When we consider the ordinary gravimetric precipitations, it is evident that if the exact strength (as regards precipitation) of the precipitating reagent

Apparatus, Reagents.—Apparatus as usual. For the preparation of the standard solution, pure silver nitrate (preferably triple crystallized) is required. For practice in this analysis the student may first estimate the chlorine in pure sodium chloride, and then in a sample of river or well water.

Method, Reactions.—If a solution of AgNO3 is added to a solution containing a chloride, a precipitate of AgCl is formed so long as any chlorine remains uncombined with the silver. The end point may be determined (in the absence of acids) by adding a few drops of K2CrO4.

AgNO3 + X,Cl = X,NO3 + AgCl
and K2CrO4 + 2AgN03 = Ag2CrO4 + 2KNO3

As soon as the silver has united with all the chlorine any excess of AgNO3 forms a red precipitate with the K2CrO4.

Preparation of the Standard Solution.—As AgNO3 is rather expensive, and as the quantity of solution required will not be great, the student need only prepare 500 c.cs. of the standard N/10 AgNO3.
N. AgNO3 should contain 107.6 + 14 + 48 = 169.6 gms. AgNO3 per litre; N/10 AgNO3, 16.96 gms. per litre.

Weigh out carefully 8.48 gms. pure AgNO3, and transfer to the 500 c.c. flask. Add about 50 c.cs. distilled water, and when dissolved make up to the mark at 16° C.
1 c.c. of this solution should be equal to .00355 gm. Cl.

Checking the Standard.—If the solution has been carefully prepared from pure AgNO3 it hardly need be checked, but the student had better check the solution as follows :—

From the burette at about 16° C. measure out 25 c.cs. of the N/10 AgNO3 solution, and carefully precipitate and estimate gravimetrically the silver contained. 25 c.cs. should contain .01076 x 25 = .269 gm. Ag.

The Analysis.—1. Of Chlorine in Sodium Chloride.—Weigh out .2 gm. sodium chloride (pure). Dissolve in a 200 c.c. beaker with about 50 c.cs. distilled water. Add three or four drops of E. K2CrO4.
Titrate with the N/10 AgNO3 from the burette until the red precipitate of chromate of silver, which is at first decomposed by the excess of silver, is just permanent.

Note the number of c.cs. used, and repeat the estimation on a fresh sample, and calculate the percentage of chlorine in the usual manner.

Note.—This titration must be performed on a cold neutral solution, as Ag2CrO4 is soluble in acids. This method reversed may be used for the estimation of silver. A modified method will be found in Furman, p. 240.

2. The Analysis of Chlorine in River or Well Water.—From an average sample of the water measure out 100 c.cs. into a 200 c.c. beaker. Add a few drops E. K2CrO4 and titrate as usual. Calculate the percentage of chlorine as usual, or estimate it as so many grains per gallon (see Water Analysis, Part III.).

If very little chlorine be present in the water it is better to take, say, 700 c.cs. and evaporate down to 100 c.cs. before titration. If 700 c.cs. be taken, then the centigrams of chlorine found give the number of grains per gallon; if 70 c.cs., the milligrams chlorine represent grains per gallon.