noun d-Table Discussion
SILICATE ANALYSIS Report of round-table discussion held by Division of Analytical Chemistry at 119th Meeting, ACS, Boston, Mass., April 1951 Moderator: ROBERT M. FOWLER, Union Carbide & Carbon Research Laboratories, Znc., iyiagara Fulls, ,V. Y . Panel: WALLACE RI. HAZEL, Norton Co., Niagara Falls, N . Y . J . PAUL HIGHFILL, New Brunswick Laboratory, Atomic Energy Commission, New Brunswick, R: J. R. E. STEVENS, U . S. Geological Survey, Washington, D . C.
N HIS introductory remarks, Fowler stated that although
and cast irons Tvith very good results. Aluminum, titanium, zirconium, and sulfuric acid interfere. The interference of aluminum is the Jvorst, although this can be minimized by working in strong hydrofluoric acid where IZzAIFsis most soluble. Iron can be complexed with oxalic acid. When applicable, it is a very rapid method, as it involves only the solution of the sample, filtration, and titration of the precipitated ICzSiF8 lvith standard sodium hydroxide.
silicate analysis was far too broad a subject to cover in a single afternoon, a t least four phases of it would be discussed by the speakers in a Tvay that he hoped would promote discussion. DETERIIINATION OF SILIC4
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In the first paper, the chairman reviewed the determination of silica. He stated that there had been few recent papers on this determination, although present methods left much to be deeired, as they were long and tedious, and had many sources of error. Fortunately, the errors are both positive and negative. The methods for the determination of silica have not changed much in 50 years, the exception being the increasing use of perchloric acid, which has many advantages, As perchlorates are very soluble salts, dehydration is smoother, there is less tendency t o bump, and the salts are much easier to dissolve after the dehydration. The dehydration is more rapid than with hydrochloric acid. Perchloric acid has disadvantages, too; organic matter must be destroyed, large precipitates of silica tends to decrepitate o n ignition, although this can be prevented by a few washes with sulfuric acid, and finally special hoods must be used or other precautions must be taken to prevent the accumulation of residues of perchlorates and organic dusts in the hoods and duct work If calcium, chromium, barium, lead, and other elements that form sparingly soluble sulfates are absent, a single dehydration with sulfuric acid nil1 usually yield more of the silica present, but sulfuric acid dehydrations tend to bump badly. Often this tendency can be reduced by the addition of oxalic acid. K i t h major amounts of silica, multiple dehydrations are required-at least two are standard practice. There have been a number of efforts to flocculate silica with gelatin and similar colloids. Probably the best procedure for gelatin is t h a t of Jenkins and Webb (8), who dissolved the fusion in hydrochloric acid, dehydrated with sulfuric acid, took up in 10% hydrochloric acid, and added 2% gelatin. They show results that compare favorably with two dehydrations. Silica flocculated with gelatin is much cleaner and easier to wash free of other salts. 8-Quinolinol forms an addition compound with silica, but this method has never been very popular. Methods based on the formation of silicomolybdates are widely used for small amounts, but are veryempirical I n some cases, they can be used with 10 to 20% of silica with fair precision. Such a method involves fusion with a carbonate-borax flux, solution of the fusion in water, careful adjustment of the pH to 1.3-1.4,addition of ammonium molybdate, then a reducing agent, usually a mixture of sodium sulfate and l-amino-2-naphthol-4-sulfonic acid, and measurement of the absorption a t 820 mp. A very old method, that is not widely used but has considerable value, is precipitation of the silica as K2SiF6, followed by titration with sodium hydroxide. This method was first 'described by Stolba in 1865. While it is a standard method in the sodium silicate industry, it is not usually found in standard texts, although recently German investigators have applied it to steels
DECOhIPOSITIOV OF REFR4CTORY SILIC4TES
Hazel discussed, first, the decomposition of refractory silicates. In his experience, the best flux for fusing these materials is a mixture of equal parts of sodium carbonate and sodium borate. This approaches a universal flux, and samples as coarse as 24mesh can be decomposed in not over 20 minutes. The sodium borate must be specially dried to prevent frothing.
r.,..
Figure 1. Flow Diagram for Rapid Analyses of Silicate Rocks The boron may be volatilized as the methyl ester, although the speaker stated that in the laboratory they did not remove boron, yet had found no interference with it in the RlOa separation. The flux is excellent for ores, as many contain fluorine which would cause losses during dehydration if borate was not present. With proper washing, the silica will not be contaminated with boron. This flux is also useful for zirconia and zirconium ores, yielding a much more readily soluble melt than either salt alone. Another useful metbod for decomposing refractory materials is heating in a sealed tube with hydrochloric acid (3). Fused alumina, alumina refractories, beryllium oxide, and many ores and minerals can be decomposed by this treatment. The procedure is very convenient for special analyses such as nitrogen in refractory nitrides or the state of oxidation of oxides, as the decomposition can be effected in an inert atmosphere. Spectrographic methods have considerable utility in refractory analysis. A special source unit giving large currents is employed ( 1 ) . The direct current arc is used for a number of determinations,
196
V O L U M E 24, NO. 1, J A N U A R Y 1 9 5 2 such as alkali in refractories, and iron, aluminum, calcium, and magnesium in refractories and abrasives. For most analyses, the finely ground sample is mixed with 3 to 4 parts of graphite and pressed into a pellet, which is placed in the crater of a special pedestal-type graphite electrode. This type of electrode is very useful for refractory materials because it prevents the heat from being carried away too rapidly. In general, a spectrograph does not replace chemists but does relieve the chemist of many tedious hours of work and results in a more efficient relation between the chemist and his employer. ANALYSIS OF BERYLLIUAI ORES
I n his discussion of the analysis of beryllium ores, Highfill described the decomposition of the ore with sodium carbonate or sodium carbonak-sodium borate although. in general, sodium carbonate is all that is required. -1ratio of 4 to 5 parts sodium carbonate to 1 of ore is satisfactory and decomposition is usually complete in 30 minutes a t 900" C. The fusion is taken up in hydrochloric or perchloric acid and double dehydrations are made for silica. Perchloric acid is faster and gives much purer silica. T h e silica is volatilized with hydrofluoric-sulfuric acid and the residue is recovered by fusion with sodium pyrosulfate. The hydrogen sulfide group is separated, and if phosphorus is present, it is separated with ammonium molybdate. The Rz03 asides are separated by a double precipitation with ammonium hydroxide a t the bromcresol purple end point, Then iron, titanium, and aluminum are separated with 8-quinolinol or if aluminum is required, iron and titanium are separated tvith cupferron a n d aluminum is separated from beryllium by precipitation with an acetic arid solution of 8-quiriolinol from an ammonium acetate solution. The speaker described a number of tests on this separation procedure, IThich shoxed that results for alumina tended t o be high by about 6 parts in a thousand TThen the precipitate was dried a t 130" to 140' C. and neighed as such. In a number of tests, no beryllium oxide !vas found in the aluminum precipitates. The beryllium oxide is determined in the filtrate by precipitation with ammonium hydroxide a t the phenol red end point. It is not necessary to remove the excess 8-quinolinol. Recoveries were good and contaminants Tvere less than 0.2 mg. of alumina and a trace of ferric oxide, but no titania or zirconia.
197 as written were faulty, the variation must be due to the varying skill of the analysts and the care taken by them in the lengthy procedures used. This was one of the incentives for starting a program on rapid procedures a t the survey; also, procedures had to be developed to meet a rapidly expanding geological program. The methods were designed to give reliable estimates \Then only estimates were needed, but the accuracy approached that of convkntional methods while requiring only about a sixth of the time to perform. The methods are shown in outline form in Figure 1. Quantitative separation of precipitates is entirely avoided, most of the measurements are made photometrically, and procedures were planned to involve a minimum of manipulation. As the chief source of error in rock analysis is in the separations, and this takes most of the time, separations were avoided as far as possible. Each constituent is determined independently-for example, silica and alumina after fusion with sodium hydroxide in a nickel crucible and solution in hydrochloric acid-silica by the molybdenum blue reaction and alumina with ferron. In a second sample decomposed by hydrofluoric and sulfuric acid, iron, titanium, manganese, phosphorus? and magnesium are determined photometrically. Calcium and calcium plus magnesium are determined by titration with disodium dihydrogen ethylenediamine tetraacetate (Versene) and the alkalies with the flame photometer. The Rz03 group is removed with ammonium hydroxide-ammonium carbonate before the alkali determinations. For a rapid determination of total iron, measuring the color in hydrochloric acid is reliable even in the presence of considerable 1)hosphate. Rapid ferrous oxide, carbon dioxide, and combined water determinations are still being sought. Stevens showed a number of comparisons of results by conventional and rapid procedures on typical rocks. These showed that, although the rapid methods did not have the inherent accuracy of the conventional procedures, the results are sufficiently accurate to meet the usual requirements of this a o r k and have a high degree of dependability because of thr simplicity of the operations involved. I n the discussion a number of questions were raised and answered by the speakers about different phases of silicate analysis.
AN4LYSIS O F GRANITE AND DIABASE
LITERATURE CITED
Stevens presented some results of a cooperative study of the analysis of samples of granite and diabase. After the results from some thirty laboratories were compared, the composition of these samples is still not known. Results for silica varied about 2%, while those for alumina varied about 3%. 4 s fer? of the methods
(1) Fetterley, G. H., and Hazel, Wm., J . Optical SOC.Am.. 40, i6-9 (1950).
(2) Jenkins, M. H., m d Webb, J. A . V.. Analyst, 75, 481-5 (1950). (3) Wichers, E., Schlecht, TV. G., and Gordon, C. L..J . Research S a t l . Bur. Standards, 33,363-81 (1944). RECEIVED October 25, 1951.
