Examine the first portions of the filtrate which pass through the filter with great care for asbestos fibers, which are most likely to be lost at this point.  Refilter the liquid if any fibers are visible.  Finally, wash the precipitate thoroughly with warm water until free from soluble silver salts.  To test the washings, disconnect the suction at the flask and remove the funnel or filter tube from the suction flask.  Hold the end of the tube over the mouth of a small test tube and add from a wash-bottle 2-3 cc. of water.  Allow the water to drip through into the test tube and add a drop of dilute hydrochloric acid.  No precipitate or cloud should form in the wash-water (Note 16).  Dry the filter and contents at 100-110 deg.C. until the weight is constant within 0.0003 gram, as described for the preparation of the filter.  Deduct the weight of the dry crucible from the final weight, and from the weight of silver chloride thus obtained calculate the percentage of chlorine in the sample of sodium chloride.

[Note 1:  The washed asbestos for this type of filter is prepared by digesting in concentrated hydrochloric acid, long-fibered asbestos which has been cut in pieces of about 0.5 cm. in length.  After digestion, the asbestos is filtered off on a filter plate and washed with hot, distilled water until free from chlorides.  A small portion of the asbestos is shaken with water, forming a thin suspension, which is bottled and kept for use.]

[Note 2:  The nitric acid is added before precipitation to lessen the tendency of the silver chloride to carry down with it other substances which might be precipitated from a neutral solution.  A large excess of the acid would exert a slight solvent action upon the chloride.]

[Note 3:  The solution should not be boiled after the addition of the nitric acid before the presence of an excess of silver nitrate is assured, since a slight interaction between the nitric acid and the sodium chloride is possible, by which a loss of chlorine, either as such or as hydrochloric acid, might ensue.  The presence of an excess of the precipitant can usually be recognized at the time of its addition, by the increased readiness with which the precipitate coagulates and settles.]

[Note 4:  The precipitate should not be exposed to strong sunlight, since under those conditions a reduction of the silver chloride ensues which is accompanied by a loss of chlorine.  The superficial alteration which the chloride undergoes in diffused daylight is not sufficient to materially affect the accuracy of the determination.  It should be noted, however, that a slight error does result from the effect of light upon the silver chloride precipitate and in cases in which the greatest obtainable accuracy is required, the procedure described under “Method B” should be followed, in which this slight reduction of the silver chloride is corrected by subsequent treatment with nitric and hydrochloric acids.]