Determination of Thiosulfate in Used Doctor Solution KARL UHRIG AND HARRY LEVIN, The Texas Company, Beacon, N. Y.
Several promising methods were investigated after it was observed that in doctor solution carbon dioxide quantitatively precipitates lead and converts sulfites to bisulfites. When sodium sulfite is added to fresh doctor solution and an excess of carbon dioxide is passed through it and filtered, the filtrate gives no evidence of lead to hydro en sulfide. When formaldehyde is added to another portion o f the filtrate and this is titrated with iodine the latter is immediately in excess, indicating the absence of sulfite.
A method for determining thiosulfate in used doctor solution is based on the observation that carbon dioxide precipitates the lead and simultaneously converts any sulfite present to bisulfite, which subsequently is bound to formaldehyde to render it inactive toward iodine. Under these conditions, thiosulfate is titrated alone.
0
NE of the finishing steps in the refining of gasoline and
other light petroleum fractions is the “sweetening” operation in which mercaptans, the compounds responsible for the “sour” odor of the product, are converted to less obnoxious disulfides. A process widely used for this purpose is the doctor treatment, in which gasoline is agitated with doctor solution (sodium plumbite in a n excess of caustic soda solution) with subsequent addition of free sulfur in carefully controlled amounts. The conversion from mercaptans t o disulfides is believed t o occur according to the following equations: 2RSH
+ NazPbOz+(RS)z Pb + 2NaOH (RS)zPb + S +RzSz + PbS
( 1)
(2)
The disulfides remain in the hydrocarbon fraction, which
Most of these procedures, though capable of giving accurate results on knowns, are too long and tedious for routine use. I n this class belong such methods as that of Autenrieth and Windaus (1) which, involving separation of sulfite as its strontium salt, requires too much time for settling and filtering. The method of Kurtenacker and Wollak (6), which is based on the fact that bisulfite is rendered inactive toward iodine by binding it to formaldehyde, formed the basis of the following method, which has been adopted for determining thiosulfate in used doctor solutions.
Apparatus and Reagents Apparatus required includes 500-ml. and 250-ml. volumetric flasks, 50-ml. pipet, 50-ml. buret divided in 0.1 ml., lOOO-ml., 500-ml., and 250-ml. Erlenmeyer flasks, glass funnels, carbon dioxide reducing valve, and Whatman a t e r paper No. 4, 11 cm. The reagents are carbon dioxide, 0.1 N iodine, 0.1 N sodium thiosulfate, c. P. formaldehyde (35 to 40 per cent), 10 per cent acetic acid, starch indicator, and phenolphthalein indicator.
is separated from the used doctor solution containing lead
Procedure
sulfide. The usual practice is to regenerate the spent doctor by oxidation (air blowing) ; the reaction proceeding, i t is believed, according to the following equation:
Pipet 50 ml. of sample into a 500-ml. volumetric flask, dilute with water to 500 ml., mix, transfer the entire solution to a 1000ml. Erlenmeyer flask, add a few drops of phenolphthalein indicator, and pass carbon dioxide through the solution until the pink phenolphthalein color is discharged. The lead is thus precipitated and any sulfite present is converted to bisulfite. As a precaution continue the passage of carbon dioxide for 5 minutes longer. Filter this solution through Whatman No. 4 filter paper and collect 250 ml. of filtrate in a 250-ml. volumetric flask. For each test put 50 ml. of this filtrate into a 250-ml. Erlenmeyer flask and add 5 ml. of 35 to 40 per cent formaldehyde, followed after 5 minutes by 20 ml. of 10 per cent acetic acid. Titrate the solution with 0.1 N iodine immediately after addition of the acetic acid, using starch as indicator. The 0.1 N iodine solution should be checked daily against 0.1 X thiosulfate.
2PbS
+ 202 + 6NaOH +2NazPbOZ + NazSzOa + 3H10
(3)
Since sodium thiosulfate, which is t h e principal by-product of this process, remains in the reactivated doctor solution and reflects its further utility for sweetening operations (8), a method for its analytical determination was desired. The effect of this increasing thiosulfate concentration is to further the tendency t o emulsification, decrease the solubility of lead, and increase the specific gravity, so that gravity is no longer a n index of the caustic concentration in doctor solution (6‘). A number of methods are available in the literature for determining it in the presence of other reducible sulfur salts, but none of these can be applied directly to the analysis of doctor solution because lead interferes. Kalmann ( 4 ) determines thiosulfate plus sulfite by titration with iodine. I n the titrated solution hydriodic acid, equivalent to the sulfite,
+ +
+
NazSOs IZ HzO = NalSOa 2HI is titrated with alkali. Thiosulfate is calculated from the two titrations. Autenrieth and Windaus (1) determine thiosulfate by titration with iodine after removing sulfite as strontium salt. A similar method, removing sulfite as barium salt, is described by Scott ( 7 ) . Giberton ( 2 ) describes a method based on rapid conversion of silver thiosulfate to silver sulfide, the sulfur of which is determined colorimetrically. Heinemann and Rahn (5)describe a method for determining reducible sulfur in caustic soda, based on reduction of such sulfur compounds to hydrogen sulfide which is determined iodometrically. 547
Calculations 1 ml. of 0.1 N iodine == 0.01582 gram of sodium thiosulfate. 20 X ml. of 0.1 N iodine X grams of thiosulfate per ml. of stand-
ard iodine = grams of sodium thiosulfate per 100-ml. sample.
Discussion The weight-volume manner of reporting results is in accord with the plant practice of expressing constituents of doctor solution in pounds per barrel. Frothing usually occurs on passing carbon dioxide through the sample, but subsides after a few minutes, during which a little shaking of the flask will prevent serious difficulty. The end point fades after a little standing b u t is sufficiently stable for recognition without difficulty. After the precipitation of lead by carbon dioxide, the used doctor solution usually is still colored and has a phenolic odor, indicating that not all phenolic compounds have been removed. These compounds, however, do not interfere with the iodine titration. This was demonstrated by comparing
INDUSTRIAL AND ENGINEERING CHEMISTRY
548
TABLE I. THIOSULFATE D ~ T I R m N A T ~ O NIN S USXI SOLUTIONS
Sample
2
3
(Known amounts of thiosulfate added) Total Thiosulfate Thiosulfate Added Found
Thiosulfate in Original Q . / i O O ml. 2.55 1.74
1.94
Q . / i O O ml. 3.12 6.23 9.34 2.49 4.88 7.48 1.25 2.49 3.74
DOCTOR
Added Thiosulfate Found
Q . / l O O ml. 5.60 8.65 11.76 4.38 6.80 9.34
Q . / i O O ml. 3.05 6.10 9.21 2.64 5.06 7.60
4.38 5.45
*
The time required for a thiosulfate determination is a p proximately one hour for the proposed volumetric method as compared to approximately 12 hours' elapsed time for the gravimetric method. The precision and accuracy of the proposed method, a p plied in a routine manner, are evident from Table I.
Literature Cited (1) Autenrieth and Windaus, 2. anal. Chem., 37, 297 (1898). Giberton, Compt. rmd., 197,646 (1933).
(2) (3)
Heinemann and Rllhn. IND. ENO. C H ~ M ANAL. .. ED.. 9.
458
(1937).
1.16 2.44 3.51
3.10
Vol. 14, No. 7
Kalmann, Bert 20, 568 (1887). (5) Kurtenaoker and Wollak, 2. amrg. allgem. Chem., 161, 201 (4)
(1927).
a
(6) Lowry,
by the propsed method with those Obtained by a long gravimetric procedure in which organic matter does not interfere. The latter involved the separation of sulfate and sulfite as barium salts and the oxidation of the thiosulfate to sulfate which was determined gravimetricauy* Results obtained by the two methods agreed very well. Obtained
U. 0. P. Booklet
(1940)
(7)
242, p. 7,
Universal Oil Products Co..
"standard Methods of Chemical An&,sisIP,
5th ed., p. 2183, New York, D. Van Nostrand Co., 1938. (8) Valentine and MacLean. Refiner Natural a b s o l i n ~Mfr., 14, 475 (1935). P R ~ S E N T Bbefore I D the Division of Petroleum Chemistry at the 103rd Meeting of the AMERICAN CHEMICAL Q O C I ~ TMemphis. Y, Tenn.
Apparatus for Continuous Concentration of a Solution under Reduced Pressure BENJAMIN L. DAVIS Bureau of Agricultural Chemistry and Engineering, U. S. Department of Agriculture, Washington D. C.
I
N THIS laboratory, i t is frequently necessary to concen-
trate large volumes of dilute sugar solutions resulting from the hydrolysis of polysaccharides, the s o l v e n t w a t e r or alcohol-being discarded. Because of the danger of decomposition at higher temperatures, distillation at reduced pressure with a maximum bath temperature of 50" C. is used. A water
3
-/lr
aspirator lowers the pressure enough to obtain rapid distillation. With the usual apparatus (consisting of two sidearm distilling flasks, one being the boiling flask and the other the receiver, cooled with a stream of water), however, the process of concentration must be frequently interrupted in order to empty the receiver and to add more of the dilute solution to the boiling flask. A modified concentration apparatus, which has been in use in this laboratory for over two years, is described here. It may be operated continuously until the concentration is completed, the receiver being automatically emptied and the dilute solution being added from time to time, or continuously, without breaking the vacuum or stopping the boiling. A cursory search of the literature shows that a somewhat similar apparatus was reported by Burger ( I ) , but it is believed that the apparatus here described necessitates much less manipulation in operation for the purpose outlined above. The receiving flask was made by sealing a iece of Gmm. tubing, A , into an ordinary %liter, sidearm, distzing flask. Another modification of this arrangement consists of a close-fitting rubber tube pushed throu h the side arm and extending to the bottom of the receiving flask While not suitable for use with all solvents, this arrangement has been used for several years in this bureau for concentrating sugar solutions. B is a fine capillary, to help prevent bumping, with a short rubber tube and screw clamp at its upper end for regulation. Replacing the usual side arm, C, of the Claisen distilling flask with a tube of larger diameter (15-mm. outside diameter) hastens the distillation. The dilute solution to be concentrated is added through !he funnel, D, either intermittently or by a continuous slow drip as the liquid boils out. E is a thermometer to indicate the bath temperature. The vapors are condensed in the receiving flask, then drawn off by the aspirator through tube A to the drain. The rest of the equipment is obvious from the drawing.
Literature Cited (1) Burger, Martin,
J. Lab. Clin. Med., 25,
1221 (1940).
AQRXCULTVRAL Chemical Reaearoh Division Contribution No. 54
