~Effect of Varying Temperature.~--The reaction goes on very quickly in the cold, and, since there is no occasion for heating, all titrations should therefore be carried out cold.
~Effect of Varying Bulk.~--In these experiments, 20 c.c. of arsenic solution were taken, 2 grams of bicarbonate of soda and 2 c.c. of starch solution added, and water supplied to the required bulk. The results were:--
Bulk 50.0 c.c. 100.0 c.c. 250.0 c.c. 500.0 c.c.
"Iodine" required 20.0 " 20.0 " 20.0 " 20.0 "
Considerable variation in bulk does not interfere.
~Effect of Varying Bicarbonate of Soda.~--This salt must be present in each titration in considerable excess, to prevent the interference of free acid. The bicarbonate must be dissolved without heating, as neutral carbonates should be avoided.
Bicarbonate added 1 gram 2 grams 5 grams 10 grams "Iodine" required 20.1 c.c. 20.0 c.c. 20.1 c.c. 20.0 c.c.
These results show that large variation in the quantity of bicarbonate has no effect.
~Effect of Free Acid.~--In these experiments, the arsenic taken, the starch, and the bulk were as before, but no bicarbonate was added. In one case the solution was rendered acid with 5 c.c. of acetic acid, and in the other with 5 c.c. of hydrochloric acid; in both cases the interference was strongly marked, and no satisfactory finishing point could be obtained. This was much more marked with the hydrochloric acid.
~Effect of Foreign Salts.~--The process for getting the arsenic into solution will exclude all metals except tin, but the solution will be charged with sodium or ammonium salts in the process of neutralising, so that it is only necessary to see if these cause any interference. The alkaline hydrates, including ammonia, are plainly inadmissible, since no free iodine can exist in their presence. Monocarbonates similarly interfere, but to a much less extent; hence the necessity for rendering the assay distinctly acid before adding the bicarbonate of soda.
With 20 c.c. of arsenic solution; and with bulk, soda, and starch as before, the results obtained were:--
"Iodine" required.
With 20 grams of ammonic chloride 20.0 c.c.
" 20 grams of sodium chloride 20.0 "
" 20 grams of sodium acetate 20.0 "
" 0.050 gram of tin, as stannic chloride 19.6 "
Without any addition 20.0 "
The interference of the stannic salt is probably mechanical, the precipitate carrying down some arsenious acid.
~Effect of Varying Arsenic.~--With bulk, starch, and soda as before, but with varying arsenic, the results were:--
Arsenic added 1.0 c.c. 10.0 c.c. 20.0 c.c. 50.0 c.c. 100.0 c.c.
"Iodine" required 1.1 " 9.9 " 20.0 " 50.0 " 100.0 "
~Determination of Arsenic in Metallic Copper.~--Put 1 gram of the copper filings, freed from particles of the file with a magnet, into a 16-oz.-flask; and distil with the ferric chloride mixture, as above described. Neutralise the distillate; acidify; add bicarbonate of soda and starch; dilute; and titrate with the standard solution of iodine.[107] Make a blank determination with 1 gram of electrotype copper, proceeding exactly as with the assay; and deduct the amount of arsenic found in this experiment from that previously obtained.
Working in this way on a copper containing 0.38 per cent. of arsenic and 0.80 per cent. of antimony, 0.38 per cent. of arsenic was found.
~Determination of White Arsenic in Crude Arsenic.~--Weigh out 1 gram of the dried and powdered substance (or 0.5 gram if rich), and digest with 10 c.c. of a 10 per cent. solution of soda; dilute to about 50 c.c., and filter. Render faintly acid with hydrochloric acid, and filter (if necessary); add 2 or 3 grams of bicarbonate of soda in solution, then 5 c.c. of starch, and titrate the cold solution with the standard solution of iodine.
The following is an example:--
1 gram of crude arsenic required 53.7 c.c. "Iodine;"
100 c.c. "Iodine" = 0.6000 gram white arsenic; 100 : 53.7 :: 0.6 : 0.3222, or 32.2 per cent.
With the test-tube method of dry assaying, this same sample gave results varying from 33 to 35 per cent. of white arsenic, which (judging from its appearance) was impure.
URANIC ACETATE PROCESS.
This may be looked upon as an alternative to the gravimetric method. It is applicable in all cases where the arsenic exists in solution as arsenic acid or as arsenate of soda. The process may be considered in two parts: (1) the preparation of the solution, and (2) the titration.
~Preparation of the Solution.~--If the arsenic has been separated as sulphide, it is sufficient to attack it with 10 or 15 c.c. of nitric acid, and to heat gently till dissolved, avoiding too high a temperature at first. Afterwards continue the heat till the separated sulphur runs into globules, and the bulk of the acid has been reduced to 3 or 4 c.c.
Dilute with 20 or 30 c.c. of water; put in a piece of litmus paper; and add dilute ammonia until just alkaline. Then add 5 c.c. of the sodium acetate and acetic acid solution (which should make the solution distinctly acid); dilute to 150 c.c., and heat to boiling. The solution is ready for titrating.
When the arsenic exists in a nitric acid solution mixed with much copper, it is separated in the way described under _Examination of Commercial Copper_ (Arsenic and Phosphorus), pages 208, 209.
If the arsenic has been separated as ammonium-magnesium arsenate, and phosphates are known to be absent; dissolve the precipitate (after filtering, but without washing) in dilute hydrochloric acid. Add dilute ammonia till a slight precipitate is formed, and then 5 c.c. of the sodium acetate and acetic acid solution; dilute to 150 c.c., and heat to boiling. Titrate.
If phosphates are present (which will always be the case if they were present in the original substance, and no separation with sulphuretted hydrogen has been made), the phosphorus will count in the subsequent titration as arsenic (one part of phosphorus counting as 2.4 parts of arsenic). It will be necessary to dissolve the mixed arsenate and phosphate of magnesia in hydrochloric acid. Add about four or five times as much iron (as ferric chloride) as the combined phosphorus and arsenic present will unite with, and separate by the "basic acetate" process as described under PHOSPHORUS in the _Examination of Commercial Copper_, page 209. Obviously, when phosphates are present, it is easier to separate the arsenic as sulphide than to precipitate it with the "magnesia mixture."
~The Titration.~--The _standard solution of uranium acetate_ is made by dissolving 34.1 grams of the salt (with the help of 25 c.c. of acetic acid) in water; and diluting to 1 litre. The water and acid are added a little at a time, and warmed till solution is effected; then cooled, and diluted to the required volume: 100 c.c. will equal 0.50 gram of arsenic.
The _sodic acetate and acetic acid solution_ is made by dissolving 100 grams of sodic acetate in 500 c.c. of acetic acid, and diluting with water to 1 litre. Five c.c. are used for each assay.
The solution of potassic ferrocyanide used as _indicator_ is made by dissolving 10 grams of the salt in 100 c.c. of water.
To standardise the solution of uranium acetate, weigh up a quantity of white arsenic (As_{2}O_{3}) which shall be about equivalent to the arsenic contained in the assay (0.1 or 0.2 gram); transfer to a flask, and dissolve in 10 c.c. of nitric acid with the aid of heat. Evaporate to a small bulk (taking care to avoid the presence of hydrochloric acid); dilute with water; add a small piece of litmus paper; render faintly alkaline with ammonia; then add 5 c.c. of the sodic acetate mixture; dilute to 150 c.c.; and heat to boiling.
Fill an ordinary burette with the uranium acetate solution, and run into the assay a quantity known to be insufficient. Again heat for a minute or two. Arrange a series of drops of the solution of ferrocyanide of potassium on a porcelain slab, and, with the help of a glass rod, bring a drop of the assay solution in contact with one of these. If no colour is produced, run in the uranium acetate, 1 c.c. at a time, testing after each addition, till a brown colour is developed. It is best to overdo the assay, and to count back. It is not necessary to filter off a portion of the assay before testing with the "ferrocyanide," since the precipitate (uranic arsenate) has no effect.
The following experiments show the effect of variation in the conditions of titration. Make a solution of arsenic acid by dissolving 4.95 grams of arsenious acid (As_{2}O_{3}) in a covered beaker with 35 c.c. of nitric acid; evaporate down to 7 or 8 c.c.; and dilute with water to 1 litre: 100 c.c. will contain 0.375 gram of arsenic. Use 20 c.c. for each experiment.
~Effect of Varying Temperature.~--It is generally recommended to titrate the boiling solution, since it is possible that the precipitation is only complete on boiling. Low results are obtained in a cold solution, the apparent excess of uranium acetate striking a colour at once; on boiling, however, it ceases to do so; consequently, the solution should always be boiled directly before testing.
In four experiments made in the way described, but with 20 c.c. of a solution of arsenic acid stronger than that given (100 c.c. = 0.5 gram As), the results at varying temperatures were:--
Temperature 15 C. 30 C. 70 C. 100 C.
"Uranium" required 18.0 c.c. 18.5 c.c. 18.5 c.c. 18.7 c.c.
~Effect of Varying Bulk.~--These experiments were like those last mentioned, but were titrated boiling, and the volume was varied:--
Bulk 50.0 c.c. 100.0 c.c. 200.0 c.c. 300.0 c.c.
"Uranium" required 14.0 " 14.0 " 14.5 " 15.0 "
Considerable variations in bulk are to be avoided.
~Effect of Varying Sodium Acetate.~--These experiments were carried out like those last noticed, but the bulk was 150 c.c., and varying amounts of sodic acetate were added in excess of the quantity used in the experiments previously described:--
Sodic acetate added 0 gram 1 gram 10 grams 20 grams "Uranium" required 14.5 c.c. 14.5 c.c. 16.0 c.c. 18.0 c.c.
It is evidently important that the quantity of this salt present in each titration be measured out, so as to avoid variation.
~Effect of Varying the Sodium Acetate and Acetic Acid Solution.~--Acetic Acid also affects the results, but in the opposite direction, by preventing the precipitation of uranium arsenate. With varying volumes of the solution now under notice, the results were:--
Solution added 0.0 c.c. 5.0 c.c. 10.0 c.c. 15.0 c.c.
"Uranium" required 14.5 " 14.5 " 14.5 " 14.0 "
Solution added 20.0 " 30.0 " 40.0 " 50.0 "