As soon as the silver has united with all the chlorine any excess of AgNO3 forms a red precipitate with the K CrO4.

Preparation of the Standard Solution.--As AgNO, 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 AgNO3.

N. 10

Weigh out carefully 848 gms. pure AgNO, 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 AgNO, 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; AgNO3

N.
10

solution, and carefully precipitate and estimate gravimetrically the silver contained. 25 c.cs. should contain 01076 × 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.

N.
10

Titrate with the 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 Ag,CrO, 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. K,CrO4 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.

(b) THE VOLUMETRIC ESTIMATION OF CYANOGEN BY SILVER NITRATE.

Apparatus, Reagents, etc.-The same as in the last analysis, except that the chromate is not required. For analysis the student may take a solution prepared by the demonstrator, or a sample of 'gold' cyanide (KCN), which, if the potassium salt, will contain from 95% to 98% KCN. This salt now frequently contains NaCN, so that if estimating in percentages of KCN the results in these terms will run above 100%.

Method, Reactions. If to a solution of KCN a solution of AgNO, be added the following reaction takes place,

KCN + AgNO3 = AgCN+KNO。

but this AgCN is soluble in KCN, forming the soluble double cyanide. KCN, AgCN, and as long as there remains in the solution some KCN not converted into this double salt no permanent precipitate forms, but immediately the last trace of KCN is absorbed to form the double salt then the slightest excess of AgNO, will produce a permanent precipitate.

2KCN + AgNO3 = KCN,AgCN + KNO2

It is evident, then, that if the strength of the AgNO, solution is known the quantity of CN present may readily be estimated, and, as is frequently required, calculated to KCN. In technical work, where the presence of ferrocyanides and other salts may obscure the end point, the addition of a few c.cs. N. NaHO and a few drops of E. KI is advisable, and even then, unless precautions be taken (see Sutton, pp. 218-226), the end point may still be indistinct, but with the materials mentioned for practice the student should have no difficulty, especially if he use an alkali and KI, in obtaining a distinct end point marked by a permanent opalescence.

N.

10

The Standard Solution.-The balance of the AgNO, used in the last estimation will serve again here.

1 c.c.

N. 10

AgNO=0052 gm. CN=013 gm. KCN

In

as by the second equation one molecule of AgNO, is required for every two molecules of KCN to form the soluble double cyanide KCN, AgCN. routine work empirical' solutions are generally used. For example, in the United States 6.535 gms. AgNO, to the litre forms the standard solution. Then 10 c.cs. KCN solution are taken for the estimation, and each c.c. of AgNO3 solution used represents 1 lb. of KCN in the ton (2000 lbs.) of solution.

Or again, as is the practice in the British Colonies, the standard solution contains 17 gms. AgNO, per litre. Then 13 c.cs. of the cyanide solution to be tested are taken, and the number of c.cs. of AgNO, used divided by ten gives the percentage of KCN in the solution.

The strength of the solution has already been checked by precipitation. The Analysis-Weigh out quickly about 10 gms. of the roughly powdered cyanide and transfer to the 1000 c.c. test-mixer. Add about 250 c.c. distilled water. Stopper and shake. When dissolved make up to 1000 C.CS. With the pipette transfer 50 c.cs. into a 200 c.c. Erlenmeyer flask. Add a few drops E. KI, and titrate with the AgNO, running in a little,

N. 10

and then rapidly swirling the contents of the flask. After a number of additions and agitation by swirling, the precipitate formed hangs much longer before dissolving. The solution is then run in drop by drop with agitation till a faint yet permanent opaline or milky tint is formed. This is best judged by placing a small square of black paper under the flask.

Note the number of c.cs. used, and repeat the estimation on another 50 c.cs. of the solution.

N.
10

Calculation of Results.-Assume that 38.2 c.cs. AgNO, are required for 50 c.cs. KCN solution, 38.2 c.cs. AgNO3=013 × 38.2=4966 gms. KCN. But 50 c.cs. contain 5 gm. of the salt, therefore 100 c.cs. will contain 1 gm. of the salt, of which 4966 × 29932 gms. are potassium cyanide, or in other words the salt contains 99.32% KCN (assuming no NaCN to be present).

Note on Testing Solutions containing KCN.-The student may test a solution if dilute (say below 5%) by measuring out 50 c.cs. and titrating as before. If a strong solution is to be tested it should be diluted to a given volume, and 50 c.cs. of the diluted solution taken and the results multiplied according to the degree of dilution. The methods used in technical work have been indicated above, and the student may adopt them when he returns to this work in Laboratory Ore Tests, Part III.

(c) THE VOLUMETRIC ESTIMATION OF CYANOGEN BY
MERCURIC CHLORIDE.

Apparatus, Reagents.-Apparatus as before. For the standard solution, pure HgCl, is required. For analysis the student may take further portions of the materials used in the last estimation.

Method, Reactions.—If to an ammoniacal solution of KCN a solution of HgCl, be slowly added, Hg(CN), is formed and is soluble in water. When this reaction is complete any further addition of HgCl, results in the formation of a white precipitate,

HgCl2+2KCN = Hg(CN), + 2KC1

HgCl+2NH,HO=NH HgCl + NH ̧Cl + 2H2O

Knowing the strength and volume of the HgCl2 solution used, the quantity of CN or KCN may be calculated.

N.
10

HgCl will contain

270.54
10

Standard Solution.-An = 27.054 gms. of the salt per litre, and according to the equation 1 c.c. is equivalent to 0052 gm. CN or 013 gm. KCN.

It will be sufficient if the student prepare 250 c.cs. of this solution. Therefore dissolve 6.7635 gms. of the salt in distilled water and make up to 250 c.cs.

The student need not at present check the accuracy of this standard, but may compare the results obtained with those from the Silver Nitrate method.

The Analysis.-Proceed exactly as in the Silver Nitrate method, weighing or measuring the same quantities of material operated on, and titrating after adding excess of NH HO(no KI) till a faint but permanent opalescence is obtained.

Run duplicates, calculate the percentage of KCN, and compare the results with those previously obtained. With pure salts this method is very accurate, but with impure salts it is not so reliable as the previous method.

Note. When instructed to estimate the percentage of KCN present, it is assumed that all the CN radical is united to K. This may or may not be so, but in Cyanide work it has become the custom to speak of the strength of a solution in percentage of KCN.

(d) THE VOLUMETRIC ESTIMATION OF LEAD BY AMMONIUM MOLYBDATE. Alexander's Method.

Apparatus, Reagents.-The usual apparatus. For the standard solution, ammonium molybdate is required [(NH), MOO,], or by a simple calculation the heptamolybdate (NH4) Mo,O,4H2O which is non-deliquescent, or the trioxide MoO, dissolved in NH,HO may be used. For an indicator a solution of 1 gm. tannin in 500 c.cs. water is used. For analysis the student may take a siliceous galena ore, containing roughly about 50% galena and 50% quartz.

Method, Reactions. When (NH),MOO, is added to a hot solution of Pb(CHO) the following reaction takes place,

Pb(C2H2O2)2 + (NH1),MoO1 = PьMοO +2NHC2H ̧O2

4

4

To determine the end point of this precipitation, an external indicator, 'tannin,' is used; a drop on a white porcelain plate giving a yellow colour with any excess of the molybdate of ammonia. Knowing the strength and volume of the standard solution used, the quantity of lead is easily calculated. The ore is broken up with HNO, and HSO4, and the lead obtained as the sulphate, which is dissolved in hot NH CHO

N.

20

Preparation of the Standard Solution.-An solution will be found most convenient. Dissolve 9.8 gms. (NH),MoO, (or equivalents of (NH) MoO24,4HO or MoO, in NH,HO) in distilled water and make up to one litre at 16° C. If the solution is not clear a few drops of strong ammonia will generally clarify it.

Checking the Standard.-Weigh out two portions, each of 2 gm. pure lead sulphate. Transfer to 300 c.c. beakers. Dissolve in a little hot 5E. NH CHO, adding a little at a time, and heating till solution is complete. Acidify with acetic acid. Dilute each lot to 200 c.cs. with hot water. Titrate with the molybdate solution, testing a drop every now and then with a drop of the tannin solution on a porcelain plate. Immediately the molybdate is in excess a yellow colour is obtained. Note the number of c.cs. used.

Assume that 13.62 c.cs. are required for 2016 gm. PbSO4. But 2016 .2016 × 207 gms. PbSo contain = 1377 gm. Pb.

=

303

Duplicates should agree within 00005 gm. Pb.

3

The Analysis.-Weigh out 5 gm. of the sampled and finely powdered galena. Transfer to a casserole or a porcelain dish with an inverted funnel placed inside. Cautiously add 15 c.cs. 16E. HNO, and 10 c.cs. 36E. H.SO. Heat just to boiling, and continue at this temperature till white fumes of SO appear. The excess of HNO3 is then expelled. Remove from the heat and wash down the lid or cover when cool. Add 50 c.cs. cold distilled water. Stir well with a glass rod, breaking up any residue. Gently boil to dissolve all soluble sulphates.

3

Wash

To the

Let settle, and pour off most of the liquid through a filter paper. by decantation twice with hot E. H2SO, and once with hot water. white residue in the beaker add a little hot NH CHO2 and heat a few minutes. Decant through the filter, having placed a clean 300 c.c. beaker under the funnel. Repeat the treatment till the PbSO4 is all dissolved. Test by removing a drop of the solution from the beaker and bringing it into contact with a drop of NH HS on a porcelain plate. Wash out the beaker with hot water and wash the filter till the lead is all through.

N.

Acidify the filtrate with acetic acid and dilute to 200 c.cs. Titrate as before with the (NH4),MOO4. Note the number of c.cs. used and calculate the percentage of lead.

20

Repeat the estimation on another sample of the ore. The duplicate should agree within 1%.

Note. This method (see the Engineering and Mining Journal, vol. lv., No. 13, 1893) is well suited for the estimation of lead in ores and mattes, being both accurate and quick. With some practice a complete estimation can be made in about half an hour.

(e) THE VOLUMETRIC ESTIMATION OF ZINC BY POTASSIUM FERROCYANIDE. (Fahlberg's Method, as modified by von Schulz and Low).

6

Apparatus, Reagents. The usual apparatus. For the preparation of the standard solution pure K,FeCN (free from K Fe,CN12) is used, and pure ZnO is employed for checking the standard. As an indicator a solution of uranium acetate is employed. In the preparation of the ore for analysis the following solutions are required :—About 100 c.cs. of a solution of KClO crystals in strong HNO, prepared by shaking the crystals and acid in a flask. Solid NH C1 and 20E. NH HO are also required, and a litre of 5 NH Cl. If copper is present, a number of small strips of thin sheet aluminium are required for its precipitation.

E.

For analysis, the student may take a zinc ore containing a fair percentage of zinc blende (ZnS).

For fuller details regarding the technical application of this and other volumetric methods the student is referred to Furman's Manual of Assaying.

Method, Reactions.-If to a hydrochloric acid solution of a zinc ore, free or freed from elements such as copper, iron, manganese, etc., a solution of potassium ferrocyanide be added, the zinc is precipitated as zinc ferrocyanide.

2ZnCl2 + K FeC N = Zn, FeCN + 4KC1

6

As

The end point of this reaction is determined by testing on a porcelain plate a drop of the solution with a drop of uranium acetate solution. long as there is not an excess of K FeC N, the drop of uranium acetate remains yellow, but as soon as the ferrocyanide is in excess it turns a light brown. Before the solution of the ore is ready for titration iron and copper must be removed, if present, as they interfere with the reaction.

Preparation of the Standard Solution. Dissolve 42.2 gms. pure K1FeCoN,3H2O in water and make up to 1 litre at 16° C.

Checking the Standard.—Weigh out in duplicate 2 gm. pure ZnO, recently ignited to free it from any carbonate. Dissolve in 5 c.cs. 10E. HCl, and add 50 c.cs. water, using a 300 c.c. beaker. Add 20E. NHHO in slight