stepwise titrated with E D T A for simplicity and accuracy. Takamoto (9) first reported the EDTA titration of antimony in the pre:ence of acetone. The antimony-EDTA complex is so weak that tartrate, citrate, or fluoride replaces E D T A to i'orm a tartrate, citrate, or fluoride complex of antimony, Thus, the amount of E D T A originally combined with antimony is released quantitatively. Bismuth can be titrated with E D T A in the presence of tartrate, which masks antimony, using lead as a back titrrtnt and Xylenol Orange or hIethylthyino1 Blue as indicator. I n the absmce of tartrate, citrate, or fluoride, antimony and other metal ions may be totally titrated with EDTA in the presence of acetone using cobalt as a back titrant and thiocyanate as indicator. The IZDTA titrations either in the same solution or in the separate aliquots give a highly-selective volumetric method for antimony. The detail? of the step$-ise E D T A titrations of antimony will be dxcribed in a separate report. When only the determination of bismuth is desired, this may be done by adding escess E D T A and enough tartrate or fluoride (masking antimony and tellurium) ; the excess of E D T A is then back-titrated with lead at p H 5 to 6. When only the determination of antimony is desired, the stepmise E D T A titrations of bismuth and antimony may be done in the presen1:e of acetone and hydrolyzed tellurous avid. In the back-
titration with cobalt, take the reading for the first end point change and continue the titration after addition of tartrate or fluoride The titer from the first end point t o the second end point is for antimony. Selenium. Bismuth telluride as a thermoelectric material usually contains small amounts of selenium. T h e selenium is better determined photometrically n i t h 3,3'-diaminobenzidine which has been proved t o be a highly selective and sensitive reagent for selenium when the masking agents and solvent extraction technique are employed. Iodine. Bismuth telluride thermoelectric material often contains small amounts of iodine which can be conveniently determined with the silver iodide turbidity method. Silver iodide is one of the most stable silver precipitates known; its turbidity may be measured in a n ammoniacal medium (a). .4 simple apparatus can be used for distilling iodine; for example, an 125-m1. Erlenmeyer flask with ground glass neck was connected to a 25-ml. volumetric pipet whose tip was dipped into 10 ml. of 1A' sodium hydroxide solution. The use of the pipet was to prevent suction of the sodium hydroxide solution into the Erlenmeyer flask when the pressure was decreased after the sample was dissolved with heating. Since the iodine is easily carried over by the nitrogen oxides generated from the dissolution of the
sample with nitric acid, the sweeping with an inert gas may be omitted. When the sample supply is limited, the determinations of iodine, tellurium, bismuth, antimony, and selenium may be carried out in a single sample. The iodine is first distilled and determined as AgI; the tellurium is precipitated as tellurous acid in the remaining solution (or an aliquot) after addition of EDTA and acetone; after separation of the tellurous acid precipitate, the filtrate is back titrated with a cobalt solution a t p H 3.5 for both bismuth and antimony and a t p H 6 for antimony alone using lead as a back titrant in the presence of tartrate or Auoroboric acid; a few drops of EDTA4are added, the selenium is then determined by diaminobenzidine after removing acetone by heating. LITERATURE CITED
(1) Cheng, K. L., A \ ~ L CHEM. . 28, 1738
(1936).
( 2 ) Ibzd., 33, 761 (1961). (3) Zbid., p. 783. (4)Cheng, I
