VOLUMETRIC ESTIMATION OF ARSENIC (Pearce’s Method)

Apparatus, Reagents —The usual apparatus. For precipitation E. AgNO3 may be used. For the standard solution NH4CNS is used, and as an indicator a saturated solution of iron alum. For the solution of the silver arseniate, pure HNO3 is required.

Method, Reactions.—The ore containing the arsenic is fused with a mixture of KNO3 and Na2CO3, the arsenic being thus oxidised to sodium arseniate and potassium arseniate. On dissolving this arseniate and carefully neutralising the solution and adding neutral AgNO3 in slight excess a brick red precipitate of Ag3AsO4 is obtained,—

Na2HAsO4 + 3AgNO3 = Ag3AsO4 + 2NaNO3 + HNO3
or NaH2AsO4 + 3 AgNO3 = Ag3AsO4 + NaNO3 + 2HNO3

To be complete, this precipitation must take place in a neutral solution; and as some HNO3 is liberated by the reactions, a second neutralisation is necessary if the amount of arsenic is large.

The solution of Ag3AsO4 is dissolved in HNO3, a little iron alum added, and a standard solution of NH4CNS run in till by excess a brownish red is obtained.

NH4CNS + AgNO3 = NH4N03 + AgCNS

From the silver found and the formula Ag3AsO4 the quantity of arsenic is calculated. As a large amount of Ag3AsO4 is formed from a small amount of As. the sample for analysis may be taken as .5 gm. ore when poor in arsenic and .2 gm. ore when rich in arsenic. Arsenical pyrites may be taken for analysis.

Preparation of the Standard Solution.—As the salt is deliquescent it cannot be weighed very accurately, therefore weigh out roughly about 8 gms. NH4CNS. Dissolve in distilled water and make up to 1 litre at about 16° C. Roughly this is an N/10 solution.

Checking the Standard. —Weigh out in duplicate about .2 gm. test silver (see Assaying). Dissolve in a flask in a small quantity of pure 5E. HNO3. When dissolved make up to about 150 c.cs. Add 5 c.cs. of the iron-alum solution and titrate with the NH4CNS till the brown ferric thiocyanate just appears.

Calculate the value of 1 c.c. in terms of silver, and from the formula Ag3AsO4 calculate the value of the solution in terms of arsenic.

Assume, for example, that 1 c.c. N/10 NH4CNS = x gms. Ag, then 1 c.c. N/10 NH4CNS= x X 324/75 gm. As.

The Analysis.– Weigh out .2 gm. (or suitable quantity) of the sampled and very finely powdered ore. Transfer to a square of glazed paper, and carefully mix with about 3 gms. of a mixture of finely powdered Na2CO3 and KNO3. Transfer to a small porcelain crucible. Gradually heat to fusion and fuse for 4 or 5 minutes. Allow to cool. Add a little warm water; set in a 10 cm. porcelain basin containing a little water. Gently warm till the mass is dissolved. Remove from the heat. Wash the upper portion of the crucible (which may have fallen over on its side) with the wash jet. Remove the crucible on a glass rod, washing it down with the jet. The crucible may now be taken in the fingers and its interior examined, and any traces of the fusion removed by further treatment with water.
Acidify the filtrate with nitric acid and boil to expel any nitrous fumes and carbonic acid.

Cool and neutralise as follows:—Float a small piece of litmus paper in the liquid, and then add drop by drop 5E. NH4HO until the litmus paper turns blue. Make slightly acid with one or two drops of 16E. HNO3. Drop in a fresh piece of litmus paper and gradually add E. NH4HO, so that on standing for half a minute the paper just turns blue.

If a considerable precipitate is thus formed, filter and wash well.
Now add a neutral solution of AgNO3 in slight excess. Stir, and note the colour of the litmus paper. If red, again neutralise.

Filter off the red precipitate of arseniate of silver and wash well with cold water. Place under the funnel a 300 c.c. flask, and dissolve the precipitate with pure 5E. HNO3. Wash the paper with distilled water acidified with HNO3, bringing the total volume of the washings up to about 200 c.cs. Add 5 c.cs. of the iron-alum solution.

Titrate as before with N/10 NH4CNS. Repeat the estimation on a fresh portion of the ore. From the value of 1 c.c. calculate the percentage of arsenic present. With some practice this method will be found both accurate and rapid, therefore well suited for the work of the metallurgical chemist.