[Note 5:  The asbestos used in the Gooch filter should be of the finest quality and capable of division into minute fibrous particles.  A coarse felt is not satisfactory.]

[Note 6:  The precipitate must be washed with warm water until it is absolutely free from silver and sodium nitrates.  It may be assumed that the sodium salt is completely removed when the wash-water shows no evidence of silver.  It must be borne in mind that silver chloride is somewhat soluble in hydrochloric acid, and only a single drop should be added.  The washing should be continued until no cloudiness whatever can be detected in 3 cc. of the washings.

Silver chloride is but slightly soluble in water.  The solubility varies with its physical condition within small limits, and is about 0.0018 gram per liter at 18 deg.C. for the curdy variety usually precipitated.  The chloride is also somewhat soluble in solutions of many chlorides, in solutions of silver nitrate, and in concentrated nitric acid.

As a matter of economy, the filtrate, which contains whatever silver nitrate was added in excess, may be set aside.  The silver can be precipitated as chloride and later converted into silver nitrate.]

[Note 7:  The use of the Gooch filter commends itself strongly when a considerable number of halogen determinations are to be made, since successive portions of the silver halides may be filtered on the same filter, without the removal of the preceding portions, until the crucible is about two thirds filled.  If the felt is properly prepared, filtration and washing are rapidly accomplished on this filter, and this, combined with the possibility of collecting several precipitates on the same filter, is a strong argument in favor of its use with any but gelatinous precipitates.]

!Method B. With the Use of a Paper Filter!

Procedure.—­Weigh out two portions of sodium chloride of about 0.25-0.3 gram each and proceed with the precipitation of the silver chloride as described under Method A above.  When the chloride is ready for filtration prepare two 9 cm. washed paper filters (see Appendix).  Pour the liquid above the precipitates through the filters, wash twice by decantation and transfer the precipitates to the filters, finally washing them until free from silver solution as described.  The funnel should then be covered with a moistened filter paper by stretching it over the top and edges, to which it will adhere on drying.  It should be properly labeled with the student’s name and desk number, and then placed in a drying closet, at a temperature of about 100-110 deg.C., until completely dry.