What Is the Weighing Form in Gravimetric Determination of Antimony(III) with Oxine ?

ANALYTICAL SCIENCES MARCH 2004, VOL. 20 543 2004 © The Japan Society for Analytical Chemistry

What is the Weighing Form in Gravimetric Determination of Antimony(III) with Oxine ?

Akiharu HIOKI

National Metrology Institute of Japan (NMIJ), National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 3-10, 1-1-1, Umezono, Tsukuba-shi, Ibaraki 305Ð8563, Japan

The gravimetric analysis of antimony(III) with oxine (8-quinolinol, Hox) was studied. The amount of antimony left in filtrate and washing solutions was corrected with the results of atomic absorption spectrometry. The weighing form,

which had not been conclusive before the present study, was determined to be SbO(ox)(Hox)2. The result (purity of antimony(III) oxide: 99.84 ± 0.05% (m/m)) of the gravimetric analysis was in good agreement with that of coulometric titration with electrogenerated iodine.

(Received September 19, 2003; Accepted January 22, 2004)

reagent grade, and analytical-reagent grade oxine were used, Introduction from Wako Pure Chemical Industries, Tokyo, Japan. The reagent 8-acetoxyquinoline was from Dojindo Laboratories, Oxine (8-quinolinol, Hox) has been extensively utilized for Kumamoto, Japan. Analytical-reagent grade chemicals were gravimetric analysis of metal ions.1Ð6 Though there have been used unless otherwise stated. Water was further purified by several reports on the gravimetry of antimony(III) with oxine sub-boiling distillation of demineralized water. since the pioneering work of Pirtea in 1939,7Ð13 the weighing Antimony(III) oxide, from Sumitomo Metal Mining Co., form has not been conclusive. In fact Pirtea himself corrected Tokyo, Japan, was the same material as used in the previous 14 the weighing form from SbO(ox)(Hox)2 to Sb(ox)3 in 1965 in study on coulometric titration of antimony(III). Antimony(III) reply to the critical description by Duval;10,12 the correction was, oxide was dried at 110ûC for 3 h before use. Approximately however, wrong. This uncertainty seems to result from lack of 2.40 g of antimony(III) oxide were dissolved in 60 cm3 of 5.9 consideration of the not-so-small solubility of antimony(III) mol dmÐ3 hydrochloric acid. The solution was slightly warmed oxinate, and/or from unclear purities of raw materials of at the last stage of the dissolution, and then diluted to 200 g with antimony(III). In the present study, I used antimony(III) oxide both water and 20 cm3 of 11.7 mol dmÐ3 hydrochloric acid by whose purity has been determined by a coulometric titration,14 using a Shimadzu balance of 0.01 g sensitivity. The solution and antimony left in both the filtrate and the washing solution was kept in a Teflon-PFA bottle and stored in a refrigerator at was determined by tungsten-furnace atomic absorption approximately 4ûC until use. spectrometry. Consequently, we are definitely able to conclude Eight grams of oxine were dissolved into glacial acetic acid of that the weighing form of the precipitate of antimony(III) as small volume as possible (19.1 cm3) and 130 g of water were 3 Ð3 oxinate is SbO(ox)(Hox)2; the present study puts an end to the added; then 5 cm of 5.6 mol dm ammonia solution were controversy. dropwise added while the solution was stirred. The forming Recently, instrumental methods of chemical analysis have white precipitate was dissolved with ca. 0.6 cm3 of glacial acetic been developed widely. Since these methods are without acid, and then the solution was diluted to 200 cm3 with water. exception comparison methods, standardization is The resulting solution (0.28 mol dmÐ3 or 4% (m/m)) was indispensable. The methods of gravimetric analysis and protected from light. coulometric titration both essentially require no reference A 0.11 cm3 portion of 1 mol dmÐ3 acetic acid and 40 Ð 60 cm3 materials except for physical standards on mass and/or of water were added to 0.4 g of oxine. After the oxine electricity, though some results should be suitably corrected by dissolved, the solution was diluted to 1000 cm3. The solution considering some minor factors. In the present study, it is also (0.0028 mol dmÐ3 or 0.04% (m/m)) was stored in a refrigerator an aim to examine whether or not the oxinate-method and was used as a washing solution while cooled in ice water. gravimetry can be a definitive method15,16 or a primary method The relative atomic mass used for Sb is 121.760(1), and the 17,18 of measurement for determination of antimony(III). chemical formula masses used for Sb2O3 and SbO(ox)(Hox)2 are 291.518(2) and 572.226(22), respectively;19 the figures in parentheses are the uncertainties following the last significant Experimental figure to which they are attributed.

Reagents Apparatus Hydrochloric acid and ammonia solution of special analytical- All weighings, unless specified otherwise, were carried out on a Mettler balance of 0.01 mg sensitivity. E-mail: [email protected] The densities of antimony(III) stock solutions were measured 544 ANALYTICAL SCIENCES MARCH 2004, VOL. 20

Fig. 1 Dependence of antimony concentration in filtrate on amount of oxine. pH 8.0 Ð 8.3; , 32ûC; , 31ûC; , 70ûC. The arrow Fig. 2 Dependence of antimony concentration in filtrate on indicates the amount of oxine equivalent to the amount of antimony. temperature of filtration. Volume of 4% (m/m) oxine, 11 cm3; pH, 8.2 Ð 8.5. using a DA-101B automatic densimeter (Kyoto Electronics, silver nitrate. The filtrate (typically ca. 42 cm3) and the Kyoto, Japan). The density of antimony(III) oxinate crystal was washing solutions (25 cm3 each of the first half and the second measured at 24.9ûC with a Hubbard type pycnometer; half) were entirely collected and stored until atomic absorption measurement and calculation were made in the same way as spectrometric determination of antimony. The crystal in the reported in the previous paper.20 Then the saturated solution of crucible was dried at 90ûC for 0.5 h or 1 h; then it was cooled in the crystal was used as a measurement medium instead of water, a silica-gel desiccator and was weighed. This drying and since the solubility of the crystal was not negligible. The weighing procedure was repeated in succession. Blank tests density of the crystal was obtained as 1.59 g cmÐ3. Buoyancy were also carried out by using 3.0 mol dmÐ3 hydrochloric acid corrections were always applied. instead of the antimony(III) solution. An SAS-725S atomic absorption spectrometer (Seiko The filtrate and the washing solutions were typically 100 Instrument & Electronics, Tokyo, Japan) equipped with a times diluted with 3.4 × 10Ð4 mol dmÐ3 tartaric acid right after tungsten-strip furnace,21Ð23 an SAS-704 power-supply unit for the filtration procedure. The amounts of antimony in those the furnace, and a simultaneous deuterium background corrector solutions were determined by the “in-boat” standard addition was used. The tungsten-strip heater was 7 mm in width, 55 mm method,24 where a tungsten-furnace atomic absorption in length, and 0.2 mm in thickness, and had a boat-shaped spectrometer was used under the following conditions: drying at indentation which could hold ca. 0.025 cm3 of solution. The ca. 75ûC for 72 s; ashing at ca. 215ûC for 12 s; atomization at temperatures of the heater at the drying stage and at the ca. 2400ûC (maximum temperature) for 2 s; wavelength, 217.58 atomizing stage were measured with a TM-1 spot thermometer nm; sheath gas, 1.0 L minÐ1 hydrogen with 5.0 L minÐ1 argon. (ITM, Tokyo, Japan) equipped with an MP type sensor and with The standard solutions for the standard addition method were an IR-P pyrometer (Chino Works, Tokyo, Japan), respectively. prepared from the above-mentioned antimony(III) stock For each pH determination, a COM-10 pH-meter (Denki solution. Kagaku Keiki Co., Tokyo, Japan) was used with a pH The procedure of the precipitation from homogeneous combination electrode (Type 6155, Denki Kagaku Keiki). solution was as follows. A 5-cm3 portion of approximately 10 g kgÐ1 antimony(III) solution was weighed into a 200-cm3 Procedure beaker. After 10 cm3 of 0.67 mol dmÐ3 tartaric acid solution and A 10-cm3 portion of approximately 10 g kgÐ1 antimony(III) 35 cm3 of water were added, 14.0 mol dmÐ3 ammonia solution solution was weighed into a 200-cm3 beaker on a Mettler was added to adjust the pH of the solution to ca. 9.5. After the balance of 0.1 mg sensitivity. After 5 cm3 of 0.67 mol dmÐ3 solution was cooled in ice water for 15 min, 3.25 cm3 of acetone tartaric acid solution were added, 4% (m/m) oxine solution containing 0.650 g of 8-acetoxyquinoline were dropwise added, (typically 11 cm3) and water (typically 9 cm3) were added to and then 40 cm3 of water were added. After being kept at ca. give a total volume of 20 cm3. The solution was heated at ca. 25ûC for ca. 150 min, the solution was heated at ca. 70ûC for ca. 65ûC unless otherwise specified and then 5.6 mol dmÐ3 ammonia 30 min. Immediately, the precipitate was filtered by means of solution (typically ca. 15 cm3) was dropwise added to adjust the suction with a 1G4-grade glass filter crucible. Because our pH of the solution on the basis of preliminary tests. After being primary purpose was to determine antimony in the filtrate, the kept at the temperature for a few minutes, the solution was beaker was not rinsed with the filtrate; otherwise, a large stood at ca. 25ûC for ca. 2 h. In the cases of filtration quantity of oxine was precipitated. The crucible was washed temperatures of 4ûC, 51ûC, and 70ûC, the pH-adjusted solutions with 10 portions of 5 cm3 of water kept at ca. 0ûC. were stood at the room temperature for 1 h followed by ca. 4ûC for 20 h, at 49 to 53ûC for 1 h, and at 68 to 72ûC for 1 h, respectively. The pale yellow crystal was filtered by means of Results and Discussion suction with a glass-filter crucible (1G4-grade). The beaker was rinsed with the filtrate, which was added into the crucible; this Dependence of Sb concentration in filtrate on amount of oxine procedure was repeated twice to complete the transfer of the The concentration of antimony left in the filtrate at room precipitate to the crucible. The crystal in the crucible was temperature did not depend on the amount of oxine beyond 11 typically washed with 10 portions of 5 cm3 of the 0.04% (m/m) cm3 of 4% (m/m) oxine added to the antimony(III) solution oxine solution kept at ca. 0ûC. This washing procedure was (Fig. 1), because the solubility of oxine is not so large.25 enough to wash out chloride ions, judging from a test using Though the solubility of oxine increased when the temperature ANALYTICAL SCIENCES MARCH 2004, VOL. 20 545

Table 1 Weights of precipitate and quantities of antimony in filtrate and washing solutionsa Weight of Sb in washing precipitate, soln., % (m/m)d % (m/m) Sb in Purity of Anal. c e b filtrate, Sb2O3, No. 3 3 SbO(ox)- % (m/m) 25 cm 25 cm % (m/m) Sb(ox)3 of of (Hox)2 1st half 2nd half 8A 101.00 97.82 1.89(31ûC) 0.06 0.00 99.77 ± 0.17 8B 102.14 98.93 0.68(31ûC) 0.03 0.00 99.64 ± 0.08 8C 102.00 98.79 0.73(31ûC) 0.04 0.00 99.56 ± 0.07 8D 102.20 98.98 1.03(31ûC) 0.05 0.00 100.06 ± 0.05 Fig. 3 Dependence of antimony concentration in filtrate on pH. 9A 101.92 98.71 0.83(24ûC) 0.04 0.00 99.58 ± 0.04 Temperature of filtration, 24 Ð 25ûC. Volume of 4% (m/m) oxine: , 9B 102.18 98.96 0.91(24ûC) 0.05 0.00 99.92 ± 0.05 3 3 11 cm ; , 10 cm . Mean 99.76 ± 0.08 a. pH, 8.1 Ð 8.3; 4% (m/m) oxine, 10 Ð 12 cm3. b. Each common first figure in the column of Anal. No. indicates a rose to 70ûC, the filtration at the temperature was not series of measurements made in parallel. advantageous, judging from Fig. 1. c. Each temperature in parentheses is a filtration temperature. d. Precipitate was washed with a cold solution of 0.04% (m/m) Dependence of Sb concentration in filtrate on temperature at oxine. filtration e. Calculated on the basis of the composition of SbO(ox)(Hox)2. Figure 2 shows the dependence of antimony concentration in Each value following ± is the combined standard uncertainty. filtrate on the temperature at filtration, when 11 cm3 of 4% (m/m) oxine were used. Under the condition, oxine itself precipitated somewhat at 25ûC. It is again ascertained that there was no advantage of raising the temperature at filtration in order Drying of precipitate to increase the solubility of oxine. On the other hand, there was The precipitate in the crucible was dried at 90ûC for 0.5 h or 1 also no advantage of lowering the temperature at filtration. The h; drying and weighing procedure was repeated in succession. fact that the solubility of antimony(III) oxinate is nearly The amount of the adsorption moisture on the surface of a glass- constant at temperatures less than 50ûC is similar to the filter crucible slightly changes according to the surrounding constancy of the solubility reported for aluminium(III) atmosphere. The change of the amount of the moisture can be, oxinate.26 however, compensated by correcting the change of the weight of a dummy crucible which includes no precipitate. In pH dependence of Sb concentration in filtrate weighing the precipitate, buoyancy correction was taken into Figure 3 shows the dependence of antimony concentration in account; the effect corresponded to 0.06%. Since the graph of filtrate on pH. Almost all experiments in the present study were weight vs. total drying time indicated a linear relationship, the carried out at pH 8.0 to pH 8.5. Therefore, in our pH range, pH weight of the crystal was estimated by extrapolating these data differences had little effect on the antimony concentration in to zero time. The weight decreased with an increase in drying filtrate. time (ca. 0.075% (m/m) per hour). Judging from a white substance which formed after long drying time, it seems that the Filtrate and washing solution decrease of the weight resulted from a decomposition of Each result in Tables 1, 2, and 3 is shown as a percentage antimony(III) oxinate, as in Pirtea’s indication.7 relaltive to equivalence of the sampling amount of antimony(III) Owing to the easiness for vaporization of oxine itself (melting oxide, when the purity of antimony(III) oxide is assumed as point: 75.5ûC; boiling point: 267ûC),27 the amount of the oxine 100% (m/m). According to the dependencies of antimony introduced from the washing solution was negligible after concentration in filtrate on quantity of oxine (Fig. 1), drying was carried out to some extent. For the same reason, the temperature at filtration (Fig. 2), and pH (Fig. 3), it is impossible free oxine coexisting in the precipitate did not cause any to find a condition under which antimony concentration in problem. filtrate is negligible. With 50 cm3 of 0.04% (m/m) oxine solution as a washing Precipitation from homogeneous solution solution (Table 1), antimony in 25 cm3 of the first half could be Since the formation of antimony(III) oxinate by precipitation suppressed within 0.1% (m/m) and antimony in 25 cm3 of the from homogeneous solution using hydrolysis of 8- second half was substantially negligible. When ca. 10Ð4 acetoxyquinoline has been reported,13 reexamination was tried. mol dmÐ3 acetic acid was used as a washing solution (Table 2), The hydrolysis of 8-acetoxyquinoline was very fast28 and the antimony(III) oxinate dissolved into the washing solution to reexamination of the literature13 resulted in the formation of a some extent, even if the washing solution was at a low black substance by over-heating; therefore, it was impossible to temperature (Nos. 4A, 4B, and 4C), except when a large follow the literature.13 Though it was tried to hydrolyze 8- quantity of oxine (No. 1A) was used in making precipitate. In acetoxyquinoline at a low temperature as done in the case of the case when a large quantity of oxine was added to magnesium,29 the antimony concentration left in filtrate and antimony(III) solution, oxine crystal itself coexisted in washing solutions could not be neglected, as indicated in Table antimony(III) oxinate. Conclusively, the existence of excess 3. This is reasonable, judging from the results of the proposed oxine in a washing solution or in precipitate suppressed the oxinate method on the antimony concentration in filtrate and dissolution of antimony(III) oxinate into a washing solution. washing solutions (vide supra). Though it has been reported in 546 ANALYTICAL SCIENCES MARCH 2004, VOL. 20

a a Table 2 Purity of Sb2O3 by oxinate method Table 3 Precipitation from homogeneous solution

Sb in washing Sb in washing d soln., % (m/m)d Precipitate as Sb in soln., % (m/m) Purity of Sb in Anal. c e Anal. Temp./ c b SbO(ox)(Hox)2, filtrate, Sb2O3, b pH filtrate, No. 3 3 No. ûC 3 3 % (m/m) % (m/m) 25 cm 25 cm % (m/m) % (m/m) 25 cm 25 cm of of of of 1st half 2nd half 1st half 2nd half 1Af — 1.40(26ûC) 0.08 0.00 — 10A 25 9.05 10.22(2.12) 0.17 0.02 10B 25 8.82 7.19(2.79) 0.34 0.05 2A 96.60 1.89(25ûC) 0.66 0.76 99.91 ± 0.25 2B 96.88 1.42(25ûC) 0.65 0.96 99.91 ± 0.18 11A 70 9.30 1.40(0.28) 0.79 0.88 11B 70 9.22 1.51(0.30) 1.04 0.69 3A 96.47 1.93(27ûC) 0.72 0.71 99.83 ± 0.24 3B 96.30 2.07(26ûC) 0.75 0.75 99.87 ± 0.19 a. The amount of 8-acetoxyquinoline used corresponded to 12.6 3C 96.13 2.57(26ûC) 0.77 0.67 100.14 ± 0.37 cm3 of 4% (m/m) oxine. Five cubicmeters of 10 g kgÐ1 Sb(III) 4A 96.61 2.37(4ûC) 0.62g 0.42g 100.02 ± 0.22 solution was used. 4B 96.58 2.15(4ûC) 0.59g 0.37g 99.69 ± 0.28 b. Each common number of two figures in the column indicates a 4C 96.54 2.35(4ûC) 0.62g 0.39g 99.90 ± 0.30 series of measurements made in parallel. ± c. Volume of filtrate was ca. 100 cm3, except for ca. 50 cm3 in No. Mean 99.84 0.05 3 Ð1 (n = 14)h 10B. If 10 cm of 10 g kg Sb(III) solution were used and volume of filtrate were 40 cm3 as the proposed oxinate method, each value a. pH, 7.2 Ð 8.7; 4% (m/m) oxine, 10 cm3. in parentheses would be expected as the amount of antimony in the b. Each common first figure in the column indicates a series of filtrate. measurements made in parallel. d. Washing solution used was water kept at ca. 0ûC. c. Each temperature in parentheses is a filtration temperature. d. Each washing solution was ca. 10Ð4 mol dmÐ3 acetic acid at the room temperature. e. Calculated on the basis of the composition of SbO(ox)(Hox)2. results of purity (Table 1) could not be explained by the Each value following ± is the combined standard uncertainty. composition of Sb(ox)3. The purity calculated as the 3 f. 4% (m/m) oxine, 15 cm . composition of SbO(ox)(Hox) was in good agreement with the g. Each washing solution was ca. 10Ð4 mol dmÐ3 acetic acid kept at 2 purity obtained by coulometric titration with electrogenerated 0ûC. 14,33 h. The mean value of 14 measurements in Tables 1 and 2. iodine (99.79 ± 0.01% (m/m)). The other results of purity calculated as the composition of SbO(ox)(Hox)2 are summarized in Table 2. The uncertainties for weighing itself and for the chemical formula masses were so small that those the literature that antimony was not detected in the filtrate,13 the uncertainties were not included in any estimation of the detection limit for antimony has not been described in the combined standard uncertainties. The mean value of 14 report. measurements (99.84 ± 0.05% (m/m)) agreed well with 99.79 ± 0.01% (m/m) by coulometric titration in which the reduction Weighing form property of antimony(III) was utilized. Though the purity value To avoid being complicated, uncertainties in Tables 1 and 2 obtained by the gravimetric analysis includes a small are shown only for the final results of purity. Each uncertainty contribution from antimony(V), that does not influence this for determinations in Tables 1 and 2 was substantially estimated conclusion. Though the determination of antimony(III) by the in consideration of only the errors for antimony determination oxinate method is not so excellent from the viewpoint of a of the filtrate and the washing solutions, because the error for gravimetric analysis itself, it is concluded that the method could the extrapolation of weighing values of antimony(III) oxinate be a definitive or primary method, provided that antimony in the was relatively small. On the other hand, each uncertainty for filtrate and the washing solutions is taken into consideration. mean values in Tables 1 and 2 is a standard deviation for a Though the weighing form was clarified and at least, an oxo mean on the basis of the variation of determinations. group in antimony(III) oxinate has been suggested from IR Both the weight of the precipitate as the composition of results,13 much interest in the structural chemistry of

Sb(ox)3 and that as the composition of SbO(ox)(Hox)2 are antimony(III) oxinate still remains. shown in Table 1. The composition of the latter is that of the former plus a water molecule. The blank value was substantially zero, though it took a few hours to vaporize oxine References in the blank which had formed in quantity. A small amount (0.04% of total antimony) of antimony(V) 1. F. Hahn, Z. Angew. Chem., 1926, 39, 1198. coexisted in the antimony(III) solution prepared from the 2. F. J. Welcher, “Organic Analytical Reagents”, 1947, Vol. antimony(III) oxide.30 It is, however, known that antimony(V) 1, D. Van Nostrand, New York, 264. forms no precipitate of oxinate under the present experimental 3. R. G. W. Hollingshead, “Oxine and its Derivatives”, 1954, conditions.8,31,32 It seems that the antimony(V) left in the filtrate Vols. I and II, 1956, Vols. III and IV, Butterworths was determined together with antimony(III) by atomic Scientific Publication, London. absorption spectrometry; therefore, the sum of antimony in 4. “Yuukisiyaku niyoru Bunribunsekihou”, ed. the Japan precipitate, filtrate, and washing solutions was the value Society for Analytical Chemistry, 1959, Vol. II, Kyouritu containing antimony(V). Syuppan, Tokyo. The sum of antimony in precipitate, filtrate, and washing 5. L. Erdey, in “Gravimetric Analysis”, in International Series solutions corresponds to the purity of antimony(III) oxide. The of Monographs on Analytical Chemistry, ed. R. Belcher ANALYTICAL SCIENCES MARCH 2004, VOL. 20 547

and L. Gordon, 1963 (Part I), 1965 (Part II), Vol. 7, International des Poids et Mesures), the 4th Meeting, 1998. Pergamon Press, London. 19. IUPAC Inorganic Chemistry Division, CAWIA: Atomic 6. K. Kodama, “Methods of Quantitative Inorganic Analysis”, Weights of the Elements 1999; T. B. Coplen, Pure Appl. 1963, Interscience Publishers, New York. Chem., 2001, 73, 667. 7. Th. I. Pirtea, Z. Anal. Chem., 1939, 118, 26. 20. A. Hioki, T. Watanabe, K. Terajima, N. Fudagawa, M. 8. I. Kudo, Bunseki Kagaku, 1956, 5, 556. Kubota, and A. Kawase, Anal. Sci., 1990, 6, 757. 9. W. W. Wendlandt and G. R. Horton, Anal. Chem., 1962, 21. N. Fudagawa, S. Nakamura, and A. Kawase, Bunseki 34, 1098. Kagaku, 1982, 31, 324. 10. C. Duval, “Inorganic Thermogravimetric Analysis”, 2nd 22. S. Nakamura, J. Anal. At. Spectrom., 1986, 1, 227. and revised ed., 1963, Elsevier Publishing Company, 23. S. Nakamura, Y. Kobayashi, and M. Kubota, Spectrochim. Amsterdam, 515 Ð 516. Acta, Part B, 1986, 41, 817. 11. L. Erdey, in “Gravimetric Analysis”, Part II, in 24. N. Fudagawa, Bunseki Kagaku, 1984, 33, 296. International Series of Monographs on Analytical 25. A. G. Asuero, Int. J. Pharm., 1989, 52, 129. Chemistry, ed. R. Belcher and L. Gordon, 1965, Vol. 7, 26. A. Claassen and L. Bastings, Analyst, 1967, 92, 614. Pergamon Press, London, 200. 27. “CRC Handbook of Chemistry and Physics”, ed. D. R. 12. Th. I. Pirtea, Fresenius’Z. Anal. Chem., 1965, 208, 411. Lide, 76th ed., 1995, CRC Press, Boca Raton, Sect. 3-309. 13. T. J. Cardwell, F. G. Nasouri, H. H. McConnell, and R. J. 28. K. Takiyama, M. Sawada, and N. Otani, Bunseki Kagaku, Magee, Aust. J. Chem., 1968, 21, 359. 1970, 19, 687. 14. A. Hioki, M. Kubota, and A. Kawase, Analyst, 1992, 117, 29. J. T. Corkins, R. F. Pietrzak, and L. Gordon, Talanta, 1962, 997. 9, 49. 15. ISO Guide 35, International Organization for 30. A. Hioki, unpublished work. Standardization, Genève, 2nd ed., 1989. 31. R. Berg, J. Prakt. Chem., 1927, 115, 178. 16. J. R. Moody and M. S. Epstein, Spectrochim. Acta, Part B, 32. K. Shiba, Tokyo Kogyo Shikensho Hokoku, 1932, 27, 1. 1991, 46B, 1571. 33. According to the new table of atomic weights (ref. 19), the 17. T. J. Quinn, Metrologia, 1997, 34, 61. uncertainty (0.01% (m/m), combined standard uncertainty) 18. CCQM (the Consultative Committee for Amount of is limited by the coulometric titration itself, though the Substance—Metrology in Chemistry) of BIPM (the Bureau uncertainty was reported to be 0.02% (m/m) in Ref. 14.