V O L U M E 2 2 , NO. 10, O C T O B E R 1 9 5 0 nately delayed and noticeably incomplete. According to Hume (3) the existence of a lead nitrate complex of appreciable stability is strongly indicated. Abandoning the use of sodium nitrate, the ionic strength of the titration solution was increased by the addition of sodium perchlorate. JVhen the concentration of this material was increased to 0.5 -11an error of 2% was observed. Thus, results of optimum accurac? can be obtained only in solutions of moderately low ionic strength, but since the method works well a t low fluoride concentrations, it, will usually be sufficient to dilute a solution of high ionic strength. An accuracy somewhat bettdr than 0.5% can be maintained under the optimum conditions defined above. The ampero-
1277 metrically determined end point coincides with the stoichiometric equivalence point within the experimental error. LITERATURE CITED (1) Haul, R., and Griess. W., 2. anorg. Chem., 259,42 (1949). (2) Hoffman, J. I., and Lundell, G. E., BUT.Standards J. Research, 3, 581 (1929).
(3) Hume, D., private communication. (4) Kaufman, s.,ANAL.CHEM., 21, 582 (1949). (5) Kolthoff,I. XI., and Lingane, J. J., “Polarography,” New York,
Interscience Publishers, 1941. (6) Langer, A , , ISD. ENG.CHEM.,ASAL.ED., 12, 511 (1940). (7) Rinck, E., Bull. soc. chim., 1948, 305. RECEITEDMarch 10, 1950.
Spectrophotometric Study of the Ruthenium-Thiourea Complex GILBERT H. AYRES AND FREDERICK YOUNG The CJnirersityof Texas, Austin, Trx.
A spectrophotometric study has been made of the blue color produced when solutions of ruthenium(1V) or ruthenium(II1) chloro complexes are treated with acid and thiourea. In hot solutions containing hydrochloric acid and ethyl alcohol the color develops rapidly and is stable. The system has a sharp absorption band a t 305 mp, arld a broad absorption band a t 620 mp. The latter wave length +I as chosen as more suitable for use, primarily from the standpoint of avoiding interference from other metals. .4 photometer with suitable light filter could be used satisfactorily. When transmittancy nieaburernents are made a t 620 mp with a Beckniaii spectrophotometer using 1-cm, cells, the optimum concentration range for the measurement is about 2 to 15 p.p.m. (micrograms per milliliter) of ruthenium. 3Iaximum attainable accuracy for the photo-
T
H E iricreasiiiy commercial importance of ruthenium is well indicated by the patent literature on its uses in high density alloys, alloys for jewelry and pen points, in spark plug electrodes, snitch contacts, and resistance wires, and as a catalyst. These uses suggest the need for a rapid, accurate method for determining ruthenium. Many of the previous methods of analysis are tedious and/or subject to considerable error. Precipitation by replacement from acid solution by active metals gives a metallic “black” with high adsorptive properties which usually contains some of the base metal used as reagent (21, p. 711). Separation as sulfide followed by ignition yields oxide residues which are not of sufficiently exact composition for weighing, and some sulfur is always retained (IO,p. 285; 21, p. 710); ignition a t too high a temperature can result in loss of ruthenium. Precipitation as hydrous oxide can be made from faintly alkaline solution ( 2 4 ) ,or from solutions of pH 6 ( 9 ) ,followed by ignition to oxide and reduction by hydrogen; the hydrous oxide is difficult to coagulate and filter, and may be considerably contaminated by coprecipitation of other substances. Ruthenium can be precipitated with thionalide (paminonaphthalide of thioglycollic acid) followed by ignition and reduction in hydrogen (18); for semimicro quantities, the results
metric process is 2,770 relatibe error per 1% absolute photometric error, or about 0.570 relative error for a precision of 0.29’0 in making the measurements. Of the other platinum metals, only palladium and osmium interfere a t 620 mp; a concentration of 7 p.p.m. of ruthenium will tolerate about 0.7 p.p.m. of palladium and about 0.2 p.p.m. of osmium. Interference tests were made with solutions of iron, cobalt, nickel, copper, and chromium; of these, only cobalt and chromium give appreciable interference. By the usual methods employed for separation of ruthenium and osmium from all other metals and from each other, cation interference can be eliminated. Anions t h a t might be introduced during the various separation methods-namely, bromide, hypochlorite, nitrate, sulfate, and p e r c h l o r a t e d o not interfere in the concentrations studied.
tend to be low by as much as 10% ( 6 ) . .A titrimetric method involving reduction of ruthenium(1V) to ruthenium(II1) with tin (11) chloride (13) gave results w-hich were always somewhat low. Xumerous color reactions of ruthenium have been reported ( 1 7 , 22, 23, 25, 26); some of these reactions are applicable only t o spot-test and microscopic identification, and are unsuitable for colorimetric determination on account of formation of insoluble products. Many color reactions, however, should be suitable for spectrophotometric determination of ruthenium; very little work along this line has been published. A reported colorimetric method for ruthenium, based on the “dark” color of its solution in hydrochloric acid to form H2RuClj ( 2 7 ) , would be subject to considerable interference from other platinum metals in solution, Breckenridge and Singer ( 5 ) studied 5-hydroxyquinoline-8carboxylic acid as a colorimetric reagent for ruthenium; spectral curves were presented, and the effect of the other platinum metals was investigated. Sandell (20) determined small amounts of osmium on the basis of the rose-red color produced by thiourea; Ayres and Wells (3) made a detailed spectrophotometric study of the osmium-thiourea system, including the effect of the blue color of ruthenium-thiourea as an interference.
A N A L Y T I C A L CHEMISTRY
1278 The work of DeFord ( 7 ) , Tvhich appeared late in 1949, gave an extensive bibliography of ruthenium, and included a study of the ruthenium-thiourea system. He gave methods for the application of the color reaction to ruthenium of various oxidation numbers and states of combination. DeFord's specification of optimum concentration range, corresponding to optical densities from 0.043 t o 0.430, is in error (2). So data were given for interference from other colored metallic ion,.
Table I. Soin.
so.
Standardization of Ruthenium Solutions
Aliqilot
taken,
e.
1111.
100.0
1
Graviiiietric X e i g h t of rutheniuin,
100 0
100.0
*elution, p p . t n . Ku
n n
0202 0203 0.0195
2
30.0 1no.o
0 Oli8 0.0351
3
250.0
0.0232
a
Concn. of
Colorimetric, Concn. of Solution. P.P.N. R u
?O"
203 19.5
A\.,
200 3.i6 351
.4v.
354
Average of 12 separate deterininarion..
!r:j
200" Used a8 spectrophotometric standard
91 b
i t t i standard deviation of 0.6
p.p.m. b
Dui.'licates.
I t is the purpose of the present investigation to make n spectrophotometric study of the ruthenium-thiourea color system, particularly with reference to rvaluation of optimum range and masimum accuracy of the photometric process (a), and also to investigate the nature and estent of possible interference from other platinum metals arid other common colored cations, as well as anions that might be introduc~edinto a ruthenium solution when it is prepared for analysis. Another paper presents a similar st.udy of the ruthenium-dithio-oxamide system (4).
hydrochloric acid. During boiling to remove chlorine, coiisidcrable ruthenium tetroxide was lost by volatilization. Attcbmpts to standardize the ruthenium solutions by reduction with metals (10, pp. 274-5) either gave inconsistent results or were too tedious to be practical. Zinc and magnesium reductions produced very finely divided residues which adhered to the walls of the vessels and always contained some of the reductant metal. Iron (reduced powder) gave good precision as a reductant; howt'ver, a t least five extractions of the residue with 6 Jf hydrochloric acid xere required to remove all of the iron and iron salts. In all rases of metallic reduction, a residue of inconstant composition x a s produced, necessitating reduction by hydrogen before weighing. Precipitation n.ith thionalide (18), using samples containing about 20 nig. of ruthenium, gave estremely poor precision resulting from the rather high solubility of the precipitate; ruthenium could always be detected both in the mother liquor sild in the wash solution. The ruthenium solutions were succcssfully standardized by the method of Gilchrist and Xichers ( 9 ) , i l l which precipit:ition from hot acid solution was effected by adding sodium bicarbonate to a pH of 6. Exercise of great csre \\-:is necessary during filtration and xashing, to prevent pc'ptizittion of the precipitate. The oven-dried precipitate was reduced by heating a t 700" C. for 20 to 30 minutes in a hydrogen atmospliere, a.nd finally weighing as metal. The results of six iiidividual determinations on three diffrrrnt solutions are shown in Tai~leI. The standard curve (Figure 2 ) was constructed from dat>aobtained by the use of aliquots of solution 2. Spectrophotometric comparison of the other solutions \vith solution 2 gave the results show1 in th6 last column of Table I.
I00
REAGESTS
Ruthenium metal powder was obtained from the Fisher Scientific Company and from the rlmerican Platinum Works. Spectrographic examination of the samples showed absence of other platinum metals. Test solutions of the other platinum metals, and the 10% thiourea solution, were prepared as described by A y e s and Wells (8). Test solutions of iron(III), cobnlt)(II), nickel(II), chromium(III), and copper(I1) were prepared from the chloride salts; chromium(V1) was used in the form of potassium dichromate. Test solutions of bromide, hypochlorite, nitrate, sulfate, and perchlorate were prepared from their alkali salts.
80
>
0
2'60
a
+ L v)
z
4
APPARATUS
Transmittancy measurements Irere made with a Beckman Model DU spectrophotometer, using Cores cells of 1.004-cm. light path. The instrument was operated a t constant sensitivity, using slit widths of the order of 0.02 to 0.10 mm., corresponding t o nominal band widths of about 1 to 1 nip. In some of the preliminary work on the development of the method and testing for interference, spectral curves were made with a General Electric recording spectrophotometer. EXPERI3IESTA L
E
40
20
0
300
Preparation and Standardization of Ruthenium Solutions. Ruthenium metal powder, 0.2 to 0.8 gram, was treated under reflux condenser with a boiling mixture of 100 ml. of 5 % sodium hypochlorite solution and 50 ml. of 2 .If sodium hydroxide. The exit of the reflux condenser was connected to a trap containing sodium hydroxide solution to absorb any ruthenium tetroxide if i t should volatilize from the mixture. Negative test for ruthenium in the trap solution confirnied the findings of Howe and hlercer (14) that no ruthenium tetroxide distills from alkaline hypochlorite solutions. The cooled hypochlorite solution was then rapidly acidified with hydrochloric acid, boiled to remove chlorine, cooled, and diluted to known volume; the final solution contained the ruthenium in the form of its chloro complexes, in 6 Af
400
500
600
WAVE LENGTH,
Figure 1.
700
mp
Spectral Transmittancg Ciirves for Ruthenium with Thiourea
Development of Color. By a preliminai y study of the variables involved the foilowing conditions \vel e established as advantageous for the rapid development of a stable colored system: solution about 6 .1f in hydrochloric acid for initial color development; large excess of thiourea; presence of ethyl alcohol to the
V O L U M E 22, NO. 10, O C T O B E R 1 9 5 0
1279
extent of 50% by volume (to increase the rate of color formation and give a more stable color); heating a t 85" C. (at much higher temperatures considerable alcohol was lost unless refluxing was used; a t lower temperatures the rate of color development was slow); final solution about 4 :If in hydrochloric acid. The procedure was as follows: -4ppropriate aliquots of the stock standard solution, to give the final conceritration desired, were added to 40 ml. ot a. 1 to 1 (by volum'e) mixture of concent,rated hydrochloric acid and ethyl alcohol; after the addition of 5 ml. of 10% thiourea solution, the mixture was heated for 10 minutes in a water bath a t 85' C. The solution was cooled, and made up to 100.0 ml. with a 1 to 1 mixture of 6 Jf hydrochloric acid and ethyl alcohol. Blanks contained the same amounts of reagents. D a t a fnr trnnsmittnncr versus wave-length ctwves were ohtailled by measuring the transmittancy a t frequeiit wive-length intervals over the range 750 to 300 mp, Typical spectral curves for various ruthenium concentrations are shown in Figure 1. All transrnitt:incy curves h a w a broad, flat minimum a t 620 mp; centered a t about 480 nip, the curves have a broad region of high transmittancy; a sharp inversion a t 305 mp is of somewhat lower transmittancy than the 620 mp minimum. Below 305 mp the transmittancy rises sharply. A plot of log transmittancy (620 nip) against concentration showed good agreement with Beer's law over the range investigated (up to 18 p.p.m.).
Table 11. T o l e r a n c e of R u t h e n i u m - T h i o u r e a S o l u t i o n for Metal Interferences Interfering Substance Osmium(1V) Palladium(I1) Iron(II1) Cobalt(I1) Nickel (11) Coyper(I1) Chrorniurn(II1)
(All solutions, 7.1 p.p.m. ruthenium) Visual Color Produced Amount % ' , Interference by Interfering SubTolerated, Relative t o stance Plus Reagents P.P.M. Ruthenium Rose-red Yellow 4 mber Blue Green Greenish-yellow Green
0; 0 , 5 0.5 20
5 1
nium-thiourea system might be expected from osmium and palladium. Interference from colored ions such as Si++,C o + + , Fe+++,Cr+++, CrzO,--, and Cu+'might be expected. In order to determine the estent of interference by the ions mentioned, solutions contairiing a constant amount of ruthenium (7.1 p.p.m., which is in the optimum range for measurement) and varying amounts of the ion in question were developed with thiourea in the usual way. The interfering substance was added in the same state as that in which it would be found in solution prepared from the metal by the same treatment as used in dissolving the ruthenium. Thus, the common metals used would be present as iron(III), cobalt(II), nickel(II), copper(II), and chromium(V1) (dichromate). I n the high concentrations of alcoholic hydrochloric acid used to develop the rutheniumthiourea color, copper gave the characteristic greenish-yellow color of the cliloro comples, and cobalt gave the blue color of the chloro and/or ethyl alcohol complex. Chromium(V1) was immediately reduced to chromium(II1) when thiourea was added to the acid-alcohol mixture; iron(II1) appeared not to be reduced by this treatment; osmiuni(VII1) reduced to osmium(1V) (chloro-osmate) and was complexed by the thiourea (5). Transmittancies of the ruthenium solutions containing the interfering substance were measured a t wave lengths between 700 and 500 mp; no shift of transmittancy minimum was observed. T h e tolerance of the ruthenium-thiourea system for the interfering substance was taken as the largest amount of t h a t substance which would give a transmittancy, a t 620 mp, not more than 0.4% (absolute) diffe:ent ( 3 ) from that of the ruthenium alone. The results of the interference tests on the various metals are shown in Table 11, along with the color of the interfering substance after treatment with the reagents.
3 10 70 7 280 70 14
Rate of Color Development. When the concentrations of reagents specified in the previous section were used, color development a t room temperature was very slow and reached a stable maximum only after a few days. When heated a t 85' C. the mixtures developed to a maximum, stable color intensity within 10 minutes. Stability of Color. Solutions containing various concentrations of ruthenium, developed by the procedure given previously, showed no measurable change in transmittancy over a period of 24 hours, and a change of only 0.2 to 0.4% (absolute) in 48 hours. A black precipitate formed in the more concentrated solutions after about 48 hours. Reproducibility.. A statistical treatment was made of the transmittancy measurements on 32 samples developed as described previously; the data, collected over a period of several weeks, included all analysis errors accumulating onward from the use of the stock standard solution, and no results were rejected. The standard deviation ( u ) of the 32 measurements was 0.21 * 0.02% absolute transmittancy (see discussion for the corresponding relative analysis error). On this basis, random discrepancies in transmittancy of 0.4% (2a) or less can be espected with a probability of 957& Effect of Diverse Ions. Qualitative tests on solutions of the other platinum metals with thiourea showed that no color reaction was given by rhodium, iridium, and platinum, but that osmium produced a red solution and palladium produced a yellow solution. These results confirmed the findings of Ayres and \Tells (3) in a spectrophotometric study of the qsmium-thiourea system; their results indicated that interference with the ruthe-
20
RUTHENIUM+THIOUREA
IO
0 2 4 6 810 20 C O N C E N T R A T I O N OF R U T H E N I U M , P.P.M.
F i g u r e 2.
C a l i b r a t i o n Curve for R u t h e n i u m w i t h T h i o u r e a at 620 m p
By the usual separation procedures for osmium and ruthenium from the other platinum metals and from each other, osmium is f i s t distilled as osmium tetroxide from nitric acid solution. Before the separation of ruthenium, the nitric acid in the residual material is removed by repeated evaporation with hydrochloric acid and fuming down with sulfuric acid. After adding sodium bromate, ruthenium is distilled as RuOa (9). When absorbed in hydrochloric acid, ruthenium tetroxide gives a solution containing
ANALYTICAL CHEMISTRY
1280 ruthenium(IV)and/or ruthenium(II1)chloro complexes(6,lS). If the ruthenium tetroxide is absorbed in hydrochloric acid saturated with sulfur dioxide, excess of the latter is removed by boiling t h e acid mixture before proceeding with the analysis (9). If absorbed in a mixture of hydrochloric acid and ethyl alcohol, ruthenium(II1) chloro complex is formed ( 1 4 ) ; this same complex is probably formed as an intermediate by the use of the acidalcohol mixture in the color development process of the present study. If alkali is used to absorb ruthenium tetroxide, alkali ruthenate ( S a 2 R u 0 4 )is formed ( 1 2 , 1 4 ) , which on acidification with hydrochloric acid gives chloro complexes of ruthenium( IV) and/or ruthenium(II1). After separation of osmium as OsOd, ruthenium can be distilled as ruthenium tetroxide from perchloric acid solution (16). When ruthenium metal is attacked by alkaline oxidizing fluxes (1, 12, f9),solution of the melt in water gives ruthenate. From alkali hypochlorite dissolution, ruthenium can be distilled as ruthenium tetroxide in a current of chlorine (f4). Considering all these methods of attack and of separation of ruthenium, the only common anions besides chloride that could have been introduced in analytical amounts in the distillate are: bromide, from reduction of free bromine by sulfur dioxide; sulfate, from oxidation of sulfur dioxide; nitrate, from incomplete removal of nitric acid before distillation; hypochlorite, from incomplete removal of this reagent or by hydrolysis of chlorine; and perchlorate. T h e study of anion interference was therefore limited to these anions. In solutions containing 4 p.p.m. of ruthenium, all the anions mentioned above were without effect up to 100 p.p.m., hence were not studied a t higher concentrations. DISCUSSION
The calibration curve for the determination of ruthenium with thiourea is shown in Figure 2, in which per cent absorptancy (100 - % transmittancy) at 620 mp is plotted against log concentration; each experimental point was established by many replicate measurements. T h e utility of this plotting method for evaluation of maximum attainable accuracy and suitable working range has been reported previously ( 2 ) . The curve has its maximum slope at about 63% absorptancy, in agreement with Beer’s law, hence a maximum accuracy corresponding to 2.7% relative analysis error per 1% absolute photometric error. T h e accuracy of the photometric process a t any point can be evaluated graphirally from the calibration curve by dividing 230 by the slope of the curve, expressed as the change in per cent absorptancy per logarithmic cycle (tenfold concentration change) of a tangent at the given point. By this method, a tangent to the curve at its steepest slope covers a change of 85% absorptancy for one logarithm cycle of abscissa; the masimum accuracy is therefore 230/85 = 2.7y0 relative analysis error per 1% photometric error. T h e maximum accuracy occurs at about 7 p.p.m. of ruthenium. Inspection of the calibration curve shows that the error is not much greater between 2 and 15 p.p.m., which would be a satisfactory working range of concentration.
If desired, the limits of the optimum range can be defined more exactly; suppose, for example, that i t is desired to find the concentration range within which the photometric error will not exceed 473 relative error per 1% absolute photometric error; inasmuch as replicate samples ran be reproduced to 0.2% absolute transmittancy, this would represent a relative analysis error of 0,8yo. A relative error of 4y0 per 1% photometric eiror corresponds t o a slope of 230/4 =
