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Vol. 3, No. 1
ANALYTICAL EDITION
Gravimetric and Direct Volumetric Determination of Cadmium' R. C. Wiley UNIVERSITY OF MARYLAND, COLLEGR PARK, MD.
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ADMIUM is often deCadmium may be determined by precipitating as for converting c a d m i u m termined gravimetricadmium molybdate and weighing as such, molybdate to cadmium. The Cadmium may be determined volumetrically by cally by weighing the results are the following: titrating with a standard ammonium molybdate solupyrophosphate or sulfate. It Cadmium present, 0.1378 gram tion, using a saturated solution of -pyrogallol in chlorois commodydetermined volu- Cadmium found: form as an indicator. metrically - b y precipitating Assulfate, 0.1380,O.1390,O.1383, 0.1385; av. 0.1384gram it as the sulfide,treating the Molybdate method, 0 1376, 0.1380, 0.1389, 0.1381, 0.1384, 0.1380, sulfide with hydrochloric acid and then titrating with iodine 0 1371, 0.1374; av. 0.1379 gram solution. This of course is an indirect method. These results show that the gravimetric molybdate is Cadmium molybdate is very insoluble in a neutral solution. It is therefore possible to determine cadmium either quite accurate. In another solution there was present 0.2061 g a m of by precipitating it as the molybdate and weighing it as such or by titrating a neutral solution of a cadmium salt with cadmium and an average of four results shows 0.2062 gram a standard molybdate solution. The end point of the titra- present by molybdate method. tion may be determined by transferring some of the soluDiscussion of Gravimetric Method tion being titrated to a spot plate and bringing it into conCadmium molybdate as precipitated forms hard white tact with a drop of chloroform saturated with pyrogallol. The solution tested must be nearly neutral. If the soluble crystals quite insoluble in water. It is, however, R little less molybdate is in excess, the aqueous drop which has been soluble in a dilute solution of acetic acid than it is in water. transferred will turn brown. Metals other than magnesium Upon drying it assumes a faint pink tinge. It may readily be or the alkali metals will interfere with either method and filtered and does not tend to clog the filter. The cryst!als are very small when first formed, resembling those of barium must be removed. sulfate. Too much suction should not be used in filtering, esCadmium Solution Used in Determinations pecially a t the beginning, for the precipitate will tend to run A cadmium nitrate solution was prepared by dissolving through if it has not been digested long enough. This 5.5150 grams of the metal in nitric acid, evaporating to precipitate is readily washed. The use of ground asbestos dryness, and diluting to one liter. The cadmium in 25 cc. in the Gooch is a precautionary measure. of this solution was determined as sulfate. No other metal Care should be used not to strike the sides or bottom of the was present. beaker with the stirring rod when making the precipitation, since the crystals of cadmium molybdate will adhere tenaThe Gravimetric Method cioudy to the beaker a t the point scratched and are detached The reagent ammonium molybdate is prepared by stirring only with great difficulty. Cadmium molybdate is stable at 120"C. At about 500" C. up 30 grams of molybdenum trioxide with 400 cc. of water and a few drops of phenolphthalein and adding ammonia it turns brown and loses about 3 per cent of its weight. with constant stirring until the oxide is dissolved. Acetic This compound is soluble in strong mineral acids and in acid is then added until the pink color disappears but the concentrated solutions of bases. It is nearly insoluble in reaction is acid to litmus. After filtering, the solution is water, but soluble enough to give a very faint color with diluted to 1 liter. One cubic centimeter will precipitate the pyrogallol indicator. 0.02 gram of cadmium. Volumetric Method Gooch crucibles should be prepared by covering the Prepare the standard as follows: Weigh out 7.4 grams layer of ordinary asbestos with a layer of ground asbestos, of molybdenum trioxide, add about 100 cc. of water and some and dried at 120" C. phenolphthalein. Dissolve in ammonium hydroxide and Procedure for Gravimetric Method neutralize with acid. Cool and dilute to a liter with disThe cadmium solution should be made just alkaline with tilled water. The solution may be standardized by disammonia and then just acid with acetic acid. It should then solving an exactly known amount of pure cadmium metal be heated to boiling and the molybdate solution added a in nitric acid and evaporating to dryness over a water bath drop at a time and with constant stirring. The presence and then titrating the resulting cadmium nitrate against it, of an excess may be determined by transferring a drop to or it may be standardized by adding to a known volume of the spot plate and testing with the pyrogallol indicator. the solution an excess of cadmium salt and weighing the The solution containing the precipitate is kept near the resulting cadmium molybdate as in the gravimetric method. boiling temperature until the supernatant is clear or nearly Procedure for Volumetric Method so, and then allowed to stand for at least two hours and prefThe cadmium solution should be so treated that very erably overnight. It is then filtered through the Gooch, little free acid is present. Ammonium hydroxide is then previously dried a t 120" C. After the precipitate has been added until a precipitate begins to form. It is heated to washed with warm water, the crucible containing it is dried boiling and the precipitate dissolved in the least possible at about 120" C., and weighed. The factor 0.4122 was used amount of acetic acid. The solution is titrated and drops tested from time to time on a spot plate, the depressions of 1 Received August IS, 1930.
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January 15, 1931
INDUSTRIAL A N D ENGINEERING CHEMISTRY
which contain a saturated solution of pyrogallol in chloroform. At the end point a deep brown color will appear. The results are the following: Cadmium present 0.1378 gram Cadmium found by volumetric method, 0.1390,0.1380, 0.1395,0.1374, 0.1378,O.1390,O.1386,O.1390; av. 0.1385gram
The above recorded results are only a few of those obtained and are given as typical. They show that cadmium may be accurately determined volumetrically. Comment and Discussion
Ammonium salts increase the solubility of cadmium molybdate. They should be kept a t a minimum. This increase in
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solubility due to ammonium salts is so small as to be negligible in the gravimetric method, but due to the great sensitiveness of the indicator it is appreciable in the volumetric method. When the end point of the titration is apparently reached it should be boiled for about 30 seconds and retested, and if necessary more standard molybdate added. Just before the end point is reached, a very faint yellow color may be noticed in the drop tested. This color is very faint and should not be confused with the much deeper color that appears when the true end point is reached. A standard solution of ammonium molybdate appears to give a sharper end point than the sodium salt, although either may be used.
Potentiometric Determination of Saponification Number of Mixtures of Asphalt and Drying Oils'
reference and indicator electrodes, respectively. After each titration the hydrogen electrode was treated with hot concentrated nitric acid and then made the cathode in the electrolysis of dilute sulfuric acid. After this treatment it was ready for the next titration. It was found unnecessary to replatinixe an electrode during the entire investigation. A Leeds and Northrup student potentiometer was used.
GILSONITE method
11 9 9 10 Mean 9
2 1 3 0 9
BERMUDEZ
Double maximum method
12 3
Blank method
8 5
26 9 28 0 27 7
9 8
27 5
8 6
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