concentration of 0.001M . Higher concentrations were not tried. Effective molar absorptivity values, taking into account the dilution of the sample solutions prior to measuring the absorbance, are presented in Table 11. The low concentration range with maximum sensitivity involves only a small dilution of the reaction solution, 5 ml to 6 ml, while the higher concentrations were diluted 5 ml to 25 ml. Beer’s law data for chloride ion are shown in Table 111. By proper choice of the solution finally measured, chloride ion could be determined in the range of 0.4 to 40 ppm and possibly higher. The absorbance values for the blank solutions, indicated in Table 111, are sufficiently reproducible. Blank corrections were always made, but this is not required. The solubility of mercuric iodate is sufficiently low in 1 :1 ethanol-water. The solubility of the iodate is estimated to be approximately 1.5 X 10-5Mfrom the absorbance values found for the blanks. This is about the same as the value reported for water (6). However, the blank readings when using water only were too high. Plots of absorbance us. chloride ion concentrations for all of these complexes were linear. The precision of these methods, as indicated by recovery data in Table IV, is reasonably good. The reproducibility is about the same for all of the complexes. Possibly somewhat better results were obtained in the higher concentration range where greater dilution is involved. Values for the blank absorptions were reasonably consistent, with maximum variations of 0.05 to 0.07 A in the low concentration range. It is very important that the solid mercury iodate be mixed thoroughly with the sample solution and that the excess solid be completely removed before adding the halide or thiocyanate solution. Reaction of Chloride with Mercuric Iodate. The chief difficulty in these procedures seems to be in getting complete reaction between the chloride ion in solution and the suspended mercuric iodate. It is important to mix the solid with the solution thoroughly so that the system appears “cloudy.” Reaction is then practically complete in 5 to 10 minutes, although more time was usually allowed in this work. It is also necessary that the excess mercuric iodate be completely removed prior to forming the mercury complex. Experiments with magnetic stirring were not very successful as it proved difficult to separate the excess mercuric iodate from the solution. Most results were obtained using only vigorous shaking for 30-60 secoiids and allowing the solutions to stand. Comparison with Mercuric Chloranilate. Mercuric iodate seemed to be superior to mercuric chloranilate from the standpoint of purity and stability. High blanks are troublesome in work involving the chloranilate, requiring extensive clean-up procedures (13, 14). Work in this Laboratory also favors mercuric iodate in comparing the ease of separation of the excess solid from the reaction solution. Also, the molar absorptivity values for the chloride and bromide complexes are considerably higher than those for the chloranilate procedure using the ultraviolet peak, so that the sensitivity for measuring chloride ion is somewhat better. Our results indicate that the sensitivity for chloride measuring the absorbance values for the iodide and thiocyanate complexes is about the same as with chloranilate ion. (13) C.F.Hammer and J. H. Craig, ANAL.CHEM., 42,1588 (1970). (14) R.E.Humphrey and W.Hinze, ibid.,43,1100(1971).
It also occurred to us that since the mercury iodide complex and the chloranilate ion absorbed at about the same wavelength (HgId2-, 322-325 nm: Ch2-, 325-330 nm) it might be possible to combine the two procedures for even higher sensitivity. By reacting chloride solutions with mercuric chloranilate, centrifuging, and adding an excess of iodide ion, two absorbing species can then be measured at the same wavelength. Our results indicated that the sensitivity for chloride ion when measuring both species was about 1.7 times as great as with chloranilate ion alone (15). However, the main problem with this method was the poor reproducibility found with the chloranilate reaction. High blanks due to the mercuric chloranilate were also troublesome. Also, the increased sensitivity was accompanied by an increased blank due to the mercuric ion in solution from the mercuric chloranilate which dissolves. Possible Interferences. The principal interferences to be expected would be those anions, such as acetate, bromide, cyanide, iodide, and thiocyanate, which form soluble nondissociated mercury compounds. Preliminary work in this Laboratory indicates that cyanide will not interfere seriously, except in the case of the HgId2-complex. This procedure has an advantage in that those anions, such as sulfide, which form insoluble mercury compounds will not show any interference. Our work also shows that sulfite ion causes no difficulty. The mercuric thiocyanate-ferric ion method will show interference in the presence of those anions which form insoluble mercury compounds as well as with those which form soluble mercury species. The principal disadvantage of the mercuric iodate method is probably the separation step involved. RECEIVED for review November 17,1971. Accepted February 15, 1972. The support of the Robert A. Welch Foundation of Houston, Texas, is gratefully acknowledged. This work was also supported in part by the Faculty Research Fund of Sam Houston State University. (15) R. R. Clark, M. S. Thesis, Sam Houston State University,
Huntsville,Texas, 1971.
Correction Electrodeposition of Alpha-Emitting Nuclides from a Mixed Oxalate-Chloride Electrolyte Regrettably an omission has occurred in the procedure of this paper by Kenneth W. Puphal and Donald R. Olsen, ANAL.CHEM.,44, 284(1972). Page 285, column 1, the last paragraph should read : “Position the cell in the electrodepositing apparatus and, while stirring, add concentrated ammonium hydroxide dropwise to the yellow end point of methyl red indicator plus 6 additional drops. Add concentrated hydrochloric acid dropwise until a red color persists for 30 seconds, then add 3 drops of 1:5 hydrofluoric acid.”
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, J U N E 1972
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