impurities other than iron and aluminum. I n well controlled m et-process phosphoric acid plants, the calcium level in the mother liquor is always well within this range. When larger aliquots were taken for samples exceptionally low in calcium, traces of gelatinous precipitate formed after addition of sodium naphthalhydroxamate and heating. However, these small amounts of precipitate had little effect on the accuracy and precision of the final results presumably because no calcium was occluded. For calcium contents below O.lSr,, absolute recoyeries of calcium tended to be a little Ion, but the percentage values were within +O.Olyoof the theoretical. I n the original procedure ( I ) , heating for 3 minutes in a boiling w a t u bath was sufficient for complete precipitation of calcium naphthalhydroxamate. I n the presence of tartaric acid, the formation of the precipitate was a little slower. By increasing the heating time to 5 minutes, complete precipitation of calcium was obtained.
Table 111.
Recovery of Calcium Added to 30y0 Wet-Process Acid
Present, Sample0
573
Found,
70
0.05 0.04 0.07 0.08 0.11 0.10 0.13 0.14 0.16 0.17 0.23 0.24 0.33 0.33 0.43 0.42 293-18 0.14 0.14 0.18 0.17 0.41 0.40 a Samples 293-4A and 293-1A contained 0.03 and O.llyocalcium before additions 293-48
were made.
and aluminum to calcium was increased for the reasons mentioned earlier. LITERATURE CITED
( I ) Banerjee, D. K., Budke, C. C., Miller! F. D., 4 N - 4 L . CHEM.33, 418 (1961). (2) Brabson, J. A., Wilhide, W.D., Ibid., 26, 1060 (1954). (3) Zbid., p. 1663. (4) Hillebrand, W. F., Lundell, G. E., Bright, H. A., Hoffman, J. I., "Applied Inorganic Bnalysis," 2nd ed., p. 261, Wiley, Sew York, 1953. (5) Khamskii, E. V., Chepelevetskii, hl. L., Zhur. Priklacl. Khain. 32 (5), 948 (1959). ( 6 j Sokolvskii, A. A,, Zhur. Khint. Prom 13, 92 (1936). (7) Volfkovich, S. I., Voskresenskii, S.K., Sokolvskii, A. A,, Remen, R. E., Kobrin, &I. &I., Trans. Sci. I n s t . Fertilizers Znsectofunaicides ( U S S R )No. 153. 12 (1940). I
Precision and recovery data obtained with synthetic samples prepared from reagent grade phosphoric acid and plant samples of 30y0 acid are shown in Tables I, 11, and 111. I n the absence of tartaric acid, low recoveries of calcium were obtained as the ratio of iron
"
D. K. BANERJEE C. C. BUDHE F. D. hfILLER
Research Division U. S. Industrial Chemicals Co. Division of National Distillers & Chemical Corporation Cincinnati 37, Ohio
Polarographic Standardization of Aqueous Histamine Solutions SIR: Several methods for the standardization of histarninc solutions have been rcported in recent liternture (3-6). Most of these are colorimetric techniques which are time consuming. both with respect to procedure and reagent preparation. Solutions of histamine salts (or histamine) and acetaldehyde, when adjusted to a suitable pH, yield a polarographically reducible imine ( 7 ) . The imine and acetaldehyde waves occur a t approximately - 1.4 volts and -1.7 volts us. S.C.E., respectively. Histamine test solutions which were 20mX or less with respect to acetaldehyde reached equilibrium within the time necessary for purging the solution with nitrogen. The measurement error was 1 to 3 relative 70 for 10-4Jf solutions. EXPERIMENTAL
Apparatus. Polarograms were recorded with a Leeds 8: Korthrup Electro-Chemograph, Type E, using a Leeds & Korthrup Polarotron as the polarographic cell. All p H measurements nere made with a Beckman Zeromatic p H meter equipped with a type E glass electrode. The dropping mercury electrode in the Polarotron had a drop time of 3.20 seconds and a n in value of 2.22 mg. per second, measured in 2.531 LiCl a t open circuit. A Precision Scientific constant temperature circulating system was used
to maintain the temperature within the polarographic cell a t 25' + 0.2" C. The calibrated pipets used in preparation of all solutions were accurate within the limits of allowable experimental error. Reagents and Chemicals. Acetaldehyde stock solutions (100mM) were prepared by measuring (semimicro buret) the desired volume of acetaldehyde (Fisher certified reagent) into a chilled 500-ml. volumetric flask, diluting to volunie n ith distilled water, and mixing properly. Previous workers (6) have shown that similar stock acetaldehyde solutions of this concentration remain stable, within experimental error, over a period of a t least 2 weeks. Histamine dihydrochloride (Fisher Reagent) and histamine phosphate (U.S.P.) were prepared by dissolving the requisite weights of the reagents in water and diluting to volume. Aqueous gelatin solutions (0.1%) were employed to suppress maxima. Fisher standard pH 10 buffer, which contains a mixture of potassium borate and potassium carbonate @), mas used as supporting electrolyte. Procedure. Pipet 25.0 ml. of Fisher standard p H 10 buffer solution into a 50-ml. volumetric flask. Add 10.0 ml. (pipet) of the histamine solution to be measured (1 to 10mM). Add 2.5 ml. (pipet) of the 0.1% gelatin solution, followed by 5.0 ml. (pipet) of stock acetaldehyde solution (lOOmM), and dilute to volume, mixing properly.
After the polarographic and presaturator cells have been rinsed with approximately 15 ml. of the prepared test solution, place about 20 ml. of the solution in the polarographic cell and the remainder in the pre-saturator. Bubble preconditioned nitrogen through the solution for 5 minutes. Record the current-voltage curve from approximately -1.1 to -1.7 volts us. S.C.E., using an appropriate instrument sensitivity. Evaluate the diffusion current of the imine by the mid-point correction technique (1). Determine the concentration of the polarographic test solution or of the original histamine salt solution by reference to a suitable, previously prepared calibration curve. RESULTS AND DISCUSSION
The formation of the imine resulting from mixtures of histamine salts and acetaldehyde was pH dependent. The diffusion current of the imine increases with pH up to the region of 9 to 9.5, decreasing thereafter. At pH values greater than 10.5, the solution became cloudy and soon yielded a yellow precipitate. Investigations of pH indicated that a constant pH of 10 would yield a solution in which the imine formation is large enough for good measurement sensitivity, while the pH is readily maintained by a commercially available buffer (Fisher Standard pH 10 buffer), which also serves VOL. 34, NO. 3, MARCH 1962
441
Table I.
Polarographic Determination of Histamine in Aqueous Solutions
Conditions: Acetaldehyde, 10mM; gelatin, 0.005%; pH maintained by addition of Fisher standard pH 10 buffer solution, 25 ml.; purged 5 minutes with preconditioned nitrogen; temperature 25" =t0.2" C. Concentrations shown are for the polarographic test solutions. Histamine Dihydrochloride Histamine Phosphate Relative Relative Found Error, Error, Taken Taken, Found ( mlw ) ( mfif ) % (a)(a) % 0.481 0.658 0.658 0.753 0.768 0.962 0.987 1.152 1.443
0.495
0.638 0.660 0.760 0.788 0.940 0.976 1.175 1.413
+2.9 -3.0 +0.3 +0.9 +2.6 -2.3 -1.1 +2.0 -2.1
Average f1 . 9
as the supporting electrolyte. Although greater sensitivity was obtained with solutions more than 20mM with respect to acetaldehyde, such solutions showed definite kinetic effects. Some of the latter solutions had not reached equilibrium for as long as 2 hours after initial mixing. Two separate calibration curves were obtained-one for histamine dihydrochloride and another for phosphate. Each curve indicated a linear relationship between concentration and dif-
0.287 0.368 0.440 0.472 0.491 0.552 0.713 0.861 1.162
0.283 0.369 0.462 0.492 0.495 0.519 0.708 0.853 1.127
-1.4 +0.5 +5.0 f4.2 $0.8 -6.0
-0.7 -0.9 -2.9 h2.5
fusion current over the test solution concentration range up to 2mM in histamine content, and both passed through the origin. The slope for the dihydrochloride calibration plot was 1.175 pa. per mdf, rvhile the corresponding plot for the phosphate yielded 0.729 pa. per m3f. Results appear in Table I. A more convenient method of current measurement employing less expensive equipment-the two-point technique (6)-was also investigated. In-
dividual diffusion current readings made a t -1.15 and -1.55 volts us. S.C.E. yielded results essentially identical with those in Table I. Histamine solutions were also investigated and gave results similar to the dihydrochloride; however, the hygroscopicity of the free base made initial concentration measurements more difficult. LITERATURE CITED
(1) Elving, P. J., Van Atta, R. E., ANAL. CHEM.26, 295 (1954). ( 2 ) Fisher Scientific Co., Fair Lawn, New
Jersey, private communication, June '
21. 1961. (3) Havinga, E., Seckles, L., Strengers, T., Rec. trav. chim. 66, 605 (1947). (4) Lyons, T. D., Bndrews, A. C., Trans. Kansas Acad. Sci., 58, 435 (1955). (5) Schmall. M.. Wollish, E. G., Galender, ' J., J . Am.'Phurm. Assoc. 41, 138 (1952). (6) Van rltta, R. E., Harrison, W. W., Sellers, D. E., ANAL. CHEW 32, 1548 (1960). (7) Zuman, P., Collection Czechoslav. Chem. Communs. 15, 839 (1950); Nature 165, 485 (1950)
D. E. SELLERS I
