.Areametric Estimation of Small Amounts of Sulfate as Barium Sulfate V. R. DAMERELL AND P. SPREMULLI Western Reserve University, Cleveland, Ohio
R
ECENTLY a method was published for the estimation of small amounts of heavy precipitates by measuring the area Of the formed when the precipitate was gathered to the center of the flask bottom (9). The present authors have used a similar technic in the case of barium sulfate,and have succeeded in working outa method which for certain types of analyses not requiring great accuracy has several advantages over methods in use a t present (8). The apparatus is simpler than that used for any of the present comparable methods, including the centrifugal (I), since only a 50-cc. Erlenmeyer flask, a dropping tube, and a millimeter rule are required. The technic is corres p o n d i n g l y s i m p l e , and smaller amounts (0.01 to 4 or more milligrams) can be determined than in the case of most other methods in use at the present time, including the centrifugal method. These qualifications make the method valuable for such routine analyses as sulfate in water, sulfate impurities in chemicals, and sulfur in certain organic materials, FIGURE 1. APPARAPrecipitation of Barium T U S USED FOR Sulfate BARIUM SULFATE PRECIPITATION A great many methods of DreciOitation tried, knd at first difficulty was encountered in obtaining a sufficiently reproducible crystal size. After some experimenting, it was found
were
rate of boiling was SO adjusted that the volume remained a t ap-
i:r%&
prsg:$i chloride solution had all run in and the resulting volume was about 10 cc., the stopper and dropping tube were removed, and the flask was a t once tightly stoppered and allowed to cool for 30 minutes or longer. If the amount of precipitate formed was about 0.2 mg. or more, it could be measured after 30 minutes. For smaller amounts than this, the authors allowed the flask to stand overnight.
Measurement of Precipitates The flask bottom was first cleaned and dried, and the flask was swirled twenty or thirty times, to bring most of the precipitate to the center of the flask. Then, after allowing a minute or two for settling, the flask was tipped, so that the edge of the liquid crept across the flask bottom nearly to the center, or to the edge of the spot of precipitate. This movement, to sweep the remaining precipitate to the center, was repeated about ten times to the left, ten times to the right, and ten times each backwards and forwards, although occasionally a precipitate adhered to the flask bottom so tightly that more tipping was necessary. When the precipitate was essentially all in the center of the flask bottom, the flask was swirled gently. In this operation the bottom of the flask was kept against a piece of black, glazed paper, and the flask was moved in small circles. This caused the precipitate to form a cone with steeper sides than could normally exist when the flask was not in motion, so that when rotating was stopped (after fifteen or twenty circles) the precipitate flowed outward from the pile, automatically adjusting the height of the pile and producing sharp, smooth edges. Infrequently a precipitate of such character was obtained that this rotation produced a scattering, and then the earlier vibration technic had to be used tal. The average diameter of these mots was estimated to the ‘nearest 0.1 mmy with a transparent millimeter rule against a white background. For the smaller spots a standard spot card, described earlier (a), was used, although this was not indispensable. The areas were rapidly obtained from the
holed, grooved rubber stbpper. OF BARIUMSULFATE SPOTS FIGURE 2. PHOTOGRAPHS Ten cubic centimeters of the solution containing sulfate were put in a 50-cc. Pyrex Erlenmeyer flask. The solution had been made neutral by RESULTS USINQPOTASSIUM SULFATESOLUTIONS WITH AND TABLBI I. REPRESENTATIV~ adding very dilute sodium hvdroxide and hvdrochloric WITHOUT IMPURITIES Sulfate Found acid Iuntil phenolphthalein Sulfate Present Impurity Present Sulfate Found Sulfate Premnt Impurity Present Me. Me. Mo. Me. just turned colorless. The stopper containing the drop1 gram MgCls 0.31 0.30 0.01 0.01 None 1 gram M Clt 0.45 0.60 0.04 ping tube was put in the flask None 0.03 0.01 I gram K& 0.03 0.08 None 0.08 (Figure 1) and the whole was 0.50 1 gram KCI 0.53 0.11 None 0.10 held by a test-tube holder and 2.0 1 gram KC1 1.9 0.27 None 0.30 1 gram CaClr 2.3 2.0 0.45 swirled over a small flame None 0.60 1 gram NH4Cl 0.05 0.03 0.72 None 0.70 until boiling started. Then 1 gram NHaCl 0.00 0.08 0.9 None 1.0 10 CC. of barium chloride0.10 1 gram NHiCl 0.14 1.9 None 2.0 0.70 1 gram NHiCl 0.69 3.0 hydrochloric acid s o l u t i o n None 3.0 3.0 1 gram NH&l 2.9 4.0 None 4.0 were put in the dropping tube, 0.40 1 gram NaCl 0.30 and the flask was swirled 0.50 I gram NaCi 0.70 1.9 and the contents were boiled 2.0 1 gram NaCl 2.8 1 gram NaCl 3.0 while the barium chloride solution dropped in. The 123
INDUSTRIAL AND ENGINEERING CHEMISTRY
124
VOL. 9, NO. 3
diameters by a numerical table. The diameters themselves could have been used instead of the a r e a s , b u t i n view of the possibility of an occasional irregular spot the areas were considered more trustworthy. The areas so obtained were then compared to those produced by known amounts of barium sulfate, completing the analysis.
due to the volatilization of a small amount of hydrochloric acid in the boiling, with resulting decreased solubility of 5 the barium sulfate. The flask was tightly stoppered a t once after boiling the solution, because this seemed largely to eliminate the formation of precipitate on the surface of the liquid. For very small amounts (0.1 to 0.01 mg.) i t was not necessary to use such care in the addition of the barium chloride Examples of spots obtained solution, because the main portion of the are given in Figure 2, and precipitate did not form until after the r e p r e s e n t a t i v e results are solution had cooled. given in Table I, including The method has several advantages results obtained in the presbesides requiring a very simple apparatus ence of large amounts of so.and technic. The determination is made dium, calcium, magnesium, in the precipitation flask, and there is no potassium, and chloride ions. transferring or filtering. No indicator or Average areas, corresponding IO 2 0 30 titration is required, and no weighing. to various weights of preMILLIGRAMS SULFATE The spot size can be easily measured in cipitates, are shown by t h e FIQURE3. AVERAQE AREASOF BARIUM SULFATE PRECIPITATES a c o l o r e d s o l u t i o n . A working time curve in Figure 3. However. of only about 15 minutes is required for the b&t results, indi: for a determination. This, added to a standing time of half vidual flasks should be calibrated with known amounts of barium sulfate, since with the swirling technic different an hour makes a total time of 45 minutes for the estimation of 0.2 mg. or more. The standing time for less than 0.2 mg. is flasks gave somewhat different areas for the same weight of barium sulfate, particularly with the larger amounts. longer. The precipitate is not destroyed in the determination, and may be used later in another way, if desired. The area Discussion of the spot can be checked as many times as desired. Finally, if an occasional precipitate forms of such a character that an Errors in the method are chiefly due to differences in crystal size and possibly in shape and not to errors in measureinaccurate measurement will result, the analyst usually becomes aware of this in handling it, and he can make a ment, as good checks could be obtained on the latter for an correction or discard the sample without blindly getting an indefinite number of times with any given sample. The authors believe that the point to watch most carefully in the erroneous result. entire procedure is the first addition of barium chloride soluLiterature Cited tion, since this governs the number of crystal nuclei formed, which in turn governs the number and size of those eventu(1) Arrhenius, O.,J . Am. Chem. Soc., 44, 132 (1922). ally formed. (2) Darnerell, V. R., and Axelrod, M.,Ibbid., 57, 2725 (1935). (3) Schroeder, W. C., IND.E m .C H ~ MAnal. . , Ed., 5, 403 (1933). The solution was boiled down to the volume occupied by the (barium sulfate-saturated) barium chloride solution. I n Presented before the Division of Phyaica RECEIVEID September 21, 1936. this way any solution of barium sulfate was overcome. Actuand Inorganic Chemistry at the 91at Meeting of the Amerioan Chemioal ally there was a small blank, about 0.005 mg., probably Society, Kansas City, Mo., April 13 to 17. 1936.
Rare Earth Salts Precipitation and pH Studies with the Glass Electrode J. A. C. BOWLES' AND H. M. PARTRIDGE, Washington Square College of New Yorlr University, New York, N. Y.
T
H E p H values a t which some of the rare earth salts are precipitated from their solutions by alkalies have been investigated to some extent by Hildebrand (4) and Britton (2) with the use of the hydrogen electrode. The present p H investigation was done by use of the glass electrode on solutions of the sulfates of lanthanum, cerous and ceric cerium, praseodymium, neodymium, ytterbium, and thorium, also the chlorides of neodymium and lanthanum. Since most methods for the separation of cerium from the other rare earths depend upon the oxidation of the cerium to the ceric state and the ultimate precipitation of the element as a basic ceric salt (nitrate or sulfate), and since this salt is sparingly soluble even in a fairly strong acid solution, a method for separating cerium by simply controlling the pH suggested itself. 1
Present address, United Drug Company, Boston, Masa.
The changes in potential a t the surface of the glass cell were measured by a vacuum-tube null-indicator used and described by the authors (6) in an investigation of the pH of normal and malignant tissues. In this work the ordinary saturated calomel electrode was used as the reference electrode. A potassium chloride agar-agar bridge connected this electrode to the solution being measured. Any contact potentials developed were not a source of error, because the whole system was calibrated at constant temperature before and after a series of measurements with buffers of known pH values. The buffer solutions were those of Clark and Lubs (3) and were compared with the quinhydrone and two hydrogen electrodes. The two calibrations, even after a titration lasting 18 hours, were never found to vary by more than two or three hundredths of a pH unit and this difference was in the low and high pH regions. The solutions were titrated in an air chamber whose temperature was 25' =t0.3' C., using a Beaver regulator in con-
