INDUSTRIAL AND ENGINEERING CHEMISTRY
38
Summary The antimony residues from citrus trees with tartar emetic and sugar can be removed from the leaves by washing with dilute tartaric acid solution and determined by titrating with standard iodine solution. The residues from orange foliage from a spray containing 1.5 pounds of tartar emetic and 2 pounds of sugar per 100 gallons were found to
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contain antimony equivalent to 6.6 to 10.7 micrograms of tartar emetic per square centimeter immediately after spraying, but these residues were greatly reduced by rain.
Literature Cited (1) Davidson, J., Pulley, G. N., and Cassil, C. C., J. Assoc. Oficial Agr. Chem., 21, 314 (1938). ( 2 ) Munger, F., J. Econ. ~ n t o m o l .35, , 373-5 (1942).
Determining an Alkali Carbonate in the Presence of an Alkali Bicarbonate A Colorimetric Method W. TAYLOR SUMERFORD WITH THE TECHNICAL ASSISTANCE OF D.IVID DALTON AND ROBERT JOIINSON School of Pharmacy, University of Georgia, Athens, Ga.
T
H E carbonates and bicarbonates of the alkali metals are important industrial chemicals and useful chemical reagents, since they are the salts of a strong base and a weak acid and are the only readily available salts of these anions which are soluble in water. It is frequently necessary to distinguish between the carbonate and bicarbonate of an alkali metal, and to determine one in the presence of the other, since solutions of bicarbonates lose carbon dioxide to revert to the corresponding carbonate and solutions of the carbonates absorb carbon dioxide to become contaminated with the bicarbonate. The p H of an alkali carbonate solution is higher than that of the corresponding bicarbonate; hence, these anions may be distinguished by their behavior with an indicator such as turmeric (Y),which is reddened by solutions of an alkali carbonate but not by solutions of an alkali bicarbonate. Mercuric chloride (6), which gives a brownish-red precipitate with solutions of an alkali carbonate and a white precipitate with solutions of the bicarbonate, has been used to distinguish b e tween these anions; as has magnesium sulfate (6),which p r e cipitates at room temperature with alkali carbonates but not with the corresponding bicarbonates.
Qualitative Procedure I n a study of the tautomerism of p-nitrosothymol and thymoquinone monoxime (9) it was observed that a solution of sodium carbonate was alkaline enough to tautomerize the colorless p-nitrosothymol into thymoquinone monoxime with the simultaneous production of a red color due to the presence of the anion of the sodium salt of the oxime:
$OH
+ NatCOs = o@a
+ NaHCOs
0
A solution of sodium bicarbonate under the same conditions produces no color or a very faint yellow color, depending on the amount of carbonate contamination in the bicarbonate sample. Thus p-nitrosothymol can be used to distinguish between alkali carbonates and bicarbonates and provides a qualitative test for the presence of an alkali carbonate in a sample of an alkali bicarbonate. p-Xtrosothymol is available from the Eastman Kodak Company, Rochester, N. Y. It can also be prepared in almost
quantitative yields by the method of Kremers and Wakeman (3). Purification of p-nitrosothymol can be accomplished by recrystallization from benzene or from diluted alcohol with the use of activated carbon.
Quantitative Procedure There are standard procedures (10) for titrating an alkali carbonate in the presence of an alkali bicarbonate by the use of selected indicators. The accuracy of these methods depends upon several factors, especially the choice of the indicator, but under no condition is it exact when the amount of bicarbonate is proportionately large (2). For determining inadmissible amounts of carbonate in official samples of the alkali bicarbonates, the U. S. Pharmacopoeia XI (11) requires that a I-gram sample of the salt be not alkaline to phenolphthalein after it has been dissolved in 20 ml. of distilled water below 15" C. and treated with 2 ml. of 0.1 N hydrochloric acid. The British Pharmacopoeial method (1) is similar, except that thymol blue is used as the indicator. While these pharmacopeia1 methods serve the purpose for which they are intended, they are not quantitative.
The qualitative test using p-nitrosothymol was investigated to determine whether the intensity of the color produced with the indicator was in direct ratio to the amount of alkali carbonate present, so that it could be used for quantitatively determining an alkali carbonate in the presence of the corresponding bicarbonate.
Experiment a1 To a series of Nessler tubes, arranged in a rack fitted with a white porcelain base, were added equal volumes of sodium carbonate solutions of graduated molarity. To each tube was then added approximately, twice the calculated quantity of p nitrosothymol previously dissolved in enough neutral acetone or neutral dioxane to give a 0.35M solution. (The p-nitrosothymol was dissolved in the solvent to facilitate its admixture with the carbonate solutions.) The tubes mere shaken for from 10 to 15 minutes, and the excess p-nitrosothymol was filtered off. These filtrates provide the reference standards. An equal volume of a solution containing an unknown amount of alkali carbonate was treated in like manner, after which its intensity of color was compared to those of the reference standards for the determination of its alkali carbonate content. By this procedure, using a blank composed of p-nitrosothymol and distilled water, it was found that the color produced by the indicator in the presence of 0.0001 M solutions of sodium carbonate could be detected with the unaided eye. From this molarity the method is applicable in concentrations up to 0.1 M sodium carbonate, above which the color is too
January 15, 1943
ANALYTICAL EDITION
intense for comparisons. Near the lower concentration i t is possible to distinguish between a 0.0001 and a 0.00015 M sodium carbonate solution, and at the higher concentration between a 0.1 and 0.09 M solution. Three different procedures were used t o check the accuracy of the p-nitrosothymol method against other methods used t o determine alkali carbonates in the presence of alkali bicarbonates. A solution of sodium carbonate of known molarity was prepared by dissolving reagent monohydrated sodium carbonate in distilled water. Measured portions of this solution were assayed by titration with standardized hydrochloric acid, using methyl orange as an indicator. Identical samples were assayed by the use of p-nitrosqthymol in the manner described above. Twelve color reference standards ranging from 0.01075 to 0.01625 M with respect to sodium carbonate were used, and each successive tube in the series differed to the extent of 0.0005 in molarity. The results by the two methods are: Molarity of solution with respect t o NazCO: True molarity As determined with HCI (average of 3 determinations) As determined with p-nitrosothymol (average of 5 readings)
The results of these comparisons are: True molarity of solution with respeot to NazCOa Molarity with respect to NazCO: Determined with fresh standard (average of 3 determinations) Determined with day-old standard (average of 3 determinations) Determined with 3-day-old standard (average of 3 determinations) Determined with 5-day-old standard (average of 3 determinations) Determined with 8-day-old standard (average of 3 determinations)
0.00525
0.00537 0.00550 0.00565 0.00567 0.00567
Discussion
0.01338
While all the foregoing experiments involved the use of only sodium carbonate and sodium bicarbonate, i t was found by preliminary experiments that the method could be applied t o the corresponding salts of lithium and potassium. For the p-nitrosothymol method to be accurate and its results reproducible, the conditions for mixing the bicarbonate solution must be kept the same as those under which the reference standards are prepared from known alkali carbonate solutions and p-nitrosothymol. There was no difference in the intensity of the color produced when 1.1 or 2 times the calculated quantity of p-nitrosothymol was added t o the alkali carbonate solutions. Addition of approximately tn-ice the calculated quantity of p-nitrosothymol provides a safe margin, especially in the case of the unknowns where the carbonate content would have to be estimated. As may be seen from the experimental data, the method is more accurate when freshly prepared reference standards are used. The reference standards change less when they are kept in a closed cupboard than when they are exposed to the light. As was expected, boiling a solution of sodium carbonate or sodium bicarbonate with the indicator gave a darker color than when solutions of the same strength were shaken with the indicator at room temperature. The p-nitrosothymol method is most accurate when only a little carbonate is present with much bicarbonate, the conditions under which the titration method is least exact.
0.01340
0.00526 0.00735 0.00612
The amount of sodium carbonate in a sample of sodium bicarbonate was determined with p-nitrosothymol in the manner previously described. To an approximately 0.15 M solution of this sodium bicarbonate were added enough 0.0175 M sodium carbonate solution and distilled water to give a solution which was approximately 0.075 M with respect to sodium bicarbonate and 0.00575 M with respect to sodium carbonate. The sodium carbonate in aliquot portions of this solution was determined by titration with standardized hydrochloric acid, using a mixed indicator composed of thymol blue and cresol red as recommended by Simpson (8). Other aliquot portions were assayed for sodium carbonate, using p-nitrosothymol in the manner described above. Twelve color reference standards which ranged from 0.00275 to 0.00825 M with respect to sodium carbonate were used, and each successive tube in the series differed to the extent .of 0.0005 in molarity. The results by the two methods are: Molarity of solution with respect to NazCO: True molarity As determined with HC1 (average of 3 determinations) As determiued with p-nitrosothymol (average of 5 readings)
which they were checked periodically with a carbonate solution of known molarity which was prepared just prior to the periodic comparisons by the same procedure used with the reference standards.
0.01325
A 0.15 M solution of sodium bicarbonate was treated with carbon dioxide until it no longer gave a color with p-nitrosothymol. To this solution were added enough sodium carbonate and distilled water to give a solution which was 0.075 fif with respect to sodium bicarbonate and 0.00525 M with respect to sodium carbonate. The amount of sodium carbonate in this solution was then determined by Kuster’s method (4), which consists of titrating the solution at 0’ C. in the presence of an alkali chloride with standardized acid, using phenolphthalein as the indicator. The amount of sodium carbonate was determined also with the use of p-nitrosothymol in the manner described above. Twelve color reference standards which ranged from 10.00275 to 0.00825 M with respect to sodium carbonate were used, and each successive tube in the series differed to the extent $of0.0005 in molarity. The results by both methods are: Molarity of solution with respect to NazCOa True molarity As determined with HCI (average of 3 determinations) As determined with p-nitrosothymol (average of 5 readings)
39
0.00575 0.00620 0.00600
Stability of Reference Standards The influence of aging of the reference standards on the accuracy of the method was determined. A series of twelve reference standards which ranged from 0.00275 to 0.00825 M with respect to sodium carbonate, with each successive tube differing to the extent of 0.0005 in molarity, was prepared by the method described above, except that the indicator was shaken with the known carbonate solutions for 30 minutes. The reference standards were kept for 8 days, during
Literature Cited (1) “British Pharmacopoeia”, p. 389, London, Constable and Co., 1932. (2) Kolthoff, I. M., and Furman, N. H., “Volumetric Analysis”, Vol. 11, p. 139, New York, John Wiley & Sons, 1929. (3) Kremers and Wakeman, “Organic Syntheses”, Collective Vol 1,p. 511, New York, John Wiley & Sons, 1932. (4) Kuster, F. W., 2. anorg. allgem. Chem., 13, 142 (1896). ( 5 ) Patein, G., J. Pharm. Chem., 25, 448 (1892): through Mellor, J. W., “Comprehensive Treatise of Inorganic and Theoretical Chemistry”, Vol. 11, p. 773, London, Longmans, Green and Co., 1922. (6) Rogers, C. H., “Inorganic Pharmaceutical Chemistry”, p. 184, Philadelphia, Lea and Febiger, 1936. (7) Schindler. R., Mag. Pharm., 33, 14 (1821); through Mellor, J . TV., “Comprehensive Treatise of Inorganic and Theoretical Chemistry”, Vol. 11,p. 773, London, Longmans, Green and Co., 1922. (8) Simpson, G., IND. ENG.CHBIM.,16, 709 (1924). (9) Sumerford, W. T., and Hartung, W. H., J. Am. Pharm. Assoc., 24, 65 (1940). (10) Treadwell, F. B., and Hall, William T., “Analytical Chemistry”, 1’01. 11, p. 512, New York, John Wiley & Sons, 1935. (11) “United States Pharmacopoeia”, 11th decennial revision, p. 336, Easton, Penna., Mack Printing Co., 1935.
