INDUSTRIAL AND ENGINEERING CHEMISTRY
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an iron solution is added during the procedure and therefore any iron already in the water should have no effect. As to the accuracy of the Levy method and its modifications there is considerable difference of opinion. The Metropolitan Sewage Commission of New Yorkl8 has used it and found close agreement with gasometric methods. Letts and Blake9 claim high results in the presence of nitrites or organic matter, while others, such as Chlopin,lg and Tiemann and Preusse,20 maintain that it gives low results, the magnitude of the error depending on the amount of time allowed for the precipitate to stand, the degree of pollution of the water and the temperature a t which the experiment is run. The longer the precipitate is allowed to stand before acidification the Jess the error. Waters run at higher temperature have given higher results than some run a t a lower temperature, which is contrary to the law of solubility of gases. With the end in view of seeing how variable amounts of iron affected the Levy method, a series of runs was made varying the amount of iron both in distilled and polluted water. The samples were tested by the Levy method, Letts and Blake modification, and also by gasometric methods.21 The results are shown in Table 111.
Vol. 15, No. 11
vey, Bull. 22, Series No. 7, p. ?. 6--Shutzenberger, Comfit. rend., 75, 879 (1872). 7-Mohr, KGnig, und Krauch, Z . anal. Chem., 19, 268 (1880). 8-Levy, Ann. de l’observatoire d e Mort-souris, 1894, p. 3101. &Letts and Blake, Proc. Roy. Dublin Soc., 9 (New Series), Pt. 4 , No. 33 (1900). lO-Linossier, J . Pharm., [5] 23, 103 (1891). 11-Ramsay, Fourth Report Royal Commission on Sewage Disposal, 4, Pts. 2 and 5 , p. 52 (1904) 12-Mackay and Middleton, J . Soc. Chem. Ind., 16, 1127 (1898). 13-Winkler, Ber., 21, 2843 (1888); 22, 1764 (1889). 14-Rideal and Stewart, Analyst, 26, 141 (1901); Standard Methods for the Examination of Water and Sewage, American Public Health Association, 1923, p. 59. 15-Hale and Melia, J . Ind. Eng. Chem., 5, 976 (1913). 16--Winkler, Z . angew. Chem., 29, 44 (1916). A m . J . Pub. Health, 3, 524 (1913); Elvove, Hygienic 17--Phelps, Laboralovy, Bull. 96, p. 15. l&Report Metropolitan Sewage Commission of New York, 1910, p. 401; 1912, p. 302. 19-Chlopin, A w h . Hyg , 32, 294 (1895). 20-Tiemann and Preusse, Be?., 12, 1784 (1879). 31.-Treadwell and Hall, “Analytical Chemistry,” p. 734.
Fractional Distillation Apparatus’
TABLE 111-COMPARISONOF LEVYAND BOILING OUTMETHODS Iron
Levy
Boiling
Levya Boilinga Cc. Cc. REMARKS Distilled Watev 1 107 7.70 7.68 5.39 5,38 2 81 7.35 7.65 5.15 5.35 3 54 7.56 7.36 5.25 5.15 Raw Sewage 0 54 0.60 Shaken with iron solu0 86 4 81 0 77 0 25 0 30 tion and allowed t o 5 54 0 35 0 43 0 20 0 15 stand for a few hours 6 27 0.28 0 22 At normal temperature and pressure.
No. P.p.M. P . p . M . P . p . M .
a
!i
There is in general a close agreement in the results by the two methods. Whether or not the criticisms of this method are false cannot be judged without a much fuller investigation. The data, however, show that the Levy method can be used in the presence of iron and will give far more accuracy than the TVinkler method, Rideal and Stewart modification. For instance, a water containing 107 p. p. m. (Table 111) which gives a correct value for the dissolved oxygen by the Levy method would give (Table I) a value about 10.6 p. p. m. too high by the Winkler method if allowed to stand some time before titration with thiosulfate. Another phase of the problem to be studied in this laboratory is the effect of time on the equilibrium of iron and hydrogen iodide. It is thought that it plays a considerable part.
CONCLUSION Determination of dissolved oxygen with the WinMer method, Rideal and Stewart modification, gives high results in the presence of iron. A correction cannot be made for iron by lowering the results in proportion to the iron present. The equilibrium Fe+++
+ H I e Fe++ +
must be considered. Reaction gives 75 to 90 per cent of iron as iodine. Thus, the presence of 1 p. p. m. iron causes a high value of 0.10 to 0.13 p. p. m. in the oxygen. The Levy method gives accurate results in the presence of iron and organic matter, while the Winkler method gives too high results.
BIBLIOGRAPHY I-Bunsen. Original-Gasometrische der Methoden, 2 Aufl., 18. Described by Fresenius, p. 232. 2--Adeney, Trans. Roy. Dublin Soc , [2] 5, 539 (1895). 3-Thresh, “The Examination of Water and Water Supplies,” 2nd ed., p. 394; J . Chem. Soc (London),17, 185 (1890). Analyst, 18, 121 (1894); Petterson and Sonden, Bev., 22, 4-Harvey, 1439 (1889). 9-Birge and Juday, Wisconsin Geological and Natural History Sur-
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