INDUSTRIAL A N D ENGINEERING CHEMISTRY
526
Substituting the Hildebrand equation once more, we can obtain the value of K in terms’of temperature. K = TA - or AT’ TB T’B
The Duhring modified relation now becomes T‘A - TA
=
- TB)
where T‘A = any temperature of the substance in question = temperature at which a reference substance has the same mol vapor concentration as the substance in question has a t T’A T A and Tn = any other temperatures a t which the two bstances have the same mol vapor con-
This means that if one knows the temperature-mol vapoc concentration curve for one liquid and one point on a similar curve for another liquid, the entire curve for the latter can be calculated. reasonable to believe, however,
Vol. 15, No. 5
that the accuracy of the results can be improved if the reference liquid is the same type of compound as the liquid the curve of which is to be determined. If vapor pressure is desired rather than mol vapor concentration, the gas laws may be assumed to hold and the pressure calculated, especially if the vapor pressures are below atmospheric. To test out the practical value of this modified relation, a few curves have been drawn from data given by Landolt and Bornsteiri, and various points calculated. The data used were vapor pressures and a value P/T was calculated (assuming the gas laws held) to represent the mol vapor concentration. I n Fig. 1 the benzol curve was taken as the reference, and the points along the hexane curve were calculated. I n Fig. 2 the curve for ethyl acetate was taken as the reference and the points on the propyl acetate, ethyl propionate, methyl acetate, and ethyl formate curves were calculated. Realiaing that the data used were not real mol vapor concentrations and considering the probable accuracy of exDerimental work, the deviations are extremely small.
Analysis of Fluorspar‘ By C. E. Gifford THE!AMERICAN ROLLINGMILL Co , MIDDLETOWN, OHIO
HERE is often occasion for the complete analysis of fluorspar (calcium fluoride), especially when the material is bought upon a specification basis, or if the presence. of any undesirable impurities, such as lead, zinc, or barium, is suspected. The published methods are found to be not only unsatisfaotory and incomplete, but, misleading in many of the statements given. For instance, D dtel’s method,* as published in Scott’s “Methods of Chemical Analysis,” states that the loss in weight found upon burning the residue insoluble in 10 per cent acetic acid less 0.0015 g. (the amount of calcium fluoride soluble in acetic acid under the condition namedl is to be reported as calcium carbonate. The‘ huthor, however, finds that the soluble material a t this point may contain the salts of iron, aluminium, calcium, and magnesium in varying amounts, depending upon the material being analyzed. No provision has been made in Dr. Bidtel’s method for the determination of objectionable constituents, such as lead, zinc, or barium, which elements are encountered in many grades of fluorspar. Martin3 states that the fluoride may be converted to the sulfate by gentle heating with sulfuric acid. It is found, however, that in commercial samples considerably more drastic treatment is necessary for the complete conversion of the fluoride t o sulfate, a t least two digestions with a strong acid often being required. The following method, which can be used for complete analysis on a single sample, has therefore been developed, and has been used with very satisfactory results in the laboratories of the American Rolling Mill Company:
T
Place 1 g. of the finely powdered sample in a small Erlenmeyer flask, then add 10 cc. of a 10 per cent solution of acetic acid. Insert a short-stemmed funnel in the mouth of the Erlenmeyer and heat for an hour on a water bath. The carbonates of calcium and magnesium as well as a portion of the oxides of iron and aluminium are decomposed and dissolved as the soluble acetates, but silica and the fluorides are only slightly affected. Filter the solution through an ashless paper and wash thoroughly with warm water. Reserve the residue. Received January 11, 1923. THISJOURNAL, 4 (1912),201 8 I b i d . . 1 (1909),462. 1
2
Acidify the acetic acid extract with sulfuric acid and evaporate in order t o convert the calcium fluoride to calcium sulfate. Dissolve in a small amount of hydrochloric acid and determine the oxides of iron and aluminium, lime, and magnesia in the solution. From the lime obtained subtract 0.0011 g. CaO, the equivalent of the calcium fluoride soluble under the above treatment. Calculate the lime and magnesia and report as the carbonates. Should the presence of lead be suspected, i t may be separated by hydrogen sulfide previous to the precipitation of iron and alumina. Ignite the residue obtained in the treatment with acetic acid, and weigh. Add hydrofluoric acid to the residue in the crucible and evaporate very carefully to dryness, best on a hot plate. Repeat the addition of hydrofluoric acid, and again evaporate to dryness. Heat to dull redness, cool, and reweigh. Report the loss in weight as silica. To the residue add 5 cc. of sulfuric acid and heat to after evaporation of the acid. Repeat this addition and evaporation to insure the complete conversion of the fluorides to sulfates, upon which the success of the analysis depends. Great care m u s t be taken at this point to insure no loss from popping. Place the cooled crucible containing the sulfates in a 200-cc. beaker and boil in water acidified with hydrochloric acid and containing 0.5 per cent sulfuric acid to effect the solution of the sulfates. Any undissolved residue shows the presence of barium, which should be filtered off, washed, ignited, and weighed as barium sulfate. As a precaution, treat this residue with sulfuric acid and ignite. Any change in weight would show incomplete conversion of the calcium fluoride. Neutralize the filtrate with ammonia, then acidify with 5 cc. of hydrochloric acid, and treat with hydrogen sulfide. Lead sulfide, if present, should be filtered off and determined in any convenient manner. Report as lead sulfide. Boil the filtrate to remove any hydrogen sulfide before oxidizing with potassium chlorate in the determination of iron oxide and alumina. After oxidizing, make an ammoniacal precipitation of the iron and alumina. Filter and determine as in the limestone determination. The ammoniacal filtrate may be treated with hydrogen sulfide a t this point to see if there is zinc present, which, if found, can be determined by any regular method. Finally, determine the lime and magnesia in their order and calculate to fluorides. To the lime obtained a t this point add 0.0011 g. previously subtracted in the determination of the calcium carbonate. It is well to dilute to a known quantity and take an aliquot part for the determination of calcium. Any iron, aluminium, zinc, or lead compounds obtained in the first part should be added to those found in the second portion for totals. Sulfur may be determined in the regular manner with precautions as giyeti. in Martin’s m e t h ~ d . ~
