INDUSTRIAL AND ENGINEERING CHEMIXTRY
April 15, 1931
The correlation in these instances is about as good as could be expected in view of the variation in the specific character of the material causing turbidity and the errors involved in the data, particularly the percentage of suspended solids. Conclusions
Although it has not been possible to test the spectrophotometric method for estimating turbidities very extensively, this method appears to have interesting possibilities. It is believed that the principle on which the method is based is satisfactory and that the method affords comparable results with various types of sugarhouse products. From a practical point of view, however, the method has a t least one drawback. The instrument used in this study is rather expensive, and as a>resultonly a few sugar laboratories are equipped with one but, since the light transmission needs
127
to be measured only a t a single wave length, it is possible that a.simplified apparatus would give satisfactory results. Literature Cited (1) Balch, Unpublished report. (2) Dawson, “Testing Apparatus and Method for Determining the Filterability of Raw Sugars,” paper presented before the Division of Sugar Chemistry at the 69th Meeting of the American Chemical Society, Baltimore, Md , April 9, 1925. (3) Honig and Bogstra, Facts About Sugar, 2 3 , 446, 470, 494 (1928). (4) Horne and Rice, IND. EKG.CHEM., 16, 626 (1924). (5) Ingersoll and Davis, Ibid., Anal. Ed., 2 , 248 (1930). (6) Lunden, Z. Ver. deut. Zuckerind., 77, 709 (1927). (7) Peters and Phelps, Bur. Standards, Tech. Paper 338 (1927). (8) Smith, Intern. Sugar J . , 26, 381 (1925). (9) Spencer-Meade, Handbook for Cane Sugar Manufacturers and Their Chemists, p. 293, Wiley, 1929. (10) Spengler and Landt, Z . Ver. deut. Zuckerind., 77, 431 (1927). (11) Wells, Chem. Rev., 3, 331 (1927). (12) Yoe, “Photometric Chemical Analysis. XI-Nephelometry,’’ Wiley, 1929.
Separation of Calcium and Magnesium by Molybdate Method’R. C. Wiley UNIVBRSITY
OF
MARYLAND, COLLEGE PARE, MD.
Calcium molybdate is quite insoluble in a solution calcium in solution was deterthat is nearly neutral. m i n e d by the m o l y b d a t e ring minerals contain Calcium may be determined accurately by precipitatmethod, and also by the percalcium and magneing as the molybdate. The presence of residual ammanganate method. The sosium. It is customary to monium molybdate in the washings from the calcium lution was about 0.1 N . separate the calcium from the molybdate does not prevent the precipitation of mag(2) A solution of magnemagnesium by making a nesium as the magnesium ammonium phosphate, and sium nitrate made by disdouble precipitation of the the presence of ammonium salts does not Interfere solving magnesium nitrate in calcium as the oxalate, evapowith the determination of calcium as molybdate. distilled water. The magr a t i n g the filtrates to a Calcium and magnesium may be determined in nesium in solutions was desmaller volume and then makmuch less time when the molybdate method for caltermined by precipitating as ing a double precipitation of cium is used than when the calcium is precipitated by magnesium ammonium phosthe magnesium. There is no the oxalate method. phate, and i g n i t i n g and question that this method of weighing in the usual way. d e t e r m i n i n g- calcium and magnesium is accurate. However, while it is accurate, it is The solution was about 0.115 N . (3) A solution of ammonium molybdate about 0.4 N , likewise tedious and time-consuming. The object of the present investigation was to find, if possible, a method whereby slightly acid with acetic acid. (4) A solution of ammonium molybdate about 0.4 h’ the double precipitations could be eliminated, thus saving a slightly alkaline with ammonium hydroxide. considerable amount of time. Smith and Bradbury (1) found that calcium molybdate Plan of Procedure for Precipitation in Alkaline Solution was only slightly soluble in water. Magnesium molybdate, on the other hand, is quite soluble in water. For these The calcium chloride solution and the magnesium nitrate reasons the writer thought it might be possible to separate these two elements by precipitating the calcium as the solution were carefully pipetted into the same beaker. The molybdate, washing the calcium molybdate well, and then solution was heated to boiling, a drop or two of concentrated precipitating the magnesium from filtrate and washings as ammohium hydroxide added, and the molybdate solution magnesium ammonium phosphate in the usual way. It was added at the rate of about one drop per second until an excess hoped that the presence of the molybdate in the filtrate would was present. The solution was then boiled until the supernot interfere with the precipitation of magnesium as mag- natant liquid was clear, and allowed to stand until cool. The precipitate was filtered from the solution by means of a tared nesium ammonium phosphate, as does an oxalate. Gooch crucible in which the layer of ordinary asbestos had been reinforced by a little ground asbestos. The suction on Solutions Used the crucible was carefully regulated. The precipitate was careThe solutions used, aside from the ordinary laboratory fully washed with ten portions of hot distilled water of about reagents, were as follows: 10 cc. each. The crucibles were then placed in an oven at (1) Carefully standardized solutions of calcium chloride 130” C. and dried for 30 minutes, then ignited and weighed. made by weighing out calcite, dissolving it in hydrochloric The filtrate was used for the determination of magnesium acid, and evaporating to dryness over a water bath, and then without evaporation. Its volume was about 100 to 125 cc. making up to the required volume with distilled water. The The magnesium was determined without double precipitation in the usual way; that is, by precipitating as the magnesium 1 Received August 16, 1930.
M
ANY naturally occur-
ANALYTIC ‘ALEDITION
128
ammonium phosphate, filtering in a Gooch, igniting, and weighing as magnesium pyrophosphate. The procedure for precipitation of calcium in an acid solution is the same as for an alkaline solution, except that a slightly acid solution of ammonium molybdate is used and no ammonium hydroxide is added. Table I gives results found when the molybdate solution used was slightly alkaline. The factor 0.2002 was used to convert calcium molybdate to calcium.
Table 111-Effect of A m m o n i u m Chloride on Calcium Found b y Molybdate Method in Slightly Alkaline Solution CALCIUM PRESENT CALCIUM FOUND Gram Gram 0.0512 0.0508 0.0512 0.0510 0.0506 0.0514 0.0514 0.0518 0.0506 Av. 0.0512 0.0511
of Precipitating Calcium a8 Molybdate in Slightly Alkaline Solution
Table IV shows results obtained with varying amounts of calcium and magnesium in a slightly alkaline solution in the presence of 3 to 5 grams of ammonium chloride.
Table I-Effect
--MAGNESIUM-I n original soln. Gram
0.0449 0.0442 0.0447 0.0446
Av. 0.0446
Found after removal of calcium Gram 0.0443 0,0445 0.0445 0.0446 0.0447 0.0447 0.0445 n 0447 0,0443
0.0445
- - - - C A L C I U M FOUND--By molybdate By permanganate method in method, original presence of soh. magnesium Gram Gram 0.510 0.0511 0.514 0.0514 0.608 0.0511 0.516 0.0512 0.0511 0.0514 0.0512 0.0515 0.0519 0.0512 0.512 0.0513
The results of the above experiment show that calcium can be separated from magnesium by the molybdate method. They show that magnesium is not adsorbed, occluded, or coprecipitated along with the calcium molybdate precipitate, and also that the results for magnesium are the same as they would be if no ammonium molybdate had been present when it was precipitated. This is in contrast to the results obtained when magnesium is precipitated in a solution containing the salts of oxalic acid. Table 11-Effect
of Precipitating Calcium as Molybdate in Slightly Acid Solution --CALCIUM
Table IV-Effect
of Separation of Varying Quantities of Calcium a n d
-------CALCIUM---Present Found Gram Gram 0 0500 0.0502 0.0504 0 0500 0.0500 0 0500 0.0501 0 0500 0.0500 0 0500 0.0498 0 0500 0.0502 0 0500 0.0500 0 0500 0.0496 0.0500 0.0500 0 0500 0.0202 0 0200 0.0199 0 0200 0 0200 0.0200 0 0200 0.0205 0 0200 0.0202 0.0197 0 0200 0.0200 0 0200 0.0200 0 0200 0.0201 0 0200 0.0201 0 0200
Magnesium ----MAGNESIUM---Difference Present Found Gram Gram Gram +0.0002 0.0125 0.0127 0.0125 $0.0004 0.0123 0.0125 0.0123 0.0000 0.0125 0.0123 $0.0001 0.0129 0.0127 0.0000 0.0129 0.0129 -0.0002 0.0129 0.0127 +0.0002 0.0129 0.0128 0.0000 0.0129 0.0123 0.0004 0.0129 0.0129 0.0000 0.0316 0.0314 +0.0002 0.0316 0.0318 0.0001 0.0316 0.0316 0.0000 0.0316 0.0316 SO.0005 +o ,0002 0.0316 0.0316 0.0316 0.0316 0.0003 0.0316 0.0316 0.0000 0.0316 0.0314 0.0000 0.0732 0.0730 +0.0001 0.0732 0.0733 +0.0001
-
-
-
Difference Gram +0.0002 -0.0002 - 0.0002 0.0002
-
- 0.0002
0.0000
- 0.0002 0 0001 - 0.0000 0.0006 -0.0002
$0 0002 0.0000
o ooon
0 0000 0.0000 0.0000 -0.0002 -0.0002 $0.0001 t
These results show a clean separation of calcium and magnesium. Comment and Discussion
The precipitate formed by ammonium molybdate and a soluble calcium salt is a white crystalline solid. The crystals are quite hard and dense and settle quickly. It is easily filtered and washed. When precipitating this compound, a stirring rod should not be placed in the beaker for, if the beaker is scratched, crystals form at the point scratched and these crystals are very difficult to remove with a policeman. The precipitate may not appear immediately. The calcium ion and the molybdate ion may both be present in 0.01 N to 0.05 N concentration for a few minutes without precipitation. The white precipitate which makes its appearance The results given in Table I1 show that it is possible to after a few minutes is rather soluble when first formed. separate calcium and magnesium by the molybdate method in After boiling for 10 to 15 minutes, however, it becomes highly a solution slightly acid with acetic acid. The results are insoluble. Sometimes the precipitate is not pure calcium almost exactly the same as those obtained when the solution molybdate but upon ignition in a Gooch it is converted into in which the precipitation is made is slightly alkaline with calcium molybdate. The color of the calcium molybdate should be pure white, as a blue color upon ignition shows ammonium hydroxide. The calcium in the solution of calcium chloride was deter- reduction. It may be ignited to constant weight in about 10 mined by the molybdate method when no magnesium was to 15 minutes in a Gooch. The white precipitate must not be present. Four determinations were made; 0.0511, 0.0513, ignited in a filter paper because it will be reduced. Water saturated with the white precipitate is less than 0.0512, and 0.0514 gram of calcium were found b’y this 0.00005 N with respect to the molybdate ion. Its solubility method. Since in most cases where calcium and magnesium are to does not increase upon ignition nor is it hygroscopic. A be determined ammonium salts are present in solution, it was Gooch crucible may be used over and over again in the decided to determine whether the presence of ammonium filtration. The crystals of this compound start to form within a chloride would exert any solvent effect upon the calcium molybdate precipitate. Therefore, portions of 25 cc. of the minute or two after the addition of the ammonium molybdate calcium solution were pipetted into each of eight beakers, and solution. If the molybdate solution is added too rapidly, 3 grams of ammonium were weighed out on a rough balance the crystals tend to stick to the sides of the beaker. They and added t o each solution. The precipitate was then may be then removed with a horn spatula or quickly detached thrown down in the usual way in a slightly alkaline solution. by rubbing a few fine crystals of ammonium chloride against them with a policeman. It has been found that it need not The results are given in Table 111. From Table I11 it will be seen that the presence of am- be more than 45 minutes from the time the molybdate is monium salts does not interfere with the precipitation of added to the solution until the crystals have grown to sufficient size to be readily filtered. calcium by the molybdate method. ----MAGNESIUM--Found after In original removal of calcium soln. Gram Gram 0.0446 0.0446 0.0447 0.0446 0.0451 0.0446 0.0448 0.0447 n n44x Av. 0.0446 * 0 0447
*
Vol. 3, No. 2
FOUND--By molybdate By permanganate method in method, original presence of soln. magnesium Gram Gram 0.0512 0.0515 0.0514 0.0513 0.0515 0.0515 0.0512 0.0516 0.0511 0.0512 0.0514
April 15, 1931
INDUSTRIAL A N D ENGINEERING CHEMISTRY
It is believed that this method should prove useful in the analysis of limestones and dolomites, for it is shown that the results obtained without any double precipitations or evaporation are as exact as could be obtained by making a double precipitation of the calcium by the oxalate method and then evaporating down the filtrates and washings and making a double precipitation of the magnesium by the usual method. The method herein described thus eliminates two precipitations and an evaporation without any sacrifice of the accuracy of the determinations and a t a considerable saving of time. Like other molybdates this precipitate is quite soluble in concentrated mineral acids and in strong bases. Therefore, care should be taken that the solution from which it is precipitated be nearly neutral.
129
The writer prefers to precipitate calcium from a slightly alkaline solution. No trace of molybdenum was found in the magnesium precipitates. Reducing gases should not be allowed te come in contact with the calcium molybdate during ignition. A large excess of ammonium molybdate shouId be avoided. Before filtering, a drop of the supernatant liquid should be tested with a saturated solution of pyrogallol in chloroform, A brown coloration denotes an excess of molybdate. This test is not decisive unless the solution tested has been boiled at least 10 minutes, for the precipitate of calcium molybdate forms rather slowly. Literature Cited (1) Smith and Bradbury, Bev., 24, 2930 (1891).
Conductometric Titration of Sulfate and Barium' I. M. Kolthoff and Tohru Kameda SCHOOLOF CHEMISTRY, UNIVERSITY OF MINNESOTA, MINNEAPOLIS, MI".
In the conductometric titration of alkali sulfate with it was thought necessary to T PRESENT, we have barium chloride, the knick point is found before the make a more systematic inneither an inside nor equivalence point. The error depends upon the divestigation on the reliability an outside indicator lution, alcohol content, acidity, and the kind of cations of the conductometric titraavailable for the detection of present. Especially in the titration of 0.1 Nsolutions are tion of sulfate with barium the end point in the direct or the results more or less irregular, the deviations being and the reverse procedure. reverse t i t r a t i o n of sulfate smaller in the titration of lithium sulfate than of powith a barium solution. A The first part of the work has tassium sulfate. been carried out in the usual potentiometric method canIn more dilute solutions, the results are reproducible way-i. e., at room temperanot be used either, for the within 1 per cent. Therefore, in series of determinareason that there is no suitture, without taking precautions of sulfate, the application of the conductometric able indicator electrode for tions to keep the temperature method can be very useful if the standardization is this reaction. Therefore, the completely constant. In ormade under the same conditions as the experiments application of the conductoder to be certain that the unwith the unknown. metric method to the titrafavorable results could not be The deviation from the theoretical results cannot be tion of sulfate with barium or a t t r i b u t e d to i r r e g u l a r attributed to a coprecipitation of sulfate. By special the reverse procedure has spechanges in temperature, all experiments it has been shown that the conductivity in cial advantages. It was first the work was repeated by perthe presence of the precipitate is distinctly higher than recommended by Dutoit ( 1 ) forming the titrations in a in the filtrates. This may be caused by the charge of in 1910. Though there is an thermostat at a temperature the barium sulfate in suspension and by its influence extensive literature on this of 25" * 0.05" C. Only this upon the cell constant. titration method, there is no part of the work will be reIn the reverse titration of barium with sulfate, the general agreement as to its ported. knick point is found nearer the equivalence point a c c u r a c y . According to Apparatus and Solutions though a small deviation of the order of 1 per cent is obseveral authors, the knick Used served. The conductometric method is very useful in point in the sulfate titration the titration of very dilute solutions in the presence of C o n d u c t i v i t y cells with occurs before the equivalence vertical electrodes were used. 30 per cent alcohol. point and the results are corThe mobility of the barium ions is decreased much They were sealed t o a heavy respondingly low; a few claim more by alcohol than that of the alkali or sulfate or chloplatinum wire, the lattermakt o find accurate results. ride ions. ing contact between the elecFrom the r e c e n t monotrode and the mercurv in the graph of Jander and Pfundt - . (f?),one gets the impression that the conductometric sulfate vertical glass tubes fused to the glass of the cell. Thi"ck coptitration under different conditions gives theoretical results. per wires dipping in the mercury made the electrical contact in However, the experimental evidence they present is too poor the circuit of the Wheatstone bridge. The biggest part of the conductivity cell dipped into the thermostat during the t o make the statement convincing. I n one of the writers' investigations, it was necessary t o titration. The microburet with the reagent was mounted make a series of analyses of the sulfate content in dilute zinc above the opening in the upper middle part of the cell. After sulfate solutions, and for this purpose the conductometric each addition of reagent, the content of the vessel was mixed method was applied. However, in testing it with solutions by careful shaking, and the final readings were made when the of known content, very disappointing results were found, the conductivity did not change further after longer standing. latter sometimes being more than 5 per cent low. Therefore In order to prevent a change in concentration by evaporation during the titration, the opening of the cell was closed by a 'Received October 20, 1930. Chapter from a thesis submitted by rubber stopper after each addition of reagent. Tohru Kameda to the Graduate School of the University of Minnesota, in All the salts used were pure substances. The potassium, partial fulfilment of the requirements for the degree of doctor of philosophy.
A
