274
ANALYTICAL CHEMISTRY relative reactivity of the hydrolyzed ceric sulfate. Ten minutes was selected as a convenient time to equilibrate the sample thermally and was constant in all determinations. It was observed, however, that the period for color development increased from 24 to 102 seconds after a sample containing 0.95 p.p.m. of fluoride ion and cerate reagent stood for 1.5 hours a t room temperature. Qualitative observations indicated that a-hydroxy acids such as tartaric completely halted the color development, while oxalic acid had a marked accelerating effect. CONCLUSION
P.P.M. F-
Figure 6. Corrections for Alkalinity by Added Sulfuric Acid, and Results Obtained after Removal of Residual Chlorine and, after several hours, more was added to give the equivalent of approximately 0.4 p.p.m. of residual chlorine. The effect of the zinc reductor on other ions possibly present, such as ferric ion, was not studied. No determinations were made on samples of fluoridated city water supplies because the variability of concentration of substances which influence the results, particularly alkalinity, would make them have little meaning. It is possible that a buffered solution would eliminate the difficulty due t o alkalinity, but that would necessitate further study, as the conditions of the method would be altered. A factor not investigated further was the effect of time on the
This brief study has indicated that although the analytical procedure described is exceedingly sensitive and capable of good precision under controlled conditions, it apparently is subject to enough influences in its present state of development to be inconvenient for routine analyses of public water supplies. Its sensitivity, however, would allow dilution of the sample by as much as 100% a t I p.p.m., which would halve the concentration of interfering substances. The method is fundamentally new and is of interest because it shows that an induced reaction type of analysis can be accurate and precise a t extremely small concentrations. Uses other than the analyses of fluoridated water supplies may be found for which this method would be more satisfactory. The need for temperature control is offset by the fact that the only measuring instrument required is a stop mratch or ordinary watch having a second hand. LITERATURE CITED
(1) Am. Public Health Assoc., Yew York, and Am. Water Works Assoc., “Standard Methods for the Examination of Water and Sewage,” 9th ed., pp. 76-9, 1946. (2) Lambert, J. L., ANAL.CHEX,23,1247 (1951). (3) Snell, F. D., and Snell, C. T., “Colorimetric Methods of Analysis,” Vol. 11, 3rd ed., pp. 743-53, Kew York, D. Van Nostrand Co., 1949. RECEIVED for review August 4, 1952. Bcoepted October 16. 1952. Contribution C 480, Department of Chemistry, Kansas State College, Manhattan, Kan.
Coulogravimetric Determination of the Halides A New Type of Indirect Analysis WILLIAM M. MAcNEVIN, BERTSIL B. BAKER’, AND RICHARD D. MCIVER Department of Chemistry, The Ohio State University, Columbus, Ohio
IhS
TANCES often arise in both coulometric and electrogravimetric analyses in which the deposition potentials of two elements, occurring together in a mixture, lie too close to allow their separate determination. It is the purpose of this paper to present a new method of indirect analysis whereby simultaneously obtained coulometric and electrogravimetric data are combined to permit the analysis of such mixtures. The principle of this new type of double measurement, conveniently called coulogravimetric analysis, is best explained by an example. Lingane and Small (3) have determined coulobrometrically-by precipitation on a silver anod-hloride, mide, and iodide when present individually, and also, by controlling the anode potential, mixtures of iodide and chloride or iodide and bromide. They were unable to determine chloride and bromide when present together, since a t the potential required 1 Present address, Southern Research Institute, 917 South 20th St. Birmingham, Ala.
for the deposition of bromide some chloride codeposited. n their work the authors have deposited together all of the chloride and bromide and have measured both the total number of coulombs required and the weight of the combined deposits. These data give the following information: Increase in weight of anode = x-eight of chlorine
+ weight of bromine + + ‘i9.92
(1)
Coulombs required = total equivalents C1 Br = 96 ,500 wt. of C1 wt of Br 35.46 (2) From these two equations, by appropriate substitution and rearrangement, the individual weights of chlorine and bromine may be calculated. Good accuracy will be obtained from indirect analyses of this type only when the equivalent weights of the two elements differ
V O L U M E 2 5 , NO. 2, F E B R U A R Y 1 9 5 3
275
Instances often arise in both coulometric and electrogravimetric analyses in which the deposition potentials of two elements, occurring together in a mixture, lie too close to allow their separate determination. This paper presents a new method of indirect analysis whereby simultaneously obtained coulometric and electrogravimetric data are combined to permit the analysis of such mixtures. The method has been found suitable for the analysis of mixtures of halides with an absolute error of around 0.4 mg. for each element. This will be a useful addition to the present rather involved methods for analysis of mixtures of chloride, bromide, and iodide, and this new type of double measurement may find other important applications.
giving results from 1 to 2% low. The reason for this was not established; but it T 5 - m probably due to traces of impurities undergoing cyclic oxidation and reduction. A mold sometimes developed in the electrolyte and this mold, or some by-product of its groFth, may have caused the trouble. Xevertheless, the hydrogen-oxygen coulometer was usually satisfactory and all of the data given in this paper were obtained by its use. Electrodes. Slomin-type platinum gauze electrodes were used. The large electrode was used as anode and was electrolytically coated with about a 5-gram deposit of silver. It was found that this anode could be used for two successive runs and then regenerated by electrolysis as a cathode to remove the accumulated halides. Replating with silver is seldom necessary.
1 ) a~ reasonably large factor. This condition is met in the case of the halogens where the equivalent weight of bromine is more
than double that of chlorine. The principal advantage of the method is that although a determination may require a total time of 3 to 4 hours, the actual operator time is short, only ahout 30 minutes. If iodide is present also, it may be removed prior to the chloride and bromide by the simple coulonietric procedure of I h g a n e and Small (3)and thus in around 5 hours (45 minutes operator time) a determination of all three halogens may be completed. APPARATUS
PROCEDURE
Potentiostat. The potentiostat used was of the design o Lingane (1) and the contacts on the galvanometer relay were adjusted to provide control to 0.01 volt. Electrolysis Cell. The cell used was a 150-ml. beaker wrapped with black tape and provided with an opaque cover t o minimize the effect of light on the silver halides. The solution was vigorously stirred by a magnetic stirring bar. Coulometer. -2 100-ml. hydrogen-oxygen coulometer of the I h g a n e type ( 8 ) was used. The electrolyte was 0.5 M sodium or potassium sulfate and the following vapor pressures were use? in correcting the barometric pressure. 2 4 O , 22.2 mm.; 25 23.3 mm.; 26O, 24.7 mm.; 27", 26.2 mm.; 28O, 27.8 mm.; 29': 29.4 mm.; and 30" C., 31.2 mm. According to Lingane this coulometer gives an observed volume of 0.1741 cc. of gas per coulomb, as compared with a theoretical 0.1739 cc. per coulomb. The value 0.1739 cc. was used and the coulometer usually was found accurate within a few tenths per cent. However, several times lox- readings mere obtained and a t these times the coulometer -*as checked against a silver coulometer and found to be
The procedure was essentially that developed by Lingane and Small (S), the only major change being the weighing of the electrodes. Lingane's article should be consulted for details, but the main points of the procedure for a chloridebromide determination are as follows: I
lor
I I I
I
I I
I I
I I !
I I
I lo-
I \
\
18-
\
\ \
\
16-
m
-
\
14-
V
\
0 0
-
D:
,
12-
1
6
~
- 2 0 -16 -12
-OS -04
0
04
08
I2
I6
20
IF
f
Figure 2. Effect of -0.2% Coulometric Error on Per Cent Relative Error at Various Weight Ratios of Chloride-Bromide
21-
I
The silver-plated anode and platinum cathode are thoroughly rinsed with water, dried 1 hour a t 250' C., and weighed. The
11
Oaurprrj,
u h -16 -12 -08 -04 0 0 4 08 12 20
6 I -20
I6
Per Cent R e l o t i v e Error
Figure 1. Effect of -0.4-Mg. Gravimetric Error on Per Cent Error at Various Weight Ratios of Chloride-Bromide
WLULIII
U L l V U l U bULl"0,ILl
UllUUllU T ."
L'LI=u+.
u1
U l C ILaillUljU,
ID
ipetted into the cell and the supporting electrolyte, 0.2 M in goth sodium acetate and acetic acid, is added until the volume totals 100 mi. The electrolysis is then begun a t 0.22 volt us. the saturated calomel electrode. (The sign of the electrode potentials used throughout this article is in accord with the European convention in which the calomel electrode is positive with respect to the normal hydrogen electrode.) When the current
ANALYTICAL CHEMISTRY
276 drops to around 10 t o 15 ma., usually after about 45 minutes, the potential is increased to 0.25 volt for the remainder of the electrolysis. The entire electrolysis requires around 1 hour and is considered complete when the current drops below 0.5 ma. The electrodes are then rinsed with water, dried 1 hour a t 250' C., and weighed.
z'r I
\
~
4 I-
\
\
b ;h
\
\
\
\ 04
d6
d8
Ib
Per Cent
1'2
lk
1'8
Ik
210
Relotive Error
Figure 4. Average Experimental Error at Various Weight Ratios of Chloride-Bromide 611
I
-20 -16
I
I
I . .
I
-12 -08 -04
0
I
I
I
I
I
04
08
12
16
20
The results are calculated from an equation derived as follows: Referring to Equations 1 and 2, let w = weight of chlorine and
rn = equiv. of C1
+ weight of bromine
(la)
+ equiv. of Br
Pa)
then m = -wt. of GI 35.46
+
w
- wt. of c1 79 92
(3)
and, rearranging, Weight of chlorine = 0.7975(79.92m
- w)
(4)
and m are corrected for transfer of silver during electrolysis. The number of equivalents, m, is calculated from the coulometer reading by the following equation: 20
V P
m = - (2.141 X 10-j) T
(5)
where
V = volume of gas in milliliters P = corrected pressure in millimeters of mercury
T
=
absolute temperature
silver halide but deposits on the cathode. Thus, if one weighs the cathode, a correction can be applied by adding the increase in weight of the cathode to that of the anode and by subtracting from the coulometer result the number of equivalents of silver transferred. The amount of silver transferred is small) usually 0.5 to 1.5 mg., but is significant and correction for it has been made in the data shown. It is desirable to choose the sample size such that nearly the full volume of the 100-ml. buret is utilized. This means that about 4.5 meq. of the halides should be present. If a constant total number of milliequivalents is maintained, an error in either the coulometric or gravimetric result \Till cause a certain constant absolute error in the x-eights of chlorine and bromine found, regardless of their ratio. Thus for a total of 4.5 meq. of halogens and an error of -0.4 mg. in the gravimetric result, a t any ratio of chloride to bromide, there will be an error of 0.3 nig. in the chloride result and -0.7 nig. in the bromide result. Because of this constant absolut,e error the per cent relative error of chloride will become greater in samples where there is a lox ratio of chloride to bromide, but in such samples the per cent relative error or In-omide will be correspondingly smaller. A second factor which magnifies this effect is that if the total niilliequivalents are kept constant, then for high chloride samples the total weight of halides is less and the relat,ive error becomes even greater. This is evident in Figure 1 which s h o w a t various weight ratios the per cent relative error of each halogen which would result, from a -0.4-mg. gravimet,ric error. Aisomewhat analogous situat,ion exists for coulometric errors and this is represented in Figure 2. However, i t has been the authors' experience that usually
RESULTS AND DISCUSSION
I n order to deposit' the chloride completely a potential of 0.26 volt is required. - i t potentials only higher, si]ver dissolves from the anode. Therefore during most of the electrolysis the potential is maintained a t 0.22 volt and is increased to 0.25 for only the last 10 or 20 minutes. However, even when the potential control is within 0.01 volt, some silver xi11 leave the anode. This silver neit,her remains in solution nor precipitates as
Table I. Approximate Weight Ratio, C1:Br 1:lO 1:7 1:5 1:4 1:2 1:l 2:l 5:l
Typical Analyses of Chloride-Bromide Mixtures Error Present, hfg. Br C1 29.4 301.0 2 9 . 5 211.3 210.8 44.2 58.9 210.7 73.6 150.5 103.0 120.4 60.2 147.3 147.3 30.1
Found, Ci 28.8 29.2 44.2 59.0 73.3 102.9 147.1 146.8
Mg.
Br
301.6 210.8 210.0 209.9 150.3 120.0 60.0 29.9
-'bsoiute* hfg. Ci Br -0.6 0.6 -0.3 -0.5 0.0 -0.8 0.1 -0.8 -0.3 -0.2 -0.4 -0.1 -0.2 -0.2 -0 2 -0.5
%
C1
Br
-2.0 -1.0
0.2 -0.2 -0.4 -0.4 -0.1 -0.3 -0.4 -0.7
0.0 0.2 -0.4 -0.1 -0.1 -0.3
V O L U M E 25, NO. 2, F E B R U A R Y 1 9 5 3
277
when low results are obtained the gravimetric and coulometric results are both low and the errors thus tend to cancel. Figure 3 represents such a combination. Because of this rather general occurrence of compensating errors the accuracy of the method is in practice somen hat better than might he predicted from an estimation of the probable error in each part of the procedure. Typical results for synthetic samples of sodium chloride and potassium bromide are shown in Table I. The average absolute error of the chloride result is 0.3 mg. and of the bromide result, 0.5 mg. The curves in Figure 4 indicate the manner in n hich the per cent relative error resulting from absolute errors of this magnitude changes as the weight ratio is varied, assuming a constant 4.5 meq. of halogen present. Figure 4 may be used to predict the average relative erior which may be expected a t various weight ratios. For example, from these curves it may be seen that chlorine to bromine iatios from 1 to 6 to 2 to 1 may be analyzed with an average relative c’rror of
no more than 0.7% for either halogen. With more extreme ratios the halogen present in greater amount may be determined with high accuracy, but the determination of the small amount of the other halogen is, of course. subject to large relative error. ACKNOWLEDGMENT
This research was supported in part from funds granted by The Ohio State University Research Foundation to the university for aid in fundamental research. LITERATURE CITED
Lingane, J. J., IND.ENG.CHEM.,AFAL. ED.,17, 332 (1945). Lingane, J. J., J . Am. Chem. Soc., 67, 1916 (1945). Lingane, J. J., and Small, L. A., ANAL.CHEX.,21, 1119 (1949). RECEIVEDfor review September 1.5, 1952. Accepted November 7, 1952. Presented before the Division of Analytical Chemistry a t the 122nd Meeting of the . ~ V E R I C A K C H E s r I C a L SOCIETY, Atlantic City, N. J.
Analysis of Sulfuric Acid and Acid Sludges from Petroleum Processes F. T. WEISS, J. L. JUNGNICKEL, AND E. D. PETERS Shell Development Co., Emeryuille, Calif., AND
F. W. HEATH Shell Chemical Corp., Pittsburg, Calif. The sulfuric acid consumed in a number of refinery processes can be more economically utilized when analytical methods are available for the determination of the components of the acid and sludge streams. .4nalytical methods have been evaluated for determination of the principal components of the “spent acid” streams from treatment of lubricating oils with sulfuric acid and from preparation of gasoline alkylate and alkyl benzene. The determinations which can be made include: total acidity, titratable acidity, ester (by difference), free sulfuric acid, sulfur dioxide, water, neutral oil and sulfonic acid content, and equivalent weight of the sulfonic acids.
T
HE present shortage of sulfur and sulfuric acid makes it
imperative for petroleum refiners t o consider means for the careful husbanding of sulfuric acid supply in refinery operations. One of the principal refinery uses of sulfuric acid is in the treatment of a number of straight-run and cracked products for improvement of stability and color and for increasing the viscosity index of lubricating oils. Considerable amounts of sulfuric arid are consumed by some refiners in the preparation of petroleum sulfonates, and in the production of alkylates in the manufacture of the alkylaryl sulfonate detergents, although in some cases the alkylation of aromatics is carried out with another catalyst. The large-scale processes for the manufacture of gasoline alkylates and polymers and for the synthesis of alkylated aromatics require the consumption of large tonnages of sulfuric acid. Important to the economical operation of these processes are analytical methods for the determination of the major and minor components of the spent arid recycle acids and of the acid sludges produced. Determinations of sulfuric acid esters, sulfonic acids, sulfur dioxide, water, and equivalent weight of the sulfonic acids are frequently required in order to evaluate process efficiency and to ascertain the fate of sulfuric acid consumed in the process. Over the course of a number of years’ study, reliable methods have been developed that are valuable for the determination of the components in various spent acid and sludge streams. The
applications of these methods to several important refinery prorwries are described and evaluated. SPENT ACID STREAMS
Spent acids from three processes have been studied: (1)treatment of lubricating oil with sulfuric acid, ( 2 ) manufacture of gasoline alkylate (C8), and ( 3 ) experimental preparation of alkyl benzene (detergent alkylate). In the first of these three processes, lubricating oil may be treated with sulfuric acids ranging in strength from 94yo to acid? heavily fortified with sulfur trioxide, depending on whether the process is being used for improvement of color and stability or the manufacture of sulfonates. The lower, spent acid phase, which is allon-ed to settle after treatment, is ralled the “acid sludge.” The commercially valuable mahogany sulfonates remaiii in the oil phase and are removed by a Rubsequent operation. The sulfonates of lower equivalent weight found in the acid sludges apparently have less commercial use. In the mariufacture of gasolinr alkylate, isobutane and butylenes are brought into contact with strong sulfuric acid catalyst t o produce a Cs alkylate. The acid stream is recycled, but because there is a gradual accumulation of organic material, a portion of the recycle stream is contiiiuously removed as spent acid and is replaced by fresh sulfuric acid. Alkyl benzene is produced as the first step in one method for the preparation of sodium alkylbenzenesulfonate, an alkylaryl sulfonate detergent. The alkyla-
