Colorimetric Determination of Fluoride in Natural Waters with Thorium and Alizarin N. A. TALVITIE Washington State Department of Health, Seattle, Wash.
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THORIUM-alizarin lake has been used, following isolation of fluoride by distillation, in the determination of fluoride by volumetric (8) and colorimetric (3)procedures. I n the method described herein this lake has been adapted to the colorimetric determination of small quantities of fluoride in potable waters, without prior isolation by distillation, in a manner similar to the use of a zirconium lake in many of the colorimetric methods (I, 4, 6') now in use. The use of thorium overcomes several of the difficulties experienced with the use of zirconium-i. e., slowness of the reaction, tendency of the lake to precipitate, and instability of the reagents. The method involves a combination of hydrolytic, adsorption, and complex-ion equilibria. A dilute solution of thorium nitrate a t pH 3.5 consists of a hydrous thorium dioxide sol stabilized by the preferential adsorption of hydrogen and thorium ions (7). Being positively charged in this condition, the oxide further adsorbs anions which in this case are nitrate ions. Alizarin monosodium sulfonate is a salt of a weak acid and, consequently, is almost completely hydrolyzed a t pH 3.5 to the yellow associated acid. The acid, however, dissociates (g) in the presence of thoria sol with the formation of a deep red adsorption complex of alizarin sulfonate ion and the positively charged hydrous oxide. This process is principally an exchange adsorption of the alizarin sulfonate ion with less strongly adsorbed anions on the oxide. Addition of fluoride to the adsorption complex converts the thoria into a fluoride of thorium, releasing alizarin sulfonate ions which reassociate into the yellow alizarin sulfonic acid. With reagents of proper concentrations, the color changes progressively from red to yellow with increasing quantities of fluoride. Thus a series of colorimetric standards may be prepared with which a sample of unknown fluoride content may be compared. Equilibrium in the thoria sol is displaced by slight changes in hydrogen-ion concentration; therefore, the pH of both sample and standards must be stabilized by means of a buffer. The system sodium formate-formic acid has a p H of about 3.5 and waa found convenient for the purpose. In order to keep the quantity of buffer to a minimum, the sample is subjected to a preliminary neutralization. Sodium sulfate, added to both sample and standards, reduces interference of the sulfate ion.
Reagents Thorium reagent, 0.001 M with respect to thorium nitrate,
and 1 M with respect to each sodium sulfate, formic acid, and sodium formate. Alizarin Indicator, 0.00025 M,0.0855 gram of alizarin monosodium sulfonate in 1 liter, Standard fluoride solution, 1 ml. equals 0.01 mg. of fluoride; 0.0221 gram of sodium fluoride in 1 liter. Nitric acid, 0.3 N .
Procedure STANDARDS. Transfer 0, 2, 4, 6, 8, 10, and 12 ml. of standard fluoride solution to a series of 100-ml. long-form Nessler tubes, each of which has been previously marked a t 110 ml., and add 5 ml. of alizarin indicator to each. Dilute to 110 ml. with distilled water and add 5 ml. of thorium reagent. Mix well and allow to stand 30 minutes.
SAMPLE. Transfer a 100-ml. sample to an Erlenmeyer flask and add to it 5 ml. of alizarin indicator. Titrate carefully with 0.3 N nitric acid to a pure yellow. Pour into a Nessler tube, dilute to 110 ml., add 5 ml. of thorium reagent, and mix well. Compare after 30 minutes with the 'standards. Should the fluoride content of the sample be beyond the range of the standards, repeat the determination with a smaller sample.
Effects of Ions and Conditions COMMON IONS.Experience with zirconium-alizarin methods (I, 4, Q and the method described has shown that, when corrections are made only for the effects of sulfate and chloride, the fluoride found in a 100-ml. sample is less than double or quadruple the fluoride found in a 50-1111. or 25-ml. sample diluted to 100 ml. This indicates the necessity of considering the combined interference of other ions tending to give low results whose individual effects may be negligible. Although the effects of the interfering ions are not strictly additive, the error introduced in applying corrections on the assumption that they are additive is of no consequence in the determination of fluoride in a sample containing no more than 100 mg. of any one ion. Approximate effects of the common ions are as follows: 100 mg. of sulfate are equivalent to +0.02 mg. of fluoride; 100 mg. of calcium, magnesium, or chloride are equivalent to -0.01 mg. of tluoride; 100 mg. of nitrate are equivalent to -0.005 mg. of fluoride. Since the ionic weight of nitrate is approximately that of bicarbonate, the quantity of nitrate introduced in neutralization of the sample may be considered identical to the quantity of bicarbonate originally present. Sodium and potassium have no effect. OTHERIONS. When phosphate or aluminum is present, the fluoride must be isolated by a distillation procedure (8) prior to determination of fluoride. Phosphate may be recognized by the turbidity of the sample caused by the precipitation of thorium phosphate. Aluminum is indicated by failure to obtain the pure yellow color of the alizarin when the sample is neutralized with nitric acid. Aluminum forms a lake with alizarin, the orange color of which is not immediately discharged by an excess of the acid. Iron occasionally may be present in sufficient quantities to give a purple or black tinge to the sample, making comparison difficult, In such case iron may be removed from the water sample by aerating and filtering. Acid waters should be made alkaline before aerating. Silica in quantities w high as 20 mg. does not interfere. COLOR. Waters containing organic coloring matter may be analyzed for fluoride by the Walpole (6)-technique by using 100ml. short-form Nessler tubes in a simply constructed comparator in which the sample tube rests above a tube of distilled water and the standard rests above a tube of a duplicate sample containing 5 ml. of thorium reagent but no alizarin. Comparison is made by viewing from the top through the lengths of two tubes. Differences of temperature between standard TEMPERATURE. and sample cause an apparent deviation of 0.002 mg. of fluoride per degree centigrade. The standards and samples are preferably brought to constant temperature in a water bath. TIME. Equilibrium is reached within 30 minutes. No visible change occurs in the standards upon standing as long as a week in darkneee. 620
ANALYTICAL EDITION
October 15, 1943
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Summary
Literature Cited
A procedure is proposed for the colorimetric determination of fluoride in natural waters containing not more than 1000 parts per tnillion of any one of the common ions. BY applying small Corrections, results may be obtained to 0.1 part per million of fluoride phosphate and . with the use of a 1 0 ( ) - ~ lsample. interfere, but their presence may be recognized in the course of
(1) Lamar, W. L., and Seegmiller, C. G., IND.ENO.CHEM.,ANAL. ED., 13,901 (1941). (1941). (2) McClendon, J. F., and Foster, Wm, c.,Ibid.,13, (3) Merwe, P. K.van der, Onderstepomt J. Vet. Sei. Animal Id.. 14,359 (1940). (4) Sanchis. J. M., I N D . ENG.CHEM., ANAL.ED.,6, 134 (1934). (5) Scott, R. D.,J . Am. Water Works Assoc., 33, 2018 (1841). (6) wdpole, &,&aJ , . , 5, 207 (1911). (7) Weiser, H.B.,“Inorganic Colloid Chemistry”, Vol. 11, pp. 273. 360,New York, John Wiley & Sons Co.,1935. ( 8 ) Willtird and Winter, IND. ENG.CHEM, ANALED.,5, 7 (1933).
the determination. The method excels previous colorimetric methods in speed, accuracy, and stability of reagents.
Modified Basic Succinate Estimation of Aluminum in Magnesium Alloys A. J. BOYLE AND D. F. MUSSER Laboratories of Basic Magnesium, Incorporated, Las Vegas, Nevada
The modification of the Willard and Tang basic succinate procedure described is rapid, simple, and accurate and is advocated as a routine as well as a referee method for aluminum. The accuracy obtained is equivalent to, or, in certain cases, greater than, that obtained with the benzoate-oxine procedure. Procedure8 are given for avoiding interference from iron, silicon, and copper.
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HE purpose of this investigation is to establish a rapid and accurate routine method for the determination of aluminum in magnesium alloys and to compare it with the recent benzoateoxine method (2), which involves preliminary separation of aluminum as henzoate from most bivalent metals. The basic succinate method submitted is o, modification of the Willard and Tang procedure (4) for the determination of aluminum by precipitation with urea. In the modified procedure as used by the laboratories of Basic Magnesium, Incorporated, the alloy is dissolved in dilute hydrochloric acid, to which ammonium chloride, succinic acid, and urea are subaequently added. This solution, diluted to the proper volume, is made just alkaline t o methyl orange by the addition of dilute, freshly filtered ammonium hydroxide or a solution of ammonium carbonate. Boiling the solution gently for 90 minutes results in the quantitative precipitation of aluminum 119 basic succinate. This procedure for aluminum is believed by the authors to be the most rapid accurate method available. It requires only a modicum of analytical skill and is advocated as a routine as well as referee method to establish the aluminum content of magnesium alloys. The precipitate of basic aluminum succinate is extremely dense, so that filtration is rapid. One precipitation is sufficient for effective separation from bivalent metals commonly occurring in magnesium alloys. This separation ip not quantitatively possible by the ammonium hydroxide procedure for aluminum, particularly in the presence of zinc (1).
Reagents Succinate reagent. 10 grams of urea, 5 grams of ammonium chloride, and 5 grams of succinic acid, diluted to 300 ml. Hydrochloric acid, c. P.; perchloric acid, c. P.; ammonium hydroxide, filtered; ammonium carbonate, c. P., 5 per cent solu-
tion.
Phenylhydrazine, c. P.; hydroxylamine hydrochloride,
c. P.; and ammonium bisulfite, 10 per cent solution.
Procedure Weigh a 1-gram sample of the alloy into a 600-ml. beaker. Treat wvith 50 ml. of distilled water and 10 ml. of concentrated hydrochloric acid. After the alloy is dissolved, filter to remove metallic copper. The interference due to appreciable amounts of iron is prevented by adding to the heated filtrate a few drops of a 10 per cent solution of ammonium bisulfite and 2 ml. of phenylhydrazine. This treatment serves to reduce the iron and to maintain it in a ferrous state during the precipitation of aluminum as basic succinate. Dilute with 300 ml. of succinate reagent and boil gently. Make the solution just alkaline to methyl orange with freshly filtered 1 to 1 ammonium hydroxide or a solution of ammonium carbonate. Continue boiling gently for 90 minutes, precipitating the aluminum as basic succinate. Gentle boiling is very effective and does not reduce the total volume quickly. No special care need be given the solution during this period. Filter through a paper of loose texture. Dissolve the small amount of precipitate which adheres to the beaker in 20 ml. of 1 to 4 hydrochloric acid and reprecipitate by addinq 1 to 1 ammonium hydroxide until just alkaline to methyl red. Boil the solution for one minute and filter through the paper containing the major portion of the precipitate. Wash the precipitate six times with hot 1 per cent ammonium chloride made alkaline to methyl red with ammonium hydroxide. Ignite the precipitate a t 1300’ C. for one hour to form nonhygroscopic Corundum, cool, and weigh as oxide. Unglazed porcelain crucibles appear to be best for this purpose since they show no change in weight. For long life of the crucible it is advisable to dry the crucible and precipitate in a standard oven before ignition. For rapid routine analysis, i t is preferable to brush the precipitate out of the crucible and weigh directly as oxide. If silicon is present in uantities greater than 0.2 per cent dissolve the alloy in 30 ml. 1 to 2 perchloric acid. Evaporate the solution on a hot plate to copious fumes of perchloric acid. Cool, dilute to 50 ml. with distilled water, and filter through B fast paper. Wash the precipitate six times with hot distilled water and discard. Copper is dissolved by this procedure. I n order to eliminate the error due to this element, add 20 ml. of the ammonium bisulfite reagent to the solution (0,or (if iron is absent) 1 gram of hydroxylamine. Add 300 ml. of the succinate reagent and continue as described previously.
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Discussion For a comparison of the basic succinate method with the benzoate-oxine method and with precipitation by ammonium carbonate (used ~ W R W it is silica-free), a standard solution of aluminum chloride was prepared by dissolving aluminum metal
