弗化物添加クローム鍍金液中の弗素の定量

抄録

In the case of chrome-plating, the addition of small amounts of the various kinds of fluorides to the baths recently prevails in order to obtain the higher quality of the plate and to improve its procedure. But it has yet been difficult to control the contents of the fluorine owing to the lack of an adequate method for determining it. In the present work this method was investigated and the satisfactory results were obtained with the sample solutions containing from 0.2 to 5mg of fluorine per liter. The analytical procedure is as follows : 5ml of the solution, which is obtained by diluting the original sample solution by 75 times, is taken and the chromium is reduced to chromic by heating with the addition of 2ml of ethanol and 3ml of N-hydrochloric acid. Excess amounts of the chromic ion is exchanged by means of Daiya Ion K. The exchanged solution is added with sodium hydroxide to the faint red color of phenolphthalein and then added with 5ml of alum solution (40γ of aluminium), 8ml of N-hydrochloric acid, 5ml of 3N ammonium acetate, 5ml of 0. 1% solution of aluminon and 0.5ml of 6N ammonium carbonate. The total volume of the solution is brought up to 100ml (pH = 4.6) and is allowed to stand in the thermostat (31°C) for 45 minutes to develop the color. The amounts of fluorine are determined by the electrophotometric method, using the calibration curve. The analytical error is 2% and the time required is 75 minutes. Silicic acid and oxalic acid interferes.