titative. However, mechanical losses invariably lead to a small reduction in the chemical yield, with the final yield dependent on the number of manipulations taking place. This is especially true with a radioisotope having a short half-life, when rapidity of handling is important. For these analyses, the chemical yield for the complete analysis was determined to be 95 f 3 % for the metals (using the centrifuge technique and one wash) and 91 += 4 % for the biological materials (using the centrifuge technique, two washes, and stripping the uranium from the organic phase with 14M HF). The use of separatory funnels for the extractions increased the yields by several per cent, indicating lower mechanical losses using this procedure. However, the rapid phase separation engendered by centrifugation was felt to be more important than a slightly higher chemical yield. RESULTS AND DISCUSSION
The results of a number of analyses using this procedure are found in Table I. For several of the samples and standards, decay curves were obtained which substantiated radiochemical purity. The gamma-ray peaks were integrated, background was subtracted, and corrections were applied for flux normalization, yield, and decay. The accuracy of the procedure can be seen by the good agreement with other analytical techniques as shown in Table I. Analysis of zirconium metal, SRM No. 1210, produced good agreement with the certified uranium concentration of 1.8 ppm. In the case of SRM 1571, Orchard Leaves, the agreement among the three analytical techniques is good, and the uranium value has recently been certified.
The copper(1) oxide-single crystal sample listed in Table I was made by NBS research scientists by oxidizing copper metal strips at high temperature and oxygen pressure. An attempt was made to introduce uranium as a dopant into this cuprous oxide at the 1 % level. Resistivity measurements showed no significant change upon addition of the uranium. Therefore, these analyses were made in an effort to verify the presence or absence of the uranium dopant. The results indicated the presence of small amounts of uranium much less than 1 %, distributed inhomogeneously. In addition, the analysis for uranium in housekeeping quality aluminum foil was attempted, primarily to determine the applicability of the separation technique. The result obtained was 0.49 ppm uranium, which agrees closely with the values obtained on similar quality aluminum foil by other scientists (11). Very good decontamination was obtained from both the aluminum and the relatively large manganese concentration in the foil using the HDEHP extraction. In conclusion, the procedure reported here is applicable to the determination of trace levels of uranium in a wide variety of sample matrices from convenient, strong acid solutions. RECEIVED for review October 29, 1971. Accepted March 14, 1972. In order to specify procedures adequately, it has been necessary to identify commercial materials and equipment in this report. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best available for the purpose.
Spectrophotometric Determination of Sulfite with Mercuric Thiocyanate and Ferric Ion Willie L. Hinze, James Elliott, and Ray E. Humphrey' Department of Chemistry, Sam Houston State University, Huntsville, Texas 77340 SOLUBL~, SLIGHTLY DISSOCIATED mercuric thiocyanate reacts with certain anions which form even less dissociated or insoluble mercuric compounds to release thiocyanate ion. A solution of ferric ion is then added and the FeSCN2+ complex formed which absorbs in the visible region. These reactions have been made the basis of a spectrophotometric procedure for chloride which seems to be one of the most common methods for that anion (1-4). This method has also been developed into an automated procedure for chloride (5). The method is not selective for chloride as many other anions, including iodide and cyanide (6),bromide (7),
Author to whom correspondence should be addressed. (1) I. Awasaki, S. Utsumi, and T. Ozawa, Bull. Cl7em. SOC.Jup., 25, 226 (1952). (2) R. P. Marquardt, ANAL.CHEM., 43,277 (1971). (3) D. M . Zall, D. Fisher, and M. 0. Garner, ibid.,28, 1665 (1956). (4) T. M . Florence and Y . J. Farrar, Anal. Chim. Acfa, 54, 373 (1 97 1). (5) R . D. Britt, ANAL. CHEM., 34,1728 (1962). (6) S. Utsumi, J. Cl7em. Soc. Jup., Pure Chem. Sect., 74, 32 (1953). (7) S. Utsumi, ibid.,73, 889 (1952).
and sulfide, thiosulfate, nitrite, iodate, and bromate (8) have also been determined spectrophotometrically using these reactions. Similar methods have been reported involving the reaction of anions with insoluble metal thiocyanates to displace the thiocyanate ion which is then complexed with ferric ion. Cuprous thiocyanate has been used for the determination of cyanide (9) and thiosulfate ( I O ) while silver thiocyanate has been used for the measurement of sulfide (12) and bromide or iodide in the presence of chloride (12). We have investigated the reaction of sulfite ion with mercuric thiocyanate as a possible method for the determination of that anion. N o reaction occurs in water alone, but thiocyanate ion is released in ethanol and methanol and in aqueous ethanol. Sulfite can be measured at rather low levels in these solvents by reaction with mercuric thiocyanate and formation of the ferric thiocyanate complex. This method may have some advantages in some instances for the spectrophotometric determination of sulfite ion or sulfur dioxide. (8) Zbid.,p 838. (9) Zbid.,74, 479 (1953). (10) Ibid.,p 526. (11) Ibid.,p 358. (12) Zbid.,p 35. ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
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RESULTS AND DISCUSSION
are presented in Table I. As is evident, the highest values are obtained in the ethanol-water mixture which was 75 volume per cent ethanol and which contained perchloric acid. The values obtained in 95% ethanol and in 1: 1 ethanol-water did not differ appreciably with nitric acid present. The sensitivity for sulfite in 3 : 1 ethanol-water, E = 7700, is considerably higher than that for chloride ion for which an effective molar absorptivity in aqueous solution of 2600 was reported (4). The molar absorptivity for chloride ion in methanol with nitric acid present is approximately 3600, estimated from literature data (2). No reaction occurred between mercuric thiocyanate and sulfite ion when water was the only solvent present. Beer's law was obeyed in all of the solvent systems studied in the approximate range of 1-20 ppm SO2. Plots of absorbance us. concentration showed some negative deviation at higher concentrations. Representative data for the 3 : 1 ethanol-water solvent are shown in Table 11. Absorbance values for the blank varied somewhat in the different solvents. These values are shown in Table I. Reproducibility was reasonably good, as shown by recovery data in Table I1 for the 3 : 1 ethanol-water solvent. Also, better sensitivity was obtained using ferric nitrate in perchloric acid solution than when ferric solutions in nitric acid were employed. Perchloric acid was also superior in the chloride analysis (4) from the standpoint of having a lower blank, although the sensitivity was about the same as with nitric acid. The molar absorptivity value based on sulfite in 1 :1 ethanol-water is about 4200 using the ferric sulfate in nitric acid and close to 6000 in the solutions with ferric nitrate and perchloric acid. The reaction between mercuric thiocyanate and sulfite ion is somewhat slow, about 15 minutes being required to obtain the maximum absorbance reading in 3 : 1 ethanol-water. Faster reaction was found in ethanol and methanol, approximately 5-10 minutes being required while about 30 minutes were needed in 1 :1 ethanol-water. Absorbance readings tended to decrease slowly with time so that it is best to take readings within a few minutes after adding the ferric solution. Fading seemed to be less in 1 : 1 ethanol-water although the sensitivity was not so good in that solvent, We found no solution to this difficulty. It has been reported that fading is not a particular problem in the chloride determination (4,13). If there is a small amount of sulfite remaining due to incomplete reaction, it is possible that some of the FeSCN2+could be lost because of reduction by sulfite. Some data indicated a loss in absorbance of the FeSCN2+species after 30 minutes when a known amount of sulfite was added to the ferric thiocyanate complex. It would seem that the excess ferric ion, rather than the FeSCN2+ species, would react with the sulfite. This was not studied in detail. The extent of reaction between mercuric thiocyanate and sulfite ion in the various solvents could be estimated by comparing the molar absorptivity values for the FeSCNz+species in the various solvents with the values calculated for the sulfite ion. Values for FeSCN2+ were obtained by using known amounts of thiocyanate ion and excess ferric ion (Table I). If it is assumed that one sulfite ion would release two thiocyanate ions, the extent of reaction would be approximately 50% in ethanol and methanol and close to 90% in the aqueods ethanol solvents. If the assumption were made that one sdfite ion released one thiocyanate ion, then reaction would be
Molar absorptivity values for sulfite ion, based on the sulfite concentration and the absorbance of the FeSCN2+ species,
(13) H. N. Elsheimer and R. L. Kocher, ANAL.CHEM., 38, 145 (1966).
Table I. Molar Absorptivities €7
Solvent
Acid
CH3OH CzHsOH CzHsOH-HzO(3 :1) CzHsOH-HzO(1: 1) CzHeOH-H*O(l:l)
HNOa HCIO, HCIO, HNO,
"03
A , Blank
0.2-0.3 0.2-0.3 0.6-0.7 0.4-0.5 0.3-0.4
€9
S 0 3 - 2 a FeSCN2+ 37005 39005 4700" 4700' 7700' 4300' 6Wd 3400d 42Wd 38Wd
Useful concentrationrange 1--20 ppm SO2. 485 nm. e 470 nm. 465 nm. ~~
Table 11. Beer's Law and Recovery Data for SOza Beer's law data Recovery data Son,PPm Ab Presentc Found Error, 3.0 3.1 6.0 5.6 12 11 15 15 a Solvent 3 :1 ethanol-water, HClOa present. * 465 nm. Values are corrected for the blank. SOz,ppm. 1.1 3.0 6.0 13
0.12 0.37 0.66 1 42
+1.6 -6.7 -8.3 0.0
EXPERIMENTAL
Apparatus. Absorption measurements were made with Beckman Spectrophotometers including the Model DB-G, the DK-2A, and the ACTA 111. Reagents. Mercuric thiocyanate was obtained from Sigma Chemical Co. Ferric ammonium sulfate was a Merck and Co. product. Ferric nitrate was a Baker Analyzed reagent chemical. Sodium sulfite was an anhydrous Baker and Adamson reagent chemical. Anhydrous methanol and 95 per cent ethanol were high quality solvents and were used as received. All other chemicals used were the best available reagent grade materials. A saturated solution of mercuric thiocyanate in ethanol or methanol was used for the work in the specific solvent employed. The sodium sulfite solutions were about 0.001M and were made up in either ethanol, methanol, or 1 : 1 ethanol-water which contained 5 glycerol to retard oxidation. These solutions were used soon after preparation. The ferric ammonium sulfate solutions which ranged from 0.15 to 0.20M were prepared in 6 M nitric acid. The ferric nitrate solution was made by dissolving 15 grams of Fe(NOJ3. 9H20 in 45 ml of 72% perchloric acid and diluting to 100 ml with water. Procedure. In the work using methanol as solvent, a measured volume of sulfite solution was put in a flask containing a small amount of methanol and 15 ml of the mercuric thiocyanate solution were added. After allowing 2-3 minutes for reaction to occur, 3 ml of the ferric solution were added and the solution was made up to 25 ml with methanol. The procedure was similar in the study involving 95 % ethanol except that only 2 ml of the mercuric thiocyanate solution in ethanol were used and the final volume was 10 ml. In the work employing 3: 1 ethanol-water and 1 : 1 ethanol-water, 2 ml of the mercuric thiocyanate solution were placed in a flask, measured volumes of ethanol and water added, followed by the volume of sulfite solution for a final volume of 10 ml. Finally, 2 ml of the ferric nitrate solution were added and the absorbance was measured within a few minutes. Absorbance is measured at the following wavelengths for the different solvents: CH30H, 485 nm; C2H50H,470 nm; CzH50H-H20(3:l), 470 nm, C2HjOH-Hz0(1: I), 465 nm.
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
essentially complete in ethanol and methanol but the results in aqueous ethanol would not be consistent. There would appear to be two possible reactions which could occur between sulfite ion and mercuric thiocyanate, although none seems to have been reported in the literature. Sulfite does form very stable complexes with mercury(I1) species, one of which is used to stabilize the ion for colorimetric estimation using the West-Gaeke procedure (14). Sulfite is also known to reduce mercury(I1) to mercury(1) under some conditions (15). It was not possible to determine, on the basis of this work, the nature of the reaction occurring. The investigation did show that no reaction took place in water alone, ethanol or methanol being required. The reaction required 15-30 minutes for completion in aqueous ethanol, and the extent of reaction was possibly in the range of 5@-90% depending on the stoichiometry assumed. This procedure is relatively simple, requires only a short amount of time and a minimum amount of manipulation. No separation step, filtration, or centrifugation, is needed as is the case with a similar reaction employing mercuric chlor(14) P. W. West and G. C. Gaeke, ANAL.CHEM., 28,1816 (1956). (15) L. M. Stewart and W. Wardlaw, J. Chem. SOC.,121, 1481 (1922).
anilate (16). The disadvantages would seem to be the somewhat low molar absorptivity of the ferric thiocyanate complex and the tendency for the color to fade. The visible absorption of the FeSCN2+ species is considerably higher than that of the chloranilate ion. The precision in both the chloranilate and thiocyanate methods is possibly somewhat lower than with other colorimetric procedures. Serious interferences would be expected if bromide, bromate, chloride, cyanide, iodate, iodide, nitrite, sulfide, or thiosulfate ions were present since each of these anions has been determined by this procedure. Since chloride is routinely measured by this method, these possible interferences apparently are not serious in that analysis. Our results indicate that sulfite does not react with mercuric thiocyanate in water only but requires the presence of ethanol or methanol. It might be possible to detect interferences by utilizing this difference, assuming that all of the other anions do react in water alone. RECEIVED for review January 27, 1972. Accepted March 17, 1972. The authors express their appreciation to the Robert A. Welch Foundation of Houston, Texas, for support of this research. ~
(16) R. E. Humphrey and W. Hinze, ANAL.CHEM.,43, 1100 (1971).
Trimethylphenylammonium Bromide as a Selective Quantitative Precipitant for Gold or Thallium W. W. White Industrial Laboratory, Kodak Park Diuision, Eastman Kodak Company, Rochester, N . Y . 14650
A RAPID SEMIMICRO METHOD is described for the gravimetric determination of gold or thallium in quantities of 10 to 60 mg by use of trimethylphenylammonium bromide (TMPB). The method represents a unique use of the reagent. The scarcity of information concerning the use of quaternary organic ammonium compounds that precipitate and provide direct weighing forms for anionic metal complexes suggests a fertile and useful area of analytical investigation. In this paper, TMPB was studied and found to be a selective quantitative precipitant for gold or thallium. The stable tetrabromo complexes of gold(II1) and thallium(II1) are precipitated with TMPB to form A B X l structures, where A is the quaternary cation [(CH3)3(C6HJNJ, B is the metal, and X is the bromide. In previous articles, White and Zuber have described gravimetric methods using trimethylphenylammonium iodide (TMPI) to determine mercury, lead, platinum ( I ) , and gold (2). Cadmium (3) and bismuth ( 4 ) have also been determined using TMPI. The speed of analysis, selectivity, flocculent nature of the precipitates, and favorable gravimetric conversion factors are the attractive features of these two reagents. (1) W. W. Whiteand J. R. Zuber, ANAL.CHEM.,39,258 (1967). (2) Zbid.,36,2363 (1964). (3) A. Pass and A. M . Ward, Aiialyst (Londo/r),58,667 (1933). (4) T. S. Burkhalter and J. F. Solarek, ANAL.CHEM.,25,1125 ( 195 3).
Beamish, in his review of gravimetric methods for the determination of noble metals, states that relatively few organic precipitants give direct weighing forms for gold ( 5 ) . The classical reduction methods (6) are time-consuming and in most cases require the removal of nitrate prior to the reduction to the metal. Most of the chemical methods for the determination of thallium require its reduction to the monovalent state. The precipitation of thallium(1) with potassium chromate has been the most frequently used gravimetric method. Many elements interfere unless steps are taken to mask or separate them from thallium. Kodama (7) gives a summary of the important chemical methods used to determine the element including the use of colorimetric methods. The proposed method for gold or thallium is simple, accurate, and requires no reduction of these elements to the monovalent or elemental state after the dissolution of the sample. Thirty-one elements were shown not to interfere with the gold or thallium determinations.
(5) F. E. Beamish, Tulai~ta,13,773 (1966). (6) W. F. Hillebrand, G. E. F. Lundell, H . A. Bright, and J. I. Hoffman, “Applied Inorganic Analysis,” 2nd ed., Wiley, New York, N.Y., 1953, pp 366-7. (7) K. Kodama, “Methods of Quantitative Inorganic Analysis,” Interscience, New York, N.Y., 1963. ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
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