Spectrophotometric Determination of Calcium in Wet-Process Phosphoric Acid SIR: The importance of the ratio of calcium to sulfate ions in wet-process phosphoric acid mother liquor in determining the formation of easily filterable gypsum crystals has been clearly established (5-7). The flame photometric method of Brabson and Wilhide using ion exchange for calcium separation was considerably faster than the double precipitation with oxalate and eliminated interference from phosphate and iron (2, 3 ) . However, due t o the separations involved, this method was not suitable for plant control. The method described here results in further saving in time and is suitable for control purposes. By using tartrate as a complexing agent for the comparatively large amounts of iron and aluminum present in wet-process phosphoric acid, calcium in the range of 0.05 to 0.5% can be determined by the naphthalhydroxamate method reported earlier (1). The color developed is stable for at least 24 hours. No separation from iron, aluminum, and phosphate is necessary. Six samples can be analyzed in 11/2 hours.
tion of reagent solutions has been described ( 1 ) . Determination of Calcium in WetProcess Phosphoric Acid. Prepare a standard curve covering the range of 0 to 300 pug. of calcium as described (1). Weigh a 5-gram sample of phosphoric acid to the nearest 0.1 mg. and transfer t o a 100-ml. volumetric flask. Add 50 ml. of water and 5 ml. of 1 to 1 HCl. Warm if necessary to dissolve any insoluble material, cool, and dilute to volume with water. Transfer a n aliquot of up to 5 ml., containing up to 200 pg. of calcium, to a clean 4O-ml. centrifuge tube. Add 5 nil. of 10% tartaric acid. Stir with a thin stirring rod. Place the tube in a beaker of boiling water and heat for 2 minutes. Remove the tube from the water, cool, and dilute to 20 ml. with water. Add 1 drop of 1% phenolphthalein and buffer solution drop by drop with stirring until the solution is alkaline. Then add a n additional 2 ml. of buffer and 5 nil. of sodium naphthalhydroxamate dropwise with stirring. Place the tube in a beaker of boiling mater for 5 minutes and stir occasionally. Remove the tube and cool. Remove the stirring rod and place it on a clean watch glass. Centrifuge the tube a t 2500 to 3000 r.p.m. for 3 minutes. Carefully decant the clear solution. .4dd about 10 ml. of wash solution to the tube and stir well. Centrifuge and decant carefully. Add 5 ml. of E D T A and 2 ml. of buffer solution and stir, Heat the tube in hot
EXPERIMENTAL
Apparatus and Reagents. A Beckman DU spectrophotometer with 1-cm. matched cells was used for all absorbance measurements. Prepara-
Table 1.
Recovery of Calcium from Synthetic 30% P 2 0 5 Solutions
0.02
Ratio" Fe A1 Ca No FA.A1
0.08
1.5 12.5 20
Calcium Sample
Present,
1
8 a
70
+
0.02 0.08 0.08 0.40 0.20 0.07 0.07 0.07
Equal amounts of Fe and A1 added to samples.
Table II.
Recovery of Calcium from
Ratios
Fe
a
Calcium Found, yo Tartaric rldded
S o Tartaric 0.02 0.08 0.07 0.40 0.15 0.03 0.01 . n.12 - --
o.ii,o . i o , o . i i
0.03, 0.04, 0.03
water until the precipitate dissolves completely. Cool, transfer to a 25-ml. volumetric flask, and dilute to volume with water. Measure the absorbance at 410 rng using 1-cm. borosilicate cells and a distilled water reference. Subtract the absorbance of a blank run through the entire procedure. Obtain the micrograms of calcium in the sample aliquot by reference to the standard curve and convert this value to per cent calcium in the sample. DISCUSSION
Initial attempts to deterniine calcium in wet-process phosphoric acid using sodium naphthalhydroxamate gave low and erratic results. The iron and aluminum present formed a gelatinous precipitate when the solution was made alkaline and apparently occluded some of the calcium which did not precipitate n h r n the naphthalhydroxamate was added. This was especially noticeable with samples low in calcium nhere a large sample weight was taken and relatively large amounts of iron and aluminum were present in the aliquot taken for ana1y.k. Hillebrand (4) also pointed out that when other elements are precipitated with NHIOH in the presence of calcium, losses of calcium will take place when much phosphate is present. The extent of calcium phosphatp precipitation is dependent on the ratio of phosphoric acid to iron and aluminum. It was therefore apparent that any precipitation of iron and aluminum would have to be avoided for good recoveries of calcium by this method. In the method as described, up to 0.25 gram of 30% P205was present in the sample aliquot. Complete recovery of calcium was also obtained in the presence of 1 gram of reagent grade 30% PzOs showing that considerable amounts of phosphate have no effect on the recovery of calcium. When a 5-gram sample of acid was diluted to 100 ml. and a 5-ml. aliquot taken for analysis, the addition of 0.5 gram of tartaric acid prevented the precipitation of iron and aluminum. This amount of tartaric acid ITas suf ficient for a range of 0.05 to 0.5yccalcium, Occasionally, after the addition of tartaric acid, the solutions became turbid when the alkaline buffer \vas added. By heating the solutions briefly after the addition of tartaric acid, clear solutions were obtained in all cases when they were made alkaline. This turbidity is probably due to unknown
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. to
Recovery of Calcium Added 30y0Wet-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 A N D 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
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