G. R. Bo n d, Jr., Houdry Proccss Corporation, Paulsboro, N . J.
I
N TH E course of recent investigations involving the use of copper soaps, it was observed that the gradual addition of a kerosene solution of cupric oleate to a sample of gasoline containing mercaptans was attended by decoloriza- tion of the dark green oleate solution and formation of a pale yellow gelatinous precipitate, followed by the production of a distinct green coloration in the solution as soon as a slight excess of the oleate had been added. This observation at once suggested a possible rapid method for the quanti
tative estimation of mercaptans. Previous methods re
ported in the literature have a number of serious drawbacks where their use for routine determination of the mercaptan content of cracked gasolines is concerned. The lamp method of Youtz and Perkins (5) and Faragher, Morrell, and Monroe (8) is indirect and moreover cannot give as accurate results for amounts of sulfur less than 0.005 gram, which are just the values of practical interest. The iodometric method of Sampey and Reid (4), while undoubtedly capable of giving very accurate results in an inert solvent, is useless in the presence of unsaturated hydrocarbons, as is the mercuric chloride method reported in the same paper. The excellent method of Borgstrom and Reid (1), involving titration with silver nitrate, while capable of giving very accurate results and apparent freedom from interfering materials, requires very careful manipulation and prolonged agitation to pre
clude high results.
The method described in this article has shown an accuracy comparable to that of Borgstrom and Reid, freedom from interference by materials normally encountered in a cracked gasoline with the exception of hydrogen sulfide, and the advantage of rapidity and simplicity of operation, an average
determination taking only 2 to 5 minutes. The completeness of the reaction has been demonstrated even in the case of Cross-cracked gasolines from Coastal crudes, which would contain most of the mercaptans ordinarily encountered.
Ma t e r i a l s
The individual mercaptans used in this investigation were n-butyl, isobutyl, isoamyl, heptyl, benzyl, and m-thiocresol, obtained from the Eastman Kodak Company and used as received without purification. Some were of indefinite age and exposure, accounting for their low purity as determined.
Tests were also run on untreated Cross-cracked gasoline.
The Ranger naphtha used in making up the mercaptan solutions had the following properties:
A. P. I. gravity at 60° F. (15.6° C.) 54.5
Initial boiling point 221° F. (104.4° C.) Final boiling point 398° F. (203.3° C.)
Peroxides None
Free sulfur Trace
So l u t i o n s
C u p r i c O l e a t e . Titrations were carried out with a crude cupric oleate solution prepared by exact neutralization of elaine oil with sodium hydroxide, precipitation in warm dilute solution with approximately the theoretical amount of cupric nitrate, and purification by washing the coagulated cupric oleate. This was found to give a material having a more intense color than that prepared from pure oleic acid.
The solution was prepared with kerosene free from oxidation products to contain about 4 grams of copper per liter. Stand
ardization was accomplished by shaking a given volume
Vol. 5, No. 4 with dilute hydrochloric acid to remove the copper, which
was then determined iodometrically. The standard solu
tion is fairly permanent if kept in the dark, showing no alteration over a period of three months, and may be used as long as it remains clear and does not develop peroxides.
A solution a year old exhibited a whitish precipitate and possessed a pronounced oxidized odor. Solutions of copper soaps other than the oleate may be used, if the reaction with the mercaptans goes to completion with sufficient rapidity and they have sufficient color to make the end point clearly visible. The reaction involves the reduction of the copper to the cuprous form with the production of cuprous mer- captide and organic disulfide and liberation of fatty acid:
4ESH + 2Cu(01), - 2CuSR + RjS, + 4H(01) From this equation it follows that 1 Cu * 2 mercaptan S.
N a p h t h a . The mercaptans were weighed by difference from a Lunge pipet directly into a weighed amount of Ranger naphtha and accurately measured volumes of this mixture were diluted with the same naphtha to the 50-cc. volume used throughout in the tests. The standard solutions were kept in the dark until used.
Me t h o d o f An a l y s i s
A definite weight or volume of the gasoline is placed in a glass-stoppered cylinder and the cupric oleate solution is added from a buret in small portions, with shaking. The end point is a pale green color, clearly visible even in the presence of considerable precipitate. It may readily be substantiated by filtering on an ordinary dry filter paper.
Addition of 0.05 cc. of the oleate solution at the end point is sufficient to impart a distinct green tint to the gasoline when viewed in a layer of reasonable thickness. For a blank, a sample of the same type of gasoline should be sweetened—for instance by alcoholic caustic—and titrated to the same depth of color. This value usually ranges from 0.10 to 0.15 cc. of the oleate solution, and is subtracted from the mercaptan titration. The percentage of sulfur is then:
(cc. Cu oleate — blank) X grams Cu/cc. X 1.000 X 100 wt. of sample
% mercaptan sulfur Further work is being done on the application of the method to dark-colored cracked distillates or oils. Preliminary' tests, based on extraction of the mercaptans with alcoholic caustic with subsequent decomposition of the alkali mer- captides in the presence of clear naphtha by means of acid and titration of the washed naphtha, have shown a recovery of about 85 per cent of the mercaptan present. Losses are largely due to the ease of oxidation of the alkali mercaptides, and it is hoped that, by reduction of the naphtha extract with zinc dust and acetic acid in the usual manner before titration, more nearly theoretical results may be obtained.
Ef f e c t o f Po s s i b l e In t e r f e r i n g Su b s t a n c e s
Addition of free sulfur to a standard mercaptan solution had no effect on the titration.
In the presence of hydrogen sulfide a dark brown pre
cipitate of copper sulfide is produced, which obscures the end point. As pointed out by Borgstrom and Reid (/), hydrogen sulfide may be removed without seriously altering the mercaptan content by shaking with acidified cadmium chloride solution (2).
Addition of phenol or o-cresol to standard mercaptan solutions was without effect on the titrations.
Addition of crude petroleum acids or purified naphthenic acids to standard mercaptan solutions did not alter the titra
tions. In fact, a copper naphthenate solution appears per
fectly feasible as a titrating agent.
Carbon disulfide was without influence on the titration.
Addition of octylene (Eastman Kodak Company) to a standard mercaptan solution apparently caused a low result.
This was traced to tlft presence of peroxides in the octylene.
Purified octylene was without any effect.
Organic disulfides have no effect on the titration.
Organic peroxides wTould not be present in a solution containing mercaptans, but might develop in the copper oleate solution because of the pronounced catalysis of oxida
tion reactions by copper salts. As wras expected, addition of a gasoline rich in peroxides to some of the copper oleate solution and utilization of this mixture to titrate a standard mercaptan solution led to a decidedly lower titration than normal. This emphasizes the precautions to be observed with respect to the standard copper solution. The solution may be tested by shaking a few cubic centimeters with potassium iodide solution, when no pronounced iodine colora
tion should develop in the aqueous layer.
Re s u l t s
Table I shows the figures obtained on titrating varying percentages of mercaptans in naphtha solution, the theoreti
cal sulfur content being based on the weight of added mercaptan, assuming it to be 100 per cent pure. The last column shows the apparent actual purity of the mercaptan based on the consumption of copper. These figures show fair agreement among themselves for all except extremely low percentages of sulfur.
T a b l e I. D e t e r m i n a t i o n o f P u r i t y o f M e r c a p t a n s i n N a p h t h a S o l u t i o n b y T i t r a t i o n w i t h C o p p e r O l e a t e
(50 cc. of naphtha used)
Me r c a p t a n Su l f u r Th e o r e t i c a l Su l f u r De t e r m i n e d
Ap p a r e n t Pu r i t t OF Me r c a p t a n
Gram % Gram % %
n-B u tyl (old) 0.0142 0.0382 0.0132 0.0 3 5 4 9 2 .7
0.00285 0.00764 0.00254 0.00682 8 9 .2
0.0142 0.0382 0.0131 0.0 3 5 2 9 2 .0
n-B u tyl (new) 0.00467 0.0 1 2 5 0.00447 0.0 1 1 9 7 9 5 .7 0.00934 0.0 2 5 0 0.0 0 8 9 4 0.0 2 4 0 9 4 .8
0.000280 0.00075 0.000212 0.00057 7 4 .7
0.00280 0.00750 0.0 0 2 5 5 0.00684 9 1 .2
0.0233 0.0 6 2 5 0.0 2 2 7 0.0 6 0 8 9 7 .4
0.00186 0.00500 0.00170 0.00457 9 1 .5
0.00467 0.0 1 2 5 0.00447 0.01197 9 5 .7
B enzyl 0.00447 0.0 1 1 9 8 0.00425 0.01140 9 5 .1
0.00894 0.02396 0.00S58 0.0 2 3 0 9 6 .0
0.001785 0.00478 0.00157 0.00422 88.1
0.000357 0.000957 0.000298 0.000799 8 3 .6
0.01788 0.0479 0.0171 0.0 4 5 8 9 5 .7
Isoam yl 0 .0 1 1 8 0 .0 3 1 7 0.00902 0.0242 7 6 .3
0.0 1 1 8 0.0 3 1 7 0.00902 0.0 2 4 2 7 6 .3
0.0 0 5 9 0 0.0 1 5 9 0.00455 0.0122 77 .1
0 .0 2 9 5 0.0791 0.0 2 2 4 0.0601 7 6 .0
0 .0 1 1 8 0.0 3 1 7 O .OOSSO 0 .0 2 3 6 7 4 .6 0.0 1 5 0 0 .0 4 0 3 0 .01113 0 .0 2 9 9 7 4 .3 0.0 1 5 0 0.0 4 0 3 0.01135 0 .0 3 0 5 7 5 .7
Isobutyl 0.00977 0.0 2 6 2 0.00944 0.0 2 5 3 9 6 .7
0.00977 0.0262 0.00944 0.0 2 5 3 9 6 .7
0.02438 0.0 6 5 4 0.02324 0.0624 9 5 .4
0.00977 0.0 2 6 2 0.00922 0.0247 9 4 .4
m-Thiocresol 0.00956 0.0256 0 .00795 0.0 2 1 3 8 3 .2
0.00956 0.0 2 5 6 0.00795 0 .0 2 1 3 8 3 .2
H eptyl 0.00738 0 .0 1 9 7 8 0.00702 0.01881 9 5 .1
0.01846 0.0 4 9 5 0.01792 0.0481 9 7 .2
Recalculating the first two columns on the basis of the approximate determined purity, the average absolute error over a range of 0.0007 to 0.06 per cent of sulfur is only about 0.0002 per cent, which far surpasses the accuracy of any other method so far published. Checks between different operators with different standard solutions fall well within 0.001 per cent. The naphtha tested after each titration was found to be doctor sweet.
In order to substantiate the presence of disulfides in the original mercaptans, which would account for less than 100 per cent purity, certain standard solutions were extracted
July 15, 1933 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 259 with alcoholic caustic to remove mercaptans, the residual
disulfides were reduced with zinc dust and acid, and the mercaptans thus produced were titrated in the usual manner.
The results shown in Table III, although only approximate, because of losses encountered during the reduction (evolu
tion of hydrogen sulfide, etc.), indicate the actual purity to be approximately as indicated by the oleate titration.
Ta b l e I I . De t e r m i n a t i o n o f t h e Ac c u r a c y o pt h e Ol e a t e
Filtration to remove the gelatinous precipitate and titration of the filtrate with alcoholic potash gave the theoretical sodium hydroxide produced a bright yellow precipitate indicative of a cuprous salt. The gelatinous nature of the cuprous mercaptide may be due either to the presence of oleic acid or to the fact that precipitation is entirely in the presence of a nonaqueous medium.
Co m p a r i s o n w i t h Ot h e r Me t h o d s
For purposes of comparison the same mercaptan solutions were analyzed by the silver nitrate method of Borgstrom (1).
Allowing only 15 minutes’ shaking on back-titration of the excess silver nitrate, results up to 125 per cent of theory were obtained, as shown in Table IV. As stated by Borgstrom, this error is caused by silver nitrate occluded by the pre
cipitate, which, in most cases, remains in the naphtha layer.
On prolonging the period of shaking to 45 minutes, results more nearly in accord with the oleate titration were obtained.
In the case of the lowrer mercaptans from Cross gasoline, the silver mercaptide remained in the aqueous phase and results checked the oleate titration almost exactly, from which it would appear that better contact between mercaptide and aqueous potassium thiocyanate solution had permitted more ready removal of occluded silver nitrate.
T a b l e IV. A n a l y s i s o f M e r c a p t a n S o l u t i o n s i n N a p h t h a observation was checked by addition of approximately four times as much butyl disulfide as mercaptan, titration of the mercaptan, removal of the mercaptide by filtration, reduction of the remaining disulfide and titration of the mercaptan formed. It was found that a quantity of disulfide sulfur approximately equal to the mercaptan sulfur had been re
moved from the solution, possibly by adsorption on the silver mercaptide. Further substantiation of this point will be sought at a later date.
A number of mercaptan solutions were likewise titrated by addition of excess standard iodine solution and titration of the excess with thiosulfate. The same difficulty was encountered as reported by Sampey and Reid (4) of a gradual return of the blue starch-iodine coloration after an end point with those obtained by copper oleate titration. Mercaptans from Cross gasoline were separated from the gasoline by
Vol. 5, No. 4 either aqueous or alcoholic caustic and again liberated in the
presence of the Ranger naphtha. In this way, unsaturated hydrocarbons were removed. Phenols or cresols were, however, still present and, although partially corrected for by a blank, may have had some influence on the titration, accounting for the high result obtained on the lower mer- captans, which solution also contained the bulk of the phenols.
All the above results are likewise shown in Table IV.
It appears that the copper oleate method possesses decided advantages over previously published methods. Its appli
cability to all types of mercaptans has not yet been estab
lished, but since doctor tests at the end of the various titra
tions have invariably shown the gasoline to be sweet, even in the case of Cross-cracked gasolines from Coastal crude oil, which would contain most of the mercaptans which should ordinarily be encountered, the method appears suffi
ciently accurate for normal plant control operation. It is planned to investigate the applicability of this method to the
determination of disulfides by establishing a method of quantitative reduction to mercaptans. Further work will be undertaken as opportunity affords.
Ac k n o w l e d g m e n t
The writer is indebted to S. Comay of the Houdry Process Corporation for several valuable suggestions, and to Russell Lee of the Vacuum Oil Company, Inc., for a number of cooperative titrations in establishing the accuracy of the method between different operators.
Li t e r a t u r e Ci t e d
(1) B orsstrom and Reid, Ind. Eno. Ch em., Anal. E d., 1, 186 (1929).
(2) Faragher, M orrell, and Monroe, I n d . E n g . C h e m ., 1 9 ,1281 (1927).
(3) Klason, P., Bcr., 2 0 , 3412 (1887).
(4) Sam pey and Reid, J . Am . Chem. Soc., 54, 3404 (1932).
(5) Y outz and Perkins, I n d . E n o . C h e m ., 19, 1250 (1927).
Received February 7, 1933.