Stable Indicator Solutions for Complexometric Determination of Total Hardness in Water EUGENE 31. DISKANT Sanitary Engineering Division, Department of Water and Power, Los Angeles, Cali$. since Biedermann and Schwarzenbach (6) described their rapid and direct complexometric titration for the determination of total hardness in water, this procedure has been gaining in popularity. It has about the same accuracy as the most accurate previously known procedure, which involved the separate determinations of calcium and of magnesium ( I ) , and has the advantage of being far simpler and more rapid. I t is even faster than the direct titration of total hardness with standardized soap solution, a procedure which was widely used because of its speed and simplicity, but was never noted for its accuracy. One drawback to the complexometric titration has been that hitherto the indicator employed has not been available as a stable solution. The dye which serves as the indicator is sold under various trade names (Eriochrome Black T, Pontachrome Black TA, Potting Black C, Diamond Blue Black EBS, Omega Chrome Black S, etc.). It is number 203 in the “Colour Index,” number 241 in Schultz’ “Farbstofftabellen,” and was referred toasF241 by Biedermann and Schwarzenbach. Chemically, it is sodium 1(l-hydroxy-2-naphthylazo)-5-nitro-2-naphtho~-4-sulfonate. It has hitherto been employed as a solution in aqueous buffer, in ethyl alcohol, or in isopropyl alcohol. These solutions are known to decompose, sometimes within a few weeks; the decomposed indicators produce indistinct end points and muddy or altered colors. Decomposition progresses slowly but steadily, so that an analyst who uses the same solution day after day may not notice the deterioration until it has become marked. Coniparison with freshly prepared indicator will accentuate the difference. Experience in this laboratory leads the author to believe that some analysts may have tried the complexometric titration and discarded it because they encountered poor end points, and werr unaware that the cause of this difficulty was the use of (stale) prepared commercial indicator solutions. Because of the instability of the indicator solutions hitherto recommended, various authors have advised that it be freshly prepared. Thus Biedermanii and Schwarzenbach say, “it is advantageous t o prepare the indicator solution freshly, as it is not stable for long.” The Hall Laboratories (8) state, “the solution normally is good for about a month”; hlarcy (9) found that the “solution is stable about a month”; Connors (7) wrote that “the stability of the dye in alcohol is. . .poor, and. . . unquestionably a frequent source of difficulty.. .”; Banewics and Kenner (2) reported that “this solution was unstable and could not be used after a few days.” Betz and No11 ( 4 ) made up their indicator solution in aqueous isopropyl alcohol with the addition of sodium carbonate and reported ( 5 )that their solution was “satisfactory after 8 mo.” Although the Bets-No11 reagent has better keeping qualities than the simple alcoholic solution, in this laboratory signs of deterioration have been observed in from 2 to 4 weeks. Betz (3) explains that he took “. . .as indication of deterioration, any decrease in the sharpness of the end point. As the indicator ages, the red and blue colors will not remain as brilliant as when the indicator was freshly prepared, but so long as a sharp end point is obtained, the solution is satisfactory for use.” The author considers any detectable alteration as indicative of deterioration, and this difference in criteria accounts in part for the apparently contradictory observations. The Sanitary Engineering Division includes reagents for the determination of water hardness in its portable field kits. These kits, which can be carried to a customer’s premises or to a reservoir for on-the-spot testing, may be used only a few times during the course of a month, and may be stored in the trunk compartment of an automobile; therefore it is desirable to eliminate from
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such kits any reagent which deteriorates quickly. In the main laboratory it is possible to prepare a fresh indicator solution at frequent intervals, but more convenient to utilize one which is stable. In outlying laboratories, where routine testing may be performed by untrained personnel, it is impractical to prepare fresh solutions frequently. For all of these reasons, a stable indicator is highly desirable. It has been suggested that the dye be ground up with an inert filler, or with dry buffer salts (a buffer must be added t o the solution which is t o be titrated, in any event) and that this dry mixture be dispensed with a scoop. Such a commercial mixture containing dye and buffer salts was tested in the author’s laboratory and proved to be unsatisfactory. Recently Porter ( I O ) has recommended that filter paper be impregnated with a concentrated solution of the dye, dried, and stored as such; he found that the dye dissolves from the filter paper into the solution to he titrated and produces “end points as sharp as those obtained with the fresh solution of the indicator.” PRESEhT INVESTIGATION
Beginning more than a year ago, the author sought solvents which would form stable indicator solutions. The dye employed was purchased from Du Pont as Pontachrome Black T 9 ; fresh solutions of this material have been found highly satisfactory. A series of solutions was prepared, consisting of 0.5 gram of dye, 0.5 ml. of a 1 N solution of sodium carbonate, 15 ml. of water, and sufficient solvent to bring the final volume to 50 ml. As solvents, Cellosolve, methyl Cellosolve, glycerol, pyridine, triethanolamine, acetic acid, dioxane, ethylene glycol, acetone, and ethyl alcohol were tried. Within the first 3 months, all of the solutions had deteriorated except those made up with pyridine or triethanolamine. Pyridine showed good promise but was discarded because its unpleasant odor made it unpopular in the laboratory. After about 6 months, the triethanolamine solution had deteriorated. Another series was prepared in which 0.5 gram of the dye was dissolved in 50 ml. of solvent without the addition of water or of sodium carbonate. As solvents, ethanolamine, diethanolamine, triethanolamine, nitromethane, formamide, acetonitrile, absolute methanol, Cellosolve, methyl Cellosolve, and tert-butyl alcohol were tried. Of these, nitromethane, acetonitrile, and tert-butyl alcohol were discarded because they dissolved the dye incompletely. Within 4 months, all of the above solutions were found to have deteriorated except those made up in diethanolamine or triethanolamine; these have remained in good condition for a t least 7 months. Moreover, these two solvents dissolve the dye quickly and completely. Because of the high viscosities of these alkanolamines, it was thought desirable to dilute them with other solvents. Bccordingly, a triethanolamine solution of the dye was diluted with four volumes of absolute methanol, or with four volumes of 95% ethyl alcohol, or with an equal volume of water. Unfortunately, within 2 months all three dilutions showed observable decomposition. Betz and No11 observed that solutions of the dye in isopropyl alcohol were stabilized by the addition of sodium carbonate, and this paper confirms the beneficial effect of a basic solvent. On the other hand, too strong a base (ethanolamine) caused the dye to assume an unusual color and to decompose. Additional stability appears to be gained by the elimination of water and of alcohols from the solution; yet the alkanolamines themselves possess alcoholic hydroxyl groups, Banewicz and Kenner cite a personal communication from Wright to the effect that solu-
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V O L U M E 2 4 , NO. 11, N O V E M B E R 1 9 5 2 tions of the dye are stable if kept out of contact with air by means of a blanket of nitrogen. The author is unable to offer a theory which would account for all of these phenomena. It is therefore empirically recommended that the indicator be prepared either with diethanolamine or with triethanolamine and that no other solvents or salts be added. There appears to be little choice between diethanolamine and triethanolamine. No advantage is gained by protecting the solutions against light or by keeping them in a refrigerator. The indicator solutions are best dispensed in prescription type dropper bottles, which should be kept tightly closed when not in use to avoid the absorption of moisture by the (hygroscopir) alkanolaminw. ACKNOWLEDGMEhT
The author is indebted to llaurice E. Selson, LouiseC.Schauer, Doroth? Llargolin, and Albert A . I'oithal of this department for testing various indicator solutions under different conditions and reporting result?: and to Joseph J Connors, East Bay Municipal
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Utility District, Oakland, Calif., and John D. Betz, Bet2 Laboratories, Philadelphia, Pa., for helpful discussions. LITERATURE CITED
(1) .Im. Pub. Health Assoc. and Am. Water Works Assoc., "Standard Methods for t h e Examination of Water and Sewage,'' 9th ed., p. 23, 1946. (2) Banewicz, J. J., and Kenner. C . T., AXAL.CHEST.,24, 1186-7 (1962). (3) Betz. J. D.. reviewer's comments. . h u . 1. 1952. (4) Betz, J. D., and Koll, C. A., J . A m y Water T o r k s Assoc, 42, 49-56 (1950). (5) Ibid., pp. 749-54. (6) Biedermann, W., and Schwarxenbach, G., Chimza, 2, 56-9 (1948). (7) Connors, J. J., personal communication, Oct. 16, 1951 (8) Hall Laboratories, Inc., Bull. 1 RE 50, 1950. (9) Llarcy, V. M., Power, 94, 105-8 (1950). (10) Porter, 3. D.. Chemzst-Analyst, 41, 33-5 (1952) RECEIVED for review July 7 1932
Accepted August 25, 1952
Preparation of Very Dilute Standard Base by Ion Exchange BESJAMIK WOLF GRUNBAUM, WOLFGANG SCHONIGER~,AND PAUL L. KIRK Diuision of Biochemistry, C;nir.ersity of California Medical School, Berkeley, Calif.
'
iX DEVELOPIXG a method for titration of microgram quanti-
ties of fatty acids from fractions of biopsy specimens of liver, a major problem was the difficulty of obtaining a constant normality of standard alkali of about 0.001 N . Solutions were carefully prepared with boiled, redistilled water and saturated caustic from which the carbonate had been allowed to precipitate. On storage of such solutions in the usual manner in bottles well coated with paraffin, and without contact with glass or rubber, the strength of the solutions was found to differ from day to day and even during the course of the day from storage, handling, or other cause. Schubert (1), describing analytical applications of ion exchange separations, quotas the suggestion of a simple means for preparing standard acid by stoichiometric release of hydrogen ions from organic cation exchange resins. The preparation and storage of very dilute mineral acid is certainly less critical than is that of very dilute alkali, but the suggestion indicated the possibility of a similar preparation of alkali by means of anion exchanging resins to obtain stoichiometric release of hydroxyl ions.
vessel with an outlet about 1 cm. below the top. Solution flowing from this outlet was led t o a drain by a rubber catheter tube. To the lower end of the vessel was attached a capillary stopcock, below which was an angular side connection and delivery tip as shown. The side connection was attached by means of a gum rubber tube to the capillary tip of a capillary buret ( 2 )which could be filled after opening the stopcock. When the stopcock was closed, the incoming standard solution was forced out of the outlet and led to the drain.
I"
Since this paper was submitted, Steinbach and Freiser ( 4 )have described a similar technique for preparation of batches of 0.1 N sodium hydroxide with Amberlite IRB-400. It is clear that the technique is of general application but n-ith special advantage in titration of microgram quantities. This idea was satisfactorily developed by means of a simple apparatus in which a strong base anion evchange resin mas reacted with 0.0010 N potassium chloride solution to provide the corresponding standard potassium hydroxide solution according to the reaction
RX-OH-
+ K f + C1- +R S + C l - + K - + OH-
The reaction apparently proceeds without attention for months and shows a remarkable constancy of normality of the solution delivered. EXPERIMENT 4 L
Ap aratus. The apparatus consisted of a glass tube, 40 cm. in lengt! and 16 mm. in outside diameter, attached a t the lower end to a 1-mm. bore heavy-wall capillary tube bent as shown in the diagram. A sintered-glass filter sealed in the larger tube served to retain the resin with which the tube was filled. The constricted end of the capillary was led to the bottom of a reservoir 1
Permanent address, Pregl Laboratories, University of Graz, Austria.
Above the column was placed a separatory funnel containing 0,0010 N potassium chloride solution which was led through a length of Tygon tubing to a tip in the top of the column. Sotches were filed in the stopcock plug to allow fine control of the flow rate which could also be altered by the height of the funnel. Procedure. The column was charged with the strong base anion exchange resin, Amberlite XE-67. The resin has been previously saturated with a 4% sodium hydroxide solution and washed with redistilled water until a neutral reaction was obtained. The separatory funnel filled with 0.0010 N potassium chloride solution was connected and about 200 ml. of solution allowed t o flow rapidly through the column to displace water, after which the rate was adjusted to about 2 ml. per hour. The
