Volumetric Determination of Calcium In Presence of Silica, Iron, Aluminum, Magnesium, Phosphorus, Titanium, and Manganese JAMES
J. LINGANE
Mallinckrodt Chemical Laboratory, Hanard University, Cambridge, Mass.
ammonium oxalate solution containing 5 grams of ammonium oxalate monohydrate were then filtered into the solution through a paper filter. A few drops of 0.1% methyl orange indicator were added, and the calcium oxalate was precipitated (at an initial temperature of about 80' C.) by dropwise addition of 1 to 1 ammonia over a period of 5 to 10 minutes with continuous and thorough mixing of the solution by swirling. The neutralization was stopped at a pH of 4.0 * 0.3, corresponding to a pinkish yellow color of the methyl orange. The proper color of the indicator can be decided easily by comparison with an equal volume of 0.1 molar potassium hydrogen phthalate (pH = 4.0) containing the same uantity of indicator. The exact pH is not very critical, provigded that it is between about 3.5 and 4.5. The solutions were allowed to stand undisturbed for only 20 to 30 minutes before filtration. The results obtained prove that precipitation of the calcium oxalate is complete within this time; longer digestion is unnecessary, and as Kolthoff and Sandell. have shown (a), it encourages postprecipitation of ma nesium. %e calcium oxalate was filtered off onto a Gooch crucible with an asbestos mat, and the precipitation flask and precipitate were wsshed thoroughly with eight to ten small portions of very cold water. Because of the appreciable solubility of calcium oxalate (0.7mg. per 100 cc. of water at 25") the total volume of wash water should be kept as small as possible (not over about 100 cc.) and it should be ice cold. The outside of the filtering crucible was washed off and the crucible was transferred back to the original precipitation flask. About 100 cc. of water and 5 to 6 cc. of concentrated (36 N ) sulfuric acid were added, and the solution was heated to about $0" and titrated with 0.1 N permanganate. The usual precaution of waiting until the first two or three drops of permanganate were completely reduced before proceeding with the titration was followed, and the temperature a t the end of the titration was kept above about 60". The permanganate solution was standardized under exactly the same conditions, and a t frequent intervals, against Mallinckrodt's Primary Standard sodium oxalate dried a t 110'. The weight of sodium oxalate used (0.3000 rem) was such that the volume of permanganate re uired in tPhe standardizations was within 2 cc. of that require2 in the determinations, and hence a blank correction for the small volume of permanganate required to produce the end-point coloration was unnecessary. The volume of permanganate used in each titration was about 48 cc., and the agreement of replicate titrations was somewhat better than one part per thousand. The titration procedure of Fowler and Bright ( 1 ) would doubtless be e ually satisfactory. The various metals were added in %e form of their chlorides, and phosphate was added as ammonium dihydrogen phosphate, immediately after solution of the dolomite samples in hydrochloric acid. All materials were of analytical reagent quality. The author is indebted to George S. Forbes for supplying a sample of pure titanic chloride, which served as the source of titanium.
A volumetric oxalate-permanganate procedure i s described which permits the direct determination of calcium in the presence of amounts of silica, iron, aluminum, magnesium, and phosphate, that are equal to or somewhat greater than the amount of calcium, and also in the presence of small amounts of titanium and manganese. The procedure employs only a single precipitation of calcium oxalate from acidic medium, duplicate determinations can be completed within two hours, and the accuracy compares very favorably with the more laborious cfassical methods which require the prior removal of most of the above elements.
T
HE principle of precipitating calcium oxalate from a slightly
acid solution, originated by Richards, McCaffrey, and Bisbee (4) for separating calcium from magnesium, has been employed by several authors for determining calcium in the presence of iron, aluminum, phosphorus, and other elements with which it is commonly associated. The most recent and systematic study of the determination of calcium under these conditions was carried out by McComas and Rieman (S),who achieved a fairly accurate determination in the presence of moderately large amounts of magnesium, iron, aluminum, and phosphate by a single precipitation of calcium oxalate a t a pII of about 3.7. Instead of employing the procedure of Richards et al., whereby the precipitation is effected by starting with a strongly acid solution and slowly neutralizing with ammonia, McComas and Rieman precipitated the calcium oxalate directly by adding oxalic acid solution to the calcim solution acidified with formic acid (initial pH about 1.5 to 2.0),and finally buffering with ammonium formate to a pH of about 3.7. I n the procedure herein described the precipitation technique of Richards et al. has been retained. The present method is as rapid and convenient, more accurate, and less sensitive to interference by iron and phosphate, than the, procedure of McComas and Rieman. EXPERIMENTAL PROCEDURE
A finely ground sample of a dolomitic limestone was used as a source of calcium in most of the experiments. This material had been used as an "unknown" for several years in a course in quantitative analysis, during which time it had been analyzed for silica, RIO,, calcium oxide, and magnesium oxide by a number of different students with closely agreeing results. In these analyses the classical procedure of separating silica and R20: was followed, and calcium was determined in the filtrate from the ammonia precipitate by the gravimetric calcium oxalate to calcium oxide method. A single precipitation of calcium oxalate was employed, starting with a strongly acid solution and precipitating by the slow addition of ammonia according to essentially the same procedure as that recommended by Richards, McCaffrey, and Bisbee (4). The average of the values of seventeen students (each of whom had run several replicate determinations) was 30.68% calcium oxide with an average deviation from the mean of =+=0.0770 calcium oxide or about 2 parts per 1OOO. This sample also contained 0.71% silica, 0.24% RIOs, and 21.41% magnesium oxide. I n all cases a 0.4000-gram sample of the dolomite (corresponding to 122.7 mg. of calcium oxide) was weighed into a 250cc. wide-mouthed Erlenmeyer flask, treated with about 5 cc. of water and 10 cc. of concentrated (12 N ) hydrochloric acid, and warmed on the steam bath for a few minutea until decomposjtion was complete. The solution was then diluted to about 59 cc. and heated almost to boiling. About 100 cc. of a hot (90')
RESULTS AND DISCUSSION
The reliability of the foregoing procedure was first tested by determining calcium oxide in two Bureau of Standards standard samples of limestones. One of these was dolomite No. 88 with a certified percentage composition as follows: CaO 30.49,MgO 21 48,Si02 0:31, AlzD: 0.07, FaOs 0.08, NaaO 0.08, ICs0 0.03, COI 47.25, Ti02 0.005, P,O, 0.003. SOI 0.035.S 0.013.MnO 0.006. SrO
