Sulfate Titration Use of Tetrahydroxyquinone in a Semimicromethod W. A . PEABODY AND R. S. FISHER Valentine's >feat-Juice Co., Richmond, Va.
A
RAPID seniimicio sulfate method 11-as needed in this
have been described previously ( I , 2 , 3 ) . The niodifications found desirahle for the semimicro scale are included below.
laboratory. With colored unknovns containing organic matter, considerable difficulty xas experienced in direct titration of sulfate with tetrahydroxyquinone used internally as recommended. Sharp end points n-ere essential in order to estimate sulfate ion in the range of 3 to 6 mg. contained in 0.1 to 1.0 nil. of original sample. Although the pale, yellowbrown color in most of the samples as diluted for titration was not so intense as to cause difficulty, a method useful in the presence of considerable color was preferred for more general application. The unknonns (liver extract fractions) had p H values of 5.8 to 6.8 and contained 14 to 35 per cent weight b y volume of organic matter, as well as small but undetermined amounts of phosphates. Highly accurate results for sulfate were not expected in these particular direct titrations, but reproducible comparative results were required to follow the course of sulfate removal. The samples, as a rule, were left unneutralized t o diminish the possibility of high results caused by phosphate and organic constituents, such as nucleic acids. The end point was less indefinite n i t h the neiver tetrahydroxyquinone (organic dispersing agent) described b y Sheen and Kahler (S),but still indeterminate over a range of 0.1 ml. in titration of the above unknowns. Kinsor (4)had described the successful use of 2-methoxyethanol (methyl Cellosolve, supplied b y the Carbide and Carbon Chemicals Corp.) as a color stabilizer in the ferric thiocyanate reaction. While the same theoretical considerations did not appear to apply t o the barium-tetrahydroxyquinoneproblem, the possibilities of 2-methoxyethanol were investigated. Use of the niethoxyethanol to replace part or all of the alcohol in the titration medium did not prove an advantage. However, the stability of an aqueous solution of the indicator was markedly increased by inclusion of sufficient methoxyethanol. Tliiq indicator solution proved admirable as an outside indicator and permitted adaptation of the method to the authors' needs. Ethanol also impeded indicator fading b u t was less effectke than the niethoxyethanol. d conipromise was necessary betn-een the proportion of metlioxyethanol needed for stability and the proportion of 11-ater needed for sufficient color intensity. T h e solution described below is still usable after 18 hour>,although the indicator d l ha1-e faded sonlev hat, nhile a purely aqiieoiic wlntion is u-eless after a fen ininiitc= The general prinriple- ant1 111ocptlui e for the macroinethod
Semimicromethod SPECIAL APPARATGS.4 semimicroburet-t he Koch calcium pipet, provided with a 50-ml. reservoir and a 2-ml. column graduated to 0.01 ml. SOLUTIONS. Indicator: 1 meahring cup (about 150 mg.) of tetrahydroxyquinone preparation (THQ Betz), dissolved in 1.0 nil. of distilled a-ater and diluted with 2 ml. of 2-methoxyethanol. Titrating solution, approximately 0.1 N barium chloride, standardized preferably by titration against about 0.1 N standard sulfuric acid (or neutral sulfate) solution, standardized gravimetrically. TABLE I. SULFATETITRATION SO4 of Cndiluted Sample, Total BaCh Based on F i n d BaCL Standardization (Less 0.02M I . B1ank)Gravimetrically Volumetrically Approximately 0.1 X 604 1mid.e THQ 1 Outside THQ 1 Graiimetric 10 Exactly0.1 N ("+SO4 Outside THQ 0.5 Outside THQ 1 Outside THQ 2 With 2 ml. additional alcohol 2 Like preceding sample, except no alcohol used 2 hctually present i n 0.1000 N salt .. Liver extract fraction, diluted sixfold Outside THQ 1 Gravimetric 12
1.24f 1.24
..
4.80f 4.80
4.92+ 4.92 4.923
0.60 1.22 2.46
4.64 4.72
4.76
4.76 4.84 4.88
2.46f
4 764-
4.88C
3.46
4.76
.. 1.56
..
4.88 4,803
36.2
37.6
37.2
STANDARDIZATION. Description of the standardization procedure essentially outlines the method. The barium chloride solution is standardized conveniently by titration against 1 ml. of 0.1 A' sulfuric acid accurately measured with an Ostwald pipet into a 50-ml. beaker. After neutralization with 0.1 N sodium hydroxide and addition of 2 ml. of ethanol, the barium chloride is added from the Koch pipet. As the end point is approached, samples of the n-ell-stirred mixture are transferred (at 0.01-nil. titration intervals) v i t h a small glass rod to droplets of the indicator solution on a suitable Epot plate until n droplet turns definitely pink almost immediately. Under these conditions, a blank of 0.01 to 0.02 ml. is deducted. The approximate end point i. previously estimated, or, in the case of unknowns, is determined b? preliminary titration. Thuc, only 3 to 5 droplets need be removed from a total wlume of approximately 5 ml., just hefore the end point iq reached i n the final titrations. Even ion of preliminary tit rations, a marked economy -quinone i; effected as compared nith internal iiw nf t h e indicator. 651
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Each titration figure in Table I was obtained from two or more titrations differing by 0.02 ml. or less. The tabulated results show that outside and inside indicators give identical values with a pure sulfate. Apparently these slightly low results tend t o be compensated by titrimetric standardization of the barium chloride solution. This is to be expected, with unknowns of equivalent sulfate-ion concentration, as the effect of probable slight incompleteness of barium sulfate precipitation in the course of a rapid titration is canceled out. For highest accuracy, the best procedure appears t o be standardization of the barium chloride b y titration, coupled with dilution of the original sample to permit use of volumes and sulfate concentrations closely similar t o those used in standardization. T h e results with ammonium sulfate, and other observations, do not entirely convince the authors that addition of alcohol is essential in titrations of this order. Reliable figures for inside indicator titration of liver extract fractions were not obtained because of the indeterminate end points previously mentioned. T h e tolerance for phosphate ion unfortunately could not be
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raised by the outside indicator method appreciably above the 60 p. p. m. limit determined b y Sheen and Kahler (2). Xumerous buffers were tried, b u t the shifting or indeterminate nature of the end points, caused by changes in p H and different buffer systems or concentrations in t h e presence of phosphates, indicated the impracticability of these procedures.
Summary A ne\v and economical tetrahydroxyquinone indicator solution is described which, used externally, greatly facilitates the direct titration of small amounts of sulfate ion in certain unashed samples containing organic constituents. T h e interference of phosphate ion is again pointed out.
Literature Cited (1) Schroeder, W. C., IND. ESG.CHEX,Anal. Ed., 5 , 403 (1933). (2) Sheen, R. T., and Kahler, H. L., I h i d . , 8 , 127 (1936). (3) I b i d . , 10, 206 (1938). (4) Winsor, H. T.V., Ibid., 9, 453 (1937). RECEIVED June 20, 1938.
Determination of Small Amounts of Potassium A Simpler and More Rapid Variation of the Sodium Cobaltinitrite Method D. S. BROWN, R. R. ROBIXSON, AND G. &I. BROWNING West Virginia Agricultural Experiment Station, Morgantown, W. Va.
T
HE procedure herein described for the determination of potassium by precipitation with sodium cobaltinitrite is simpler and more rapid than any of the many similar procedures which have been proposed since the original work of Adie and Wood (1). The potassium is precipitated in a relatively short time at room temperature. A centrifuge is used t o separate the precipitate, as in the procedures of Kramer and Tisdall (7) and others (5, 8, 9 ) , and obviates the filtration employed elsewhere (1-4, 11, 13). The precipitate is washed only once, whereas two or more washings are required in other procedures ( 1 , 2 , S,6-9,11, I S ) . T h e use of ceric sulfate (4) instead of potassium permanganate for the determination of the nitrites in the precipitate is advantageous. The end point with ceric sulfate is very sharp. When potassium permanganate is used, often a precipitate of hydrated manganese dioxide is formed, which necessitates the addition of a n excess of sodium oxalate to effect its solution, after which the end point is reached by an additional titration with permanganate. This difficulty is, of course, not encountered when ceric sulfate is used. The procedure is satisfactory for the determination of potassium in amounts ranging from 0.2 t o 1.0 mg., the error, in general, not exceeding 2 per cent. It is especially applicable t o the determination of potassium in plant material and in soil extracts. Reagents Precipitating Reagent. Mix together 46.2 grams of sodium cobaltinitrite, 18.9 grams of sodium acetate, 120.0 ml. of distilled water, and 18.0 ml. of glacial acetic acid. Prepare this solution 48 hours before using. Keep stoppered and in a cold, dark place. Before using, centrifuge to remove any precipitate. Ethyl Alcohol. 95 and 70 per cent by volume. Ceric Sulfate. Dissolve about 9 grams of anhydrous ceric sulfate in 500 ml. of distilled water to which have been added 30 ml. of concentrated sulfuric acid. Make up to 1liter. This solution, which is approximately 0.02 N , may be standardized with sodium oxalate.
Ferrous Ammonium Sulfate. Dissolve 8 grams of FeSO4(KH4)&304.6H,O in 500 ml. of distilled water to which have been added 10 ml. of concentrated sulfuric acid and make up to 1 liter. Sulfuric Acid. Concentrated sulfuric acid diluted 1 to 1. Indicator. 0.025 M o-phenanthroline ferrous complex.
Procedure To 1.5 ml. of 95 per cent ethyl alcohol in a 15-ml. centrifuge tube add a 5-ml. aliquot of the potassium solution. Mix thoroughly. Add dropwise, with continuous shaking, 2.0 ml. of the precipitating reagent. Allow to stand for 1 hour at a temperature of from 20 to 25' C. Centrifuge for about 10 minutes at about 2000 r. p. m., so that the precipitate is firmly packed in the bottom of the tube. Pour off the supernatant liquid and allow the tube to'drain for about 5 minutes. Wash the precipitate with 5 ml. of 70 per cent alcohol, breaking up the bulk of the precipitate by forcing the wash solution in a fine stream from a pipet. Centrifuge for 5 minutes and drain as before. Dry the precipitate for 0.5 hour at 80" to 85' C. to remove all the alcohol. Add 5 ml. of the ceric sulfate reagent and 1 ml. of 1 to 1 sulfuric acid. Heat in a water bath at 90" t o 100" C. until all the precipitate is oxidized, as indicated by its disappearance (usually within about 5 minutes). Maintain an excess of ceric sulfate throughout the reaction (5 ml. of 0.02 N ceric sulfate are sufficient for precipitates containing no more than 0.5 mg. of potassium). Cool to room temperature and titrate the excess ceric sulfate with ferrous ammonium sulfate, using one drop of o-phenanthroline ferrous complex as indicator. The end point is very sharp, the color of the solution changing from pale blue to red. CALCELATION. Milligrams of K = ml. of Ce(SO4)l used in oxidation of the precipitate X normality of Ce(SO& X 6.52. O
Discussion The precipitating reagent is similar in composition to that used b y Adie and Wood ( I ) , but is more easily prepared. It involves one solution only, whereas t h a t used in some other procedures (1, 2, 3, 5-8, 11) involves two solutions. Experience has shown that, after its preparation, i t is best to let the reagent stand for 2 days before using. The results will be high if i t is used too soon after being prepared. There is no deterioration of the reagent within 2 or 3 weeks if
