over a Meker burner. The mass does not fuse to a liquid but sinters together."

Removal of Silica. To the cold crucible add a little water and allow to stand for a few minutes. If the sintered mass can now be detached from the crucible, transfer it to a casserole and wash out the crucible, first with hot water and then with 6 molar hydrochloric acid. Wash off the crucible cover in the same manner. Make the solution in the casserole acid with 6 molar hydrochloric acid and evaporate it to dryness on the water-bath. Place the casserole and residue in an oven set at 115°-120° for 30-40 minutes in order to dehydrate the silica. Any salts of the iron and aluminum groups will be more or less in the basic form, and in order to obtain their solution it is necessary to treat the dried residue with 20-25 c.c. of 6 molar hydrochloric acid and digest on the steam bath for 10-20 minutes, and then, but not before, dilute with 80-100 c.c. of water prior to filtration. The filtration should be conducted through filter paper and the siliceous residue washed with 0.1 molar hydrochloric acid until free of all salts (test for Fe+++ by 0.1 M KCNS). If silica is to be determined, proceed as directed in § 374.

Removal of Iron, Aluminum, Phosphorus, etc. To the filtrate obtained above a few drops of bromine are added and the solution boiled until all the bromine is expelled. Sufficient hydrochloric acid is now added so that magnesium will not precipitate when the solution is made alkaline with ammonium hydroxide. A few drops of methyl red solution are now added and ammonium hydroxide added, first concentrated and finally dilute, until the solution turns yellow. The solution is now brought to the boiling point and kept there for a few minutes. The precipitate is al

There are two other methods for the decomposition of limestones which are as follows: Method No. 2. Ignite one gram of the rock powder in a platinum crucible, gently at first with the Bunsen burner and finally at bright red heat in the Meker burner. Decompose with HCl as described above in the text under the caption "Removal of Silica." This method with the one given above can be used on typical limestones and substances of like composition such as cements, which are low in silica, iron and alumina and high in calcium. Method No. 3. This method is employed when the insoluble residue is to be analyzed separately. Treat one gram of the rock powder with 2 molar hydrochloric acid in a covered beaker until effervescence ceases, warm if necessary. Filter and wash. Ignite the filter and residue in a platinum crucible and fuse with sodium carbonate in the usual manner. Decompose the fusion with HCl and analyze in the same way as the main portion of the sample obtained by the first treatment with HCl.

lowed to settle, then filtered 10 and finally washed two or three times with hot water. The filtrate and washings are set aside to be subsequently joined with the filtrate and washings from the reprecipitation of the hydroxides. The filter paper and its contents are transferred to the beaker in which the first precipitation was made and a little 12 molar hydrochloric acid added. The paper is broken up into a pulp by means of a stirring rod. A little water is added, the beaker warmed and turned so that the solution dissolves any hydroxide that may have adhered to the walls. After the precipitate has dissolved, the solution is diluted to about 300 c.c. and heated to boiling. The combined hydroxides are reprecipitated in the same manner as before and washed with 0.01 molar ammonium hydroxide solution. It is important to add the wash solution in such a manner that the precipitate will be thoroughly stirred from the bottom of the filter paper. After washing the precipitate and filter paper free from chloride, reject the precipitate and the filter paper" and combine the filtrate and washings with those obtained in the previous precipitation. This combined solution now contains all the calcium and magnesium.

Determination of Calcium. The filtrate from the iron and aluminum hydroxide precipitation, after concentration to a volume of about 200 c.c., is heated to boiling and treated with an amount of 0.25 molar ammonium oxalate solution equivalent to all the calcium and magnesium present plus an excess of ten percent of this amount. Ammonium hydroxide is added in slight excess and the solution allowed to stand on the hot-plate or on a water-bath for about an hour or until the crystals are large enough to be retained by the filter paper. Decant the solution through a small filter paper leaving as much of the precipitate as possible in the beaker. Place the beaker containing the precipitate under the funnel and pour a little hot 3 molar hydrochloric acid through the filter to dissolve any precipitate that may have been transferred in decanting the solution. Wash the paper with hot water until the washings show no test for

10 Too long a time must not elapse between the precipitation and filtration, as otherwise calcium and magnesium carbonates might precipitate due to absorption of carbon dioxide from the air by the ammoniacal solution.

11 If the combined oxides of iron and aluminum are to be determined, see § 375.

calcium, then heat the calcium oxalate precipitate with enough 3 molar hydrocholoric acid to effect its solution. Dilute to about 150 c.c. and add 3 molar ammonium hydroxide slowly, drop by drop, until the solution is alkaline; then add a few c.c. of 0.25 molar ammonium oxalate solution. Digest and filter as in the first precipitation, washing the precipitate with hot water and adding a drop of ammonium oxalate solution each time the funnel is filled. Combine the filtrates containing the magnesium and concentrate them by evaporation to a volume of about 100 c.c. Transfer the filter paper and precipitate to a platinum crucible that has been weighed with its cover. Ignite in the usual way, and finally over the Meker burner. Cool in a desiccator and weigh the calcium oxide quickly and with the cover in place so that the amounts of moisture and carbon dioxide absorbed shall be negligible. Ignite to constant weight, and calculate the percentage of calcium as CaO.

Determination of Magnesium. Assuming that the combined filtrate containing the magnesium has been concentrated to a volume of about 100 c.c., allow to cool to room temperature, 12 and add about 5 c.c. of 3 molar hydrochloric acid and then 3 molar ammonium hydroxide until slightly alkaline. If a precipitate of magnesium hydroxide forms, repeat the addition of hydrochloric acid and ammonium hydroxide until the solution can be made ammoniacal without the formation of a precipitate. Make the ammoniacal solution acid to methyl orange with 3 molar hydrochloric acid, heat to boiling and add 0.25 molar di-sodium phosphate solution until present in forty percent excess over the amount theoretically demanded by the stoichiometrical equation. Now add dropwise from a burette 15 molar ammonium hydroxide until the solution is just ammoniacal, and then enough more to make the final concentration of ammonium hydroxide 1.5 molar. Allow the solution to stand at room temperature for four hours, and complete the determination as detailed in § 191. Calculate the percentage of magnesium as MgO.

12 If ammonium salts are present in such quantity as to crystallize from the solution on cooling, they may be gotten rid of by adding 20-30 c.c. of 16 molar nitric acid from time to time during the evaporation and continuing the evaporation to near dryness.

207.

CHAPTER XIII

DETERMINATION OF ZINC GRAVIMETRICALLY AND
VOLUMETRICALLY

There are several methods in use for the determination of zinc. It can be determined gravimetrically by precipitating it as zinc sulphide, converting the zinc sulphide to zinc sulphate and weighing the latter; or by precipitating it as zinc ammonium phosphate monohydrate, converting the monohydrate to the anhydrous salt by drying it at 100°-105° and weighing the anhydrous salt. It can also be determined volumetrically by precipitating it as zinc potassium ferrocyanide, Zn,K2[Fe(CN)6]2, using uranyl acetate as an outside indicator.

Table 8

Precipitation of zinc as

Allows presence of

Requires absence of

ZnS

Iron, aluminum, nickel and Silicic acid, cobalt and those manganese; members of the metals whose sulphides have alkali and alkaline earth groups a smaller solubility product than that of zinc sulphide, i.e., members of the copper, tin and silver groups

ZnNH,PO4 H2O Only ammonium and sodium Silicic acid, potassium and

salts and the latter in
quantities only

Zn3K2[Fe(CN)6]2 Aluminum and lead in

small most every metal of the alkaline earth and the other groups because of the insolubility of their phosphates

small Silicic acid and practically

quantities and members of the all metals of the alkaline

alkali groups

earth and other groups (except as noted) because of the insolubility of their ferrocyanides

1 The precise determination of zinc is not an easy matter. The reader is referred to § 58, which gives a summary of the report of the Committee on Uniformity in Technical Analysis, J. A. C. S. 26, 1648 (1904). Their report showed results from forty-two analysts ranging from 12.20% to 39.22% on a sample of zinc ore containing 18.16% of zinc.

With respect to the precision desired, the first method is the most precise, the second less so, and the third least of all. As regards the amounts of zinc that can be handled by the respective methods, the first can be used for quantities ranging from 2 to 200 mg. figured as zinc, the second for quantities from 20 to 200 mg., while the third is at its best advantage for quantities between 100 and 200 mg. With reference to the presence of interfering substances, the table on p. 245 summarizes the conditions that must be observed.

208. Whichever of the foregoing methods is to be used for the determination of zinc, the following sequence gives the general order of procedure that must be observed in the removal of the interfering substances: (1) silicic acid, (2) copper, (3) remaining metals of the copper and tin groups, (4) members of the iron and alkaline earth groups. With respect to the handling of these several groups, particularly the iron group, it is to be remarked that many shortened methods of separation are possible, depending on the absence of certain of the interfering substances. For such methods the student is referred to works on technical analysis, such as Griffin, Lord and Demorest, etc., loc. cit., § 13. The scheme of procedure presented in the following paragraphs 2 is offered not alone because of its general applicability but because it isolates the zinc in the form of zinc sulphide, thereby allowing the student the option to complete the determination of the zinc by whichever of the three methods he or the instructor may elect.

Silicic acid will generally only be encountered in the case of ores. It is best gotten rid of by adding about 20 c.c. of 9 molar sulphuric acid to the solution of the sample and evaporating same in a casserole until copious fumes of sulphur trioxide have come off for five minutes or so. The solution is allowed to cool, carefully diluted with 40-50 c.c. water, then heated to dissolve any basic salts. It is next filtered while hot and the filter paper washed with hot water. The filtrate can be used at once for the removal of copper if this element is present.

Unfortunately copper must not be precipitated as copper sulphide in the presence of zinc ion because copper sulphide will invariably drag down zinc even though the solubility product of

2 Essentially this scheme is that due to Waring, J. A. C. S. 29, 265 (1907).