~Effect of Foreign Salts.~--The conditions of these experiments were the same as before. The salts were added and dissolved before the addition of the potassium iodide. Using 5 grams (or in the case of the acids, 5 c.c.), the results were as follows:-- dilute Acetic Salt added -- H_{2}SO_{4} acid NaAc NaCl "Hypo" required 20.0 c.c. 20.0 c.c. 20.1 c.c. 19.3 c.c. 20.1 c.c. Salt added KNO_{3} Na_{2}SO_{4} AmCl Am_{2}SO_{4} "Hypo" required 20.2 c.c. 18.7 c.c. 20.0 c.c. 19.9 c.c. The low result with the sulphate of soda was evidently due to the formation of a sparingly soluble double salt, which removed copper from the solution; on adding a little acetic acid the full amount of "hypo" was required. The effect of the presence of certain metals is important. The method of determining it was to add the substance to the solution containing the copper, and partly precipitate with soda solution; then treating with 1 c.c. of acetic acid, adding the iodide, and proceeding as before. Substance Added. "Hypo" Required, - 20. c.c 0.050 gram arsenic as As_{2}O_{5} 20.0 " 0.050 " antimony as SbCl_{5} 19.8 " 0.050 " lead as Pb(NO_{3})_{2} 20.1 " A similar experiment with 0.050 gram of bismuth nitrate could not be determined, the end-reaction being masked. Bismuth iodide is soluble in potassic iodide, forming a brown solution, the colour of which is very similar to that produced by iodine; and although it does not strike a blue colour with starch, "hypo" has an action on it. A similar experiment with 0.050 gram of iron as ferric chloride required 22.3 c.c. of "hypo," and the colour returned on standing. This shows that ferric acetate liberates iodine under the conditions of the assay. Trying to counteract this, by adding to a similar solution 0.5 gram of phosphate of soda dissolved in a little water, 19.7 c.c. of "hypo" were required instead of 20.0, but the assay showed signs of returning colour. In standardising, the same result was obtained, whether the copper was present as nitrate or sulphate before neutralising. ~Effect of Varying Copper.~--With the same conditions as before, but with varying amounts of copper and a proportionally increasing quantity of iodide, the results were:-- Copper present 1.0 c.c. 10.0 c.c. 20.0 c.c. 50.0 c.c. 100.0 c.c. "Hypo" required 1.0 " 10.0 " 20.0 " 50.0 " 100.0 " showing the results to be exactly proportional.[58]