The filter contains the tungsten as WO, and the SiO2 more or less contaminated with chromium and iron oxides. Dry the filter and contents, ignite in a platinum crucible until all carbon is removed, cool and weigh. Add several cubic centimeters of HF, evaporate to dryness, ignite carefully and again weigh. The loss in weight is the SiO2.

The filtrate contains a small amount of tungsten which can be recovered with cinchonine. (See page 140.)

Add to the impure WO, in the crucible 5 grams of sodium carbonate and a few milligrams of tartaric acid and fuse the mixture until all the tungsten is dissolved. Cool, digest the cake with 100 c.c. of water until no hard lumps remain and the sodium tungstate is completely dissolved. Filter through a small filter paper and wash with hot water until the washings are non-alkaline when tested with phenolphthalein. Burn off the filter paper, ignite and weigh the residue of Fe2O3 and Cr2O3. Subtract the weight of this residue from the last weight of the impure WO3; the difference is WO3, which, multiplied by 0.7931, gives the weight of tungsten in the sample.

Analysis of Ferrotungsten.-Ferrotungsten is not soluble in acids without the use of HF, hence this alloy must be fused with 10 times its weight of sodium peroxide in a nickel crucible if it is desired to determine the silicon in the tungsten. If the tungsten only is desired, the ground sample is dissolved with 5 c.c. of HF in a platinum dish. When action is over, add HNO3, a few drops at a time, until decomposition is complete, add 10 c.c. 1:1 H2SO4 and proceed as above directed, using cinchonine to recover the tungsten in the filtrate as directed in the notes below.

When the

Notes on the Tungsten and Silicon Determinations. steel is dissolved in strong hydrochloric acid as above directed, the tungstic acid does not form until the iron is dissolved and the precipitate is not much contaminated with iron. Since a trace of tungstic acid remains in solution after the first evaporation, also some silica, it is necessary to evaporate the filtrate to dryness again to render both insoluble by dehydrating the tungstic acid and silicic acid forming the insoluble anhydrides, WO3 and SiO2. But a fairly accurate determination of tungsten may be made by omitting the second evaporation to dryness.

Tungstic anhydride is volatile if ignited at a much higher temperature than a dull red, hence the necessity for heating carefully.

There will always be a little WO, which adheres to the casserole and cannot be removed by a policeman. It can be removed as follows: When the casserole is well washed out and the filter paper and precipitate thoroughly washed, wet a piece of filter paper with NH4OH and wipe out the casserole with it. This will remove all the tungstic acid adhering to the casserole. Add the paper to the other two papers and ignite them all together.

If the chromium in the filtrate is not desired and a tungsten determination alone is wanted, it is not necessary to evaporate the the filtrate from the WO, to dryness. Proceed as follows: Add to the filtrate 20 c.c. of 1:1 hydrochloric acid solution containing 0.5 gram of cinchonine. The cinchonine precipitates the tungsten completely. Heat the solution and filter and wash with a dilute acid solution of cinchonine. Ignite the precipitate with the rest of the precipitate of WO3. When the filtrate from the WO, is evaporated to fumes of sulfuric acid, the HCl is expelled. This is necessary before the chromium is determined. The sulfates will dissolve slowly, but it is not necessary to wait until they are all dissolved before adding the permanganate to oxidize the chromium to chromic acid. The reactions involved in the chromium determination are given under the determination of vanadium and chromium.

If the presence of molybdenum is suspected, the filtrate from the fusion of the WO, should be tested for molybdenum as directed on page 144. If molybdenum is found it should be determined.

When a sulfuric acid evaporation is used a small amount of tungsten goes into solution, the amount dissolving increasing with the amount of phosphorus in the alloy, and cinchonine must be used to precipitate this small amount. Add 10 c.c. of HCl containing 0.5 gm. of cinchonine, heat and filter immediately. Wash with dilute cinchonine solution 10 times, ignite very slowly and weigh the WO3. This weight is added to the weight of the main portion of the WO3.

Cinchonine Solution.-Dissolve 125 grams of cinchonine in 1,000 c.c. of 1:1 HCl.

Volumetric Method for Tungsten.-Wash the precipitate of WO, obtained as above directed with a hot dilute hydrochloric acid solution until the iron salts are gone, then wash with a 5 per cent solution of KNO, until the washings are free from acid. It is not necessary that the WO3 be all removed from the casserole as directed for the gravimetric method. Put the filter and

contents in the casserole, run in from a burette about 60 c.c. of a N/10 sodium hydroxide solution and stir the paper about until all the WO, is dissolved. Then add a few drops of phenolphthalein and titrate the excess of soda with N/10 HCl until the pink color disappears. The number of cubic centimeters of soda used minus the number of cubic centimeters of acid used multiplied by 0.0092 divided by the weight of the sample used and multiplied by 100 gives the percentage of tungsten.

The titration reaction is WO3+2NaOH = Na2WO4+H2O. If the WO3 is very impure with iron the end point of the titration is unsatisfactory. (See LINDE and TRUEBLOOD, J. Am. Chem. Soc., 29, 477.)

CHAPTER XIII

DETERMINATION OF MOLYBDENUM IN STEELS, FERROALLOYS AND ORES

Molybdenum is found in ores chiefly as the minerals molybdenite (MoS2) and wulfenite (PbMoO4). Metallic molybdenum, as ferromolybdenum, is added to steel to produce one of the many varieties of high-speed steel in conjunction with tungsten, chromium, vanadium and, occasionally, cobalt. It is also added in small percentages to special steels where especial resistance to wear is demanded, such as in gears.

The determination of molybdenum is satisfactorily accomplished by both gravimetric and volumetric methods. The volumetric method, most widely used, depends upon the same reactions as those used in the Emmerton process for the determination of phosphorus, as given on page 56, whereby the molybdenum is reduced from hexavalent to trivalent condition and then reoxidized by standard permanganate.

Molybdenum is determined gravimetrically by precipitating as sulfide and converting the sulfide to oxide, which is weighed, or by precipitating as lead molybdate, which is weighed as such.

Molybdenum can be separated from most interfering elements by precipitation with H2S in a slightly acid solution, or by fusion with sodium peroxide, forming sodium molybdate, which can be extracted from iron, titanium, etc. with water.

Process of Analysis of Steels.-Dissolve 2 grams of the sample in 100 c.c. of water and 50 c.c. of 1:1 H2SO4, heating until everything is dissolved. When solution is practically complete add 1 gram of ammonium persulfate and boil until the persulfate is decomposed. Cool and nearly neutralize with ammonia, heat to boiling and pass hydrogen sulfide through the solution at a rapid rate for 15 minutes. Keep the solution hot for several hours while a slow stream of H2S is passing. Filter off the sulfides of molybdenum, arsenic and copper and wash with a 2 per cent H2SO4 solution saturated with H2S. The filtrate will contain traces of molybdenum and should be allowed to stand in a pressure bottle for several hours after saturating with H2S.

Gravimetric Method. Wash the precipitate of sulfides into a 400-c.c. beaker, set the beaker under the funnel and dissolve the sulfides remaining on the filter by washing with 20 c.c. of hot 1:1 HCl, alternating with bromine water. Dilute the filtrate to 100 c.c., add 20 c.c. of HCl and 3 grams of sodium chlorate and boil until the molybdenum sulfide is completely dissolved. Filter and wash well.

To the filtrate add 20 per cent NaOH solution until decidedly alkaline, boil, allow the precipitate to settle and filter off ferric hydroxide, etc. If the precipitate is large, dissolve in hot HCl and reprecipitate, adding the filtrate to the main filtrate. To the filtrate, having a volume of about 200 c.c., add 1:4 HCl until 5 c.c. excess is used, add 50 c.c. of 50 per cent ammonium acetate solution, heat to boiling and slowly add lead acetate solution prepared as directed. Use 1⁄2 c.c. of lead acetate 12 solution for each milligram of molybdenum present and an excess of 5 c.c. This can be best accomplished by adding the lead acetate slowly and with stirring until further addition produces no increased cloudiness, then add 5 c.c. excess, boil the solution for 10 minutes, and allow the precipitate to settle until the solution is clear. Filter and wash 15 times with hot water, transfer the paper and precipitate to a porcelain crucible and ignite very slowly at first and, finally, when the carbon is all gone (use a red-heat), cool and weigh. The weight of the lead molybdate multiplied by 0.2614 gives the molybdenum.

Lead Acetate Solution. Add 10 c.c. of glacial acetic acid to 990 c.c. of water and dissolve in the solution 10 grams of lead actetate.

Volumetric Method. Wash the molybdenum sulfide as obtained above into a 250-c.c. beaker with 50 c.c. of 1:1 HNO3 containing free bromine. Ignite the paper (with a trace of residue) transfer the ash to the beaker, add 5 grams of sodium chlorate and 25 c.c. of 1:1 H2SO4 and boil until copious fumes of SO3 are evolved. Cool, add 5 c.c. of HNO3 and again evaporate until the SO3 is copiously evolved (it is essential that the nitric acid should be completely expelled). Cool, add 100 c.c. of water and heat nearly to boiling, which should give a clear solution. Add 2 grams of pure granulated zinc and boil until the zinc is nearly dissolved. This treatment partially reduces the molyb