=267. Differences in Crystalline Form.=—Winton has also observed a distinct difference in the crystals of potassium platinochlorid when obtained from concentrated and dilute solutions.[216] When platinic chlorid is added to a concentrated solution of potassium chlorid, a large part of the salt which is formed is precipitated in a pulverulent state, the remainder being deposited on evaporation. After treating with alcohol, filtering, and drying, the double salt is found in the state of a fine powder which, when examined under the microscope, is found to consist largely of radiating crystals. The characteristic form is one having six arms formed by the intersection, at right angles, of three bars. Numerous globular cavities in the crystals are observed in which mother liquid is enclosed. For this reason the salt is not easily dried at 100°, but when so dried loses additional moisture at 130°, and still more at 160°. The total additional loss, after drying at 100°, from this cause may amount to as much as six-tenths per cent of potassium chlorid. When, however, the solution of the potassium salt is so dilute that no precipitate at all is formed on the addition of platinic chlorid, the double salt is all deposited, as well as formed slowly, during the evaporation and occurs exclusively as octahedra. These octahedra are comparatively free of cavities, and give up practically all their moisture when dried at 100°. A method of procedure therefore for potash determination, based on the above principle of the addition of the reagent to dilute solutions, and drying the double salt produced upon evaporation, after washing with ninety-five per cent alcohol at 100°, and using the factor 0.30688 for potassium chlorid and 0.1939 for potassium oxid, gives good results and is regarded as better than any of the methods which prescribe the addition of platinic chlorid to highly concentrated potash solutions. =268. Factors for Potash Estimation.=—The factor now in use by the official chemists to convert potassium platinochlorid into potash (K₂O) is 0.19308, and for potassium chlorid 0.3056. Wolfbauer gives the differences which may arise by computing the potash from its platino-double chlorid by the different values assigned to the atomic weight of platinum.[217] The common factor used to obtain potassium chlorid from potassium platinochlorid is based on the atomic weight 197.18 and is derived from the formula: 2(39.13 + 35.46) 149.18 ------------------------------ = ------ = 0.30557. 2 × 39.13 + 197.18 + 6 × 35.46 = 488.20 The variations arising from taking other assigned values for the atomic weight of platinum are shown in the following table: