CHAPTER XXIII

INTRODUCTION TO SYSTEMATIC ANALYSIS

ANALYSIS OF SILICATE ROCK.

DETERMINATION OF POTASSIUM AND SODIUM

369. While it is general practice in many analyses where the separation of constituent elements is unnecessary or can be avoided, to determine the various constituents on separate portions of the sample, there are, however, cases where it is necessary to separate and determine many of the constituents on one weighed portion. A conspicuous example in this regard is the analysis of earthy material commonly known as rock or of manufactured materials which are similar to rock in composition, namely, cements and slags.

370. A rock may be defined as a more or less complex mixture of the different mineral species which go to make up the earth's crust. Rocks may be classified according to their composition as silicate, carbonate, phosphate, and sulphate or according to their formation as igneous and sedimentary, the latter comprising the sandstones, cherts and sinters, carbonates, slates and shales, and clays and soils. The average composition of rocks is given in the table on page 408, the data of which are taken from Clarke's The Data of Geochemistry, pp. 28-29.1

371. — Of the foregoing elements it is customary to have the student determine SiO2, Al2O3, total iron as Fe2O3, MgO, CaO, Na2O, K2O, H2O at 110°, and possibly TiO2, MnO, and P2O5, SO that we shall limit our discussion to these constituents and refer the student for all further detail to the primary authority on rock analysis, W. F. Hillebrand, The Analysis of Silicate and Carbonate Rocks (§ 13). At the very outset it cannot be too strongly emphasized that a complete and accurate rock analysis requires extreme skill and care on the part of the analyst. The mere detail is intricate and taxes one's patience to the utmost. The

1 Frank W. Clarke, The Data of Geochemistry. Bulletin 695, U. S. Geological Survey, Washington, 1920, 4th ed.

beginner is warned that for even approximate determinations of the comparatively few constituents mentioned above, the analysis will be found trying and will require his greatest care. The example that we shall select will be the partial analysis of a silicate rock, because this is the most difficult of the various kinds of

rocks to analyze and the methods that apply to it apply with but slight modification to the other kinds of rocks as well as to slags and cements. In the prosecution of a rock analysis, evaporations should be made only in platinum or less preferably porcelain vessels, while glassware should be avoided because of its solubility. The purity of reagents is an all-important matter, particularly the purity of those which are chiefly used, namely, sodium carbonate, ammonium hydroxide, ammonium chloride, and hydrochloric acid.

Partial Analysis of Silicate Rock

372. Exercise No. 36.- A material crushed so as to pass a sieve 30 meshes to the centimeter will serve for the analysis. The powder should be air-dried. The partial analysis for the constituents named in the preceding paragraph requires for the beginner in the neighborhood of ninety hours' time for completion. The results should add up to 99.75%-100.5%.

H2O at 110°. Weigh accurately into a previously weighed platinum or porcelain crucible about two grams of the rock powder and dry it for two hours in the electric oven at 110°; allow to cool in the desiccator and weigh. Repeat the heating and weighing until the loss in weight is constant. Report the loss in weight as water at 105°-110°.

Note. The loss in weight at 105°-110° is due mainly to hygroscopic water and also essential water. In some cases constant weight is difficult to obtain due to the slow loss of essential water. If extreme accuracy is not sought, the determination may be stopped when the weights of successive weighings differ by not more than 0.2-0.3 mg. For the determination of water above 110°, see W. F. Hillebrand, loc. cit., § 13, pp. 64–89.

373. Decomposition of the Rock. Make an intimate mixture of one gram of rock powder with 5 or 6 grams of anhydrous sodium carbonate by stirring carefully in a weighed platinum crucible with a short glass stirring rod. Remove any rock powder from the rod by brushing with a camel's hair brush. Cover the crucible and fuse, at first very carefully over a low Bunsen flame, then gradually over the full Bunsen flame, and finally over the full flame of a Meker burner until quiet fusion results and decomposition is complete. While the melt is still liquid, grasp the crucible with a pair of tongs and rotate it so as to spread the molten mass around the inner walls of the crucible, and allow it to cool.

Note 1. Sodium carbonate is used instead of the more readily fusible equimolecular mixture of sodium and potassium carbonates because the higher melting-point of the sodium carbonate is a distinct advantage in the effective decomposition of some of the refractory rock constituents.

Note 2. The mixture is gradually heated with the Bunsen burner because carbon dioxide is evolved as a result of the decomposition of the rock. If a strong flame were used the action might become violent and the

evolution of carbon dioxide so rapid as to throw out of the crucible much of its contents. The Meker burner or in special cases the blast lamp is employed to complete the decomposition, which is evidenced by the further evolution of carbon dioxide.

Note 3. At the end of the fusion the result should appear as a viscous liquid occasionally clear though in general more or less turbid.

Note 4. The alkaline earth metals, iron, aluminum, magnesium and manganese are not present in the melt as carbonates, and manganese may not necessarily be present as the manganate.

Note 5. The fusion must not be carried out in a muffle furnace because the platinum crucible appears to be unduly attacked under this condition. Note 6. Other methods of decomposing rocks are by means of hydrochloric acid under pressure in a sealed tube; by fusion with boric oxide as the flux, and by treatment with hydrofluoric acid. For a discussion of their relative merits, see W. F. Hillebrand, loc. cit., § 13, pp. 89-94.

374. Determination of Silica. To the cold crucible add a little water and allow to stand for a few minutes. If the fused mass can now be detached from the crucible, transfer it to a 500 c.c. 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. If the fused mass cannot be removed from the crucible, place the crucible on its side in the casserole, cover with a watch-glass and add sufficient water to cover the crucible. Allow to digest for some time and then add 6 molar hydrochloric acid in excess and allow to digest until the mass is completely decomposed. When the mass can be separated from the crucible, the latter is removed from the casserole, washed with hot water and any film of silica removed by wiping with a small piece of ashless filter paper. This paper is kept to be added to the main portion of the silica to be obtained later. Place the casserole on a steam-plate and heat. When the liberation of carbon dioxide ceases, remove the watch-glass and wash the solution from its under side into the casserole and continue the evaporation after placing some glass bends around the edge of the cassero e to support the watch-glass. The solution may be further protected from dust by supporting a large inverted funnel over it by means of a ring stand and clamp. Evaporate the solution to dryness on the steam-plate and then place the casserole in an oven at 110°-120°, or on a hot-plate at this tem

perature, for twenty minutes to dehydrate the silica. Moisten the residue with a few c.c. of water, then with 10 c.c. of 12 molar hydrochloric acid, turning the casserole so that the hydrochloric acid comes in contact with all portions of the residue. Add 20-30 c.c. of hot water and heat the casserole on the steam-plate for a few minutes, stirring every now and then until all soluble salts are dissolved; then allow the silica to settle and decant the solution through a 12 cm. filter paper. Add 2-3 c.c. of 12 molar hydrochloric acid to the silica in the casserole, then 10-20 c.c. hot water, decant through the filter and repeat until the addition of hydrochloric acid to the residue in the casserole gives no yellow color due to iron. Transfer the silica to the filter and wash alternately with cold water and cold 3 molar hydrochloric acid, and finally with hot water until all soluble salts are removed from the filter. The thin film of silica which adheres to the casserole need not be removed at this time. The filtrate is returned to the casserole and evaporated to dryness on the steam-plate as before. Treat the residue exactly as before and filter through a 7 cm. filter paper, removing all the silica from the casserole. If the volume of the filtrate is greater than 500 c.c., evaporate it in a casserole so that it will be ready for the operation of § 375 by the time the remaining operations of this paragraph are completed. The last thin film of silica which adheres to the porcelain is removed as much as possible by wiping the inside of the casserole with a small piece of ashless filter paper and adding this paper to the main portion of the silica. Ignite the silica precipitates in a weighed platinum crucible as described in § 48. In the ignition of the silica, in order to dehydrate it completely, it is necessary to heat the crucible to the highest temperature of the Meker burner for at least thirty minutes after all the carbon of the filter paper has been oxidized. After constant weight has been secured, moisten the silica with water, add two drops of 18 molar sulphuric acid and then 10-15 c.c. of hydrofluoric acid. Evaporate to dryness in an air-bath as described in § 40, and ignite the residue over a Bunsen burner. The loss in weight represents the major portion of the silica present in the sample though a small amount still remains to be recovered from the iron and aluminum oxides to be obtained later. The crucible