Analysis by “Internal” Electrolysis I. Determination of Bismuth and Copper in Lead Alloys Containing Antimony and Tin BEVERLY L. CLARKE, L. A. WOOTEN,
AND
C. L. LUKE, Bell Telephone Laboratories, New York, N. Y.
E
LECTROANALYSIS by the spontaneous discharge of a galvanic cell in which a solution of the alloy to be analyeed constitutes the catholyte, has been named “internal” electrolysis (8, 9), emphasizing the distinction from ordinary electroanalysis in which the e. m. f. is externally applied. The earliest reference t o this method is by Ullgren ( I I ) , who plated small quantities of copper from sulfate solution by means of zinc-platinum, cadmium-platinum, and aluminumplatinum cells. Later work includes that of Franpois (6), Hollard (6),Tutundzic (IO), Sand ( 8 ) ,and Collin (I-4). I n view of the inherent advantages of “internal” electroanalysis, it is surprising that it has not been more widely used. This method is unique in that the cathode potential is automatically limited. Since platinum anodes are not employed, separations may be made in chloride solution in the absence of an anodic depolarizer. In “internal” electroanalysis iron does not interfere with the plating of metals from chloride or sulfate solution because the anodes are shielded. The method is rapid and is therefore suitable for routine analysis; the apparatus required is inexpensive. The method is subject, however, to certain limitations and involves a number of experimental difficulties. I n general, its application is limited to the determination of small quantities of metals. Thus it has its greatest usefulness in the separation of impurities from the bulk of the base metal, as of bismuth and copper from lead. Frequently the method serves only for a preliminary separation, other methods being more suitable for the final determination. There are also certain limitations imposed upon the choice of the anode metal which in turn limit its applicability: The cell will not function unless there is free and uninterrupted dissolution of the anode-i. e., the anode must not become passive in the anolyte. The value of the anode potential must be such as to cause rapid and complete deposition of the metal being determined. Collin (I), working with Sand (8), proposed the leadplatinum cell for the determination of bismuth and copper in lead and lead-antimony alloys. The present authors have modified the Sand and Collin method and have extended its applicability to lead-tin alloys. Certain improvements in apparatus have been introduced. The chief differences between the authors’ apparatus and method and those of Sand and Collin are as follows: A new electrode assembly has been designed. Norton alundum shells (7) have been substituted for parchment paper as anode envelopes. I n this laboratory parchment paper was found t o stretch and become increasingly porous in hot nitric acid solution. The bismuth-copper plate is weighed directly. After solution, the copper is titrated iodometrically and the bismuth calculated by difference. This obviates the unsatisfactory basic carbonate separation of bismuth and copper. Silver is removed as iodide rather than chloride because of the lower solubility of silver iodide, a necessary measure when separating the small quantities of silver usually found in commercial lead. Another objection to chloride is that its presence contributes to the production of a bismuth plate physically unsuited to direct weighing. The authors have shown that the method can be used in 411
nitric-hydrofluoric acid mixtures, thus extending its applicability to lead-tin alloys.
Apparatus The apparatus is shown diagrammatically in Figure 1. It consists of two lead anodes enclosed in alundum shells (19 x 90 mm. RA No. 84 or RA No. 360, R. Norton and Co., 7 )and a single platinum gauze cathode (2.5 cm. in diameter and 4 cm. in length), arranged to fit into a 400-ml. beaker. The three electrodes are connected to a single binding post so that good electrical contact is made. The electrode support is shown in .. detail in Figure 2. The anodes are made by winding pure lead wire (2.5 mm. in diameter X 70 cm. in length) around glass tubing (5 mm. in diameter) in the form of a-compact helTx, leaving&ough wire at the top to act as a lead to the binding post. The surface of the anode should in general be as large as is practically possible t o ensure rapid plating. A glass corkscrew stirrer with tungsten shaft is used to stir the catholyte. It is driven by a motor capable of 1000 r. and geared so that the solution is drawn in towards the catl%g: An anolyte reservoir is provided which ermits the flushing out of the anode chambers during an electrorysis. In this way any platable metal ions which may find their way through the shells can be returned t o the catholyte. Flushing of the anode chamber also tends to prevent anodic concentration polarization, although this effect proves to be negligible if the anode area is large. THE ANODE SHELLS.The porosity of the alundum thimbles used as anode shells plays a very important role in the success of the method. When a shell of this composition (alumina with aluminum silicate as a binder) is subjected to treatment with hot nitric acid, a certain amount of hydrolysis of the aluminum silicate takes p l a c e . The gelatinous hydrolytic products tend to- reduce the porosity of the shell. In an electrolysis the formation of such a precipitate in the anode shells will increase the internal resistance of the cell to such an extent that the cathodic plating will be prohibitively s 1o w . Obviously, then, in devising a method in which an i n t e r n a l electrolytic separation is made from RUBBER TUBING nitric acid solution, the procedure used GLASS TUBING must be such that hydrolysis of the material of the anode shells is a v o i d e d . Thus, in the method HOLE FOR FLUSHING described below the temperature of the ALUNDUM SHELL ONTAINING ANODE s o l u t i o n is n o t allowed to go above 75°C. The method further specifies that tartaric acid be added t o the catholyte in all electrolyses, e v e n though its presence is not required in the solution of the alloy. E x p e r i e n c e has shown that new alunFIGURE 1. DIAGRAM OF APPARATUS
412
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dum shells are nearly always easily clogged Selenium interferes with the determinaby hydrolysis. Continual use in any internal tion, but preliminary experiments indicate electrolysis will gradually wash out the hydrothat very small quantities of tellurium lyzed material t o such an extent that quantitative plating from nitrate solution can be obmay be present, provided it is in the hexatained. In order t o accelerate the process of valent form. opening up new or contaminated shells a preSIZEOF SAMPLE.The method is subject liminary acid treatment has been devised (see below). Even after subjecting the shells to to certain l i m i t a t i o n s as regards the the vigorous "opening" treatment, however, amount of bismuth that can be handled. the plating may be slow for the first three or Bismuth alone plates out in loose granular four electrolyses. form. Under these conditions lead and If, during the electrolysis of a copperbismuth solution, the shells become clogged, other metal salts may be mechanically the addition of a little chloride will immedioccluded in the plate, thus causing high ately open the shells up and permit rapid results. When copper is plated with the deposition of the metals. bismuth, however, the deposit is much more The optimum temperature for plating from lead solutions containing chloride is 80" to compact and adherent; as much as 10 mg. 90" C., for at lower temperatures the internal of bismuth can be handled if the amount of resistance of the cell may be increased by the copper present equals or exceeds that of the formation of lead chloride on the anodes or bismuth. If less copper than bismuth is in the shell walls. The use of chloride in the solutions may make it necessary to dissolve present the plate may be contaminated and L.J and replate the de osit. in such a deposit the occluded impuritiesDespite the digculties mentioned above e. g. antimony, arsenic, iron, etc.-may the authors have found that alundum thimbles cause an appreciable blank in the copper provide the most satisfactory means of shielding the anodes. Two types of shells have titration. been tested: the dense RA No. 84 shells and I n order, then, to ensure correct results the less dense RA No. 360 shells of R. Norton for both copper and bismuth in an analysis and Go. Both types are satisfactory for use of a lead or lead-tin alloy which contains in the electrolysis, provided they are open. The plates obtained (especially those which more bismuth than copper, the analyst FIGURE 2. DETAIL OF ELECcontain much bismuth) are better when the TRODE SUPPORT must either dissolve the first plate and reNo. 84 shells are used because the plating is A . Cast-metal supporting arm electrolyze from pure nitric acid-tartrate slower. On the other hand, the No. 360 shells B. Hard-rubber board are more easily opened and kept open, and solution (using 3 per cent nitric acid soluC. Metal bindin post (stainless steel or oxidiced%rass) the time of electrolysis is shorter. tion as the a n o l y t e ) , or add a known CI. Cathode suonort ._ -. CONDITIONING AND TESTING OF THE ANODE Cz,Ca. Anode s h o r t s amount of copper to the solution of the SHELLS. The following procedure is recomD. Removable anode support alloy before electrolysis to make the bisE. Holes for glass tubes which support mended for opening up or cleaning alundum anodes anode shells : muth-copper ratio less than unity. The H. Hole for stirrer 1. Draw 400 ml. of hot concentrated 5. Removable threaded screw latter procedure is preferable as requiring hydrochloric acid through the shell into a less time, and is therefore recommended flask by means of suction. Remove and in the method below. reserve the solution. 2. Draw 400 ml. of hot nitric-tartaric acid solution (40 ml. of SOLUTIONOF THE SAMPLE.Alloys containing arsenic, concentrated nitric acid plus 20 grams of tartaric acid dissolved antimony, or small amounts of tin are soluble in nitric acidin 360 ml. of water) through the shell. tartaric acid mixture. For larger amounts of tin the tartaric 3. Repeat the chloride wash of paragraph 1. Then draw acid is replaced by hydrofluoric acid. warm distilled water through the shell t o remove all chloride. (The chloride must be removed since its presence may invalidate It is obviously desirable to use as little hydrofluoric the conclusions drawn from the testing of the shells by acid as possible. On the other hand, the amount of this acid paragraph 4.) used must be sufficient to ensure solution of the tin during 4. Prepare a solution containing 8 ml. of nitric acid, 1 gram the vigorous boiling previous to electrolysis (see below). of tartaric acid, and 10.0 mg. of copper as copper nitrate in a volume of 150 ml. Boil t o expel oxides of nitrogen. (If these Procedure B is suitable for the analysis of alloys containing fumes are not expelled the plate may be dark.) Dilute t o 250-ml. as much as 50 per cent of tin. With alloys containing less than volume. Coolto65" C. Electrolyzeat 65Oto 75' C. as described 10 per cent of tin the amount of acid used can be reduced. below in Procedure A, paragraph 3, using as an anolyte a 3 per Once the tin has precipitated, however, it is extremely difficult cent nitric acid solution. Weigh the plate and compare the result with the amount of copper added. to redissolve it by adding more fluoride. Once opened, the shells usually require no further treatment. When tin is dissolved in nitric-hydrofluoric acid solution it To prevent an accumulation of salts in the walls, it is advisable tends to remain in the bivalent state unless nitrous acid is to remove and wash the shells with distilled water after use. produced in the solution to ensure oxidation. When dealing with 50 :50 lead-tin solders there is usually sufficient nitrous The Method acid produced to catalyze the solution and oxidation of the The sample is dissolved in a mixture of either nitric and tin. With alloys of low lead content it is advisable to add tartaric acids or nitric and hydrofluoric acids. Antimony sodium nitrite. and arsenic are oxidized to their higher valence states with STANDARDIZATION OF SODIUM THIOSULFATE SOLUTION.Prepermanganate. Silver is removed as iodide. Bismuth and pare a standard copper nitrate solution containing 0.5 gram of copper are plated out with a lead-platinum couple and the copper per liter. Pipet a sample from this solution which will give approximatelythe same size thiosulfate tJiteras that given hy combined deposit is weighed. The plate is then dissolved the unknown sample. Transfer t o a 250-ml. wide-mouthed flask. and the copper titrated iodometrically. Bismuth is calculated Add 1 ml. of (1 t o 1) sulfuric acid. Take just to fumes of sulfur by difference. trioxide twice to expel all nitrates. Cool. Wash down the sides of Such elements as calcium, cadium, and zinc do not interfere. the flask and add 25 ml. of water. cool t o room temperature. Add 5 ml. of a freshly repared 40 er cent solution of potassium Iron in amounts usually present in lead does not interfere. iodide and mix well. A)dd 5 ml. o?a clear, freshly prepared 20 Antimony and arsenic do not interfere when present in the per cent solution of sodium thiocyanate and titrate immediately pentavalent form in nitric-tartaric or nitric-hydrofluoric acid with 0.025 N sodium thiosulfate. When the end point is apsolution. Quadrivalent tin does not interfere even when proached add 5 ml. of clear 1 per cent starch solution and then present in large quantities in nitric-hydrofluoric acid solution. complete the titration.
NOVEMBER 15, 1936
ANALYTICAL EDITION
413
A. PROCEDURE FOR ALLOYSSOLUBLEIN NITRIC ACIDTARTARIC ACIDSOLUTION.(This procedure is suitable as written for 1 per cent antimony-lead alloy containing less than 0.05 per cent of silver and 0.005 to 0.1 per cent of bismuth.) 1. Weigh out 10 grams of the sample and transfer to a 400-ml. beaker. Add 1 gram of tartaric acid and 80 ml. of nitric acid (20 per cent). Cover with a watch glass and heat gently until solution is complete. Boil for 1 minute to expel most of the nitrous acid fumes. Dilute to 150-ml. volume with cold water. Cool to 40" C. Add potassium permanganate (1 per cent solution) while stirring until the solution acquires a violet tint which lasts a minute or two. Let stand or heat gently to dispel the color. 2. (In case silver is known to be absent this paragraph can be omitted.) Add 2 ml. of potassium iodide (1 per cent solution) while stirring. Allow t o stand 5 to 10 minutes at 35' to 40" C., avoiding direct sunlight. (If a t any time during the precipitation or filtration of the silver iodide there is evidence that the potassium iodide is being oxidized, the solution can be decolorized by adding a drop or two of freshly prepared sulfurous acid solution.) Filter with suction through asbestos into a second 400-ml. beaker. Wash with water or with 3 per cent tartaric acid solution in the event that the antimony content of the alloy is high. Discard the precipitate. Dilute the filtrate to 250-ml. volume. Place 2 or 3 glass beads in the beaker. Cover and heat to boiling. Boil vigorously until the solution has become colorless and then for an additional 10 minutes. 3. Dilute the solution to 250 to 300 ml. (Standard copper nitrate solution should be added a t this uoint if the ratio of bismuth to copper in the alloy is greater than unity.) Cool to 65" C. Add 0.2 to 0.3 gram of urea. Electrolyze with rapid stirring (800 to 1000 r. p. m.) at 65" to 75" C. with a lead-platinum couple. (Anolyte for the lead-platinum couple: a 3 per cent nitric acid solution containing 50 grams of pure lead nitrate per liter.) Flush out the anode chambers, wash down the sides of the beaker and anode shells, and add small amounts of urea, once or twice during the electrolysis. When the plating is completed (as indicated by failure to plate on new surface when level of solution is raised) wash away the acid-lead nitrate solution with approximately 1liter of distilled water by siphoning. (If there is any reason to suspect that the plate contains other
metals besides copper and bismuth, proceed as follows: Dissolve the plate from the electrode in 8 ml. of nitric acid. Dilute to 150 ml. Boil to expel fumes. Dilute to 250 ml. Add 1 gram tartaric acid. Cool to 65" C. Electrolyze as described above using as anolyte a 3 per cent nitric acid solution.) Remove the beaker and rapidly replace with another containing distilled water. Detach the electrode, wash in alcohol and ether, and dry with an electric fan. Weigh. 4. Place the electrode in a 250-ml. wide-mouthed flask and dissolve the plate with 5 ml. of 3 per cent hydrogen peroxide and 1ml. of 1to 1sulfuric acid. Wash the electrode well and remove. Take the solution to fumes of sulfur trioxide to destroy all traces of peroxide. Titrate the copper with thiosulfate as directed above under standardization of the latter solution. (In the titration of samples containing bismuth the starch solution must be added after the thiocyanate and before the beginning of the titration.) B. PROCEDURE FOR ALLOYSSOLUBLEIN NITRIDHYDROFLUORIC ACIDSOLUTION.(This procedure is suitable as written for a 50 per cent tin-lead alloy containing less than 0.05 per cent of silver and 0.005 to 0.1 per cent of bismuth.) 1. Place 10 grams of the sample in a 600-ml. beaker containing 20 ml. of nitric acid, 15 ml. of hydrofluoric acid (48 per cent), and 100ml. of distilled water. Cover and heat gently (water bath) until solution of the alloy is complete (caution). Boil for 1 minute to expel most of the nitrous acid fumes. Dilute to 300 ml. with cold water. cool the solution to 40" C. Add 1 per cent potassium permanganate solution until the solution remains ermanently colored or fades but very slowly. 2. (Ifsilver is known t o be absent omit this paragraph.) Add 2 ml. of 1 per cent potassium iodide. Just decolorize the solution by adding dropwise freshly prepared sulfurous aoid solution. Digest cold for 5 to 10 minutes, avoiding direct sunlight. Decolorize again if necessary. Filter through asbestos. Wash with cold water. Discard precipitate. Dilute filtrate to 350 ml. 3. Drop 2 or 3 glass beads in the filtrate, cover, and heat to boiling. Boil vigorously until the solution is colorless and then for 5 to 10minutes longer. 4. Dilute to 350- to 400-ml. volume. Color again with potassium permanganate. Add 2 grams of tartaric acid. Add standard copper nitrate solution if necessary. Proceed as in paragraph 3 of Procedure A.
TABLE I. DETERMINATION OF BISMUTH AND COPPERIN THE PRESENCE OF LEADAND COMMON IMPURITIES OF LEAD
Experimental
Lead Present Urame
Copper Present
Bismuth Present
MO.
Mo.
Copper and Bismuth Present Found
Mo.
TABLE 11. DETERMINATION OF BISMUTH AND COPPERIN TEE PRESENCE O F LEADAND COMMON IMPURITIES O F LEAD" Bismuth Copper and Bismuth Copper Present Found Present Found Present Found Mg. Mo. Mo. Mo. Ivf~. Mu. 28.3 14.7 14.8 28.5 10 13.7 13.7 16.6 2.9 2.7 16.4 13.7 10 13.7 5.7 2.9 2.9 5.7 2.7 10 2.8 11.4 11.2 5.9 5.8 5.4 10 5.5 7.0 5.9 5.9 7.0 10 1.1 1.1 11.4 11.3 5.9 5.8 10 5.6 5.5 11.4 11.3 5.9 5.8 5.5 10 6.5 5.7 2.9 5.6 10 2.8 5.7 2.9 5.6 2.8 10 2.8 2.7 8.6 8.8 5.9 6.0 10 2.8 , 2.7 11.4 11.6 6.9 10 5.5 6.1 5.6 (1 No tin was added in these experiments. Lead Present Grams
...
...
OF BISMUTH AND COPPER I N SYNTABLE 111. DETERMINATION THETIC LEADSOLUTIONS AFTER SEPARATION OF SILVER
Copper Present Found Mg. Mg. 11.0 10.7 4.2 4.4 4.4 4.3 4.4 4.4 4.4 4.2 4.4 4.3 4.4 4.4 11.0 10.9
Bismuth Present Found Mg. MQ. 5.1 5.0 5.0 5.0 5.1 5.0 2.5 2.5 5.2 5.0 5.2 5.0 1.1 1.0 10.4 10.0
Bismuth and AntiCopper mony Silver Present Found Present Present Mg. Mo. Mo. Mg. 6 15.8 100 16.0 1 100 9.2 9.4 ... 5 9.4 9.4 1 6.9 50 6.9 .. 9.4 9.4 ... 9.5 9.4 100 5 5.4 5.5 100 5 21.3 21.0
...
..
Synthetic solutions containing 6 ml. of nitric acid, 1 to 2 grams of tartaric acid, 100 mg. of antimony, 10 mg. of arsenic, 5 mg. of iron, 10 mg. of tin, 5 mg. of cadmium, and known amounts of lead, copper, and bismuth, all as nitrates, were diluted t o 250-ml. volume and analyzed as described in Procedure A. The anolyte was lead nitrate solution containing 3 per cent of nitric acid in which the lead concentration approximated that of the catholyte. The results are shown in Tables I and 11, Synthetic samples containing 15 grams of lead nitrate, 8 ml. of concentrated nitric acid, 1 gram of tartaric acid, and known amounts of copper, bismuth, silver nitrate, and tartar emetic were prepared and diluted t o 150 ml. The solutions ~~
~
TABLE IV. DETERMINATION OF BISMUTH AND COPPERIN LEADTINALLOYB FOLLOWING REMOVAL OF SILVER Copper Present Found MQ. Me. 11.0 11.2 11.0 11.2 4.4a 4.4 11.0a 10.9 4.46 4.5 5.0 4.7 5.0 4.8 4.9 5.0 5.1 5.0 12.5 13.0
a
b 0
Bismuth Present Found Mg. Mg.
... ...
5.0 5.0 2.5 5.0 5.0 5.0 2.5 5.0
... ... 5.0
5.3 2.7 5.0 5.7 4.8 2.6 5.0
Copper and Bismuth Silver Present Found Present
Mu.
Mu.
9:4 16.0 6.9 10.0 10.0 10.0 7.5 17.5
4:4 16.2 7.2 9.7 10.5 9.7 7.6 18.0
..
..
Mu.
.. 5 5
..
3 .. ..
.... ..
100 mg. of antimony as tartar emetic added with the standard copper. 10 mg. of tellurium (dissolved in HNOa) added with the copper. 100 mg. of arsenic trioxide added with the copper.
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414
TABLEV. PRECISION OF METHOD APPLIEDTO TYPICAL SAMPLES Deviation from Mean
Deviation Bismuth from Mean
Sample Designation
Copper
Commercial lead Av.
0.065 0.063 0.064
4-0.001 -0,001 *0.001
0.014 0.015 0.0146
-0.0002 -0,0002 -0,0002
Av.
0,056 0.056 0.056 0.057 0.0562
0.124 0.124 0.122 0.122 0.123
%
%
No 1 i%'antimony lead
+0.0008
+=0.0004
-0.0005 +0.0005 t0.0005
fO.001
+0.001
-0.001 -0.001 tO.001
fO.001
0.064 0.060
Av.
0.059 0.059 0.0593
fO.0007 -0.0003 -0.0003 *0.0004
0.065 0.063
4-0.002 h0.002
-0.001 +0.001 *0.001
0.038 0.034
Av.
0.022 0.024 0.023
+0.002 -0.002 t0.002
No. 2 l%'antimony lead
0.060
40-60 lead-tin solder
0.036
-0.003
VOL. 8, NO. 6
The data presented in Tables I to V indicate that there are no significant constant errors in the method. The precision of Procedure B, in which hydrofluoric acid is used, is somewhat lower than Procedure A because of the poorer character of the plates obtained in this solution. I n Procedure B there is also more danger of loss of metal by re-solution during the washing of the plate. The precision limits of Procedure A are as follows: bismuth, *50 parts per thousand when the bismuth content of the sample is 0.05 per cent; copper, *30 parts per thousand when the copper content is 0.05 per cent. The precision limits of Procedure B are as follows: bismuth, * 100 parts per thousand when the bismuth content is 0.05 per cent; copper, *75 parts per thousand when the copper content is 0.05 per cent.
Literature Cited were boiled to remove nitrous acid fumes, and were then diluted to approximately 300 ml., cooled to 65' C., and analyzed as described in Procedure A. The results are shown in Table
111. Ten grams of a 50:50 lead-tin alloy were dissolved in a nitric-hydrofluoric acid mixture as recorded in Procedure B. The solution was diluted, 2 grams of tartaric acid were added, and the solution was electrolyzed a t 65" to 75' C. in the leadplatinum cell. The plate was discarded. Known amounts of copper, bismuth, and silver as nitrates were added to the purified solution. Copper and bismuth were then determined as described in Procedure B. (See Table IV.)
(1) Collin, Analyst, 55, 312 (1930). (2) Ibid., 55, 495 (1930). (3) Ibid., 55, 680 (1930). (4) Ibid., 56,90 (1931). (5) Francois, Ann. chim., 191 12, 178 (1919). (6) Hollard, Bull. soc. chim. de Paris, [3]29, 116 (1903). (7) R.Norton and Co., Worcester, Mass. (8) Sand, Analyst, 55, 309 (1930). (9) Schleicher, 2. anal. Chem., 83, 127 (1931). (10) Tutundzic, 2. anorg. allgem. Chem., 190, 59 (1930). (11) Ullgren, 2. anal. Chem., 7,442 (1868). RECEIWD August 11, 1936. Presented before the Division of Physical and Inorganic Chemistry at the 91st Meeting of the Amerioan Chemical Society, Kansas City, Mo., April 13 to 17, 1936.
Separation of Strontium, Barium, and Lead from Calcium and Other Metals By Precipitation as Nitrates HOBART H. WILLARD AND EDWIN W. GOODSPEED, University of Michigan, Ann Arbor, Mich.
I
3 T H E quantitative separation of strontium from calcium
there has always been considerable difficulty due to the fact that most of the properties of strontium are very closely related to those of calcium. Thus, although the determination of either strontium or calcium alone is easily carried out, the task of determining them when taken together presents greatly increased difficulty. By extracting the mixture of anhydrous nitrates with absolute alcohol, in which strontium nitrate is not very soluble, Stromeyer (5) separated strontium from calcium. Rose (4) improved this method by using a mixture of equal parts of absolute alcohol and anhydrous ether, in which strontium nitrate is much less soluble than in alcohol alone. With certain modifications introduced by Fresenius (I), this method is the one most used today. Rawson (3)suggested the use of concentrated nitric acid as the extracting solvent. Since precipitation methods are known to be superior to extraction methods, both in accuracy and ease of manipulation, it seemed desirable to develop such a method for the separation of strontium from calcium and other metals.
Preparation and Standardization of Solutions c. P. strontium nitrate was dissolved in water and filtered to remove any insoluble impurities. The strontium was precipitated as the nitrate by the slow addition, with stirring, of enough 70 per cent nitric acid to bring the acid concentration of the solu-
tion up to about 40 per cent. The strontium nitrate was filtered off, washed with 40 per cent nitric acid, dissolved, and then reprecipitated a second time in a similar manner. The solution of strontium nitrate was standardized by pipetting 10-ml. samples into weighed crucibles, adding a few drops of nitric acid, and evaporating to dryness on a very low-temperature hot plate. The strontium nitrate was then dried 2 hours at 130' to 140' C., and weighed. This standardization was checked by precipitating 10-ml. samples as the sulfate with a tenfold excess of sulfuric acid from a 50 per cent alcohol solution. After standing 12 hours, the precipitate of strontium sulfate was filtered throu h a Gooch crucible and dried 1 hour in a muffle heated to 500" The results obtained by the two methods agreed within 0.1 mg. Solutions of barium nitrate and lead nitrate were purified in a similar manner and standardized by evaporating to dryness and weighing as the nitrates. The calcium nitrate solution was prepared from reagent quality calcium carbonate by dissolving it in dilute nitric acid. It was purified by adding 100 per cent nitric acid until the acid concentration was 80 or 81 per cent, the volume being such that a small amount of calcium nitrate was precipitated. After standing 0.5 hour the precipitated calcium nitrate (along with any barium or strontium nitrates that might have been present) was filtered off and discarded. The filtrate was evaporated t o dryness t o remove the nitric acid, and the calcium nitrate was then dissolved in water. The solution was standardized by precipitating IO-ml. samples as oxalate, followed by ignition at 500' C. to the carbonate. Although fuming nitric acid as purchased could be used SUCcessfully for obtaining the desired acid concentration, in most of this work freshly prepared 100 per cent nitric acid was employed. This was prepared by distilling a mixture of c. P. sodium nitrate
8.
