Concentrations. Using a constant amount of platinum (10 p.p.m. in the final solution, which is about in the middle of the optimum concentration range), solutions were developed with reagent concentrations varying within wide limits. Transmittancies of both the aqueous and the extracted solutions were measured over the range 450 to 345 mp (to detect any possible shift in the minimum). The hydrochloric acid produced little change in the transmittancy, provided that the amount.added was greater than about 6 ml. of concentrated acid per 100 ml. of final solution. Below this amount, with decreasing .hydrochloric acid concentration the transmittancy of the aqueous solution decreased rapidly and that of the organic layer increased somewhat less rapidly. Tin( 11)chloride reagent used in amounts from 5 to 30 ml. gave nearly constant transmittancies in both phases. If palladium is present and 10 ml. of concentrated hydrochloric acid are used, at least 20 ml. of tin(I1) chloride should be added to prevent extraction of the palladium color into the organic solvent. With increasing concentration of hydrochloric acid, larger amounts of tin(I1) chloride are necessary to prevent palladium interference. Ammonium chloride was added to prevent cloudiness in the evtracted layers; 25 ml. of 20% solution were adequate. The transmittancy of the solutions was not influenced by the addition of ammonium chloride up to 30 ml. On the basis of the results of these tests, the procedure described under “Color Development” was adopted; the final solution was about 2 molar in hydrochloric acid, 1 molar in ammonium chloride, and 0.2 molar in tin(I1) chloride. The transmittancy of a portion of the aqueous solution was measured. A measured volume of the aqueous solution was extracted with an equal volume of amyl acetate (containing 1% resorcinol) by shaking for 1 minute; the organic layer was separated and dried for 5 minutes over silica gel, and its transmittancy was measured, the blank consisting of a similar extract of the aqueous blank. Both the aqueous and the organic solutions showed a small increase in
V O L U M E 23, NO. 2, F E B R U A R Y 1 9 5 1 transmittancy with increase in temperature; hence they were allowed to stand in the spectrophotometer for 10 minutes to reach thermal equilibrium before transmittancy readings were made. Stable readings %ere obtained after this equilibration period. Reproducibility. Samples of the same concentration gave transmittancy readings. on t.he aqueous solution, which seldom differed by more than 0.10;. To attain this precision, it wgs found advisable to clean the absorption cells with chromic acid cleaning mixture ai leapt daily: a I-minute treatment with the cold cleaning mixture wm satisfactory. Solutions heated to boiling anti cooled after color development but before dilution to final volume were reproducible to 0.1% transmittancy. However, if the solutions were boiled for several minut'es, the final 3olutions showed a decreaw of about 1%absolute transmitt.ancy. Leutwein (9) has shown that dilute solutions of the platinum metals stored in glass bottlw underwent a significant change in concentration. This elTect was confirmed by the aut.hors; dilute platinum solutions (0.001 M )which had been stored for several weeks (in borosilicate glas-stoppered bottles), then color developed tiy the standardized procedure, had transmittancies corresponding to 1 to 2% relative decrease in platinum concentration. For this reason, solutions freshly diluted from the stock standard polut,ion wcre used for calibration data.
60
2
L
,? 5G CL
a:
:: 40
m
30 20
301 Table I.
Transmittanoies of Platinum(1V)-Tin(I1) Chloride Solutions
Concentration of platinum, p.p.m.
50
60
70
BO
90
100
CONCENTRATION OF PLATINUM, P.P M.
Figure 3
Kith the organic extracts, the most careful attempts to reproduce conditions indicated larger indeterminate errors than for the aqueous solutions; the reproducibility for the extra& was about. 0.5% absolute transmittancy. Larger errors may be introduced through instability of the resorcinol in the amyl acetate extractant. When freshly prepared the solution was pale yellow, and gradually changed to a red tint on standing; once started, this chmge accelerated rapidly, and serious errors resulted if the mixture was prepared more than one week in advance. By simultaneous preparation of sa,mples and blank with fresh solution, no interference from this source was encountered. Effect of Temperature. The transmittancy of a color-ded o p e d sample containing 10 p.p.m. of platinum was measured over the range 20" to 50" C. With increasing temperature, the t ransmittancy increased a t a rate of 0.0770 absolute transmittancy per 1' C.; the effect, was completely reversible. Effect of Platinum Concentration. Solutions containing a final concentration of platinum from 1 to 25 p.p.m., in suitable increments. were developed by the standardized procedure, and
2
5
10
15
26
tion a t 403 mp
91.7 8 2 . 7 62.3 a8.s 23.8
9.2
92.8 8 4 . 3 6 4 . 4
9.7
% transmittancy of amyl acetate solution a t 348 mp
39.6 2 4 . 2
the transmittancies of both the aqueous solution and the organic extract were measured over the range of 325 to 650 m p . Minimum transmittancy occurred at 403 mp for the aqueous solutions, and a t 398 mp for the amyl acetate solutions. The transmittancies at, these wave lengths are shown in Table 1. The data for the aqueous solutions are shown in Figure 2, in which per cent absorptancy (100 - % transmittancy) is plotted against log concentration. The concentration can be extended to higher values, with ail increase in accuracy, by the differential method ( I , 4), in which the transmit'tance of a sample is compared with the transmittance of a reference standard of slightly lower concentration instead of the customary blank solution. Aqueous solutions containing 55 to 100 p.p.m. of platinum were compared against a reference standard containing 50 p,p.m. of platinum, the measurement,s being made over a wave-length range around 403 mp. The plot, of per cent absorptance (at 403 rnp) against log concentration is shown in Figure 3. The transmittance ratios showed deviation from Beer's law when the sample solution was more concentrated than about 70 p.p.m.; above this concentration the posit,ion of minimum transmittancy shifted slightly in the direction of longer wave lengths. Extraction Efficiency. In order to determine t8heextraction efficiency. color-developed samples containing 100 and 250 p.p.m. of platinum were prepared. One portion of each solution was extracted yith an equal volume of amyl acetate; another portion of each solution was extracted with only one fifth of its own volume of amyl acetate. The platinum remaining in the aqueous layer was determined spectrophot~metricallyat 403 mp; the color density in the organic solvent was too great for direct measurement, and was obtained by difference. The extraction coefficient was calculated on the assumption that no volume changes occurred in either of the phases. The results are shown in Table 11. Table 11.
0
1
% transmittanoy of aqueous solu-
Extraction Efficiency
Original concentration of platinum (aqueous), p.p.m. 100 Aqueous solution used, ml. 25 . h y l acetate used, mi. 25 Transmittancy of aqueous layer, 70 96.8 concentration of platinum in aqueous layer, p.p.m. 0.4 Concentrrition of platinum in amyl acetate layer (calcd.), p.p.m. 99.6 Ratio of platinum ooncentration in aqueous layer-amyl 0 0040 acetate layer Platinum extracted from equal voluiiies, Yo 99.6
100 50
250 25
260 50
10
25
10
84.1
92.6
67.2
0.8
4.2
1.9 490 0.0039
99.6
249 0.0032
99.7
1230
0.0034
99.7
The follo~ingexperiments were performed to test the possibility of increasing the flexibility of the method by extracting the color from large volumes of aqueous solution into relatively small volumes of organic solvent, so that the method would be applicable to determination of minute absolute amounts of platinum.
A 10-ml. portion of solution containing 10 p.p.ni. of platinum was extracted with 10 ml. of amyl acetate, and the transmittancy of the extract was measured. Another 10-nil. portion of the same solution was diluted to 50 ml. with the blank solution, and the diluted solution was extracted with 10 ml. of amyl acetate; a
ANALYTICAL CHEMISTRY
302 similar blank was prepared simultaneously; the transmittancy of the organic extract was measured. The above rocess was repeated, using aqueous-organic ratios of 10 to 1. TEe transmittancies a t 398 mu are shown in Table 111.
Table 111. Concentration by Extraction Ratio of Aoueous Volume Volume lo:] 5:1 5:l 1O:l 1:1 5:1 l0:l 5:1 34 2 35.8 33.0 33.4
~- t o Orpa&
I n test solution I n blank Transmittancyoforganicextract, tG
1:l 1:l 37.7
Effect of Diverse Ions. The following were the observed react,ions of the platinum metals when a test solution containing 1 mg. of metal was treated with 5 nil. of concentrated hgdrochloric acid and 5 nil. of tin(I1) chloride and diluted to 25 ml.. giving a final concentration of 40 p.p.m. of metal.
PLATINUM. The test solution was very pale yellow; on addition of tin( 11) chloride an intense yellow-orange color developed, reaching its maximum intensity with less than 1 ml. of the reagent. The colored material m-as completely extractible into ether, et,hyl acetate, or amyl acet,ate. Aqueous and organic solut,ions left in cont'act showed no apparent change over 12 days. Addition of 5 grams of ammonium chloride before the addition of tin( 11) produced no immediate precipitation of ammonium chloroplatinate even in solutions containing up to 200 p.p.m. of platinum; solutions containing 20 p.p.m. gave a slight precipitate after 24 t o 48 hours; the precipitate dissolved slowly after the addition of tin(I1) chloride. OSMIUM. The test solution was amber; no change was observed on adding the reagents. There was no evidence of extraction. IRmIunr. The test solution was orange; on addition of the tin(I1) chloride, the color changed to pale yellow. Part of the colored material was extractable, giving a pale yellow color to the organic solvent. The solutions were stable for a t least 12 hours. RUTHENIUX.On addition of tin(I1) chloride to the dark orange solution, the color changed t'o pale blue. Within about 12 hours the color had changed to pale yellow. There was no evidence of extraction into ether, ethyl acetate, or amyl acet,ate. RHODIUM.The test solution was light red in color. Addition of the reagents produced a light yellow color. On standing, the solution slowly turned to a deep raspberry red. Both the yellow :md the red solutions extracted to give a pale yellowish green solution in ether or the esters. The extracted solution was stable for a t least 12 hours. No apparent differences were produced b y adding ammonium chloride. PALLADIUM.The test solution was light orange. An intense red-orange color developed with the first few drops of tin(I1) rhloride; on further addition of the reagent, the color changed to yellowish green, then to nearly black, and finally to dark olivr green. The color partly ext,racted into ether or the acet'ates to give a red color to the organic layer and leave a green aqueous layer. Both colors were unstable, the organic solution turning to :L lighter color and the aqueous solution turning reddish brown within 12 hours. The addition of ammonium chloride was without apparent effect. A further study showed that the amount of palladium extracted depended upon both the concentration of hydrochloric acid and t,he concentration of tin(I1) chloride. Using the same amount of palladium as before, but only one fourth as much hydrochloric acid, addition of tin(I1) gave an intense yellow color, a large amount of which was extractable. As more tin(I1) reagent was added, the color changed to emerald green, then to dark blue green, and finally to olive green; further addition of tin(I1) chloride produced no further changes. The final olive-green material was extractable only to a slight extent. This material is possibly the colloidal metal; in a dialysis test there was no evidence that it passed through a collodion membrane. Chloroform, carbon tetrachloride, and xylene were without' effect in all the extraction t,ests. The metallic ions selected for detailed study of interference were osmium(IV), iridium(IV), ruthenium(III), rhodium(III), palladium(II), gold(III), iron(II), cobalt(II), nickel(II), copper(II), chromium(III), and tellurium(1T'). Ahionsincluded were bromide, sulfate, nitrate, and perchlorate. The diverse ions were added, individually, to solutions containing 10 p.p.m. of platinum (in the final solution), in amounts expected to give a t least mod-
erate interference; the color was developed in the usual way. The object of these tests was to determine the concentration of the added substance which would give interference corresponding to 1% relative error on the concentration of platinum. At thip voncentration a relative error of 1 % corresponds t,o 0.4% absolute transmittancy ; because the experimental error is relatively large with respect to 0.4% transmittancy, the diverse ions were added i i i amounts that would give absolute transmittancy changes of 1% 01'more. Readings were made on solut,ions of a t least three con(acintrat,ionsof interfering ion, and t,he value for 0.4% absolute t rmsmittancy difference was obtained graphically from a plot of t h e change in transmittancy against concentration of interfering agent. The interferenee graphs are shown in Figures 4 and 5 . ('opper, iron, cobalt, nickel, and all t,he anion8 gave relatively *ni:ill intrrfermces and were not plotted.
TRANSMITTANCY OF IO P P M PLATINUM w 0 2 4
I
0
AQUEOUS -6 -
SOLUTIONS ~
O
4
CGNCENTRATIOK
8 OF
12 INTERFERING
16 20 ELEMENT, PP.M
Figure 4 The platinum solutions to which other platinum metals were added underwent rather rapid initial changes in color after thc xddition of the tin( 11) reagent; therefore, transmittancy measurements on the aqueous color were not made until equilibrium was attained-Le., when there was no further change in transmittancy at 403 mp. This usually required about 30 minutes, but in some cases required about an hour. Because of the rapid initial color change of the aqueous solution, a portion of the solutiori was extracted immediately, for measurement of the organic extract. In the case of gold, the interference in the extracted phase seemed to be due largely to coprecipitation of platinum; because this might change with time, portions of the platinum-gold miuture were extracted immediately. In all other cases of the diverse ions studied, the final aqueous colors were stable, hence were c,\tracted when convenient, usually after about 4 hours. The concrntrationF of the diverse ions required to produce a
Table 11'.
Effect of Diverse Ions
(All solutions contained 10 p.p.m. of platinum) Aqueous Solution Organic Extract Concn., % Conon., % p.p.m., for relative p.p.m., for relative 170relative to 170relative to Interfering Substance error platinum error platinum 15 1.5 > 50 > 500 4a 11 110 4.8 Iridium(1V) 14 1.4 40 400 KutheniumUII) a 0.1 1 Rhodium(II1) 0.8 2.3 23 0.4 Palladium(I1) 0.5 0.7 7 Gold(II1)" 0.1 1 3 0.3 Telluriurn(1V) a 90 930 100 10 Chromium(II1) > 200 400 40 S i r kel ( 11) 2 x loa 2 x 10: Iron(I1) 2 x 103 2 x 10 Cobalt(I1) 2 x 103 2 x 103 Copper (11) > 2 x 10' Sitrate > 6 X 104 Perchlorate > 2 x 104 Sulfate > 2 x 104 Bromide > 2 x 10' u All substances produced a decrease in transmittancy, except that in only the extracted phase gold and tellurium produced a n increase.
+
V O L U M E 23, NO. 2, F E B R U A R Y 1 9 5 1 transmittancy difference corresponding to 1yo relative error in measuring 10 p.p.m. of platinum are shown in Table IV. DISCUSSION
The platinum( 1V)-tin(I1) chloride color system conforms to Herr’s law up to a concentration of a t least 30 p.p.m. of platinum whvn samples are measured against a reagent blank. I n the diffcwnt’ial method, using 50 p.p.m. as the reference standard, the *ystcxin follows Beer’s law up to about 70 p.p.m. Deviations from the law a t higher concentrations appear to be due a t least i l l p u t to a shift in the position of minimum transmit,tancy tonxrd longer wave lengths. I n the calibration curve, Figure 2, pt’r cent absorptancy (100 - % transmittancy) a t 403 mp for thti :iqueous solutions is plotted against log concentration. Reftxrriice to Table I shows that a similar plot of the organic extract? ivould be almost coincident with the curve of Figure 2; hence thts following considerations would apply equally n-ell to either phasc’. Thts inflection point in the curve occurs a t 63y0absorptancy, and r h c s slope of the curve a t this point corresponds to a relat,ive erroi’ of 2.7% per 1% absolute photometric error, in conformity nith derivations from Beer’s law ( 1 ) . For a precision of 0.2% absolute t.ransmit.tancy (aqueous solutions), the minimum relative tbrror is therefore 0.5Y0; a precision of 0.5% absolute transmittancy in measuring the organic extracts corresponds to a minimum relative error of 1.4%. Minimum error occurs at about 10 p,p.m. of platinum, akhough the error js not appreciablv greater in the range of 5 to 20 p.p.m.; in order t,o keep the relative error within 1%, for aqueous solutions the Concentration range must l)e within the limits 3 to 25 p.p.m. of platinum.
E F F E C T OF INTERFERING ELEMENTS ON TRANSMITTANCY OF IO P P M PLATINUV EXTRACTED SOLUTIONS ~
0
4
~
~ ~ _ _ _ _ _
12
8
CONCENTRATION
16 20 24 OF INTERFERING E L E M E N T , P P M
Figure 5
Hy the differential method (mrasurement against a reference htandard instead of a blank), the range can be extended to higher concentrations; by this inethod the accuracy can be increased (1 ) From Figure 3 i t can be shown that a t 60 p.p.m. the relativr error is only 0.5% per 1% absolute photometric error, or a relati\,(, crror of 0.1% for a precision of 0.2% absolute in reading the trarisniittancy; a t about 85 p.p.m.. the relative error is about Z.i70 per 1% absolute photometric error, which is the same as the iicruracy attainable in the range of 5 to 20 p.p.m. when samples arc’ measured against a blank solution. The range could not be e x t c d c d to much higher concentrations, inasmuch as 50 p.p.m of platinum is approaching the limit of color density of a reference bt:tndard which can be balanced in the instrument used. KO attempt was made to apply the differential method to organic extracts, because the stability of the extracted solutions decreased 1%ith increasing platinum concentration Although in general the aqueous solutions proved to be superior to the evtracted solutions for measurement, the extraction prowdure provides a method of concentrating the material being (letcwnined The high tiiqtrihution cocxficient of the colored
303 system between organic solvent and aqueous solution enhanceh the applicability of this method. 4 large number of factors may influence the intensity of the extracted color and cause transmittancy variations when the relative volumes of the two phases are varied; two factors of considerable importance are the distribution coefficients of colored species and of reagents, and the mutual solubilities of the two phases-Le., changes in volumes of thr layers on mixing. The reagent concentrations in the aqueoub phase were selected on two criteria: first, that slight changes in reagent concentrations produce little change in transmittancy ; and second, when possible the reagent concentrations were made high to decrease the solubility of the organic solvent and to minimize the reagent distribution effects. It is possible, of course, to correct for volume changes in the organic layer by carefully separating the two phases and diluting the organic solution to definite volume; this would require additional time, and when the extraction is used, to concentrate the color into a small volume would be impractical. -4t the reagent concentrations selected, the transmittancies of the developed solutions (either aqueous or organic phase) are iiot sensitive to small changes in amounts of reagents; errors of i t s much as 1 ml. in measuring the hydrochloric acid, tin(I1) chloride, or ammonium chloride were without effect on the transmittancy. The data of Table I11 show the effect of changing the ratio of aqueous to organic phase on the transmittancy of the organic extract; both the sample and the blank were affected. These results indicate that a separate standard curve is required for each volume ratio that would be used; otherwivise considerable wror xvould be introduced. In cases where only a small amount of platinum is present, or the volume of the solution is large, thr vstraction method may prove advantageous. Bpproximately 3 ml. of solution are required to fill the absorption crlls used; thr colored material may therefore be extracted with as little as 5 ml. of amyl acetate. From a volume of 25 ml. of aqueous solution, the platinum can be concentrated by a factor of five by such a11 rxtraction. The interference by palladium in the amyl acetate extract ir unique, in that interference increased up to a concentration of about 5 p.p.m., after which it remained constant up to concentrations of 50 p.p.m. or greater. For determination of platinum in the presence of small amounts of palladium, interference from the latter could be prevented by the use of a calibration curve for platinum solutions to which palladium had been added in an amount greater than 5 p.p.m.; addition of a t least 5 p.p.m. of palladium to the unknown platinum sample would ensure measurements on the plateau of the palladium interference, and reference to the special calibration curve would cancel the effect of the palladium. A simpler alternative would be the addition of at least 5 p.p.m. of palladium to both the sample and the blank solution. These procedures would be subject to some crror, because the cxtracted color of palladium is very sensitive to reagent concent rations. Table I V shows that the extracted solutions have more selectivity than the aqueous solutions, for the amount of interfering substance to produce a given relative error. With the exception of rhodium, and to a degree tellurium, extraction increased the tolerance of the platinum systcm for the other substances. I t is unfortunate that the greatest improvement by extraction is fc r those substances which are most easily separated from platinumnamely, osmium and ruthenium; these elements are separated, by volatilization as their t e t r o d e s , from the other platinum metals. For platinum determination in the presence of rhodium, however, use of the aqueous solution rather than the organic extract is indicated; this would probably be the case even in the piesrnce of palladium, because palladium is more easily separated from platinum than is rhodium. The tolerances for palladium, rhodium, gold, and telluriuni leave much to be desired. However, for a given relative error in the platinum determination, the spectrophotometric method
ANALYTICAL CHEMISTRY
304 does not require as complete a separation from these elements as is necessary for gravimetric determination. The spectrophotometric method offers an especially advantageous means for determining platinum in the presence of the more base metals of the acid hydrogen sulfide group; determination of platinum through the sulfide ( 3 , p. 289) requires absence of all other elements that are precipitated with hydrogen sulfide in acid solution. Wohler and Spengel ( 1 6 ) suggested that the color produced in the reaction of platinum(1V) with tin(I1) chloride was due to colloidal platinum. This suggestion is probably erroneous, in view of the fact that the colored material readily passes through semipermeable membranes such as collodion. Further evidence that the colored species is not the colloidal metal is shown by the rapid and complete extraction into organic solvents. The color has been attributed also to platinum(I1) (6). However, the authors found that when the platinum(1V) chloride solution (dilute chloroplatinic acid) was evaporated to fumes with sulfuric acid to expel hydrochloric acid, and the sulfuric acid solution was treated with tin(I1) sulfate, no color developed. Simple reduction to platinum(II), therefore, cannot account for the color reaction. Furthermore, addition of hydrochloric acid to the colorless platinum and tin sulfate solution immediately produced ax1 intense yellow color. I t appears, therefore, that chloride ion ia one of the requisites for color formation. The color is not due to chloroplatinous acid, as has been claimed (12, I d ) . h solution containing 10 p.p.m. of platinum as chloroplatinous acid (potassium chloroplatinite in 10% hydrochloric acid solution, prepared so as to prevent any possible atmospheric oxidation) was essentially colorless. Upon the addition of tin(I1) chloride to this solution, an immediate yellow color developed, comparable in intensity with that produced from 10 p.p.m. of platinum(1V). A solution containing 1000 p.p.m. of platinum as chloroplatinous acid had a color intensity comparable to that produced by 10 p.p.m. of platinum(1V) in the tin(I1) reaction. The spectral curve (transmittancy versus wave length) of the 1000 p,p.m. solution of chloroplatinous acid was of the same general shape, below 420 mp, as the curve for the solutions from the platinum(1V)-tin(I1) chloride reaction (Figure l), except that the maximum occurred a t 360 m r (instead of 353 mp), and the minimum occurred at 390 mp (instead of 403 mp); in addi-
tion, the chloroplatinous acid had another minimum a t 475 q. It is obvious, therefore, that the color is not due simply to chloroplatinous acid, but in some way involves the tin. A further study of the chemistry of the reaction process is under way in this laboratory, and will be made the subject of a later report. ACKNOW LEDGMEYT
Much of the work reported in this paper was made possible by a grant from the University of Texas Research Institute to Gilbert H. .\yes; this assistance is gratefully acknowledged. LITERATURE CITED
(1) Ayres, G. H.,ANAL.CHEM.,21, 652 (1949). (2) Figurovskii, N. A., Ann. secteur platine, Inst. chim. yen. (U.S.S.R.), KO.15,129(1938). (3) Hillebrand, W. F., and Lundell, G. E. F., “Applied Inorganic Analysis,” New York, John Wiley & Sons, 1929. (4) Hiskey, C. F., ANAL.CHEM.,21, 1440 (1949). (5) Hopkins, B.S., “Chapters in the Chemistry of the Less Familiar Elements,” Chap. 21, p. 39, Champaign, Ill., Stipes Publishing Co., 1940. ( 6 ) Hunter, D., Milton, R., and Perry, K. 11. A., Brit. J . I n d . .Wed., 2,92 (1945). (7) Karpov, B. G., and Savchenko, G. S., Ann. secteur platine.I m t . chim. gen. (U.S.S.R.), No. 15, 125 (1938). (8) Langstein, E.,and Prasunitz, P. H., Chem.-Ztg., 38,802 (1914). (9) Leutwein, F., Zentr. Mineral. Geol., 1940A,129. (10) Poluektov, N. S., and Spivak, F. G., Zavodskaya Lab., 11, 398 (1945). (11) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” pp. 358-60, New York, Interscience Publishers, 1944. (12) Scott’s “Standard Methods of Chemical Analysis.” S . H. Furman, ed., 5th ed., Vol. I, p. i13, New York. D. Van Nostrand Co., 1939. (13) Thompson, S. O.,Beamish, F. E., and Scott, M., INO.ENG. CHEX.,ANAL.ED.,9,420(1937). (14) Treadwell, F. P., and Hall, W. T., “Analytical Chemistry,” 6th English ed., Vol. I, p. 284, New York. John Wiley & Sons, 1921. (16) Wohler, L.,Chem.-Ztg., 31,938 (1907). (16) Wobler, L.,and Spengel, A.. Z. Chem. I d . Kolloide. 7 243 (1909). (17) Wolbling, H., Ber., 67B,773 (1934). RECEIVED June 5.1950.
Spectrophotometric Determination of Molybdenum with Phenylhydrazine Hydrochloride GILBERT H. AYRES A N I BARTHOLOMEW L. TUFFLY T h e Unizersity of Texas, Austin, Tex.
T
H E most widely used photometric method for the deterniination of molybdenum is based upon the amber to orange color produced by treatment with thiocyanate and a reducing agent, usually tin(I1) chloride; ordinarily, the colored complex is extracted into butyl acetate for the color measurement (IO). The color intensity is dependent upon a number of variables ( 6 ) ; one of the serious disadvantages of the method is fading of the color, and the rather rigid control necessary t o minimize this effect. By using water-acetone solutions and tin(I1) chloride reduction, Grimaldi and Wells ( 4 ) eliminated the extraction procedure, and found the acetone to exert a stabilizing effect on the color. Recently, Ellis and Olson ( 2 ) used acetone as the reducing agent, and found that this increased both the color stability and the sensitivity; extraction into organic solvents was not recommended. Among the several other color reactions of molybdenum that have been observed, i t appeared that the reaction with phenylhy-
drazine might prove useful for photometric analysis. Spiegel and Maass (9) observed that molybdates in acid solution reacted with phenylhydrazine to produce a blood-red color or a red precipitate. The reaction was later studied by Montignie (S), and reported t o be specific for molybdenum; color formation was said to involve the Oxidation, by molybdate, of the phenylhydrazine to a diazonium salt, which then coupled with the excess phenylhydrazine and molybdate. The method has been used as a spot test for molybdenum ( S ) , but appears not t o h a w been studied for application t o quantitative spectrophotometric analysis. I t was the purpose of this investigation to study the molybdenum-phenylhydrazine reaction to establish the best con ditions for color development, to evaluate the optimum range and the accuracy of the photometric process, to determine the nature and extent of interference from diverse ions, and to test the applicability of the method to the spectrophotometric determination of molybdenum in steel.
