The quantity of iodin present in a phosphate is rarely more than one or two-tenths of one per cent. It can be determined as a silver salt, in the absence of chlorin or by any of the standard methods found in works on qualitative analysis. Iodin is quite a constant constituent of Florida phosphates. For a quantitative determination, the sample is treated with an excess of strong sulfuric acid in a closed flask and during the decomposition a stream of air is aspired through the flask and caused to bubble through absorption bulbs containing sodium hydroxid in solution. The temperature of the decomposition may be raised to about 200°. After the solution of the sample the sodium iodid formed is oxidized by heating with potassium permanganate, acidulated and mixed with a solution of potassium iodid to hold the free iodin in solution. The free iodin is determined in the usual way by titration with standard sodium thiosulfate solution. The reactions preparatory to the titration are represented by the following formulas: 2KI + H₂SO₄ = K₂SO₄ + 2HI. 2HI + H₂SO₄ = 2H₂O + SO₂ + 2I. 6I + 6NaOH = NaIO₃ + 5NaI + 3H₂O. NaI + 2KMnO₄ + H₂O = NaIO₃ + 2KOH + 2MnO₂. HIO₃ + 5HI = 6I + 3H₂O. The titration is represented by the following reaction: 2Na₂S₂O₂ + 2I = 2NaI + 2NaI + Na₂S₄O₄. The decinormal solution of sodium thiosulfate may be used. Grind the crystals of the salt to a fine powder, dry between blotting papers, and use 24.8 grams of the dried salt per liter. The quantity of iodin found in phosphates is so minute that it is hardly worth while to make a quantitative determination of it. =122. Occurrence of Chromium in Phosphates.=—In some phosphates a small quantity of chromium has been found. In a sample of phosphate from the Island of Los Roques in the Caribbean Sea, Gilbert found three-fourths per cent of chromium oxid (Cr₂O₃). The phosphates containing chromium have a greenish color and are characterized by great insolubility in solutions containing organic acids. The chromium is to be determined by the usual methods described in mineral analysis.