Quantitative Spectrographic Studies of Co- Precipitation I. Magnesium in Calcium Oxalate STEPHEN POPOFF, LOUISWALDBAUER, AND D. C. MCCANN,State University of Iowa, Iowa City, Iowa HE quantitative separaQUANTI T A TI VE spectrographic analysis Both long-fiber and micro asbe+ tos were used in equal quantities. where its tion of calcium and magis reliable and rapid in such The long fibers were separated nesium has always been use is justified. The method is here shown to and sorted by hand, This asa time-consuming and rather uncertain procedure. The large be applicable throughout a range Of to bestos showed no loss on several per cent magnesium and may, by further refineignitions, with or without internumber of papers pertaining to ments, be extended to even lower concentrations. mediate washings. the separation of calcium as the It is ,found that magnesium is SOLUTIONS.The stock soluoxalate, and the equally large tion of calcium chloride was number of opinions expressed are dependent primarily upon the time of digestion prepared by we"ighing such a ample proof that the problem deserves further study. after the solution is alkaline, and somewhat upon quantity of the liquid CaC12,Previously, i n v e s t i g a t o r s , the time of precipitation and the amount of 6H20 as would give 0.40 gram of calcium oxide per 50 cc. This in their e s t i m a t i o n s of conammonium oxalate present, but is practically independent of ihe ammoniumchloride content. amount was found convenient tamination, have relied on loss of weight with a second preto handle The complete precipitation of calcium is deThe sol;tions used in checkcipitation or a gravimetric deammonium ing the method of analysis were termination of the magnesium pendentWonthepresenceOf oxalate for both calcium and magnesium. prepared from the spectrographioxidein the DreciDitate. Because of the incomplete precipically pure CaC12.6H20. The tation of calcium in many cases and the very small amounts method of drying was the one used by Richards in the deterinvolved, such methods have yielded a variety of conclusions. mination of the atomic weight of calcium ( 5 ) . The decomWith the hope of eliminating as many of the foregoing position of the chloride was found to be negligible. Two difficulties as possible, a study has been made by means of grams of the salt were found to require only 0.12 cc. of 0.1 N quantitative spectrographic methods. Gravimetric deter- hydrochloric acid for neutralization. The anhydrous calminations have been made for each condition in order to cium chloride prepared in this manner was made by weight determine the variation from the weight taken. Spectro- to a known concentration similar t o that used in the regular graphic determinations of the per cent of magnesium were analyses. made on a sample precipitated under identical conditions. The spectrographic standards were prepared by weight from the same CaC12.6H20. The solution was then checked MATERIALS USED gravimetrically. From the solution of known concentration, Pure distilled water, showing no trace of magnesium spec- a range of standards was prepared from 0.0001 to 1.0 per cent trographically, was prepared by redistillation from alkaline magnesium by introduction of measured volumes of a dilute potassium permanganate in an all-Pyrex still. All crys- solution of magnesium chloride. Each solution was of the tallizations and solutions were made from this water. same calcium and hydrochloric acid content as those prepared Constant-boiling hydrochloric acid was prepared by dis- from the precipitates. tillation of 6 N hydrochloric acid in an all-Pyrex still. The The precipitates for spectrographic analyses were obtained first and last portions were rejected. from a third sample made simultaneously with the two graviAmmonium hydroxide, 6 N , was prepared by distillation of metric determinations. The precipitate was caught in a concentrated ammonium hydroxide into distilled water, small Hirsch funnel, using a filter paper. This method proved cooled in ice. The ammonium hydroxide was prepared in quite satisfactory, and a t the same time eliminated posliter quantities as used, and kept in paraffined bottles. sibility of contamination from asbestos, which might have Pure ammonium oxalate, (NH,),C20d.HzO, was prepared resulted from the use of the gravimetric precipitates. The by crystallizing Riedel and de Haen's material three times calcium oxalate thus obtained was ignited to calcium oxide from distilled water. under the same conditions as the others, using a platinum The stock solution of magnesium chloride was prepared by crucible. Solutions were obtained by dissolving 0.35 gram dissolving magnesium metal in constant-boiling hydrochloric of precipitate in 4 cc. of constant-boiling hydrochloric acid. acid. The solution gave no test for calcium. The magAPPARATUS.A mechanical stirrer was used for all prenesium chloride for spectrographic standards was prepared by cipitation. The stirring was accomplished a t a slow rate, uscrystallizing three times from redistilled water. ing a large S-shaped stirrer. A drilled cover for the beaker The hexahydrate of calcium chloride was prepared from prevented entrance of dust particles. A short funnel, fed Riedel and de Haen's CaC12.6H~0,found to contain very from a separatory funnel, delivered the precipitating reagent small amounts of magnesium initially. The hexahydrate was directly to the liquid in the beaker. recrystallized three times from redistilled water. Each Pyrex glassware was used throughout. Several attempts crystallization was carefully prolonged for a period of 3 days. to use quartz beakers, bottles, and stills gave serious magThe final product showed less than 0.001 per cent magnesium nesium contamination, apparently from a scale which came by spectrographic analysis. from the surface of the quartz. Asbestos was prepared by washing with dilute hydroDigestion was carried out on an electrically heated hot plate chloric acid and ammonium hydroxide, and finally with water. designed to maintain a temperature of 90' C. Oaool
43
ANALYTICAL EDITION
Precipitates were ignited in an electric muffle made by the Barkmeyer Electric Laboratories. This furnace was capable of maintaining temperatures as high as 1300" C. and was found to be an invaluable means of ignition. Platinum Gooch crucibles were used for filtration and ignition. No plates were used in making the matte. Three separate layers of asbestos were added, carefully packing each with a stirring rod. A fourth was then added and deposited evenly by settling. The crucible was dried for an hour a t 110" C. and then ignited. Desiccators filled with fresh anhydrone and potassium hydroxide were used throughout the work. Calcium chloride gave values of the order of 2 mg. too high. Cs,
Cs,
Vol. 4, No. 1
was quite turbid (100 drops). The precipitate was then digested. If the precipitation was to be completed in one step, the total amount of ammonium hydroxide was added, omitting the intermediate digestion after 100 drops. Final or complete precipitation was made with constant stirring within a prescribed length of time, as determined by a calibrated tip for the separatory funnel containing the precipitating reagent. Alkalinity was determined by the use of a few drops of phenolphthalein. Precipitates were washed twice by decantation, using a hot dilute solution of ammonium oxalate and ammonium hydroxide. After transfer to the filter, the calcium oxalate was washed free from chlorides. All precipitates were placed in the cool furnace, heated to
Ca '1
BY SPECTROGRAPHIC METHOD FIGURE 1. DETERMINATION
A Hilger E2 quartz spectrograph was used, giving a dispersion of 83 8. per cm. a t 2500, and 285 8. per cm. a t 4000. The arc was operated on 110 volts d. c. Electrodes were of Acheson graphite, 2 inches (5.08 om.) long, 6/16 inch (0.79 cm.) in diameter, vertically opposed. The electrodes were held in position by screwing them into an aluminum disk, which also served to cool them to some extent. The electrodes were found to contain relatively minute traces of magnesium, as purchased. The upper electrode, the negative pole, was cut in the form of a very blunt chisel, whose wedge-shaped end was set normal to the axis of the collimating lens of the spectrograph. The lower electrode, the positive pole, was drilled to a depth of 1 cm., using a l/s-inch drill. The length of the arc was 3 mm. Each drilled electrode was burned in the arc 30 to 60 seconds to obtain a uniform, high porosity. The plates used were Eastman 33, which were found t o give very clear photographs in the region of investigation. GENERAL METHODS OF ANALYSIS Portions of calcium chloride solution to be used for analysis were weighed into 600-cc. beakers. To this amount sufficient hydrochloric acid was added to prevent the formation of a precipitate on addition of the ammonium oxalate. Magnesium ion in a 50-cc. volume was next added, if contamination was to be observed. The solution was then diluted with water to give a volume of 200 cc. and heated to first signs of boiling. The ammonium oxalate was then added in a 50-cc. portion. If two digestions were to be made, 3 N ammonium hydroxide was then added drop by drop until the solution
1075" C. (approximately 0.5 hour), and maintained at this temperature for 1 hour. More specific details accompany the experimental data. Creeping was prevented by keeping the side of the beaker above the surface of the solution dry, save for the water which condensed, and by forming all precipitates within the solution rather than along the sides. It was for this purpose that hydrochloric acid was used to prevent a precipitate forming on addition of ammonium oxalate, and that a short funnel dropped the precipitating reagent directly into the solution. For spectrographic determination, electrodes and solutions were prepared as previously described. Six drops of the solution to be analyzed, or of the standard, were placed in each electrode. The electrodes so prepared were dried in an oven at 110' C. for at least 6 hours. When not completely dry the arc does not burn well. For the photography, the arc was set a t the proper position as determined by the condensing lens and the spectrograph. The condensing lens should be mounted on a support clasped directly to the spectrograph. On the same support should be mounted a pointer showing the exact position of the arc. The arc was struck by shorting the poles with a carbon rod, the shutter on the slit being open so that the exposure began a t once. After each 30 seconds the rheostat was adjusted to maintain a current of 10 amperes. Each exposure was timed a t exactly 1.5 minutes. During the entire time, the arc was adjusted as a unit in a line normal to the axis of the collimating lens, so that the image fell directly on the slit. No effort was made to adjust the individual electrodes during the exposure. Nineteen photographs were taken on each plate. Final analysis of the plate was made visually by
January 15, 1932
INDUSTRIAL AND ENGINEERING CHEMISTRY
light reflected from a white sheet of paper (Figure 1). Although this method was none too accurate, i t was felt that visual estimation of intensities gave values as reliable as the method seemed to warrant. No claim is made that determinations are accurate beyond one significant figure. The spectrographic method as described is essentially that given by Nitchie (4). Comparison with standards was made by photographing a series covering the expected range of precipitates, at the top of the plate, followed by the spectra for analysis. This method was found to be as reliable as that of photographing the unknown between several standards. Fortunately, the per cent of magnesium oxide in the precipitates was within the range which allowed the use of the five lines of wave lengths 2783, 2781, 2780, 2778, 2777 A., where they were very weak. Comparison is much easier under such conditions.
EXPERIMENTAL RESULTS Attention was first directed to possibilities of precipitation of magnesium oxalate. A few brief experiments were made to determine some characteristics. I n each case 0.26 gram of magnesium ion, 1 gram of ammonium oxalate and ammonium hydroxide to alkalinity were used. The total volume was 225 cc. The results follow: 1.
AT R O O M TEMPERATURE; DIGEST, NO STIRRINQ
HRS. 0 3 20
TURBIDITY None None None 11.
AT 100° C.; DIQEBT, NO ETIRRING
None None. Tu&d Turbid Very turbid
0 3 20 44
0 2 2.5 3
111.
AT 100'
C . ; DIGEB'$
None; stirred None: stirred Slightly turbid Very turbid
Judging from these experiments and those of Fischer (W), the determinations were started with a variation in the time of digestion. TABLEI. EFFECT OF VARIATION IN TIMEOF DIGESTION ON CALCIUM OXALATEPRECIPITATE (One digestion; wt. taken, 0.3963 gram of CaO) W T . WITH GAIN OVER TIMEOF WT. WITH Mg EQUIVA- GAIN OVER WT. WITH MgO IN PPT. D I G ~ S T I ONo N Mg LENT TO C s WT. TAKEN No Mg Min. Gram Gram % % % 12 00. HC1 (6 N ) , ppt. (1) 1.2 z. (NH~)zCZO~.HZO. .. in 3 A n . 15 0.3957 0,3972 0.27 0.4 0.8 0.3961 0.3976 30 0.3958 0.3960 -0.02 0.05 0.8 0.3961 0.3963 60 0.3962 0.3951 -0.12 -0.12 0.4 0.3964 0.3963 0.3954 0.3962 (2) 2.4 g. (NHa)zCzO&.HzO,12 00. HC1 (6 N ) , ppt. in 7 min. 0.1 0.1 0.1 15 0.3961 0.3964 0.3964 0.3971 -0.02 0.1 0.3 30 0.3961 0.3964 0.3963 0.3969 0.6 0.7 1.0 60 0.3957 0.3996 0.3961 0.3978
One and two-tenths grams of ammonium oxalate gave an excess of approximately 0.2 gram. Twelve cubic centimeters of hydrochloric acid (6 N ) were required to keep the precipitate in solution. The per cent of magnesium oxide in the last column was determined by means of the spectrograph. This value should be equal to the per cent gain over the weight taken, if the calcium is completely precipitated. If such is not the case, the method is not suitable for a double precipitation. The values in the last three columns must approach zero if the method is suitable for a single precipitation.
45
Part 1 of Table I shows considerable magnesium oxide contamination. It is also evident that the calcium is not completely precipitated. A few determinations made with a magnesium-ion concentration one and a half times that of calcium did not increase the magnesium oxide contamination but further prevented precipitation of calcium. Part 2 shows that a digestion over 15 minutes is unsatisfactory. However, twice the required amount of ammonium oxalate is seen to remedy the incomplete precipitation of calcium. In this case, a magnesium-ion concentration one and a half times that of calcium made no appreciable difference. Table I is in keeping with Blasdale's suggestion that the oxalate ion is removed by formation of the little-ionized magnesium oxalate (1). I n the experiments tried so far, the precipitates were exceedingly fine-grained, and did not settle well. The next series (Table 11) represents an attempt to form seed crystals by digestion in an acid solution, and subsequent formation of precipitates on these same crystals. TABLE11. EFFECT OF VARIATION IN TIMEOF FIRST DIGESTION ON CALCIUM OXALATEPRECIPITATE (Two digestions; wt. taken, 0.3963 gram of CaO)
TIM. OB DIQESTION
GAIN GAIN W T . WITH OVER OVER WT. WITH Mg E Q ~ V A -WT. W T . WITH MpO IN N o Mg L E N T T O Ca TAKEN N o Mg PPT.
2nd Hour Gram Gram % % % (1) 1.2 g. (NH4)zCzOaHaO. 12 00. HCl (6 N ) , ppt. in 3 min. 2.6 0.25 0.3963 0.3968 0.06 0.02 0.3 0.3966 0.3962 1.6 0.25 0.3969 0.3967 0.04 0.12 0.3 0.3961 0.3966 0.3966 0.3965 0.5 0.25 0.3954 0.3961 0.0 0.22 0.07 0.3953 0.3965 0 0.25 0.3957 0.3972 0.27 0.4 0.8 0.3961 0.3976 (2) 1.2 g. (NH3aCzOa.Hz0, 12 00. HC1 (6 N ) , ppt. in 7 min. 1.5 0.25 0.3960 0.3962 -0.04 0.0 0.1 0.3963 0.3960 0.6 0.25 0.3957 0.3963 0.05 0.05 0.2 0.3961 0.3959 0 0.25 0.3958 0.3961 0.02 0.05 0.3 0.3963 0.3963 (3) 2.4 g. (NH4)zCzO&.HzO,12 00. HC1 (6 N ) , ppt. in 7 min. 1.6 0.25 0.3960 0.3960 -0.02 -0.02 0.07 0.3963 0.3962 0.3966 0.3962 0.3963 0.6 0.25 0.3957 0.3961 0.02 0.12 0.2 0.3960 0.3967 0.3961 0.3967 0 0.25 0.3961 0.8964 0.1 0.1 0.1 0.3964 0.3971 1st
NOUP8
TABLE111. EFFECT OF VARIATION IN TIMEOF SECOND DIGESTION ON CALCIUM OXALATEPRECIPITATES (Two digestions; wt. taken, 0.3963 gram of CaO) 1.2 g. (NH~)ZCZOI.H~O, 12 00. HC1, ppt. in 3 min. GAIN GAIN
TIMEOF
DIGESTION 2nd Hour Hours 0.25 1.5 1st
1.6
0.5
1.5
1
W T . WITH
WT.WITH Mg EQUIVAN o Mg
OVER
OVER
WT. W T . WITH MgO IN No Mg PPT. L E N T T O CTAKEN ~
Gram
Gram
%
%
%
0.3959 0.3961 0.3966 0.3961 0.3963 0.3965
0.3967 0.3965 0.3958 0.3960 0.3965 0.3965
0.05
0.12
0.3
0.0
0.3
0.02
0.6
-0.1 0.04
The formation of seed crystals proved to be useless in Part 1 of Table 11, probably because of too rapid precipitation. Parts 2 and 3, however, produced precipitates which settled out very quickly where the first digestion was 1.5 hours. Microscopic examination disclosed aggregates of crystals. Interestingly enough, decreasing the time of the first digestion increases the magnesium oxide contamination in every case, except in Part 3. The addition of double the amount of ammonium oxalate again brings the results into closer accord.
46
ANALYTICAL EDITION
The increase in time of precipitation appears as an intermediate step between Parts 1 and 3. A variation in the time of the second digestion (Table 111) gave results somewhat analogous to Part 1of Table 11. Here again the data point t o the fact that a digestion of more than 15 minutes after the filtrate has been made alkaline gives progressively poorer results. Another possibility for variation was the ammonium chloride content. The variation was produced by addition of hydrochloric acid. Contrary to the usually accepted ideas, the amount of ammonium chloride has no appreciable effect. If anything, it is undesirable, as in Part 4 of Table IV. TABLEIV. EFFECTOF AMMONIUM CHLORIDE ON CALCIUM OXALATE PRECIPITATES (Wt. taken, 0.3963 gram of CaO; PFrts I, 3, and 4, two digestions; Part 1, one digestion) WT. WITH Mg GAINOVER MgO IN TIMEOF DIGESTION NHaCl E Q I J I V A L ~TON T Ca WT. TAKBN PPT. Hours Grams Gram % % (1) 2.4 g. (NHn)zCzOrHzO, ppt. in 7 min. 1st 2nd 0.07 1 . 5 0.25 4 0.3963 -0.02 0.3960 8 0.3961 0.2 0.1 0.3967 ppt. in 3 min. ( 2 ) 1.2 g. (NH~)ZCZO~-HSO, 0.3962 -0.12 0.4 1 4 0,3964 0,3965 0.0 0.5 8 0.3961 ppt. Z Oin , 7 min. (3) 1 2 g. ( N H ~ ) ~ C Z O ~ . H 0.3960 -0.04 0.1 1 . 5 0.25 4 0.3963 0.3949 -0.35 0.1 8 0.3948 (4) 1.2 g. (NH4)zCzOcHzO,ppt. in 3 min. 0.07 0.2 1.5 0.25 4 0.3959 0 3961 0.3944 -0.4 0.7 8 0.3951
'
A further possibility presented itself in the addition of ammonium oxalate to the alkaline solution rather than the ammonium hydroxide to the solution containing the oxalate. Although the reverse method, whose results are given in Table V, is unsatisfactory for analysis, it is interesting to note that the contamination is practically constant. Such results are to be expected where the solutions in which the precipitates are formed are very similar. With the completion of the part of the work in which no magnesium was used, the method using two digestions of 1.5 and 0.25 hour, shown in Part 3, Table 11, was chosen as the most convenient method, giving the highest value and the most granular precipitate. It was therefore used in the analysis of the known solution of calcium chloride prepared by Richards' method of drying. The results shown in Table VI prove the method, and it was subsequently used to standardize the stock solution. METHODS ON CALCIUM TABLEV. EFFECTOF REVERSE OXALATEPRECIPITATES ( W t . taken, 0.3963 gram of CaO; one digestion except in last part) WT. WITH Mg GAINOVER TIMEOF DIQESTION EQUIVALENT TO c a W T . TAKBN MgO IN PPT. Hoars Gram % % 1.2 g. (NH3zCz04.Hz0, 12 cc. HCI (6 N ) , ppt. 3 min. 0.0 0.5 0.25 0.3960 0.3964 1.2 g. (NH4hC104.Hz0, 12 cc. HC1 (6 N ) , ppt. 3 min. 0.6 1 0.3961 0.2 0.3967 1.2 g. (~XH~)ZCZO~.HZO, 12 cc. HC1 (6 N ) , ppt. 7 min. 0.25 0.3946 -0.35 0.5 0.3953 2.4 g. (NH~)ZCZO~+HIO, 1 2 CO. HC1 (6 N ) , ppt. 7 min. 0.25 0.3984 0.6 0.6 0.3999 2.4 g.' fNHhCzOcHz0, 12 ac, HC1 (6 N ) , ppt. 7 min. 1st 2nd 1 . 5 0.25 0,3972 0.27 0.5 0.3977
Vol. 4, No. 1
TABLEVI. TESTOF METHOD BY STANDARD CALCIUM CHLORIDE SOLUTION EXPJRIYJNTAL WT. Gram 0.3545 0.3547 0.3927 0.3930
I I1
CALCD.WT. Uram 0.3646 0.3930
Finally, regarding the data as a whole, there are only two methods satisfactory for either single or double precipitation. These are the methods tested by the known solutions (Table 11, Part 3), and the 0.25-hour digestion in Table I, Part 2. I n either case, the error of a single pfecipitation is only 0.0004 gram for a magnesium content as high as one and a half times the equivalent of calcium. TABLE VII. TESTBY SINGLEAND DOUBLE PRECIPITATION OF CALCIUM OXALATE (One digestion in first part; two in second) MgO BY DIFF. TIMEOB DIGESTION WT. TAKEN 1 PPTN. 2 PPTNR. IN WT. Hours Cram Gram Gram % 2.4 g. ( N H ~ ) ~ C ~ O ~ * 12 H S00. O ,HCI (6 N ) , .ppt 7 min. 0.26 0.3729 0.3734 0.3727 1.8 0.3735 0.3728 2.4 g. (NH~)zCZO~.HZO, 12 oc. HC1 (6 N ) , ppt. 7 min. 1--. Rt
2nd ___
1 . 6 0.25
0.3930
0.3928 0.3930
0.3927 0.3931
0.0
As a further check, values, given in Table VII, were obtained using both single and double precipitations, again using a magnesium concentration equivalent to that of calcium. The only essential difference between the two is the 1.5hour digestion in acid solution preceding final precipitation. The shorter method does not give as granular a precipitate as the other, and is therefore less easily filtered. The accepted method is as follows: Start with a sample which will give about 0.35 gram of calcium oxide. Adjust the volume of solution t o 200 cc., and heat to boiling. Add twice the necessary amount of ammonium oxalate in a 50-cc. volume, preventing precipitation by addition of hydrochloric acid. Then add ammonium hydroxide (3 N ) , drop by drop, until the solution is quite turbid. About 100 drops are necessary. Digest the precipitate for 1.5 hours at 90" C. Then complete the precipitation during a period of 6 to 8 minutes with constant stirring. Digest for 0.25 hour longer, then filter and wash at once. Ignite the recipitates in platinum to a temperature of 850' C. or more. d o l in a desiccator containing anhydrone or Pz05, and potassium hydroxide. Efforts to discover the way in which magnesium contaminates the precipitates met with little success. By means of a cataphoresis cell, all the particles were found to be charged negatively before washing, and possessed no charges after washing. This points to occlusion of C1- or Cz04-- rather than Mg+f. The crystals were too small to be of value under the polarizing microscope. Those in which magnesium occurred were in the order of fifteen times larger, however, and appeared to be monoclinic in some cases. Groth (3) states that crystals formed from a warm concentrated solution are the monoclinic calcium oxalate. Magnesium oxalate likewise appeared to be monoclinic. The data, however, are so very doubtful that one can only suggest that the magnesium is isomorphous with calcium, and that the larger crystals are due to the crystal habit of Ca, Mg(C2O4)2X HzO.
LITERATURE CITED (1) Blasdale, J. Am. Chem. SOC.,31, 1918 (1909). (2) Fischer, 2. anorg. allgem. Chem., 153, 62 (1926). (3) Groth, "Chemische Kristallographie," Vol. 111, Wilhelm Engleman, Leipzig, 1910. (4) Njtchie, IND.ENC.CHEX.,21, 1 (1929). (5) Richards, J. Am. Chem. SOC.,32, 25; 1577-90 (1910). RECJIVED June 22. 1931. Based on a thesis submitted by D . C. MoCrtnn in partial fulfilment of the requirements for the degree of master of science at the University of Iowa.
