(Ethylenedinitri1o)tetraacetic Acid Titration of Calcium in Electrolytic Bath in Aluminum Reduction Cell SIR: The coniplexometric deterniination of calcium is fast replacing the conventional oxalate method (4) for the determination of calcium fluoride in an aluminum reduction cell bath. At present, the two indicators used in the determination of calcium fluoride are Calcein, described b y Diehl and Ellingboe ( f ) , and the zinc salt of Eriochrome Blue Black R. which is 1 - ( 2 - hydroxy - 1 - naphtliylazo) - 2 naphthol-4-sulfonic acid. The trade name is Pontachrome Blue Black R. Cheng ( 1 ) has assigned it the name Zinchrome R and recommends its use for the titration of water hardness. The color change of this indicator at a pH of 11.0 to 11.5 is very sharp. The end point is easier to detect, in the authors’ opinion. than the color change of Calcein. This observation agrees n ith that of Hildebrand and Reilley ( 3 ) .11ho recommend it for the titration of total cnlciuni in the presence of
Table 1.
Comparative Calcium Fluoride Analyses
Calcium Fluoride, yo EDTA Oxalate
Difference
7.6 7.6 7.2 7.3 6.6 6.6
7.4 7 5 7 2 7 3 6.6 6.6
+o +o
7.5 7.5 7.5 7.4 6.8 6.8 8.2 8.0
7.5 7.5 7.4 7.4 6.9 6.8 8.3 8.1
0.0 0.0 +O.l 0.0 -0.1 0.0 -0.1 -0.1
7.6 7.6 7.5 7.4 7.9 7.8 7.4 7 4 7 4 7 3
7.6 7.7 7.6 7.6 7.9 7.8 7.5 7 3 7 4 7 3
0.0 -0.1 -0.1 -0.2 0.0
2 1 0 0 0 0 0.0 0.0
0.0 -0 1 +o 1 0 0 0 0
magnesium at a p H of 12.3. Calcium can be satisfactorily titrated in the presence of small amounts of magnesium (0.1%) a t a p H of 11 to 11.5 to a sharp end point. The reduction cell bath consists of about 85yo cryolite, 5% aluminum oxide, and 8 to 10% calcium fluoride. Ions in the reduction cell bath that interfere 1%-ith the determination of calcium are aluminum and fluoride. These ion interferences (6) are eliniinated b y evaporating the sample with hydrochloric acid, and adding 2,2‘,2”-nitrilotriethanol before p H adjustment. EXPERIMENTAL
Reagents. Hydrochloric acid, C.P. 1 t o 1aqueous solution. Sodium hydroxide solution, 30% aqueous solution. 2,2’2”-Nitrilotriethanol, technical grade, 50% aqueous solution. Indicator. Grind 1 gram of the zinc salt of 1-(2-hydroxy-l-naphthylazo)-2-naphthol-4-sulfonic acid and 100 grams of C.P. sodium chloride to a homogeneous mixture. Standard Calcium Solution. Keigh 3.0000 grams of dry calcium carbonate and transfer to a 500-ml. volumetric flask containing approximately 75 ml. of distilled water. Dissolve the salt by adding slowly 1 t o 1 hydrochloric acid, and dilute to volume with distilled water. E D T A Solution. Dissolve 17.1 grams of the disodium salt of (ethylenedinitri1o)tetraacetic acid (EDTA) in approximately 500 ml. 01distilled water and dilute to 1 liter. T o standardize transfer a 10-ml. aliquot of the standard calcium solution t o a 250-ml. Erlenmeyer flask, and dilute to approximately 120 ml. with distilled rvater. S d d 15 ml. of 2,2’,2”-nitrilotriethanol,5 ml. of sodium hydroxide solution, and 1 scoop (approximately 0.2 gram) of indicator to the flask. Titrate with standard E D T A solution to a pure blue end point. Grams of calcium fluoride equivalent to 1 ml. of E D T A = 0.0468 A
where
A
= nil. of E D T A t o titrate the 10ml. aliquot
Procedure. TTeigh 0.5000 gram of finely ground b a t h (100-mesh) sample into a 250-ml. Erlenmeyer flask. Add 25 ml. of 1 t o 1 hydrochloric acid, a n d evaporate t o dryness on a hot plate (180’ t o 190’ C.), H e a t for 10 minutes t o ensure evaporation of fluorine. Remove flask from source of heat, and let cool for 2 minutes. Kash down sides of flask with 120 nil. of hot, distilled water. Bring t o a boil t o dissolve calciuni salts. Cool to io0111 temperature in a mater bath. Add in order 15 ml. of 1 to 1 2,2’,2”-nitrilotriethanol, 5 ml. of 30y0 sodium hydrouide, and 1 scoop of indicator (0.2 gram). Titrate the sample Kith E D T A until the color changes from wine-red to pur e blue. DISCUSSION
This method, a modification of the methods of Hildebrand and Reilley ( 3 ) and Cheng ( I ) , was developed for use in routine control bath analysis. It is as precise and accurate as the oxalate permanganate titration (Table I). Since its development, six analysts have run 300 bath samples with a n average difference of O . O l ~ o . I n addition, the use of filter paper, and the filtration and washing steps, v-hich are required in the oxalate method, are eliminated. LITERATURE CITED
(1) Cheng, K. L., Chemist Analyst 45, 79-81 (1956). ( 2 ) Diehl, H., Ellingboe, J. W.,ANAL. CHEM.28.882 (1956). (3) Hildebrknd, ‘G. P., Reilley, C. K., Ibid.,29,258-63 (1957). (4) Scott, W.W.,‘IStydard Methods of Chemical Analysis, Vol. I, p. 211, Van Nostrand, Princeton, N. J., 1939. ( 5 ) R7elcher, F. J., “Analytical Uses of Ethylenediaminetetraacetic Acid,” p. 72, Van Nostrand, Princeton, N. J., 1958.
ALLAND. BEHEL,SR. JAMES PENDLEY Listerhill Reduction Plant Reynolds Metals Co. Sheffield, Ala.
VOL. 32, NO. 8, JULY 1960
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