Electrometric Titration of Boric Acid L. V. WILCOX,Bureau of Plant Industry, Riverside, Calif.
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H E direct determination of boron in dilute aqueous solutions as described by Foote (a) calls for a highly sensitivemethod of titration. The Cavanagh (1) electrometric method has been successfully used for this titration. The electrometric method is rapid and accurate and is especially well adapted to measuring the small amounts of boron found in irrigation waters. The observation made by Foote and utilized in the method here described is essentially the following: A dilute acid solution of mixed salts containing boron as boric acid is titrated to a neutral point near pH 7; the solution becomes acid upon the addition of mannitol and the quantity of alkali required to titrate the solution back to the same neutral point is an accurate measure of its boron content. By this procedure the pH a t the initial and end point of the boron titration is the same. Cavanagh makes “application to electrometric titration of the long-known principle of the Helmholz cell or cell without liquid junction. A familiar example of this type of cell is that consisting of a hydrogen electrode and.. .a silver chloride electrode in one and the same solution.” The e. m. f. of this cell a t the temperature T is:
mm. silver wire 10 cm. long. This is half submerged and made the anode (a piece of platinum wire, the cathode) in a 0.1 N sodium chloride solution and a current of about 2 milliamperes passed for an hour. Cavanagh states that a number of these electrodes may be prepared a t a time and stored for a week or more in the dark in distilled water. I n use, the two electrodes are held parallel from binding posts and about 1 cm. apart, The galvanometer is of the pointer type and sensitive t o 0.25 X 10-6 amperes per division, with a shunt and tap key in the circuit. The latter is preferably located on the floor to be operated with the foot, A small mechanical stirrer completes the equipment.
PROCEDURE Transfer 250 cc. of the solution to a 400-cc. beaker. This should not contain over 1 mg. of elemental boron. (Larger quantities of boron can be titrated, but a more concentrated standard alkali solution should be used, as the total volume used between the initial and final points should not be greater than about 5 cc.) Add to the sample sufficient sodium chloride solution (12.5 cc. of 2 N sodium chloride) to bring the chloride concentration to 0.1 N . The small amounts of 2RT E = Eo F log, c chloride usually found in irrigation waters can be neglected as the absolute chloride concentration is not critical. Howwhere Eo is a constant and C is the concentration of hydro- ever, the chloride concentration should be altered as little chloric acid times its activity coefficient. as possible between the initial and final points, hence the At a temperature of 16” C. the e. m. f. of a solution contain- use of a small volume of titrating solution and adjusting the ing hydrogen and chloride ions is expressed by: aliquots to its concentration. Add a few drops of bromothymol blue (phenol red or sofnol red may be used) and acidify E = Eo 0.05735 loglo (CE X CcJ with 1 N sulfuric acid, adding about 0.5 cc. in excess. Boil 3 minutes to expel carbon dioxide. Cool and add carbon where CH and C C ~are “corrected concentrations” or acdioxide-free sodium hydroxide to approximate neutrality. tivities. Using a silver chloride-quinhydrone cell and various con- Add sufficient quinhydrone to saturate the solution (approximately 0.2 gram). Introduce into the solution the two centrations of the two ions, Cavanagh obtained a mean value which are connected through the shunt and tap key of 0.4740 for EO. At the end point of the titration, the equa- electrodes to the galvanometer. Add carefully carbon dioxide-free tion may be written: sodium hydroxide until a null point is indicated on the galvanometer. The solution should be stirred for a short time E = 0.4740 0.05735 loglo (H) 0.05735 loglo (GI) to make certain that equilibrium has been reached. This is With this value for E,, E vanishes in a solution with a pH of the initial point of the titration and should be a t about 7.62 and a chloride concentration ( X the activity coefficient) pH 7.3 a t 16” C. Add 10 grams of neutral mannitol. Boric of 0.209 equivalent per liter. Likewise, in a solution with a acid in the presence of mannitol titrates as a reasonably strong p H of 7.3 and a chloride concentration ( X the activity co- monobasic acid. The acidity of the solution will therefore efficient) of 0.108 equivalent, E vanishes. At a lower pH, for increase and in case the tap key is closed a t this point, the instance, 5.62, E vanishes when the chloride concentration galvanometer should be protected by the shunt. Add stand(x the activity coefficient) equals 0.0022 equivalent per ard alkali until the null point is again reached. This is the end point of the titration, The standard alkali used for the liter. The fact that the pH a t which Eo vanishes can be predeter- titration is carbon dioxide-free 0.0231 N sodium hydroxide, mined by the adjustment of the chloride concentration makes of which 1 cc. is equivalent to 0.25 mg. of boron, or with a it possible to adapt this electrometric titration to the Foote 250-cc. aliquot, to 1 part per million of boron in the original solution. The buret should be of such accuracy that the procedure for the titration of boric acid. volume of alkali used may be estimated to 0.01 cc. From APPARATUS the gross volume a blank is deducted. This blank is deterThe quinhydrone electrode i s made from platinum wire mined by substituting distilled water for the sample and 1 mm. in diameter and 6 cm. long. The wire is cleaned in proceeding as indicated above. The sodium hydroxide is hot sulfuric-chromic acid and “blanked” in an alcohoI flame, standardized by titrating known amounts of pure boric as suggested by Cavanagh. It is sealed into a glass tube and acid. external connection made by means of mercury. The quinRESULTS hydrone, supplied by the Eastman Kodak Company, is used without further purification. Pure boric acid was ground to a fine powder and dried to The silver chloride electrode is made from a, phce of 1- constant weight over calcium chloride. From this, a solution
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INDUSTRIAL AND ENGINEERING CHEMISTRY
January 15, 1932
39
of boric acid was prepared, containing 0.1077 mg. of elemental boron per cc. Aliquots of this solution were titrated by the above described method with the results shown in Table I.
The results of the boron determinations of a series of natural waters are reported in Table 11. The figures represent duplicate determinations on separate aliquots. To a third aliquot of each sample 1.21 mg. of boron as boric acid were added. Column 4 shows the recovery of this added boron TABLEI. TITRATION OF SOLUTION OF PURE BORICACID expressed in milligrams of boron. BY ELECTROMETRIC METHOD The method here described has been used on 300 samples BORON PER cc. BORICACID EQUIVALBNT0.046 N SODIUM HYDROXIDEOF SODIUM of irrigation water in comparison with the modified Chapin SOLN. BORON Total Less blank HYDROXIDE method (3) with satisfactory results. The boron content cc . Mg. Cc . cc . Mg. of such waters as determined by direct titration is consistently 0 (blank) 0 0 0 (blank) 0.07 0.06used as blank slightly higher than that obtained by the distillation method, 0 (blank) 0 0.08 O'O4/ 2.5 0.269 0.60 0.54 0.498 but since it is known that by the latter method only 90 to 95 2.5 0.209 0.59 0.53 0.508 per cent of the boron is recovered, it is believed that the results 2.5 0.269 0.81 0.55 0.489 5.0 0.638 1.13 1.07 0.503 by the direct-titration method are nearer the truth, 5.0 0.538 1.13 1.07 0.503 The direct-titration method may not be dependable for 1.11 1.05 5.0 0.538 0.512 1.077 2.25 10.0 2.19 0.492 use with solutions containing appreciable quantities of phos1.077 2.15 2.09 10.0 0.515 phates, such as the nutrient solutions used in the plant cul10.0 1.077 2.20 2.14 0.503 tures, or with solutions containing relatively high concentrations of silicates. It is recommended for use with irrigation ON IRRIGATION TABLE11. BORONDETERMINATIOXS and drainage waters in which the range of hydrogen-ion conWATERS BY ELECTROMETRIC-TITRATION METHOD BORON,DUPLICATE DETNS. BORONRECOVERED centration is usually between p H 7 and 8, and in which the LAB.No. content of phosphates and silicates is low. P.p . m. P. p . m. IlilO. I
4519 4522 4525 4528 4531 4534 4537 4540 4543 4546 4549 4552 4555 455s 4564 4567 4570 4573 Av.
0.21 0.02 0.04 0.10 0.06
0.17 0.04 0.05 0.05 0.04
1.19 1.23 1.24 1.23 1.22
0.60 0.22 0.07 0.11 1.22 0.35 0.22 0.64 0.31
0.63 0.15 0.05 0.09 1.25 0.29 0.22 0.65 0.30
1.20 1.23 1.20 1.20 1.21 1.15 1.21 1.24 1.214
ACKNOWLEDGMENT The author wishes to acknowledge his indebtedness to Francis Scofield, who first pointed out the possibilities of the Cavanagh procedure in the titration of boric acid, and also to Albert P. Vanselow and Frank M. Eaton for suggestions during the early part of this work. LITERATURE CITED (1) Cavanagh, B., J. Chem. Soc., 1927, 2207-16. (2) Foote, F. J., IND. ENG.CHEM., Anal. Ed., 4, 39 (1932). (3) Wilcox, L. V., Ibid., 2, 358-61 (1931). RECEIVED September 5, 1931.
Determination of Boron in Waters Method for Direct Titration of Boric Acid FRED J. FOOTE, Limoneira Co., Santa Paula, Calif. OST procedures for the determination of boron necessitate the absence from the solution of all other substances titrating between a p H of about 5 and a pH of about 8.4. To eliminate these substances from a sample containing less than one part per million of boron without the loss of some boron or the addition of a varying amount with the reagents used has been found to be a troublesome and time-consuming process. In the past year a method has been developed and used for titrating boron in irrigation waters and water extracts of soils which overcomes this difficulty, and determinations made by it are rapid and capable of great accuracy. This method should be applicable to any aqueous solution of boron, although modification may be necessary in some instances. Results by the direct method have been compared with results by the distillation method formerly used in this laboratory, and the new method found to be more reliable (1). Hundreds of determinations have been made on waters and soil extracts without encountering any trouble. The principle of the determination is that with no mannitol present, boric acid is so weak that it is only partially neutralized at a pH of 7.6, but in the presence of sufficient man-
nitol it is completely neutralized a t this pH. No other acids or bases encountered or tested are affected by mannitol, so that the boric acid can be accurately titrated by bringing the solution to a pH of 7.6, adding mannitol, and titrating back to the same pH. The alkali used for titrating is standardized against a known amount of boric acid in the same manner, so that the small amount neutralized up to pH 7.6 is taken care of in the standardization factor. The amount so neutralized is about 12 per cent of the total boric acid present. Since there is no pH change involved in the two end points, there is no interference from other substances in the solution. It has been found, however, that the solution must be practically free of carbon dioxide, or equilibrium is difficult to attain and low results are obtained. Curves illustrating the titration of boric acid are shown in Figure 1. The initial point for the direct titration is A , and the end point is R. DETERMINATION OF PH For the determination of pH, either colorimetric or electrometric apparatus may be used. The colorimetric method has been used for most of the determinations and found en-
