In titrating the hydrochloric acid solution of the metal oxine,the author found that the amount of the bromate-bromide solution to be added is of vital importance in obtaining is correct result. In the bromization of oxine with the bromate-bromide solution, care should be taken not to add much excess of the solution, otherwise, by the subsequent addition of potassium iodide, insoluble iodine separates out together with a chocolate brown precipitate of an iodine addition product. After the back titration with thiosulphate, the blue colour of the starcd indicator returns again after standing the solution for a while, and this is liable to give too high a result in the analysis. The phenomenon is best explained by assuming that in the presence of dibrom-oxine, the solubility of iodine in potassium iodide solution is greatly diminished. The method, therefore, differs from an ordinary sense of the back titration, for the excessive bromate-bromide solution is restricted within a comparatively narrow limit.
In order to avoid too much excess of bromatebromide solution, the author prefers methyl orange instead of indigo carmine as shown in Table 3. The best plan to perform the titration proposed by the author is as follows: Add 2 or 3 drops of methyl orange to the hydrochloric acid solution of metal oxine and bromize with the bromate-bromide solution; if the colour of methyl orange fades away, add some drops of the methyl orange and continue to bromize. If, on the addition of methyl orange indicator, red colour fades away, stop the bromization.Dilute the solution to about 300 ml, add about 6 gr of potassium iodide, and the solution should be clear brown,free from precipitate, titrate with thiosulphate,using starch solution as an indicator toward the end.
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