Amperometric Titration of Styrene with Potassium Bromate Using the Rotating Platinum Electrode I. M. KOLTHOFF
AND
F. A . BOVEY, University of Minnesota, Minneapolis, Minn.
A convenient, rapid, and accurate method for the determination of small quantities of styrene in water (or any other polar solvent which is inert to bromine) is described. The method involves titration of the styrene in acidified methanolwater solution at 10" C. with potassium bromate-bromide solution, using the amperometric titration technique, with a rotating platinum wire electrode to indicate the end point. Amounts of styrene of 0.2 to 1.0 mg. in 100 ml. of solvent can be determined with an accuracy of 3 to 5%; amounts as large as 5 to 10 mg. can be determined nith an accuracy of about 0.5 to 1.0%. The method is suited to routine analysis, and is applicable to any substance which reacts quantitatively with bromine at a rate comparable to or greater than that of styrene.
I
bromine present. As a result of the formation of tribromide ion, t h concentration of free bromine decreases as the concentration of bromide ion is increased. However, a high concentration of bromide ion is desirable in order to obtain a convenient rate of formation of bromine from the reaction between bromide and bromate ions. I n practice i t is found that the rate of bromination of styrene is sufficiently rapid for titration purposes when the methanol-water mixture is saturated with respect to potassium bromide.
S CONNECTION with an investigation of the rate of poly-
merization of styrene in water solution, it became necessary to have an accurate method for determining trace quantities (0.02% or less) of styrene in water. For this purpose, a colorimetric method has been proposed (4) which depends on the formation of a brown coloration when aromatic hydrocarbons react with a formaldehyde-sulfuric acid reagent. The accuracy of this method is described by the authors as *lo%. Measurement of the ultraviolet absorption gives somewhat better accuracy than this in a concentration range of about 0.02%, but is not suitable for lower concentrations. This paper describes a method for the direct determination of small quantities of styrene by amperometric titration with potassium bromate solution, in the presence of acid and potassium bromide. For the detection of the end point, a rotating platinum wire electrode is used. The use of a rotating platinum electrode to indicate the end point in titrations with bromine was first described by Sernst and Merriam (5), and has been applied by Laitinen and Kolthoff (3) to the determination of arsenite. If the bromination of a substance is rapid and quantitative, the amperometric titration method can be used for its accurate determination at high dilutions. Styrene satisfies these requirements, and the amperometric method furnishes a convenient, rapid, and accurate method for its determination even in trace quantities, provided no other substances 1% hich react with bromine are present. The titration is carried out in strongly acid medium, using a stable standard aqueous potassium bromate-bromide solution. As discussed below, it is necessary that the titration medium be a good solvent for styrene, as otherwise serious errors occur, due to volatilization of styrene in the course of the titration. This loss is promoted by the vigorous agitation of the solution by the rotating electrode. Methanol has been found to be a satisfactory solvent, since it does not react with bromine during the titration. Even when methanol-water is used, it is recommended that the titration be carried out a t 10" to 15' C. to minimize losses by volatilization. The stoichiometrical equation may be represented by: CaH5CH=CH2
APPARATUS AND MATERIALS
The apparatus employed is identical to that described by Kolthoff and Harris ( 2 ) , except that the customary saturated calomel reference cell is used instead of the mercury-mercuric iodide halfcell. The saturated calomel electrode produces a potential sufficiently negative to yield the diffusion current of bromine. The platinum wire electrode used in this work had a length of about 1 cm. and a diameter of about 0.6 mm.; a t about 1000 r.p.m., i t gave a current of 4.5 microamperes when 0.005 millimole per liter of free bromine \vas present. The styrene employed was a specially purified sample supplied by the Dom Chemical Co. I t had a purity of over 99.0%, and wras distilled in vacuo before use, to remove polymer and inhibitor. The methanol used had a boiling range of less than lo, and showed only a very small blank when titrated with 0.00178 M
P
+ Bra +CoH5CHBrCH2Br Figure 1. Amperometric Titration of 5.87 Mg. of Styrene with 0.00178 M Potassium Bromate
Actually, as Bartlett and Tarbell have shown in the case of stilbene (I), the product may be in part, a t least, the methoxy bromide, but this does not affect the stoichiometry. The rate of bromination of the styrene is dependent upon the concentration of free
.
100 ml. of 75 to 25 methanol-water ao oolvent a. b.
498
Residual current Blank
499
J U L Y 1947 potassium bromate. Methanol giving a laige blmk car1 br purified by treatment with excess bromine, followed by removal of the bromine with zinc duqt, decantation from the zinc dust, and hnally by distillation, rpjerting the lait 10 t o 1 5 5 .
Table I . Titration of Styrene w-ith Potassium BromateBromide in 75 to 25 \Iethanol-Water
Mg.
Volume of Solution ,161.
11.75 11 75 11.75
100 100 100
Titration MI. 20.98 21.05 21.10
5.87 5 87 5.87
100 100 100
10.60 10.50 10.65
0.05 0 0.5 0.05
5.85 5,80
5.88
-0.3 -1.2 +0.2
2.93 2.93 2.93
100
ion ino
5 25 325 5.25
u.05 0.05
0.05
2.88 2.88 2.88
-1.6 -1.6 -1.6
u 587
50 75
10.90 10.85
0.50 0.50
0.576 0.574
-1.7 -2.1
Styrene Present
PROCEDI R E
liito a 250-ml beaker pour about 75 ml. oi niethanul a i d 5 in1 of concentrated hydrochloric acid. Cool in an ice bath to 5" to IO" C'. Add the water bolution of styrene from a pipet. The quantity of water added should not exceed 25 ml. (If a solution of styrene in some solvent other than water is used, the quantity of styrene added should not exceed 10 mg.) Cool for about 5 minutes more, until the tempeiature of the contents of the beaker is about 10' C. (The mixing of the ivater and methanol evolves considerable heat.) Then insert the salt bridge and the rotating electrode, add about 1 gram of potaqsium bromide, and titrate with approximately 0.002 M potabsium bromate, which is made ahout 0.1 M in potassium bromide
0.587
Blank ,
MII. 0.05 0.05 0.05
Styrene Found x g. 11.60 11 65 11.67
Error
% -1.2 -0.7 -0.5
RESULTS
w a
0
0
" " ' ~ ~ " ' ' '
I
2
3
4
5
6
7
ml. KBrO3
8 9 IO SOLUTION
II
I2
Figure 2. kniperometric Titration of 0.587 Mg. of Styrene with 0.000178 -If Potassium Bromate 50 m l . ( A ) and 75 ml. ( E ) of 75 to 25 methannl-water a a solvent a. Residual currents b . Blank
Ueiure the end poiiit a constant residual current uf U.2 t o 0.5 microampere is observed. -4fter each addition of potassium bromate solution, the needle of the galvanometer deflects slightly to the right, but returns almost immediately t o the residual current reading. As the end point is approached, the return of the needle becomes somewhat sloxer. Plot. two or three points before the end point is reached, and three points after the end point, a t intervals of 0.05 t,o 0.10 ml. As soon as the first excess of bromine is added, a diffusion current corresponding to the concentration of free bromine present is observed. The steepness of the slope of the curve obtained deprnds upon the volume of the solution being titrated, the molarity of the bromate solution, and the area of the platinum mire electrode. Under the conditions of the titrations corresponding to the graph shown in Figure 1, a line with a very steep slope is obtained after the end point. Accurate extrapolation to the value of the residual current is t h w po.wible, and thc ticpivalence point can be readily determined. Khcn very small quantities of st,yrene are tu be determined (less than 1 mg.), the volume of methanol used can be reduced if necessary, since the vapor pressure of styrene will be very low. In Figure 2 are shown curves for the titration of 0.587 mg. of styrene in 50 ml. and 75 ml. of solution (75 to 25 methanol-water by volume) with 0.000178 X potassium bromate. When working at thme extreme dilutions, it is necessary to plot points for only a .;mall volume (0.2 to 0.4 ml.) beyond the end point, since the current-roncentration curve is appreciably nonlinear and extrapolations over a large range cannot be made. .\ blank titration must be run under the same conditions as the tktermination, and subtracted from the rolume of rragrnt used in the titration of' the sample. Calculation.
Y C 5tyrellr i+-hrreA B
.If
=
ia
-
- B)
x M x -3 g 2 T-
= ml. of titratioii = ml. of blank = concentration of KHr03sohitioil, 1111)le$per iitei
T 7 = volume of $ample
In Table I are shown some results obtained with various quantities of styrene. The smallest quantities, shown a t the bottom of the table, were determined by titration with 0.000178 M potassium bromate, and the others with 0.00178 1M potassium bromate. The solvent was 75 to 25 methanol-water in all cases. A purity of 99.5% is assumed for the styrene. In general, results tend to be slightly low. The accuracy obtained is sufficient for all practical purposes. Even R hen no interfering substances are present, serious errors will occur in these titrations if due allowance for the volatility of styrene is not made. The vapor pressure of stjrene over a saturated aqueous solution a t 25 O C. is 6.3 mm. By bubbling nitrogen through such a solution, it has been found possible to remove nearly all the styrene in 10 to 15 minutes. I t can therefore be readily understood that a considerabl? proportion of the styrene may be lost from a TTater solution during the ordinary manipulations necessary in carrying out a titration. It was found during this investigation that reproducible results in the titration of styrene by this method could not be obtained when water alone was the solvent. Low and erratic results were always obtained. .4ddition of a considerable volume of methanol was found to improve the results greatly, since the vapor pressure of the styrene is thereby greatly reduced. It was found desirable to work a t about 10" C., rather than at room temperature. Even when 50% by volume of methanol is present, some volatilization occurs. Thus when 5.87 mg. of styrene were determined in 100 ml. of 50 t o 50 methanol-water cooled to 10" C., quantitative results were obtained if the solution ~ i a titrated s immediately, but if the solution was stirred with the rotating electrode for 15 minutes and then titrated, only 68% of the original quantity of styrene was found. However, if 75 to 25 methanol-water was employed, reiults were essentially quantitative even after 15 minutes' stirring. Results tended to be someivhat l o v (1.0 to 1.5%) R hen amounts of styrene as large as 150 to 200 mg. in 100 ml. of 75 to 25 methanol-water were titrated, probably because of volatilization. This method is of course subject to interference by any subbtances other than styrene which absorb bromine a t an appreciable rate, such as phenol, aniline, phenyl-&naphthylamine, other unsaturated hydrocarbons, etc. Substances like persulfate ion, which are reduced a t the rotating electrode but do not react with bromine, cause no interference if the diffusion current M hich they produce is constant, since this current serves as the reference to Ti-hich the bromine current is to be extrapolated. OTHER APPLlCATIONS
This method can be applied to the determination of any substance which reacts quantitatively with bromine, and a t a rate a t least equal to that of styrene. Many substances do not fulfill this latter requirement. Thus, stilbene reacts too slowly even a t con-
500
VOLUME
19,
NO. 7
LITERATURE CITED
centrations of 200 mg. per 100 ml. of methanol-water to give a satisfactory end point. $-vinyl cyclohexene (butadiene dimer) gives low results. Oleic acid can be titrated, but reacts somewhat slov~lya t the end point. The method should be well suited to the determination of the enol content of substances such as 0-keto aldehydes, acetyl acetone, acetoacetic ester, etc., by the Meyer method, since with these compounds bromination is very rapid even a t the low temperatures necessary to retard tautomcsiization to the keto form. The first excess of bromine is at onm’ indicated when the amperomrtric techhique is employed.
Bartlett, P., and Tarbell, D., J..4m. Chem. Soc.. 58, 466 11936). ( 2 ) Kolthoff, I. M.,and Harris. W. E.. IND.ENG.CHERT., ~ A L ED., . 18, 161 (1946). ( 3 ) Laitinen, H. .1..and Kolthoff. I . 11..J . Ph,ys. Chem.. 45, 1079 (1941). (4) Morris, H. E., Stiles, K.B., and Lalie, If-. H., IKD. EXG.CHEM., A1x.kx.. ED.,18, 294 (1946); Lane, W.H., Ibid., 18, 295 (1946). ( 5 ) Nernst, W., arid Merriain. E., Z. ghysik. C‘hem..5 3 , 235 (1905). (1)
THISinvestigation was carried out unilrr t h e sponsorship of t h e Office of Rubber Reserve, Heconstruction Finance Corporation. in connection with thr Gowrninent Synthetic, Hilhhrr. Proarain.
Application of Reflectance Spectrophotometry to Quantitative Microanalysis JULIUS SENDROY, JR. Department
of
Experimental ,Medicine, Loyola University School of Medicine, and Mercy Hospital, Chicago, I l l . AND
WALTER C. GRANVILLE’ Research Laboratories, Interchemical Corporation, New York, N. Y .
Reflectance spectrophotometry may be used in quantitative microanalysis by measuring changes in spectral refiectance corresponding to changes in color of a solid highly reactive chemical reagent in the form of an opaque-type surface coating. Limited application to cyto- and histochemistry is indicated.
A
LTHOUGH reflectometry has frequently been employed in industry for the descriptive evaluation of colorants and pigments in both wet and dry states (3, 5 ) , its greater use in quantitative chemistry has been long overdue, Chemical reagents have long been used t o bring about reactions resulting in color, the intensity of which serves as a measure of the amount of one of the reactants, and hence, directly or indirectly, of the amount of the substance to be determined. However, in such systems of quantitative analysis, the intensity of color has been determined in solutions or a liquid medium by light transmittance measurements. Although the intensity of the color of a solid can similarly be evaluated by light reflectance measurements, analytical chemists have paid little attention to the development of methods of analysis in which the color produced by, and hence indicative of the extent of, a reaction, is ultimately confined to a solid medium or surface. Steps in this direction have recently been taken, a i appears from the work of two laboratories. Compton, Granville, Boynton, and Phillips (1)used spectral reflectance measurements to evolve the quantitative relationship between variation in the green color of untreated, native apple leaves and their nitrogen content. Kienle, Steams, and Van der Meulen ( 2 ) determined the Hammett reaction constant for a photochemical reaction in which the azo bond of certain dyes was destroyed by light. This reaction was followed by quantitative spectral reflectance measurements for the compounds involved. The authors wish to call attention to the possibility of such B type of quantitative analysis by measurements of changes in spectral reflectance corresponding t o changes in color of a solid, highly reactive chemical reagent in the form. of an opaque-type surface coating. This interesting application occurred in the development by the senior author ( 4 ) of a simplified, smsitivr, Present address, Color Laboratorles Division, Container Corp America, Chicago, I11 1
uf
visual method for the detection and quantitative determination of carbon monoxide in air, based on its well-known reaction with palladium chloride. (In part because of wartime restrictions, publication of the method and of the spectral reflectance data presented here has necessarily bcen delayed.) The nature of the colorations involved made it necessary to identify and measure the changes produced, by an objective method, the results of which could be applied to the accurate reproduction of a standard color scale.
A
B
7 0 - REAGENT 66/50
C
D w
K
E
P
20
-
I
400
I
1
500 600 WAVELENGTH mu
1
700
Figure 1. Spectrophotometric Reflectance Curves forAir Samples Containing Carbon Monoxide A , hlank; B, 0.0012fh; C, 0.0031%; D , 0.0044%; E, 0.010%
