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V O L U M E 22, NO. 5, M A Y 1 9 5 0 DISCUSSION
If fructose is absent from the sample, the initial treatment of the sample with 90% ethyl alcohol may be omitted, as well as the evaporation of a liquid sample to dryness. The analysis may be started by shaking a solid sample of proper weight with 20 ml. of 50% ethyl alcohol or by adding 10 ml. of absolute ethyl alcohol t o 10 ml. of a liquid sample, and then proceeding as directed for the separation and identification of lactose. If fructose is present, it is essential t o remove it first. Therefore a sample size must be chosen to permit the removal of fructose with 50 ml. of 90% ethyl alcohol solution. At 20" C., 50 ml. of 90% ethyl alcohol dissolve 1.7 grams of anhydrous fructose. However, if water is present in a liquid sample consider.able amounts of other sugars will be removed by the 90% ethyl alcohol treatment and may result in negative tests later in the analysis. If fructose is not completely removed, it will give an erroneous test for glucose, because their osazones are identical. I t is necessary to wash the residue containing lactose with additional portions of 50% ethyl alcohol to remove any maltose that may remain. Maltose behaves like lactose in the formation of an osazone, but their osazones are not identical and can be differentiated under the microscope. The residue remaining after lactose is dissolved by water contains starch and dextrins. This may be dissolved in boiling water and tested for starch or dextrin with a solution of iodine in potassium iodide. If any starch or dextrin dissolves with the lactose it is converted to glucose by the acetic acid, but this does not interfere with the test for lactose, because the glucosazone is insoluble a t 87" C. and will be filtered out before the osazone of lactose precipitates.
The presence of maltose in a sample is often indicated by its characteristic odor and its marked tendency t o absorb water and cake. When a mixture of maltose and glucose is treated with acetic acid and phenylhydrazine base, the mixture becomes cloudy a t once if maltose is present. It remains clear for several minutes if glucose alone is present. I t is essential to evaporate the solution of sucrose, glucose, and maltose in 50% ethyl alcohol solution t o half its volume in order t o remove excess alcohol. The presence of ethyl alcohol delays the formation of the osazones beyond 45 minutes. The mixture cannot be held a t 87" C. longer than 45 minutes because sucrose may hydrolyze and give a false test for glucose. It does not interfere under the conditions of this scheme. The tests given in this scheme detect 5 mg. of fructose or sucrose and 200 mg. of glucose, lactose, and maltose. The Raybin test for sucrose is stated by Raybin (3) to detect 50 mg. of sucrose in 5 ml. of solution. The scheme is not a complete scheme for all sugars, because no provision is made for the less common pentose sugars or for mannose and raffinose. However, these are not usually found in food products. LITERATURE CITED
(1) Assoc. Offic.Agr. Chemists, "Official and Tentative Methods of Analysis," 6th ed., p. 562, 1945. (2) Molisch, H., Monatsh., 7, 198-209 (1886). (3) Raybin, H. W., J . Am. Chem. SOC.,55,2603 (1933). (4) Whitehead, T. H.. and Bradbury, W. C., ANAL. CHEM..21, 1430 (1949). RECEIVED December 5, 1949. Presented in part before the Meeting-inMiniature, Georgia Section, AMERICAN CHEMICAL SOCIETY, Atlanta, Ga., November 18, 1949.
Conductometric Titrations with Organic Reagents Determination of Magnesium with Calcium Saccharate in Presence of Calcium J. F. CORWIN, A. P. DRESEL', A N D G . E. OSUCH, Antioch College, Yellow Springs, Ohio
The use of calcium saccharate for the volumetric determination of magnesium has been found suitable for titrating magnesium either alone or in the presence of limited amounts of calcium. The conductometric method of analysis was used and the end point determined by calculation.
C
ALCIUM sacchautte has been found satisfactory for the direct conductometric titration of magnesium. This reagent and saccharic acid have been used in the gravimetric determination of magnesium in limestone ( 3 , s )and in the detection of magnesium in the presence of barium, calcium, and strontium (4). When a standard solution of calcium saccharate is added t o a solution containing magnesium, the insoluble precipitate of trimagnesium saccharate is formed and calcium ions replace the magnesium ions in solution. The replacement of the magnesium ions in solution by the faster-moving calcium ions causes a slight decrease in resistance before the equivalence point is reached. After the end point, further addition of concentrated calcium saccharate solution causes a sharp decrease in resistance. Typical titration curves are shown in Figure 1. Magnesium can be titrated either alone or in the presence of calcium, provided that the amount of calcium does not exceed the amount of magnesium present. Other metals of the alkaline earth group and those that appear in small quantities in the usual limestone analysis do not interfere with the reaction, but as in most conductometric procedures, high concentration of conducting ions must be avoided. 1
Present address, Eastman Kodak Company, Rochester. N. Y.
APPARATUS
The 60-cycle, Leeds & Korthrup, industrial conductivity equipment was used. The dip cell for high conductivity solutions was modified by removing the glass shield. A 300-ml. beaker served aa a reaction vessel. The selection of concentrations that allowed a t least 200-ml. volume of solution in the reaction vessel to not over 10 ml. of titrant added eliminated the necessity of volume corrections. The reaction vessel was kept a t constant temperature. SOLUTIONS
The magnesium chloride was prepared by adding approximately 17 grams of magnesium chloride hexahydrate to 2 liters of water and was standardized gravimetrically by the precipitation of the magnesium as magnesium ammonium phosphate. The calcium saccharate solution was prepared by adding 54 grams of pure calcium saccharate, obtained from the Fisher Scientific Company, to 500 ml. of boiled water. Because calcium saccharate reacts with the carbon dioxide in the air, the solution was stored in a glass bottle with an Ascarite absorption tube on the inlet and a pinch clamp on the outlet. Nevertheless, o w h g to the slow decomposition of the salt, the solution was standardized before each series of titrations or, a t least, every other day. PROCEDURE
The standardization procedure consisted of pipetting 25 ml. of the magnesium chloride solution into a 300-ml. electrolytic
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ANALYTICAL CHEMISTRY
Table I. C a l c i u m S a c c h a r a t e us. a S y n t h e t i c M i x t u r e of M a g n e s i u m Chloride and C a l c i u m N i t r a t e hIg Taken Gram 0.0261 0.0232 0 0232 0.0237 0.0237
Calcium Saccharate Used M 1. 7.60 6.80 6.86 6.96 6.96
Mg Recovered Gram 0,0260 0.0232 0.0234 0,0238 0.0238
Difference Gram 0.0001 0.0000 0.0002 0.0001 0.0001
Ca Added Gram 0.0200 0,0200 0.0200 0.0200 0.0200
In order to see if this procedure would be effective in the presence of calcium, calcium nitrate was added to the magnesium chloride solution. As long as the amount of calcium WBS less than the amount of magnesium, it had no apparent effect on the reaction. Typical titration data are listed in Table I for a series of determinations which contained the maximum amount of calcium allowable. The results in this table were obtained by calculation rather than graphically. By selecting the linear values of resistance and titrant before and after the equivalence point, respectively, and substituting them in the equation for a straight line, a pair of equations may be obtained that when solved simultaneously will give the equivalence point. A complete description of the calculation has been given in a previous publication ( I ) . DISCUSSION
Less than 1% error is obtained by this method of determining magnesium and it is not necessary to separate calcium from the magnesium if the amount of calcium is less than the amount of magnesium. The titration can be made in 1 hour and the calculations can be completed in 10 minutes. CONCLUSION
A method for the volumetric determination of magnesium is described, the accuracy of which varies from 0.4 to 0.8 mg. of magnesium per 100 mg. of magnesium taken. The method is fairly rapid and can be carried out in the presence of limited amounts of calcium. F i g u r e 1. C a l c i u m Saccharate us. Magnes i u m S o l u t i o n C o n t a i n i n g C a l c i u m Ions beaker and diluting with water to make approximately 200 ml. of solution. The beaker was placed in a constant temperature bath and the calcium saccharate was added in about 0.5-ml. portions from a microburet. Because the magnesium saccharate complex is somewhat soluble, 1 ml. of titrant was added and the solution was stirred constantly for 10 minutes before the fist resistance reading was taken. All subsequent readings were made when the resistance became constant or a t 10-minute intervals after each addition.
LITERATURE CITED
(1) Corwin and Moyer, IND.ENG. CHEM.,ANAL. ED., 18, 302 (1946). (2) Shead and Heinrich, Ibid., 2, 388 (1930). (3) Shead and Valla, Ibid., 4,246 (1932). (4) Tananaev and Lovi, J. A p p l i e d Chem. (U.S.S.R.), 10, 1112 (1937).
RECEIVED November 28, 1949. Work done under Grant-in-Aid from t h e Frederick Gardner Cottrell Fund, Research Corporation, New York, N. Y.
Application of the lead Reductor to Determination of Uranium WILLIAM D. COOKE', F R E D HAZEL, A N D WALLACE M. MCNABB University of Pennsylvania, Philudelphia, Pa.
A
NUMBER of metallic reducing agents have been proposed for the determination of uranium. Reduction of uranium (VI) in a Jones reductor gives a mixture of uranium(1V) and uranium(II1). The solution is aerated to oxidize uranium(II1) to uranium(1V). Birnbaum and Edmonds (1)used asilver reductor for reduction to the quadrivalent state. This procedure necessitated a high temperature, 60" to 90" C., and controlled acidity, 4 N hydrochloric acid. However, at least in small amounts, the extent of reduction depends upon the temperature and the rate of passage through the column ( 6 ) . Someya ( 2 ) found that uniform reduction of uranium could be carried out in strong acid solutions by means of liquid lead and bismuth amalgams. Koblic (4)reduced uranium quantitatively to the quadr,ivalent state with lead. However, his procedure is inconvenient because the solutions have to be boiled in an inert atmosphere of carbon dioxide in the presence of hydrochloric acid for a t least 0.5 hour. 1 Present address. Frick Chemical Laboratory, Princeton University, Princeton, N. J.
Treadwell (II) used a lead reductor instead of a Jones reductor for various metals but did not include uranium. The use of the lead reductor probably has been limited because of the difficulty of applying i t to sulfuric acid solutions. When sulfuric acid solutions are reduced with lead, an adherent film of lead sulfate is formed which soon decreases the efficiency of the reducing agent. The formation of the lead sulfate film can b e prevented by the presence of hydrochloric acid. If the concentration of hydrochloric acid in the solution is greater than 2.5 N , no lead sulfate is formed even after continued use. The successful application of the lead reductor to the determination of uranium is described below. Solutions of uranium(V1) were reduced to uranium(1V) i n both hydrochloric and sulfuric acid solutions. The acidity was not critical and could be varied within wide limits. However, when sulfate ions were present, it was found necessary to add sufficient hydrochloric acid to prevent the formation of lead sulfate. The reduced uranium was caught in a solution of ferric sulfate.
