Part 16 (1/2)
Transfer the moist precipitate to a weighed porcelain or platinum crucible and ignite, using great care to raise the temperature slowly while drying the filter in the crucible, and to insure the ready access of oxygen during the combustion of the filter paper, thus guarding against a possible reduction of the phosphate, which would result in disastrous consequences both to the crucible, if of platinum, and the a.n.a.lysis. Do not raise the temperature above moderate redness until the precipitate is white. (Keep this precaution well in mind.) Ignite finally at the highest temperature of the Tirrill burner, and repeat the heating until the weight is constant.
If the ignited precipitate is persistently discolored by particles of unburned carbon, moisten the ma.s.s with a drop or two of concentrated nitric acid and heat cautiously, finally igniting strongly. The acid will dissolve magnesium pyrophosphate from the surface of the particles of carbon, which will then burn away. Nitric acid also aids as an oxidizing agent in supplying oxygen for the combustion of the carbon.
From the weight of magnesium pyrophosphate (Mg_{2}P_{2}O_{7}) obtained, calculate the phosphoric anhydride (P_{2}O_{5}) in the sample of apat.i.te.
[Note 1: The ionic change involved in the precipitation of the magnesium compound is
PO_{4}^{---} + NH_{4}^{+} + Mg^{++} --> [MgNH_{4}PO_{4}].
The magnesium ammonium phosphate is readily dissolved by acids, even those which are no stronger than acetic acid. This is accounted for by the fact that two of the ions into which phosphoric acid may dissociate, the HPO_{4}^{--} or H_{2}PO_{4}^{-} ions, exhibit the characteristics of very weak acids, in that they show almost no tendency to dissociate further into H^{+} and PO_{4}^{--} ions.
Consequently the ionic changes which occur when the magnesium ammonium phosphate is brought into contact with an acid may be typified by the reaction:
H^{+} + Mg^{++} + NH_{4}^{+} + PO_{4}^{---} --> Mg^{++} + NH_{4}^{+} + HPO_{4}^{--};
that is, the PO_{4}^{--} ions and the H^{+} ions lose their ident.i.ty in the formation of the new ion, HPO_{4}^{--}, and this continues until the magnesium ammonium phosphate is entirely dissolved.]
[Note 2: During ignition the magnesium ammonium phosphate loses ammonia and water and is converted into magnesium pyrophosphate:
2MgNH_{4}PO_{4} --> Mg_{2}P_{2}O_{7} + 2NH_{3} + H_{2}O.
The precautions mentioned on pages 111 and 123 must be observed with great care during the ignition of this precipitate. The danger here lies in a possible reduction of the phosphate by the carbon of the filter paper, or by the ammonia evolved, which may act as a reducing agent. The phosphorus then attacks and injures a platinum crucible, and the determination is valueless.]
a.n.a.lYSIS OF LIMESTONE
Limestones vary widely in composition from a nearly pure marble through the dolomitic limestones, containing varying amounts of magnesium, to the impure varieties, which contain also ferrous and manganous carbonates and siliceous compounds in variable proportions.
Many other minerals may be inclosed in limestones in small quant.i.ties, and an exact qualitative a.n.a.lysis will often show the presence of sulphides or sulphates, phosphates, and t.i.tanates, and the alkali or even the heavy metals. No attempt is made in the following procedures to provide a complete quant.i.tative scheme which would take into account all of these const.i.tuents. Such a scheme for a complete a.n.a.lysis of a limestone may be found in Bulletin No. 700 of the United States Geological Survey. It is a.s.sumed that, for these practice determinations, a limestone is selected which contains only the more common const.i.tuents first enumerated above.
DETERMINATION OF MOISTURE
The determination of the amount of moisture in minerals or ores is often of great importance. Ores which have been exposed to the weather during s.h.i.+pment may have absorbed enough moisture to appreciably affect the results of a.n.a.lysis. Since it is essential that the seller and buyer should make their a.n.a.lyses upon comparable material, it is customary for each a.n.a.lyst to determine the moisture in the sample examined, and then to calculate the percentages of the various const.i.tuents with reference to a sample dried in the air, or at a temperature a little above 100C., which, unless the ore has undergone chemical change because of the wetting, should be the same before and after s.h.i.+pment.
PROCEDURE.--Spread 25 grams of the powdered sample on a weighed watch-gla.s.s; weigh to the nearest 10 milligrams only and heat at 105C.; weigh at intervals of an hour, after cooling in a desiccator, until the loss of weight after an hour's heating does not exceed 10 milligrams. It should be noted that a variation in weight of 10 milligrams in a total weight of 25 grams is no greater relatively than a variation of 0.1 milligram when the sample taken weighs 0.25 gram
DETERMINATION OF THE INSOLUBLE MATTER AND SILICA
PROCEDURE.--Weigh out two portions of the original powdered sample (not the dried sample), of about 5 grams each, into 250 cc.
ca.s.seroles, and cover each with a watch-gla.s.s (Note 1). Pour over the powder 25 cc. of water, and then add 50 cc. of dilute hydrochloric acid (sp. gr. 1.12) in small portions, warming gently, until nothing further appears to dissolve (Note 2). Evaporate to dryness on the water bath. Pour over the residue a mixture of 5 cc. of water and 5 cc. of concentrated hydrochloric acid (sp. gr. 1.2) and again evaporate to dryness, and finally heat for at least an hour at a temperature of 110C. Pour over this residue 50 cc. of dilute hydrochloric acid (one volume acid (sp. gr. 1.12) to five volumes water), and boil for about five minutes; then filter and wash twice with the dilute hydrochloric acid, and then with hot water until free from chlorides. Transfer the filter and contents to a porcelain crucible, dry carefully over a low flame, and ignite to constant weight. The residue represents the insoluble matter and the silica from any soluble silicates (Note 3).
Calculate the combined percentage of these in the limestone.
[Note 1: The relatively large weight (5 grams) taken for a.n.a.lysis insures greater accuracy in the determination of the ingredients which are present in small proportions, and is also more likely to be a representative sample of the material a.n.a.lyzed.]
[Note 2: It is plain that the amount of the insoluble residue and also its character will often depend upon the strength of acid used for solution of the limestone. It cannot, therefore, be regarded as representing any well-defined const.i.tuent, and its determination is essentially empirical.]