Constant-(Current Potentiometry in Coulometric Determination of Bromine Numbers FREDERICK BAUMANIN and DON D. GILBERT California Research Corp., Richmond, Calif.
b A coulometric bromine number is described. The voltammetric behavior of the platinum indicator electrodes is discussed in detail, The method is particularly applicable for low level bromine numbers and for samples of limited supply. Relative standard deviation of &2% has been obtained on a variety of materials with bromine numbers of 0.006 to 300.
B
as indications of olefinic unsaturation have been determined coulometrically by several workers. The essential difference between the various methods is one of end point detection. Leiriey and Grutsch (8) and Walisch and Ashworth (14) used dual electrode amperometric detection systems; Miller and De Ford, a spectrophotometric technique (10); and Bratzler and Kleeman, a vkiual iodine starch end point (a). Electrometric end point systems are not influenced by colored bodies, and are generally applicable to a wide variety of samples. Constant-current potentiometry was first used aa an indicator electrode system by Van Name and Fenwick (IS). Reilllsy, Cooke, and Furman (11) gave a n excellent exposition of the technique in 1951. There are definite advantages i n the constantcurrent potentiometric system over other electrometric methods. Electrodes reach equilibrium potentials ROXINE NUMBERS
Table 1.
2,4,4-Trimethylpentene-l. 2,4,4-Trimethylpentene-2
4-Vinylcyclohexene-1 2-Methylbutadiene-l,3 0
0
EXPERIMENTAL
Apparatus. A Fisher Coulomatic Titrimeter (Fisher Catalog No. 9312-500) was used for all bromine number determinations. The titrator comprises two units. One serves a s amplifier and indicator for the indicator electrodes, and t h e second supplies constant current for the indicator electrodes and for the generation of bromine and times the generation period. T h e generation current and timer are stopped simultaneously a t a preselected potential difference between the indicator elec-
thomine Numbers of Pure Olefinic Hydrocarbons
Compoundo Cyclohexene 1-Hexene 2-Hexene0 cis-4-Methylpentene-2 4-Methylpentene-1
rapidly, in contrast to many zerocurrent potentiometric systems; and, constant-current potentiometric systems are not so seriously affected by fluctuations in applied potential, temperature, or rate of stirring as are amperometric techniques, Because platinum electrodes adopt fairly positive potentials in a bromidebromine system, formation of oxide films can occur. These films can seriously affect indicator electrode benavior and lead to irreproducible results in electrometric indicator systems. This work presents current-potential data to explain reactions at the indicator a constant-current electrodes of potentiometric system in bromine number determinations. Experimental methods are given by which reproducible indicator electrode behavior can be obtained.
Purity, mole % 99 99
++
95+ 95+ 99+ 95f
+ 99 + 95+ 99
Theory 195 190 190 190 190 142 142 296
470
Bromine no. Thie __ASTM D 1159 workb HgC12 NO HgC12 195 189 187 187 188 151 159 293
198 191 190 194 298
249
240
193 181 189
190 176 137
189
... ...
Phillips Petroleum Co. Average of three or more replicate determinations, HgClt present. Mixed cis- and trans4somers.
141 :a. 210
(
236
trodes. Two platinum foil electrode8 with approximate geometric surface areas of 2.7 sq. cm. were used as the indicating pair. A platinum foil-platinum wire combination mas used to generate bromine, with a glass frit separating the wire cathode from the bulk of the solution. Current-potential curves were recorded automatically with a Leeds and Northrup Electro-Chemograph, Type
E. Potentials of the indicator electrodes were measured with a General Radio d.c. amplifier electrometer, Type 1230-A, with a n input impedance of loll ohms. Indicator currents were measured with a Weston d.c. microammeter, Model 622, with an internal resistance of 100 ohms on the 0-50 pa. scale. All potentials were measured with respect to the saturated calomel electrode (S.C.E.). Reagents. Analytical reagent grade or C.P. chemicals were used to prepare all solvent systems. The electrolyte consisted of 400 ml. of chloroform, 400 ml. of methanol, 180 ml. of water, 5 grams of potassium bromide, and 12 grams of mercuric chloride diluted t o 2 liters with glacial acetic acid. The current-potential curves were obtained in a titration medium which contained 8 ml. of 36N sulfuric acid per 2 liters. Some bromine number determinations (Tables I and 11) were carried out in the absence of sulfuric acid with the same results. The electrolyte must fulfill three requirements. It must be suitable for electrogeneration of bromine; the rate of bromine addition reactions must be faster than the bromine generation rate; and hvdrocarbons must have moderate solubility in the electrolyte. The ability of metal salts to catalyze the bromine addition reaction is well known (3, 9). Du Bois and Skoog found HgCl:, to be a good catalyst (4). The ASTM Committee D-2 removed the catalyst from the electrometric bromine number method (D 1159-61) because higher than theoretical results Fere reported in the presence of easily bromine-substituted materials. Values obtained in this laboratory indicate that although certain samples give high results in the presence of HgCl,, other results by the coulometric method mnre closely approach the theoretical bromine number. Because there is no large excess of bromine a t any time in this coulometric method, the possibility of bromine substitution reactions is minimized. VOL. 35, NO. 9, AUGUST 1963
1133
Table II. Bromine Numbers of Petroleum Mixtures Bromine no. ASTM 1159-61b
This worka Kerosene Kerosene Light fuel oil Fuel oil (inhibited) Light straight run naphtha Mid-continent light - straight - run naphtha Experimental motor gasoline Experimental motor gasoline Experimental motor gasoline Catalytic reformate (low severity) Catalytic reformate (high severity) Coker gasoline blend Motor gasoline blend Commercial premium motor gasoline Commercial premium motor gasoline Commercial premium motor gasoline Commercial motor gasoline Catalytic cracked gasoline Light catalytic cracked gasoline Catalytic cracked heavy naphtha
0 .57d
1.25 5.03-11, Id 5.7d 0 .62d 0.94 19.7 2.38
51.3 1.56 1.86 13.6 33.7 33.3 60.0 41.1 14.2 98.ad
120d
29.5
Calresearch
n..x -_ 0.8 1.6
2.6 0.7
0.8 19.5 2.5 46.4 1.4
1.8
10.8
30.4 36.1 63.6 43.4 16.4 74.4 83.7 25 .O
ASTM av.c 0.39 0.83 1.7-12.8 2.70 0.61 0.70 19 .i 2.37 47.3 1.28
1.65 10.4 29.6 36.6 64 .O 41.7
16.7 76.1 84.0 25.0
HgCh present, average of three or more replicate determinations. Amperometric end point, HgC12 absent. Average of duplicates by seven laboratories. Substitution reactions indicated by intermittent titration; values calculated from time of first cessation of titration. a
The electrolyte is sufficiently ionic to allow the generation of bromine and to aid the catalyzed bromine addition reaction which undoubtedly takes place via a n ionic intermediate ( I ) . The ratio of Hg(I1) to total halide is optimized so that formation of tribromide, a poor bromination reagent (e), is avoided. At the same time, some free HgXz (X = chloride or bromide ion) is present t o form HgXsBrz with the generated bromine. Mercuric halide complexes of bromine are more reactive in bromine addition reactions than is free bromine (6). A standard cyclohexene solution was prepared by diluting 1.00 ml. of Phillips Research Grade material (99.84- mole yo)to 1 liter in chloroform. The bottle was stored in the dark at 3" C. after opening. The cyclohexene yielded a bromine number of 195 (theoretical 194.6), indicating that peroxides were not present. If peroxides should be present, as shown by a significantly lolver than theoretical bromine number, the cyclohexene can be purified by the procedure given in the ASThl D 1159-61 standard method. Pure olefinic hydrocarbons (Phillips Petroleum Co.) were used without additional purification and were stored in the dark a t 3" C. PROCEDURE
Current-Potential Curves. Current-potential data were obtained by impressing a linearly variable potential across a n indicator electrode and a saturated calomel electrode and recording the resultant current with the Leeds and Korthrup Electro1134
ANALYTICAL CHEMISTRY
Chemograph. Electrodes were not connected t o t h e Fisher Titrimeter for these experiments. The electrolyte was stirred a t an arbitrary but fixed rate with a magnetic stirrer. Bromine Numbers. One indicator electrode is preanodized and the other precathodized for 20 minutes i n 10% (v./v.) sulfuric acid b y connecting the electrodes t o a 4.5-volt d.c. source. The electrodes are washed thoroughly in distilled water, then the preanodized electrode is made the positive indicator electrode (anode) and the precathodized electrode, the negative (cathode) one. The electrodes should retain the same polarity in any later treatment. The Fisher Coulomatic Titrimeter is calibrated so that the 0 to 1 volt scale measures from -0.5 to +0.5 volt. It is then set to stop the bromine generation and timer when the cathode indieator is -0.09 volt with respect to the anode indicator electrode (0.41-volt scale setting). Both indicator and generator electrodes are immersed in a 150-ml. titration beaker filled with about 100 ml. of titration solvent. A magnetic stirrer holds the beaker in position. The indicator constant-current source is turned on and the potentials of the electrodes are allowed to reach equilibrium. A solvent blank is obtained using a generation current of about 5.0 ma. The timer is cleared, and 0.500 ml. of cyclohexene standard is pipetted into the pretitrated solvent. This solution is titrated and both generation current and time are noted. When the electrodes are first treated, three to five titrations (about 200 seconds each) may be necessary before
reproducible results are obtained. When two consecutive titrations of cyclohexene agree to within zt2 seconds a t 5.0 ma., a sample is added to the same solvent and titrated. When titrating successive samples of cyclohexene, it is not necessary to replace the titration solvent, but changing it after each sample will prevent accumulation of species which might undergo substitution reactions. The end point of bromine addition is denoted by a n abrupt cessation of bromine generation. If relatively slower substitution reactions take place, there is an intermittent titration, evidenced by regular pulses of bromine generation following completion of the more rapid bromine addition reaction. The titration is considered complete following the first definite cessation of bromine generation. The bromine number is calculated bv the formula (0.0820)(i)(t) Bromine no. = W
where
i
= generation current, ma. t = titration time, sec. w = weight of sample, mg.
The constant, 0.0820, has the dirnensions of the equivalent weight of bromine and the Faraday. Use of the above equation assumes that (i)(t) is known to a n accuracy of better than 2%. Current may be accurately determined by measuring potential drop across a 15-ohm precision resistor in series with the generator electrodes. DISCUSSION
Titration Curves. The constantcurrent potentiometric end point detection system is based on a change in potential difference between a pair of indicator electrodes at which electrolyses are occurring. Reilley, Cooke, and Furman (11) and Duyckaerts (5) discussed the interpretation of various constant-current potentiometric curves. Using the Fisher Coulomatic Titrimeter as described, a current of about 0.7 pa. is passed through the indicator electrodes to the equivalence point. There the indicator current increases to about 1 pa. By altering the circuitry of the instrument increase in indicator current could be eliminated, but it presents no problem in bromine number determinations or in investigation of electrode reactions. The electrolyses occurring a t the indicator electrodes during various stages of titration can be explained by the current-potential curves reproduced in Figure 1 Coordinates of Figure 1 and other current-potential figures are consistent with polarographic literature-Le., anodic and cathodic currents are negative and positive, respectively. Broken constant-current lines represent current that flows through the indicator elecI
-50
I
I
+IO
toa
to6
to4
tor
0.0
-02
POTENTIAL, VOLT VERSUS SCE.
Figure 1 . Current-potential curves in titration solvent Neither HgC12 nor KBI present, electrode precothodized B. Both HgClz and KBr present, electrode precathodized C. Same as B but a differerlt electrode D. KBr, HgClz, and Brz present, electrode used for several bromine number determinations and not precothodized
A.
trodes when they arcs used with the Fisher Titrimeter. Points a t which these lines intersect the current-potential curves define the reactions which then occur a t the indicator electrodes, Before curves A , B, 2nd C were obtained, the platinum electrode was precathodized for 20 minutes in 10% (v./v.) sulfuric acid as described above and a +0.40-volt potential was applied until only a residual current flowed. Curve A is the currentpotential (c-p) curve taken in the electrolyte, but with both potassium bromide and mercuric chloride absent. The anodic discharge., beginning a t about f l . 0 volt, is !,he evolution of oxygen and the large (bathodic current, the evolution of hydrogen. Curve B is the c-p curve obtained in the electrolyte used with the bromine number determination; both potassium bromide and mercuric chloride are present. Anodic and cathodic! electrode reactions-Le., when the indicator current is flowing-are oxidation of bromide and reduction of mercury(.-I), respectively. Curve C was obtained under the same conditions as B , but with a different platinum electrode. The cathodic dissolution pattern, which can also be obtained in a medium free of both bromide and mercury(II), is the reduction of a platinum oxide film. Therefore, while the anodic reaction a t 0.7 pa. of indicator current is the oxidatioii of bromide, the cathodic reaction is the reduction of platinum oxide. Curve D , obtained with the same electrodl: used in C with no further treatment, is the e-p curve taken in the presence 01' excess bromine. h theoretical titration curve can be constructed from Figure 1. Curves B and C represent alternative situations before the equivalence point, and D, after the equivalence point. With the indicator current flowing, and in the
absence of excess bromine, anodic and cathodic indicator electrode reactions give rise to a relatively large potential difference. As B and C show, magnitude of the initial potential difference is dependent primarily on the cathodic reaction, which may be the reduction of either platinum oxide or mercury(I1). Whichever cathodic reaction occurs, potential difference of the indicator electrodes remains large until the appearance of excess bromine, when reduction of bromine occurs a t the indicator cathode. The cathodic potential becomes increasingly more positive with increasing bromine concentration. Eventually, bromine concentration is sufficiently high so that the cathodic potential remains constant. Potential of the indicator anode remains essentially constant, becoming only slightly more positive with the presence of excess bromine. At the equivalence point, then, there is a diminution of potential difference between the indicator electrodes. A theoretical titration curve constructed from current-potential data ( B and C, Figure 1) is shown as B of Figure 2; curve A is the potential difference titration curve actually measured during typical bromine number determinations. The actual titration curve shows a gradual increase in potential difference between starting point and equivalence point because the indicator cathode assumes a less positive potential during titration (curve C, Figure 2). If the cathodic indicator reaction is the dissolution of platinum oxide, the trend toward less positive potentials may be due to the fact that the dissolution of oxide is nearing completion, I f mercury(I1) reduction is the cathodic reaction, increased activity of the film of reduction product being deposited on the electrode is probably responsible for the decrease in cathodic potential. The possibility of a mixed potential of the two possible cathodic reactions exists.
1
EQUIVALENCE POINTLI
END FO!NT
I
2
u)'
-900 v) v) 3
-
-
6
>
i J -
5
-loo c Y z -
0 c
-600
a
-
0 1
,
,
50
IO0
!,
j500
52
0
0 L 2
u I50
200
TIME, SECONDS
Figure 2. A.
Titration curves
Real titration curve B. Theoretical titration curve based on data from Figure 1 C. Potential of the indicator cathode
,
-50 t IO
, 108
,
,
106
,
, to4
,
,
,
t02
I,,
00
-0.
POTENTIAL, VOLT VERSUS S.C E.
Figure 3. Current-potential showing presence of oxide film
curves
A.
flectrode anodized 5 minutes in 10% (v./v.) HzSOi 8. Electrode cothodized several 20-minute periods in Has04 C. Electrode cathodized 2 0 minutes in HzS04 Different electrodes were used for each curve All curves obtained in the titration solvent
Platinum Oxide Films. Curve A , Figure 3, is the c-p curve of a n electrode in the titration electrolyte which had been preanodized for five minutes in 10% (v./'v.) sulfuric acid. The large cathodic peak a t about +0.35 volt is the dissolution of the oxide film. Curves B and C mere also taken in the titration solvent, but with different electrodes and following a cathodization treatment. In both of the latter cases, a potential of $0.40 volt was applied after the cathodization until only a residual current flowed, thus removing dissolved hydrogen. These two curves show a basic difference in behavior of the two electrodes, One electrode, that for curve B, readily forms a dissoluble platinum oxide a t more positive potentials. The curve C electrode, however, does not form an easily dissoluble film after the cathodization treatment. After some time, the electrode used for curve C did show an oxide formation, but another cathodization treatment restored the electrode's original behavior (curve C). Two hours of hydrogen evolution does not alter curve B . There is, then, a significant difference in the behavior of different platinum electrodes. As shown in Figure 1, a small amount of oxide will not hinder the bromine number determination; a large amount will make the determination impossible. Curve A , Figure 3, shows that an initial potential difference of 0.09 volt is not possible if a large amount of oxide is on the cathode and the Fisher instrumerit will not begin titration hen operated by the procedure given abovp. Cathodization treatment d l restore satisfactory behavior of indicator electrodes by removing the accumulation of easily dissoluble platinum oxide. Figure 2 shows that the end point VOL. 35, NO. 9, AUGUST 1 9 6 3
1135
chosen for titration is significantly beyond the equivalence point. The exact initial potential difference between the indicator electrodes cannot be accurately predicted. This, coupled with the usual increase in potential difference before the equivalence point, makes it impossible to stop titration automatically a t a preselected potentid difference just beyond the equivalence point. Thus, by adding excess bromine until there is a 0.09-volt potential difference between the indicator electrodes, there will be a titration error if the same excess of bromine is not in solution from the titration of the standard cycloliexene reagent before the sample is added. Should an oxide film form on the indicator cathode, or the amount of oxide already there increase significantly between successive titrations, an irreproducible excess of bromine will be added to the solution because the equivalence point potential difference decreases with an increasing amount of oxide. The titration of standards-e.g., cyclohexene-is essential when using the preselected end point procedure to check the reproducibility of electrode behavior. APPLICATIONS
Pure Hydrocarbons. Bromine numbers of some unsaturated hydrocarbons determined with constantcurrent potentiometric indicator systems are shown in Table I. Results are compared with those obtained by the ASTM D 1159 volumetric method in the presence and absence of the mercuric chloride catalyst. Only the branched chain pentenes gave very high bromination results due to concomitant substitution reactions in the presence of the mercuric chloride catalyst ( 7 ) . Conjugated diolefins (for example, 2-methylbutadiene-1, 3) are known to add only slightly more than 1 mole of bromine rapidly, the bromination of the second double bond being much slower than the first (12). Petroleum Mixtures. Bromine numbers of petroleum mixtures from a n ASTM cooperative study were determined by this coulometric method and are tabulated in Table 11. Results are compared with averages from seven laboratories using the ASTM volumetric method without the cata-
1136
ANALYTICAL CHEMISTRY
T a b l e 111.
Material Alkylbenzene Cetane Polybutene Polymerized C11 a-olefins
Typical Bromine N u m b e r Determinations
Sample weight, - . mg.
Current, ma.
Time, sec.
Bromine no.
431.6 8G3.1 767.3 383.5 6.486 6.486 13.878 13.878
4.56 4.56 4.60 4.60 4.60 4.60 4.55 4.55
77.2 155.8 403.3 189 .O 129.7 126.2 150.2 144.7
0.0669 0.0675 0.198 0.186 7.54 7.36 4.04 3.89
lyst (D 1159-61). Column 3, Table 11, shows results obtained in this laboratory by the ASTM procedure with the same sample used for the coulometric determination. Generally, good agreement is obtained between the two methods. I n some cases, substitution reactions were indicated by intermittent generation of bromine following a definite cessation of the faster addition reaction, The ASTM procedure yields somewhat lower results if readily substituted compounds are present because the mercuric chloride catalyst is omitted. The rate of generation of bromine in an automated coulometric titration must be less than the bromination reaction rate. At a generation current of 5.0 ma., the catalyst is necessary for most titrations. To omit the catalyst when titrating substances known to substitute readily, a smaller generation c u r r e n b e . g . , 1 0 ma.-should be used Miscellaneous Applications. The coulometric method is most applicable for determination of traces of olefinic contamination. Typical results for alkylbenzene and cetane are given in Table 111. Recent experiments using a 0.50-ma. generation current showed t h a t bromine numbers of alkylbenzenes a t the 0.003 level can be determined with a relative standSamples ard deviation of *2%. which have limited solubility-e.g., polybutenes and po!ymerized a-olefins-are also particularly suited for coulometric bromine numbers. Olefinic content of automobile engine exhausts has been determined by this technique. With an understanding of indicator
electrode behavior, it is possible to determine bromine numbers of a wide variety of materials with relative standard deviations of &29$ or less. ACKNOWLEDGMENT
The authors thank P. D. E. Xelson for his assistance in this work. LITERATURE CITED
(1) Bartlett, P. D., Tarbel, D. S., J. A m Chem. BOC.58,466 (1936). (2) Bratzler, K., Kleeman, H., Erdoel Kohle .7, 559 (1954). (3) Dams, H. S., Crandall, G. S., Higbee, W. E.. Jr.. IND.ENG. CHEM..ANAL. ED. 3.'108 (1931). (4) Du Bois,". fi., Skoog, D. A., ANAL. CHEW20, 624 (1948). (5) Duyckaerts, G., Anal. Chim. Acfa 8, 57 flQ5.11 --,. (6) Kanyaev, N.P., J. Gen. Chcm. USSR (Eng. Tmnsl.) 26, 3037 (1956); C . A . 51, 72952' (1957). (7) Kaufman, H. D., 2. Unterwch. Lebensm. 51, 3 (1926). (8) Leisey, F. A., Grutsch, H. F., ANAL. CHEM. 28, 1553 (1956). (9) Lewis, J. B., Bradstreet, R. B., IND. ENQ.CHEM.,ANAL.ED. 12, 387 (1940). (10) Miller, J. W., De Ford, D. D., ANAL. CHEM.29, 475 (1957). (11) Reilley, C. N., Cooke, W. D., Furman, N. H., ANAL. CHEM.23, 1223 1 .
\-l
(14.51 ,-I--,.).
(12) Unger, E. H., ANAL. CHEM. 30, 375 (1958). (13) Van Name, R. G., Fenwick, F., J. A m . Chem. SOC.47, 19 (1925). (14) Walisch. W.. Ashworth. M. R. F.. ' Mikrochim: A& 1959,497.'
RECEIVED for review November 19, 1962. Accepted June 12, 1963. Division of Analytical Chemistry, 144th Meeting ACS, April 1963, Los Angeles, Calif.
