Separation of Rhodium( Ill) from Iridium( IV) Spectrographic and Spectrophotometric Evaluation GILBERT H. AYRES and CHARLES M. MADDIN’ The University of Texas, Austin, Tex.
methods were the same as given by Ayres and Berg ( 3 ) . A sharp rise in film contrast generally occurs for radiation of wave lengths greater than about 3400 A. (7’). Film calibration curves for the 3400 to 3500 A. region were found to be in very close agreement with the curves for the 2600 to 3400 A. region, over which the film calibration curve is generally valid. Spectrophotometric measurements of iridium solutions were made with a Beckman Model D U spectrophotometer, operated a t constant sensitivity. Matched Corex absorption cells of 1.00-cm. light path were used for samples and blank. Potentiometric measurements were made with a Beckman Model H-2 p H meter, using a standard calomel-platinum electrode system.
Rapid and specific methods of analjsis were required for the evaluation of the sharpness of separation of rhodium(II1) from iridium(1V) by selective reduction of the rhodium(II1) to the metal by titanium(II1) chloride. Spectrographic methods of analysis were developed, using the porous cup electrode technique with a high voltage spark source and the Rh 3434.9/Co 3455.2 and Ir 2849.71Co 3455.2 line pairs. Relative analysis and the lower limits of detecerrors were about 2.87~~ tion were 0.5 p.p.m. for rhodium and 50 p.p.m. for iridium; smaller amounts of iridium were determined spectrophotometrically. Precipitations of rhodium (111) as metal were made from boiling solutions; the amount of rhodium failing to precipitate was determined spectrographically, while the iridium which coprecipitated with the rhodium was determined spectrophotometrically. Evaluation of separations of disproportionate amounts of rhodium(lI1) and iridium(IV) showed a separation of about 99.7% for each element. The amount of rhodium failing to precipitate and the amount of iridium coprecipitating were both independent of the quantity of the other element present. The results indicated that, in a gravimetric separation of rhodium from iridium, there exist compensating errors of small magnitude.
REAGENTS
Rhodium(II1) chloride tetrahydrate, obtained from A. D. Mackay, Inc., was tested for impurities by direct current arc analysis and found to contain traces of silver, gold, copper, lead, and platinum, but no iridium. A small quantity of concentrated hydrochloric acid was used to dissolve 2.735 grams of the salt; the solution was diluted with distilled water to 1 liter, making a standard solution containing 1000 p.p.m. of rhodium. Iridium(1V) chloride, stated by the supplier (hfackay) to be of purity 99.5% or higher, was tested for impurities by direct current arc examination; principal impurities were lead, silicon, platinum, and rhodium. The rhodium concentration was estimated to be O . l % , a quantity which would impair the use of the iridium solution for preparation of spectrographic standards for determination of small amounts of rhodium in the presence of large amounts of iridium. The rhodium and platinum impurities %ere removed by precipitation with titanium(II1) chloride (9). A4small quantity of concentrated hydrochloric acid was used to dissolve 3.486 grams of the iridium(1V) chloride, then 100 ml. of concentrated sulfuric acid were added, and the solution was heated until heavy fumes of sulfuric acid appeared and until a clear green solution resulted. The solution was diluted to about 500 ml. and heated to boiling, then 20% titanium( 111) chloride solution was added dropwise until the pqtentiometric equivalence point was reached. The mixture v a s heated for 2 minutes, and filtered after the addition of paper pulp to gather the precipitate; the precipitate was washed 10 times with hot 2.5% sulfuric acid. The filtrate (including washings) was heated until dense fumes appeared. .2fter cooling, the solution was transferred to a 2liter volumetric flask and diluted to volume with distilled water, making a standard solution containing 1000 p.p.m. of iridium. By extensive concentration of some of this solution, the rhodium content of the purified iridium was estimated, by direct current arc examination, to be not greater than 0.01%. Cobalt(I1) sulfate heptahydrate (Merck) was found to be spectrographically free from all platinum metals and to contain only traces of iron and nickel. A solution containing 8000 p.p.m. of cobalt was prepared by dissolving 38.154 grams of the salt in distilled n-ater and about 10 ml. of concentrated sulfuric acid, then diluting to 1 liter. This solution was used as an internal standard. Titanium(II1) chloride was obtained as a 20y0 solution. Direct current arc analysis showed absence of platinum metals. This solution used as the precipitant to effectselective reduction of rhodium(II1) to the metal in the presence of iridium(1V). Titanium dioxide (General Chemical CO.)was found to be spectrographically free from all platinum ( h m n t r a t e d SUIfuric acid, 100 ml., was added to 5.17 grams of titanium dioxide and heated dense fumes appeared for 10 minutes, The solution was then cooled, diluted to about 400 ml., and traces of residue mere filtered off. After concentrating the filtrate to about 200 ml., it was transferred to a 250-ml. volumetric flask and diluted to volume, giving a final concentration of about 12,500 p.p.m. of titanium. This solution was used as a diluent for spectro!PPhic standard solutions, to approximate the concentration of titanium Present in solutions to be a n a l ~ z e d . Acids used, nitric, hydrochloric, Bulfuric, phosphoric, and perchloric, were of purity in every way satisfactory for use in this study.
T
H E methods of separation of the platinum metals from each other and t.heir quantitative determination, originally given by Gilchrist and Wichers ( 6 ) , have been generally adopted ( 1 , 8). I n a previous paper ( S ) , some of the difficulties inherent in the precipitation separations and gravimetric determinations were outlined, and Ppectrographic methods for the determination of palladium, platinum, iridium, and rhodium were proposed as a means of evaluating the sharpness of the separations. These methods have been used i n studying the separation of palladium from platinum, iridium, arid rhodium with dimethylglyorime
(4). The present report conceriii a similar study of the separation
of rhodium from iridium by reduction of the rhodium compound in solution t o elemental rhodium, using tit>anium(III)as the reducing agent. Preliminary work indicated that the rhodium line 3323.1 and the iridium line 3220.7 used by hyres and Berg (S), could not be used in the present study on account of interference from lines produced by the titanium reagent; other lines (more exactly, line pairs with caobalt as internal standard) were therefore sought The amount of rhodium failing t o precipitate a t the stoichiometric point Kas within the range t o be adequately evaluated by the porous cup electrode technique, but the amount of iridium contaminating the rhodium precipitate was too small t o be eva]uated by this method. It was found that a slight modification of the spectrophotometric method given by Ayres and Quick ( 5 )was patisfactory for determining the coprecipitated iridium. APPARATUS
The spectrographic equipment used, the preparation of porous cup electrodes, the exposure conditions and film developing procedures, the densitometer measurements, and the film calibration Present addresq, Douell, Inc., Tulsa, Okla.
671
672
ANALYTICAL CHEMISTRY
1000- I I
Rh 3434.9/Co 3455.2 Concentration ~ ~ l of Rhodium, P.P.M. Taken Found error, 25.0 6 0 26 5 30.0 9 3 32 8 35.0 2.0 35 7 50.0 50 0 0 0 55.0 5.5 0 0.0 60.0 59 0 91.7 65.0 1 5 66 0 70.0 2 9 72 0 1.3 75.0 76 0 .i 1..
Ir 2849.7/Co 3455.2 Concentration ~ of t Iridium, i ~ P.P.hI. ~ ~ ~ Taken Found error, % 250 266 6 4 290 3 3 300 " 9 350 340 442 450 1 8 518 500 3 6 550 560 1 8 600 0 0 600 7 00 0 0 700 1 1 720 710
2 8
" 4
I I I II
I
-
Table I. .4ccuracy of Rhodium and Iridium Determinations
I
-
l
~
50 0
~
i
~
~
250
i Q 0:
2-
0 l-
a
E c
IO 0
EXPERIMENTAL
Preliminary work shon ed that the line pairs rhodium 3323.1 / cobalt 3354.4 and iridium 3220.7/cobalt 3354.4, used by A4yres and Berg ( S ) , could not be used in the presence of titanium, because of interference with rhodium 3323.1 by titanium 3322.9, with cobalt 3354.4 by titanium 3354.6, and with iridium 3220.7 by titanium 3320.5. Although the rhodium 3323.1 line could be used in analyzing the dissolved precipitate for rhodium, it would be useless in determining unprecipitated rhodium in the filtrate containing titanium. Interference with the iridium line was not serious if the titanium concentration was kept below 1200 p.p.m., but in order t o make the iridium determination highly specific, it was decided t o use lines free of interference, because a filtrate containing * iridium will necessarily contain titanium in varying amounts depending upon the amount of rhodium precipitated. Evaluation of New Line Pairs. Because of the geneial low spectral sensitivity of the platinum group metals, the use of other lines for rhodium and iridium was limited to their other persistent lines: rhodium 3396.9 and 3434.9, and iridium 3513.6, 3437.0, 2924.8, and 2849.7. All cobalt lines appearing in the 3200 to 3500 A. range a t a concentration of 3200 p.p.m. of cobalt and the aforementioned rhodium and iridium lines, were checked for interference from each other and from the titanium reagent. This study showed iridium 2849.7, rhodium 3396.9 and 3434.9, and eight cobalt lines t o be free of interference. I n addition, the lines rhodium 3323.1 and iridium 3220.7 and 2924.8 were eyamined to determine their behavior relative t o the cobalt lines pelected. The cobalt concentration was varied to bring the desired cobalt lines into a moderate range of absorbance; nine replicate spectra were recorded for each concentration used, in order t o obtain a statistical variation of a given intensity ratio. ill1 possible intensity ratio combinations were calculated; on the basis of the results thus obtained, the line pairs rhodium 3434.9/cobalt 3455.2 and iridium 2849.7/cobalt 2355.2 were selected for further study. The optimum working range was determined t o be 20 to 80 p.p.ni. for rhodium, and 200 to 800 p,p.m. for iridium; the loryer limits of detection were 0.5 p.p.ni. and 50 p.p.m., respectively. For the preparation of spectrographic standards, suitable aliquots of the rhodium stock solution were fumed down with sulfuric acid, to approximate more nearly the conditions under a hich the method 11-ould be used; the iridium standard solution had already been converted t o the sulfate form. Rhodium and iridium standards v ere prepared in the above working ranges, nith the addition of cobalt(I1) t o give a final concentration of 1600 p p.m. and sulfuric acid t o give a final concentration of 10% by volume. Triplicate exposures were made of each solution. Logarithms of average intensity ratios were plotted against logarithms of the corresponding concentrations. The working curves are illustrated in Figure 1. These curves can be resolved into normal a-orking curves (45 a slope on logarithmic coordinates) by applying background corrections. The suitability of the selected
50
25
.25
.50
I
2
4
INTE NSlT Y RATIO Working Curves for Spectrographic Figure 1. Analysis of Iridium and Rhodium
line pairs was verified by pel forming analyses in triplicate on nine standards; the results are shown in Table I. Incidental t o this study was the determination of the utility of other line pairs not suited for use in thii particular problem. The line pairs most suitable, and their corresponding average relative analysis errors, were: rhodium 3434.9/cobalt 3354.4, 1.5 %; iridium 3220.7/cobalt 3354.4, 2.7%; iridium 3220.7/cobalt 3455.2, 2.1 %; iridium 2924.8/cobalt 3354.4, 2.3%; iridium 2924.8/cobalt 3455.2, 2.9%; and iridium 2849.7/cobalt 3354.4, 1.8%.
Table 11. Reproducibility of Rhodium Data
spectra X O of Concentration, P.P.11. Rh Ir Co Ti 0 20 0 0 1600 40 0 0 1600 0 0 1600 80 0 0 1 0 625 400 80 2 0 625 400 80 2 5 5000 1600 400 3 0 400 1600 5000 1600 10 0 5000 400 1600 20 0 5000 400
Intensity Ratio,.\leas- Rh 3434.9,'Co 3455.2 ured Average Btd. der. 0 015 4 0 703 0 010 5 0 997 1 53 0 05 6 1 039 0 001 3 0.009 1 076 0 003 0 339 0 020 0 389 0 043 0 472 0 593 0 015
concn,, 4% 2.5 1.5 2.5 3
20 4
15
23
7 5
The precision with n hich the rhodiuni/cobnlt intensity ratio could be reproduced is illustrated in Table 11; the evaluation includes day-to-day and film-to-film data used for standard curves for determining small amounts of rhodium in the presence of iridium and titanium. For solutions containing 2 p.p.m. or less of rhodium, maximum opening of the grating doors allowed maximum exposure for a given time; the cobalt concentration of 80 p.p.m. was advantageous in giving intensity ratios near unity, thereby minimizing film calibration differences. Preliminary work had shown that the spectral sensitivity of iridium was too low for the determination of small amounts of coprecipitated iridium; hence, Table 111 shows the reproducibility of the iritlium data only in the optimum concentration range.
673
V O L U M E 2 6 , NO. 4, A P R I L 1 9 5 4 Spectrophotometric Determination of Iridium. For the determination of coprecipitated iridium, a modification of the method of Ayres and Quick ( 5 ) was used. The method is based upon a purple color formed when iridium in solution is heated with a mixture of phosphoric, perchloric, and nitric acids, hereafter designated as "mixed acid reagent." iilthough .4yres and Quick did not use sulfuric acid in the reagent finally selected, it seemed advisable to study the stability and reproducibility of the color in solutions of high sulfuric acid content, because the solutions to be analyzed in the present study contained relatively large amounts of sulfuric arid used for dissolution of the rhodium precipitate.
Table 111. Reproducibility of I r i d i u m D a t a Concentration, P.P.11. Ir co Ti 200 1600 0 1600 400 0 1600 800 0 200 1600 62.5 400 1600 625 625 800 1600 1600 200 5000 1600 400 5000 1600 800 5000
K O . of spectra
Intensity Ratio, J I ~ ~Ir ~2849.7/Co 3455.2 ured Average Std. Dev. 0 0 1 0 0
668 954 41 638 824
n
703
1 15 0 556
i ,009
0 0 0 0 0 0 0 0 0
040 026 04 042 070 06
046 015 024
Der'
&,!:.,
7C
'0 1
13 20 10
17 4 4
Standard iridium solutions containing from 20 to 60% sulfuric acid (in addition to the mixed acid reagent) were found to have a characteristic absorption maximum a t 556 mp, whereas solutions containing no sulfuric acid RhoNed maximum absorption a t 564 mp; for a given amount of iridium, solutions prepared by the two methods had the same absorbancy a t theee wive lengths, respectively. Iridium solutions containing 40% sulfuric acid developed maximum color intensity by heating a t 150" C. for 40 minutes, and showed no appreciable change in aborbancy over a period of 50 hours. On the basis of these findings, the folloaing standardized procedure was adopted: An aliquot was taken such that the final iridium concentration would be from 5 to 100 p.p.ni., and sufficient mixed acid reagent and sulfuric acid were added to give a final concentration of 20% and 40%, respectively. The resulting solution was heated a t 110' C. for 1 hour to eliminate volatile components; the temperature was raised slowly to 150" C. and held a t that temperature for 40 minutes. The solution was cooled, transferred to an appropriate volumetric flask, and diluted to volume with 1% nitric acid. Absorbancy measurements were made at 556 mp. Separation of Rhodium(lI1) from Iridium(1V). In the procedure of Gilchrist and Wichers ( 6 ) , the solution containing rhodium and iiidiuni is fumed down a i t h sulfuric and nitric acids to destroy the excess dimethylglyoxime remaining from the previous separation of palladium. T o the hot eolution, titanium(II1) solution is added slowly until it is present in slight excess as shonn by the purple color. The precipitated rhodium metal is filtered off and u aqhed with 2.5% sulfuric acid, and the filter paper and rhodium are fumed down with nitric and sulfuric aridq; any metal remaining unattacked is filtered off, and again tieated with the a d s . In the preliminary work of the present study, obcervation of the end point of the reduction [purple color of excess titanium(II1) reagent] n a s ohcured by the finely divided black rhodium precipitate that coagulated only s l o ~ ~ l ythe ; purple color in solution was not observed until after the addition of amounts of titanium(II1) far i n exceqs of the stoichiometric requirements for the amount of rhodium present. Furthermore, the precipitate always contained notable amounts of iridium These difficulties were eliminated by potentiometric detection of the end point of the reduction. Difficulties were also experienced in completely dissolving the metal precipitate, even by repeated treatments with nitric and
sulfuric acids. Direct current arc examination of the insoluble residues showed them to be platinum (impurity in the rhodium and iridium source materials) and rhodium; these residues were found to be completely soluble in aqua regia. On the basis of these findings, the following procedure was adopted. Aliquots of the rhodium( 111) and iridium(1V) standard solutions were heated with 3 ml. of nitric acid and 10 ml. of sulfuric acid. After strong fuming for 1 minute, the solution was cooled, 20 ml. of distilled water were added, and the fuming was repeated. The solution was cooled and diluted to about 200 ml., then heated to boiling, and 20% titanium(II1) chloride solution was added dropwise until a reversal in the sign of the potential was observed, using a standard calomel-platinum electrode system. The amount of titanium(II1) reagent required for a given amount of rhodium always agreed closely with the theoretical amount. The electrodes were rinsed, and any adhering metal was wiped off with small pieces of filter paper which were added to the filter to be used later. The mixture was boiled for two minutes, and a small amount of paper pulp was added to gather the precipitate. After filtering, the precipitate was washed 10 times with hot 2.5% sulfuric acid. The filter paper and contents were treated with 10 ml. of sulfuric acid and 3-ml. portions of nitric acid a t a low heat until the paper was completely decomposed, and the mixture was then fumed strongly for about 1 minute. After cooling, 20 mi. of distilled water and 20 ml. of aqua regia (1 part of nitric acid to 3 parts of hydrochloric acid) were added, and the mixture was heated gently until evolution of brown oxides of nitrogen ceased. The mixture was then fumed strongly for 1 minute, cooled, and the treatment was repeated. The solution was transferred to a 25-ml. volumetric flask and diluted to volume. Suitable aliquots were taken for the spectrographic determination of rhodium and for the spectrophotometric determination of iridium. The filtrate from the original separation was evaporated to fumes of sulfuric acid, cooled, and transferred to a 25-ml. volumetric flask; the proper amount of cobalt(I1) internal standard was added, and the mixture was diluted to volume. This solution was used for spectrographic analysis for rhodium and iridium content; aliquoting and diluting procedures were used as necessary. RESULTS AND DISCUSSIOY
Following the above procedure, separations of varying amounts and ratios of rhodium and iridium were performed in quadruplicate. Analyses for the gross amounts of the elements, rhodium in the precipitate and iridium in the filtrate from the separation, indicated that the separation was complete within the limits of the analysis errors of the analytical methods employed (see Table I); because this inherent error is about 2.8%, on this basis alone one can say only that the separations were a t least 97.2% complete. However, a more accurate evaluation is obtained from the analysis for the amount of rhodium unprecipitated-Le., in the filtrate with the iridium-and the amount of iridium coprecipitated with the rhodium. The result$ are illustrated in Table IV, in which each entry is the average of closely agreeing quadruplicate analyses.
Table IV.
Separation of R h o d i u m f r o m I r i d i u m dmount Found,
Amount Taken. l f g . Rh Ir 10.00 10.00 10.00 100.0 100.0 10.00 100.0
100.0
Rh in filtrate 0 02 0.03 0.51 0 36
Mg.
Ir In, preclpltate 0.02 0.30 0.04 0.30
Separation, % Rh Ir 99.8 99.7 99.5 99.6
99 8 99 7 99 6 99.7
The results indicate that, a t least within the tenfold differences in the amounts of rhodium and/or iridium taken, the absolute amount of iridium which coprecipitated with the rhodium was independent of the amount of the latter taken, but was dependent upon the amount of iridium in the solution; also, the absolute amount of rhodium lost in the filtrate was dependent upon the amount of rhodium taken. In terms of percentage separation, the values all ranging from 99.5 to 99.8% indicate
674
ANALYTICAL CHEMISTRY
the errors to be essentially proportional rather than additive. The data of Table IV show that there is some compensation of errors in the separation-Le., some rhodium is lost in the filtrate and some iridium is found in the precipitate. A second precipitation of the rhodium, to effect a more complete separation from iridium, appears to be unnecessary unless results of the highest accuracy are required. Spectrographic errors were evaluated from the reproducibility of the intensity ratios. For rhodium in the lowest concentration range, the average reproducibility corresponds to a relative error of about 6%. Errors in the spectrophotometric determination of iridium, evaluated from the standard curve as recommended by Ayres (@, correspond to a relative analysis error of about 5% for the smallest amounts of iridium determined. Much larger relative errors than these could be tolerated without affecting the final results. ACKNOWLEDGMENT
This investigation was supported jointly by The University of
Texas and the United States Atomic Energy Commission, under the terms of Contract No. AT (40-1)-1037. LITERATURE CITED
(1) Alstodt, B. S., and Benedetti-Pichler, A. A,, IXD.ENG.CHEM., ANAL.E D , 11, 294 (1939). (2) Awes. G. H.. ANAL.CAEM..21. 654 (19491. (3j Ayres] G. H.1 and Berg, E.'W.,'Ihid.; 24, 465 (1952). (4) Ibid., 25, 980 (1953). (5) Ayres, G.H., and Quick, Quentin, Ibid., 22, 1403 (1950). (6) Gilchrist, R.,and Wichers, E., J . Am. Chem. Soc., 57, 2565 (1935). (7) Harvey, C. E., "Spectrochemical Procedures," p. 72, Glendale, Calif., Applied Research Laboratories, 1950. (8) Miller, C. C., and Lowe, A. J., J . Chem. Soc., 1940, 1263. (9) Wada, I., and Nakaaono, T., Sei.Papers Inst. Phys. Chem. Research (Tokyo), 1, 139 (1925). RECEIVED for review September 19. 1953. rZccepted January 15, 1954. Condensed from a dissertation submitted by Charles M. Maddin to the faculty of the Graduate School of The University of Texas in partial fulfillment of the requirements for the degree of doctor of philosophy, May 1953.
Determination of Vinylidene Cyanide WILLARD P. TYLER, DONALD W. BEESING, and SEWARD 1. AVERILL The B. F. Goodrich Research Center, Brecksville, Ohio
Studies of the preparation and properties of vinylidene cyanide required a method for its determination and also some kinetic studies on the reaction of vinylidene cyanide with anthracene. A method of determination of vinylidene cyanide has been developed based on reaction with an excess of anthracene and determination of excess anthracene by differential colorimetry at 359 mp. The equilibrium constant of formation of a complex existing between anthracene and vinylidene cyanide in nonpolar solvents has been estimated for one set of conditions. Some kinetic data have been obtained for the anthracene-vinylidene cyanide reaction in benzene. The method is useful as an analytical and a kinetic study tool and should be applicable to the determination of other dienophiles.
T
H E preparation of vinylidene cyanide, 1,l-dicyanoethylene, as described by Ardis and colleagues (2-6) has created a need for a method of determining this substance both in crude reaction product and in purified samples. The extreme ease with which vinylidene cyanide is polymerized by water and many other substances indicates the need for careful handling and suggests a possible gravimetric method based on polymerization in water. Such a method will give approximate results on purified samples but is of little value on crude samples, particularly those containing solvents immiscible with water. Vinylidene cyanide functions as a dienophile in Diels-,llder reactions ( 4 , 6),for example, with butadiene, cyclopentadiene, and anthracene. None of the reactions tried yielded adducts sufficiently insoluble in the possible solvents to be useful in a gravimetric method. The reaction with anthracene is quantitative under proper conditions.
Vinylidene cyanide can be determined by a spectrophotometric measurement of the excess anthracene, since the adduct has no absorption in the region of the 360 mp absorption band of anthracene. The principle of this method can undoubtedly be a p plied to the determination of other dienophiles which react quantitatively with anthracene or with other dienes whose concentration can be readily measured. The spectrophotometric method used for determination of the unreacted anthracene is based on the differential or precision colorimetry method described by Bastian et al. ( 7 , 9) and Hiskey et al. (12, IS). APPAR4TUS AND RE4GENTS
The spectrophotometer used in developing this method was a Beckman Model DU instrument, using either a tungsten lamp or hydrogen discharge tube and I-cm. Corex or quartz cells. Although the scattered radiation filter is recommended for the wave length used, it was advantageous to omit it as a higher light intensity was thereby obtained. Spectrophotometers with a wider pas8 band than the Reckman have been used but with some loss in precision. Anthracene, Eastman Kodak S o . 480-X (blue-violet fluorescence) was used in developing the method and in routine testing. Less pure grades introduce errors. The reaction solvent was anhydrous toluene. A grade of toluene which does not discolor upon standing over phosphorus pentoxide is necessary. Removal of water by azeotropic distillation followed by storage over phosphorus pentoxide for 24 hours before use is advisable. The azeotropic distillation may be omitted for some better grades of toluene. Keep stored over phosphorus pentoxide. Do not use other drying agents. The diluting solvent was commercial or reagent grades of toluene or benzene. The solvent must have no absorption in the 360 m9 region. CALIBRATIOS
Prepare the reference standard by exact weighing of anthracene so as to give 45 mg. (within a few tenths of a milligram) per liter upon proper dilution Rith toluene or benzene. Prepare a series of known solutions containing increments above 45 mg. per liter in steps sufficient to get as accurate a curve as 1s required. The upper limit for routine control work is about 65 mg. per liter and for precise work about 55 mg. per liter. Measure the absorbance of each known solution against the 45-mg.-per-liter reference standard. Plot the calibration curve. The wave length used should be that of maximum absorption in
