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When the percentage of chromic oxide in an ore or mineral is more than about 0.2 per cent, and vanadium has not also to be estimated, much time may be spared by at once taking the color of the original extract from the sodium carbonate fusion after insuring complete reduction and removal of manganese, and perhaps concentrating. But if the chromium is much less than this amount, and especially if several grams of powder have been operated on, it becomes difficult or impossible to obtain a filtrate of sufficiently small bulk to show a decided color. Therefore, in such cases, and when vanadium is likewise to be estimated, it is necessary to precipitate as above with mercurous nitrate in order to eventually have a small bulk of sufficiently colored solution. Even then, for very minute amounts, it is necessary to use Nessler tubes exactly as in ammonia estimations,
If niter has been used in the fusion and the crucible has been attacked by it, a yellow color of the filtrate may be due to dissolved platinum, but neither the proportion of niter nor the temperature of the blast should ever be high enough to permit the crucible to be attacked.
THE COLORIMETRIC APPARATUS AND ITS USE.
The glasses employed were of approximately square section, about 12 centimeters high and 3.3 centimeters inside measurement, with exactly parallel sides, and of course as nearly alike as they could be obtained. One pair of opposite sides of each should be blackened. With glasses of the thickness mentioned it is generally advisable to use from 5 to 10 cubic centimeters of standard at a time and to so regulate the strength of the solution to be tested that it shall contain more than 2 milligrams chromic oxide in 100 cubic centimeters, which is about the limit of distinct visibility in a thickness of 3.3 centimeters.
In order to exclude the effect of side light in this and other similar methods (titanium, for instance), it is very convenient to have a simple light box that can be easily held in one hand, about 35 centimeter long and 13 to 14 centimeter square, painted black inside and out and with one end closed by a piece of ground glass, the other open. For a space equal to the width of the glasses the cover is removed at the top next the glass end to permit of the insertion of the glasses, side by side, in such a way that no light shall penetrate around their sides or between them. A stiffly sliding, black cardboard shutter is movable up and down immediately back of the glasses, so that all light can be cut off except that which comes through the liquid.
Precautions of this kind are necessary if accurate results are to be counted on. Except for mere traces this simple combination of glasses and darkened box insures greater accuracy and rapidity of work than Nessler tubes and is preferable likewise, so far as my own experience goes, to expensive instruments like the colorimeter of Soleil-Duboscq, etc. The glasses should, of course, have exactly parallel sides and equal diameters, though they may perhaps be made with advantage rather narrow in one direction to permit of using smaller bulks of liquid.
In making the color comparisons the box is best held close to a window, so as to get a full, strong light. The condition of the light seems to make an appreciable difference in the accuracy of the comparisons, all of which were made by daylight.
ACCURACY OF RESULTS.
Especially for comparatively and very small amounts of chromium the method gives exact results, better than can ordinarily be hoped for from any gravimetric method, considering the fact that as a rule other substances are present which it is extremely difficult to remove completely.
A few comparisons between colorimetric and gravimetric determinations of chromium in a few rocks are here given to show the order of agreement, the former having been made several months and even years after the latter:
.083 .013 .0086
The outcome was somewhat of a surprise, for it was scarcely to be expected that the long and laborious gravimetric separations should have resulted so well as they are shown to have done. It should be mentioned that for the gravimetric tests but 1 or 2 grams at most were used, which accounts for the reported absence of chromium in two instances, this report being based on the lack of color in the aqueous extract of the alkaline fusion after removal of manganese.
No experiments have been made with high percentages, but the tables give with certainty the degree of accuracy even then attainable. Whether the chromic oxide to be estimated is 0.1 per cent or 100 per cent, the percentage result is the same, provided the dilution is alike in each case. For instance, if 99.5 per cent is found in a total solution of 100 cubic centimeters containing 0.01 gram chromic oxide, the same percentage holds if the total volume were 10 liters holding 1 gram chromic oxide.
It is probably inadvisable to increase the strength of the standard much above that of No. 2 above, containing 0.2 milligram chromic oxide in form of potassium chromate to the cubic centimeter.
No tests have been made with other alkaline solutions than sodium carbonate, in order to ascertain if there is a difference in the color intensities for like dilution, nor is it probably important to do so.
VOLUMETRIC ESTIMATION OF VANADIUM IN PRESENCE OF SMALL AMOUNTS OF
CHROMIUM, WITH SPECIAL REFERENCE TO THE ANALYSIS OF ROCKS AND ORES.
By W. F. HILLEBRAND.
OUTLINE AND LIMITATIONS OF THE METHOD.
When chromium and vanadium occur together and the former has been estimated colorimetrically as detailed in the preceding paper, the vanadium can in many instances be at once estimated without separating from the chromium by the well-known method of titration with potassium permanganate in sulphuric acid solution after reduction by sulphurous acid and expulsion of all excess of the latter.
The application of the method is subject to one limitation-the chromium must not be present above a certain moderate amount. This limitation is due to the considerable amount of permanganate required to produce a clear transition tint when titrating in a hot solution, as is necessary with vanadium. In a cold solution of chromic sulphate much less permanganate is needed to produce the peculiar blackish tint without a shade of green, which affords a sure indication of excess of permanganate, but in a hot and especially a boiling solution the oxidation of the chromium' itself takes place so rapidly that a very large excess of the reagent may be added before a pronounced end-reaction is obtained. Nevertheless, as will be shown, quite satisfactory determinations of as little as 1 or 2 milligrams of vanadium pentoxide can be made in presence of as much as 30 milligrams of chromic oxide.
Inasmuch as chromium is almost never a prominent constituent of the clays, coals, iron ores, and rocks in which vanadium has been thus far reported, the method promises to be of great advantage in rock and ore analysis, where these elements probably frequently occur together, generally with phosphorus and sometimes arsenic. It is not directly applicable in presence of molybdenum, and arsenic must first be removed by hydrogen sulphide if present.
Storer, through Gmelin-Kraut, Vol. II, pt, 2, p. 290.
TESTING THE METHOD,
A solution of sodium vanadate was prepared by fusing 0.937 gram of pure vanadium pentoxide with sodium carbonate and diluting to 1 liter. Each cubic centimeter contained 0.000937 gram vanadium pentoxide. Of this, sufficient amounts were mixed with varying volumes of a standard solution of potassium chromate. After in some cases determining chromium colorimetrically, sulphuric acid was added, sulphur dioxide gas' introduced for a few moments, and the excess of the latter expelled by boiling in a rapid current of carbon dioxide, without which its complete expulsion is difficult. Wben cooled to from 700-800 C. the titration was made with very dilute solution of permanganate corresponding to about 1 milligram vanadium pentoxide to the cubic centimeter as calculated from its equivalent in iron. By repeating the reduction and boiling, any number of check tests can be rapidly made in the same solution. The solutions ranged in bulk from 50 to 200 cubic centimeters according to the amounts of chromium added.
With the larger chromium contents the differences in the quantity of permanganate used, according as titration was effected at practically boiling temperature or a somewhat reduced one, were very apparent. In one case, at nearly boiling heat, 10 milligrams of chromic oxide were largely oxidized to chromium trioxide, as shown by the change of color from dark green to yellow. Even at 700_800 with this amount of chromium the results for vanadium will be too high unless the precaution is taken to ascertain the needed correction by adding permanganate to an equal bulk of hot chromic sulphate solution containing approximately the same amount of chromium. When this is done, however, the results are satisfactory even when as much as 30 milligrams of chromic oxide are present and only 1 or 2 milligrams of vanadium pentoxide.
The following tables contain the results of a considerable number of tests, those in Table II being tabulated separately in order to show the degree of accuracy attainable with a large excess of chromium by applying the correction above mentioned and also the amount of this correction.
The direct use of a solution of sulphur dioxide or of an alkaline sulphite is inadmissible unless these have been freshly prepared, since after a lapse of time they contain other oxidizable bodies than sulphurous acid or a sulphite. The sulphur dioxide is best obtained as wanted by heating a Nask containing a solution of sulphur dioxide, or of a sulphite to which sulphuric acid has been added