Aug., 1919
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
surrounded by boiling water for z hrs. Remove, loosen the papers in the funnels, let stand for half an hour. in the weighing room, and weigh. The empirical factor t o lead is 0.6375. If any variation of this method is used, i t should be standardized against pure lead and a factor calculated. The theoretical factor will give results too high when paper filters are used. If preferred, the lead sulfate may be separated by the use of alcohol and weighed as such. The filtrate from the lead is electrolyzed for copper a t about zoo cc., after adding 5 cc. nitric acid. This can be done best with a coil and cylinder, so t h a t t h e electrolyte can be saved without dilution. I n removing the electrodes more care should be used t o avoid diluting or spilling the solution t h a n t o avoid dissolving the copper, for the copper can be recovered. Pass hydrogen sulfide into the electrolyte and filter on an ashless paper, with a little pulp. I n some cases it may be necessary t o add sulfuric acid t o prevent the precipitation of zinc. The precipitate will consist of copper.and tin. I n most cases i t is accurate enough t o burn and weigh, and guess a t t h e proportion of each. Or the cupric oxide may be dissolved away from the stannic oxide in the crucible after ignition with a little hydrochloric acid, as described above. The weights of tin and copper are added t o the major amounts found. I n some determinations, particularly when the tin is low a n d is found largely in the electrolyte, it is best t o reserve t h e tin precipitate until this second portion is recovered, before ignition. The tin will then be weighed all a t once, and the copper found with i t can be determined separately. The separation is conveniently made with hydrogen sulfide, leaving the iron in condition t o add t o the main portion. Wher the separation has proceeded thus far, t h e filtrates will contain all the manganese, iron, aluminum, nickel, and zinc. I t is convenient t o oxidize the iron by adding hydrochloric acid while boiling out the hydrogen sulfide, as some nitric acid is present. As the amount of nickel is small, the basic acetate separation can be made almost a t neutrality. Bring the solution just t o the acid side with ammonium hydroxide and hydrochloric acid, and then add 2 0 per cent ammonium hydroxide, using a wash bottle, until a slight cloudiness of iron remains after stirring. Add a gram or so of sodium acetate, boil, settle, filter, and wash well with water containing ammonium sulfate. Dissolve, and precipitate the iron and aluminum with ammonium hydroxide, mixing some ashless paper pulp with thc precipitate before filtering. This makes the ignited oxide soft and easily fusible. As the aluminum is generally present only in traces, i t is not safe t o take i t by difference. After weighing the combined oxides they may be fused two or three times with sodium carbonate and the aluminum precipitated by nearly acidulating the filtrate with hydrochloric acid, or the fusion may be made with acid potassium sulfate, the solution made just acid with sulfuric acid, using ammonium hydroxide t o neutralize it, and the iron removed electrolytically, using a mercury cathode. The precipitate of iron and aluminum is likely t o con-
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tain silica, and expulsion before weighing, using a drop of sulfuric acid with the hydrofluoric acid, is a wise precaution. Titration of the iron is still better. Precipitate the manganese in the filtrate from the iron b y the use of ammonium persulfate, and determine i t as usual. The nickel may now be separated from the zinc by dimethyl glyoxime, and filtered and weighed on a Gooch filter. As the solution by this time contains too much interfering acid t o titrate the zinc directly, i t should be made just acid with acetic acid and the. zinc precipitated with hydrogen sulfide. Filter, but do not wash. P u t paper and all back into the beaker, pour in I O cc. water and 8 cc. hydrochloric acid, warm for a few minutes, dilute, and titrate with ferrocyanide. I n the case of brass it will be necessary t o take a suitable aliquot before precipitating the zinc. Instead of the glyoxime separation, the zinc may be precipitated first by Treadwell’s salting out method. This method is laborious and tedious, but i t makes possible a practically complete analysis of one weighed portion, and the results are very accurate, in spite of the multiplicity of corrections. Complete analyses generally foot up within a few hundredths of I O O per cent. 48 TONNELB AVENUE JERSEY CITY, N E W JERSSY
THE ALKALIMETRIC DETERMINATION OF SMALL AMOUNTS OF MAGNESIUM By P. L. HIBBARD Received January 9, 1919
A method for determining magnesium by titration of ammonium magnesium phosphate was published by Bruckmiller in 1917.‘ The principles of his method had been known for some time, but he improved the technique considerably. While endeavoring t o use the method for determining magnesium in soil extracts, the writer has further changed the procedure so t h a t it is now very convenient and exact for the estimation of quantities of j mg. or less, down t o 0.1 mg. The principal changes introduced are: 1-Use of the Gooch crucible for filtration whereby it is possible to wash the precipitate in the most efficient manner, with the least quantity of wash solution. z-Use of neutralized alcohol followed by water solution of ammonium magnesium phosphate for washing. 3-Use methyl red instead of methyl orange as indicator, giving a much sharper end-point. Nearly all of the experimental work was in connection with the second point, finding a suitable wash liquid. Among those tried were water, alcohol, ether, in various concentrations and mixtures, I per cent water solution of ammonium sodium hydrogen phosphate, and a water solution of the salt which was t o be purified, ammonium magnesium phosphate. T h e last named and pure alcohol were the only ones permitting good results, so it is not necessary t o give the results with others. Ordinary alcohol is not neutral, but when properly neutralized i t has very little solvent effect on ammonium magnesium phosphate. However, if left in contact with the salt for some time ammonia is taken up b y the alcohol so t h a t 1
“Titration of Magnesium,” J
A m Chem. Soc., 39 (1917), 610
THE JOURNAL 0 F . I N D U S T R I A L A N D ENGINEERING CHEMISTRY
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the alkalinity of the precipitate is changed. It was t o avoid this difficulty t h a t other wash solutions were tried. A saturated solutions of ammonium magnesium phosphate in water has practically no effect on the precipitate so long as the temperature of saturation does not differ much from the temperature a t which it is used. The alkalinity of this solution is so slight t h a t the small amount of i t remaining in the precipit a t e is negligible, or a t any rate may be allowed for . by a blank determination. The solution is easily prepared by agitating some of the salt with carbon dioxide-free water for several hours, then filtering. I t must not be heated, as hot water hydrolyzes the salt and causes separation of a basic compound. Addition of ammonia t o the solution again precipitates ammonium magnesium phosphate. For this reason i t cannot be used t o remove excess of ammonia from the precipitate. Hence the use of alcohol t o remove the main portion of the excess of ammonia. Then the washing is completed with the ammonium magnesium phosphate solution. Precise details are given in the description of the method following. The procedure is essentially the same as if the magnesium were t o be determined by weight up t o the operation of filtering. This and the titration require only a few minutes. No special apparatus is required, though i t is well t o use for the washing solution a wash bottle operated by a rubber pressure bulb in order t o avoid the entrance of carbon dioxide. The following table illustrates the degree of accuracy attained. The quantities are stated in milligrams of magnesium. FOUND
TAKSN
0 53 0 55 0 50 0:98’0:98’ 1.00 2.00: 1.98: 2 . 0 2
0.5
I .o 2.0 METHOD
The method is alkalimetric, depending for success on obtaining the magnesium in a definite compound, ammonium magnesium phosphate, the alkalinity of which may be accurately determined, by means of standard acid. I. PREPARATION OF THE soLuTroN-The Solution must be free from other bases precipitable by phosphate ion in alkaline solution. I n most cases i t is convenient t o use the solution from which calcium has been separated as oxalate. The volume should be small in order t o effect complete separation of the magnesium in case there is very little present. For amounts less than 1 mg.. . . .. . . . . . . . . . . , . . 5 c c . volume For amounts of 1 to 3 mg.. , , . . . . . . . . . . . . . . . . . 10 cc volume For amounts over 5 mg... . . . . . + . . . . . . . . , . . . 20 c c volume Other soluble salts increase solubility of the precipitate, hence should be absent if possible. Ammonium salts should be removed by evaporation and ignition, or by evaporation with nitric and hydrochloric acids. The quantity of ammonium salts formed in making the separation of calcium as oxalate ordinarily will not interfere with precipitation of ammonium magnesium phosphate. 2. ( a ) PRECIPITATION I N THE coLD-The solution, in a 2 0 0 cc. Erlenmeyer flask, should contain one- to two-tenths of one per cent of ammonium chloride and
Vol.
11,
No. 8
enough free ammonia t o give the odor distinctly, but not strong. Add, dropwise, a 1% per cent solution of ammonium sodium hydrogen phosphate, I cc. for I mg. of magnesium, shaking constantly. If there is much magnesium, the precipitate forms a t once; if there is very little, i t will not appear until after addition of ammonia. After I O min., add one-third the volume of strong ammonia, cover, and let stand 2 hrs. or more. ( 6 ) PRECIPITATION HOT (SCHMITZ METHOD) ‘-If the solution contains much ammonium salts which it is not convenient t o remove, this plan gives very good results. Acidify slightly with hydrochloric acid, heat t o boiling, add the necessary amount of ammonium sodium hydrogen phosphate, then one-third the volume of strong ammonia, cover, let stand.2 hrs. or more. If there is much magnesium present, a flocculent precipitate forms a t once. On standing and cooling this becomes crystalline and has the same composition as t h a t formed by Method ( a ) , ammonium magnesium phosphate. 3.
FILTERING AND
WASHING
THE
PRECIPITATE-
Prepare a Gooch crucible with a moderate thickness of paper pulp as filtering medium. Asbestos is not so good. Filter with moderate suction. The principal part of the soluble salts may be washed out with z per cent ammonium hydroxide or this may be omitted and washing with alcohol begun a t once. Wash two or three times with 95 per cent alcohol, neutral t o methyl red. I n order t o prepare the alcohol, measure I O cc. into 50 cc. neutral water and determine the amount of standard acid or alkali necessary t o neutralize it. From this, calculate the acid or alkali required for the volume of alcohol t o be used. It is impossible t o neutralize the alcohol by titrating directly on account of failure of the indicator t o ionize in strong alcohol. After washing with the alcohol, wash four times with 5 cc. portions of a saturated water solution of ammonium magnesium phosphate. Rinse inside of crucible once with the same wash. Each time the wash should be entirely sucked out before adding the next portion. Return the felt with precipitate t o the flask, rinse crucible into flask with water, and titrate. 4. TITRATION OF THE PRECIPITATE-Add a few drops of I per cent alcoholic solution of methyl red indicator, run in acid, while shaking, until it turns red, then about 5 cc. in excess. Shake well t o insure complete solution of the precipitate. If the crystals are large, considerable time will be required t o dissolve them. If not entirely dissolved before back titration, the end-point will be unstable and results will be low. Titrate back with alkali of the same strength as the acid until the color changes through pink almost t o yellow. 1 c c . N/50 acid = 0 24 mg magnesium 1 cc N/10 acid = 1 2 m g magnesium
When more than 2 mg. magnesium are present, the color change is not sharp. DIVISION OF AQRICULTURAI. CHEMISTRY UNIVERSITY OF CALIFORNIA BERKELEY, CALIFORNIA 1 Z. anal. Chem., 1906, 512. Chemistry,” 2, 67.
Method given in Tseadwell’s “Analytical
