Note on the Effect of Old Filter-paper on Iodate-free Potassium Iodide
Journal of the American Chemical Society
The figures are grams of thioblue color with starch solution. sulphate, equivalent to I cc. of iodine solution. Volume of iodine solution. Weight of sample. cc. Standard of solution. 0.8833 27.45 0.03228 0,7288 22.57 0.03227 0.9319 28.90 0.03224 0.6807 21.07 0.03229 The standard of the same iodine solution, as determined by the direct method with a weighed quantity of iodine, was 0.03228. Three days later the following determinations were made with the same substance in order to see if there
... to see if there was any deterioration of the anhydrous salt within that time, due, perhaps, to absorption of moisture or other cause. Volume of iodine solution. Weight of sample. cc. Standard of solution 0.9404 29.30 0.03210 0.8338 25.92 o.oj217 0.8518 26.47 0.03218 0.6100 18.92 0.03224 Two determinations by the direct method, made at the same time, gave 0.03217 and 0.03221. Except for the first determination, the agreement is all that could be desired. There is more than likely a small experimental error in the first determination. Note on the Effect of Old Filter-paper 0% Iodate-free Potassizrnz Iodide.-In the course of some experiments with potassium iodide it develaped that a dilute solution ( I in IO) of iodate-free salt, after filtering through the ordinary filter-paper of the laboratory and acidification with a few drops of dilute sulphuric acid after the manner of the United States Pharmacopeia test for iodate,l oftentimes showed a distinct brownish coloration after 1 The Pharmacopoeia of IW recommends the use of starch paste to render the liberation of iodine evident by the formation of the characteristic starch-iodine blue. We find that the yellow coloration of the free iodine is as delicate and as easily seen in a colorless clear solution as the starch reaction. Hence we omitted the starch paste. We worked with I gram of potassium iodide in IO cc. of water instead of I gram in 20 cc. as directed in the Pharmacopoeia. In all these tests it isnecessary to r u n a blank on the iodide solution along with the teat since as is well known the acidified potassium iodide solution becomes brownish fr&m thioxidation of the hidriodic acid by the oxygen of thf air. Ifthe time of standing be short, say three minutes, the blank solutions will be practically waterwhite, whereas the solutions treated with old filter-paper will be. distinctly yellowish. The coloration corresponds, of course, to a very small weipht of iodine : For io cc. of the solution, it re uired only two or three drops of N/IW thiosulphate solution to cause a complete bleaaing.