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THE WHEAT PROBLEM:

Based on Remarks made in the Presidential Address to the British Association at Bristol in 1898.

REVISED WITH AN ANSWER TO VARIOUS CRITICS

By SIR WILLIAM CROOKES, F.R.S.
SECOND EDITION.

ITH PREFACE AND ADDITIONAL CHAPTER, BRINGING THE
STATISTICAL INFORMATION UP TO DATE.

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United States, by MR. C. WOOD DAVIS, of Peotone,
Kansas, and the HON. JOHN HYDE, Chiet Statistician
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THE

265

minutes, and filtered into a 250 cc. flisk. The insoluble matter was washed with cold distilled water till acid free. The filtrate and washes were then made up to 250 cc., and well mixed. Each 50 cc. represents I grm. of pyrites. The method of Pelouze was again employed to determine the nitric acid, if any, in this solution. 50 cc. were treated in a 200 cc. flask with 10 cc. strong HCI, 1.16 sp. gr., and 10 cc. of a standard (approximately 1/10) solution of FeSO4 boiled for five minutes, cooled, diluted to about

IN PYRITES.

By T. J. I. CRAIG, D.Sc.

(Concluded from p. 255).

Experimental Investigations.

FOR use in the following experiments a set of volumetric apparatus was provided, each vessel being carefully tested and regraduated where necessary.

I. Elimination of Nitric Acid.-It is well known that nitric acid can be completely removed from aqua regia | solution of pyrites by taking to dryness once or twice, after adding strong hydrochloric acid. This is always done in the gravimetric determination of the sulphur as BaSO4. The same means were adopted to render the aqua regia solution free from nitric acid preliminary to carrying out the volumetric determination by titration with

standard alkali.

To obtain a solution which would provide a number of experimental portions, 20 grms. of pyrites, ground to pass through a 100-mesh to the inch sieve, were dissolved in 600 cc. of aqua regia (3HNO3: 1HC), evaporated to dryness overnight, treated with strong HCl, again taken to dryness, heated at 110° C. for one hour, dissolved in cold distilled water, filtered, and washed into a 500 cc. flask, and made up to the mark (each 25 cc. = 1 grm. pyrites).

The method of Pelouze was employed to determine the nitric acid, if any, in the solution (Journ. Prakt. Chem., xl., 34).

Standard solutions were made of N/10 KMnO and FeSO4, such that 2 grms. of the latter were equivalent to 4'575 cc. of the former-the standardisation being carried out under the same conditions as in the test. 25 cc. of the above pyrites solution were boiled with 2 grms. FeSO4 solution, 10 cc. HC, and 25 cc. water down to half the bulk of the mixture. On cooling and titrating with the permanganate 4'55 cc. were used. In another experiment carried out in the same way, 4'6 cc. KMnO4 were used.

The average of these two trials is 4'575, which is exactly the amount for 2 grms. of the FeSO4 solution, thus proving absence of nitric acid.

Removal of Nitric Acid from the Dilute Nitric Acid and Bromine Method of Dissolving Pyrites.-Five grms. of pyrites (ground to pass through 100-mesh per inch sieve) were treated in a covered 800 cc. beaker with a mixture of 50 cc. strong HNO3, 142 sp. gr., and 50 cc. distilled water. An action with generation of much heat at once set in, and continued until all the pyrites was decomposed and dissolved with the exception of a small amount of sulphur. On cooling to 50° C. and adding 3 cc. bromine, and shaking gently round, the bromine gathered up all the sulphur. A further 20 cc. of strong nitric acid were now added, and the vessel and contents were warmed up on the steam-bath, and finally boiled on a flame for a few minutes. All the sulphur was oxidised and dissolved. The solution was now transferred to a porcelain basin and taken to dryness on the steam, moistened with water, and dried again, and finally heated in the air-bath for one hour at 110° C. About 50 cc. of distilled water were then added and heating on the steam applied for half-an-hour to thoroughly hydrate and dissolve the residue. The solution was cooled, treated with another 50 cc. of cold distilled water, stirred for a few

with n/10 KMnO4. The FeSO4 solution was standardised by taking 10 cc. of it, adding 50 cc. distilled water, 10 cc. strong HCI, boiling for five minutes, cooling, diluting to 500 cc. and titrating with n/10 KMnO4.

By carrying out the analytical test and the standardisation under the same conditions, any disturbing influence will be practically the same in each case, and will therefore not materially affect the result.

The FeSO4 solution keeps very well if it is slightly acidified with H2504, and a few strips of metallic

aluminium are immersed in it.

It was found in the blank experiment that to cc. of the FeSO4 solution were equal to 9'9 cc. π/10 KMnO4. In the analytical test for HNO3 10 cc. of the FeSO4 solution also used 99 cc. of the /10 KMnO4, thus showing the absence of HNO3. A repetition of this experiment gave a similar result.

As it takes some considerable time to thoroughly dry the pyrites solution on the water-bath, it will be found quite suitable when a quick result is desired, to take the dried residue as soon as it ceases to give off visible fumes of HNO3 and proceed with the solution and filtration of it. The small amount of HNO, remaining in the solution may then be easily determined and allowed for; each cc. /10 KMnO4 corresponds to o'000533 grm. S. 6FeSO3+2HNO3+6HCl=3Fe2(SO4)2Cl2+4H2O+2NO, 10FeSO4+2KMnO4 +16HCl=

=5Fe2(SO4)2Cl2 + 2MnCl2+2KCl+8H2O.

It is obvious from these equations that

2KMnO4 are equivalent to 3:33 HNO3, 2KMnO4=315.6 and 3.33 HNO3 = =210. One cc. 1/10 KMnO4 contains o'003156 grm. KMnO4. Therefore

I cc. 1/10 KMnO4 is equivalent to o'0021 grm. HNO3, 2HNO, are equivalent to S (2HNO3 + S = H2O+2NO+SO3), 126 grms. HNO3 are equivalent to 32 grms. S. Therefore

0'0021 HNO3 corresponds to 0'000533 grm. S; that is

each cc. /to KMnO4 corresponds to 0.000533 grm. S.

II. Experiments on Removal of HCI from Aqua regia Solution of Pyrites.-The HCI remaining in the first pyrites solution described under I. was determined by taking 25 cc., treating with 2 grms. pure CaCO3, heating on the steam, filtering and washing on the pump. 1595 cc. of /10 silver nitrate solution were used. This is equivalent 1595 cc. of n/1 alkali.

25 cc. pyrites solution evaporated to dryness and the Cl determined as above described took 13 4 cc. /10 AgNO3. 25 cc. dried, taken up with 10 cc. distilled H2O, and dried again, took 11'45 cc. n/10 AgNO3.

25 cc. dried, dissolved in 10 cc. distilled H2O, dried again, then once more treated with 10 cc. distilled water and dried, used 9 65 cc. 1/10 AgNO3.

25 cc. treated with 10 cc. n/1 H2SO4 and taken to dry. ness used I cc. n/10 AgNO3.

25 cc. treated with 5 cc. strong nitric acid and dried on the steam-bath took 6:05 cc. n/10 AgNO3.

25 cc. pyrites solution treated with 5 cc. strong nitric acid, dried, and heated at 110° C. in the air-oven for three-quarters of an hour took o'6 cc. n/10 AgNO3.

It is obvious from these experiments that it is difficult to eliminate HCl by evaporation from the pyrites solution.

The best plan therefore is to bring the HCI as low as possible by the evaporation at the end of the aqua regia attack when removing the nitric acid. It may then be determined as already described and subsequently allowed for. Or, alternatively, the remaining HCI may be removed from the pyrites solution by means of a slight excess of pure moist silver oxide in the cold. The precipitate of silver chloride along with some unacted on silver oxide is of a dense nature, easy to filter off and wash. If desired, the silver chloride precipitates may be preserved and silver oxide recovered for re-use by a treatment with hot caustic potash solution, filtering and washing free from alkali and chlorine. Silver oxide removes all the HCl, and the precipitate of AgCl was proved to be free from sulphate by treating it with pure boiling caustic potash, filtering, acidifying with HCl, and adding BaCl2. No trace of BaSO4 was produced.

III. Investigations Regarding the Arsenic.-Experiments were made with the solution of pyrites hereinbefore described. This solution was made from a pyrites con taining o'02 per cent As, which is fairly low. In order to study also the behaviour of the arsenic when testing an arsenical pyrites, arsenic acid was added to the portion taken for analysis to bring up the arsenic to o'5 per cent As on the pyrites.

25 cc. pyrites solution were diluted with 300 cc. distilled water in a porcelain basin, a little phenolphthalein solution

Aliquot portions (one-tenth) of the precipitate solution and of the filtrate were each tested for arsenic in the manner recommended by the Committee for the purpose of ascertaining the best method for determining small quantities of arsenic, and described in the Journ. Soc. Chem. Ind. (xxi., 94).

Two further tests were made similar to the last one, but with the addition of arsenic acid to the pyrites solution to bring the amount of arsenic in each group up to to 0.005 grm. As per grm. of pyrites (ie., 0.5 per cent on the pyrites).

The results of these experiments are seen in the photograph (Fig. 1) of the arsenic deposits, and it is clearly shown that practically all the arsenic is removed from the solution by the precipitated ferric hydrate, and does not therefore interfere with the test.

IV. Determination of the Volume occupied by the Precipitate of Ferric Hydrate produced when the Aqua regia Solution of Pyrites is treated with Standard Alkali.—In titrating ferric salts with standard alkali and a suitable indicator such as phenolphthalein, it is not possible to obtain satisfactory results when completing the titration with the ferric hydrate precipitate still present. The latter obscures the end-point, and for accurate work the precipitate must be removed. This means making up to a known volume and filtering off an aliquot part for titration. The precipitate occupies a certain volume which should be taken into account and allowed for. To determine this volume a few experiments were made as follows:

25 cc. pyrites solution were diluted with 200 cc. distilled water, neutralised with / caustic soda in slight excess, boiled for a few minutes, and the ferric hydrate filtered off and washed. It was then mixed with a little distilled water, and a specific gravity bottle of known weight was filled with part of the mixture, and the bottle and contents were weighed. The weight of the ferric hydrate in the bottle was then determined by dissolving it in HCI, and titrating with standard titanous chloride and sulphocyanide as indicator. The bottle was refilled with distilled water and again weighed. From these weights it was possible to arrive at (1) the weight (and thus the volume) of the water in the bottle, (2) the weight of water plus ferric hydroxide in the bottle, and (3) the volume of the ferric bydroxide.

was added, and then standard alkali until a permanent red colour was produced in the liquid. 4 cc. additional alkali were then added, and the liquid was brought to the boil. After boiling for about a minute the whole was transferred to a 500 cc. flask, cooled to the room temperature, and made up to the mark.

The precipitate of ferric hydrate was filtered off, washed, dissolved in arsenic free H2SO4, and the solution made up to 100 CC. The filtrate was preserved without the washings.

H2O accompanying Fe2(OH)6..

=

=

=

=

16.1478

The space occupied by the o*1491 grm. Fe2(OH) 6 therefore 16 1967-16 1478 00489 cc. The pyrites contained 41'94 per cent Fe = 80'14 Fe2(OH)6. Therefore, if o 1491 grm. Fe2(OH)6 00489 cc. 08014 grm. Fe2(OH)6 from I grm. pyrites 0.268 cc. By similar method o 1449 grm. of Fe2(OH)6 occupied 00499 cc., and therefore by proportion the o'8014 grm. occupies o 276 cc.

=

The average of these two last results is that o‘8014 grm. Fe2(OH)6 occupies o 272 cc. Therefore an allowance of 0 27 cc. should be added to the volume to make up for the space occupied by the ferric hydroxide from grm. of pyrites containing 42 per cent Fe. For practical purposes an allowance of from a quarter to one-third cc. will answer quite well.