Colorimetric Determination of Alkyl Benzene Sulfonates JAY C. HARRIS, Monsanto Chemical Company, Central Research Department, Dayton, Ohio
agents, until a color change just ap ears. Make the tube up to volume and compare with the blani. From the volume of unknown added, and the fact that 1p. p. m. of alkyl benzene sodium sulfonate is required for a color change, calculate the strength of solution.
Simple colorimetric quantitative method for the estimation of alkyl benzene sodium sulfonates is described which is sensitive to one part per million of pure sulfonate. The analysis may be made visually or photometrically.
Used in the manner described above, the method is suitable for determining the strength of dilute solutions by visual observation where photometric equipment is not available.
ETHODS for the determination of the active ingredi-
,
ent content of commercial surface-active agents, including alkyl benzene sulfonates, have been the subject of several papers (3, 4, 6). Few of these methods are entirely satisfactory, because they include impurities such as sodium sulfate, or are inoperative with dilute solutions of surface-active agents. The determination of surface tension depression is easily accomplished, but does not lend itself to strictly quantitative estimation. The need for a rapid method for the quantitative estimation of alkyl benzene sodium sulfonates in dairy cleansing processes is described by Scales and Kemp (6), who outline a qualitative test based upon the formation of a blue coloration when a solution of alkyl benzene sulfonate is added to a solution of o-tolidine and sodium hypochlorite. However, not all detergents undergo this reaction, since they found that soap, Turkey red oil, lauryl sodium sulfate, and three related compounds failed to produce the iodo-blue color characteristic of this test. Cinder their conditions a flocculent precipitate obscured the test, so that it was not considered sufficiently accurate for quantitative purposes. Re-examination of their data and further investigation proved that the test could be made quantitative in character.
Experimental
The experimental information developed for the visual method is applicable to use in photometric equipment. The several variable factors involved were investigated. Benzidine and diamidine were tested to find whether they would produce a more suitable color for analytical purposes, but o-tolidine produced the most satisfactory results. Tests were made to determine the effect of quantities of reagents upon the color formation. To show the effect of available chlorine, a standardized solution of 300 p. p. m. of sodium hy ochlorite was used, and known increments were added to 50-mf Nessler tubes containing 30 ml. of distilled water, 5 ml. of o-tolidine solution, and 0.5 ml. of 1 per cent alkyl benzene sodium sulfonate solution. The volume was made up t o 50 ml. and the colored solutions were examined. The results, shown in Table I, indicate optimum color formation at 6 p. p. m. of available chlorine in the final test solution. To determine the optimum amount of o-tolidine solution, 1-ml. increments of o-tolidine solution were added to aqueous solutions containing 300 p. p. m. of available chlorine and 0.5 mi. of a 1 per cent solution of alkyl benzene sulfonate. The 1-ml. increment produced a brownish blue coloration, with very little precipitate; 2 ml. gave a deep blue; and increasing amounts of o-tolidine produced a greenish blue color with a greater amount of precipitate. The order of addition of reactants had a profound effect upon the formation of the undesirable suiface precipitate. Addition of the alkyl benzene sulfonate solution just before making up to volume minimized this difficulty.
Visual Method EQUIPMENT.50-ml. Nessler tubes (or 50-ml. graduates), pipets, and a 10.0-ml. buret graduated in 0.05-ml. divisions. SPECIAL SOLETIOM. o-Tolidine Solution. Add 1.0 gram of otolidine to 5 ml. of 20 er cent hydrochloric acid (100 ml. of hydrochloric acid, spec& gravity 1.18-1.19, to 500 ml. of distilled water) and grind to a thin paste, Add 150 to 200 ml. of distilled water to produce a solution, Transfer to a I-liter graduate and make up to 505 ml. with distilled water, then make t o 1 liter by adding the balance of the 20 per cent hydrochloric acid solution. Store in an amber bottle out of direct sunlight. Sodium Hypochlorite. Prepare a solution of sodium hypochlorite by slurrying 5 grams of high test calcium hypochlorite and 4 grams of anhydrous sodium carbonate in 334 ml. of distilled water. Let the insoluble matter settle, then decant the solution through a filter. Analyze it for available chlorine content according to the A. 0. A. C. (8) arsenious oxide titration method. Prepare a standard solution containing 300 p. p. m. of available chlorine. Alkyl Benzene Sodium Sulfonate, or Unknown. Prepare a 0.05 per cent solution of the alkyl benzene sulfonate by suitably diluting a stronger solution with distilled water. Dilute a wash solution 1 to 5 or more, depending upon the approximate concentration of the alkyl benzene sulfonate. This solution must be as dilute as indicated, or if stronger must be measured accurately from a suitably calibrated buret or pipet. PROCEDURE. Transfer 40 ml. of distilled water to each of two Nessler tubes. Add 1 nil. of 300 p. p. m. sodium hypochlorite solution to each tube, and invert to mix. Add 2 ml. of o-tolidine solution to each, and invert to mix. Make the blank up to 50 ml. with distilled water and invert several times t o mix thoroughly. Add the unknown solution of alkyl benzene sulfonate of definite dilution carefully in known amounts to a fresh mixture of re-
AMOUXTOF AVAILABLECHLORINE TABLE I. OPTIMUM (Varying increments of NaOCl solution, 5.0 ml. of o-tolidine solution, 0.5 ml. of 1% alkyl benzene sulfonate solution, q. s. water) NO Chlorine 7 Available Chlorine in Test Solution (Control) 1 p. p. m. 3 p. p. m. 6 p. p. m. 24 p. p. m. Yellow solu- Yellow pre- Blue solution, Deep blue so- Brownish blue tion cipitate precipitate lution, presolution, large on surface cipitate on amount of surface precipitate on surface
Utilizing the optimum amounts of available chlorine (300 p. p. m.) and o-tolidine solution (2 ml.), the desired dilution of alkyl benzene sulfonate was ascertained with varying amounts of pure alkyl benzene sulfonate solution: 0,0005 gram of sulfonate produced a suitable coloration, whereas 0.004 gram resulted in a deep brown color. This indicated that somewhat less than 80 p. p. m. of alkyl benzene sulfonate should be present in the test solution. Time (up to 15 minutes) had no effect upon the end point, but it is preferable to make the test shortly after all the ingredients have been mixed. Tap water (ca. 300 p. p. m. hardness) produced no visible effect. 254
April 15, 1943
ANALYTICAL EDITION
TABLE11. EXAMINATION OF SOLUTION FOR OPTIMUMWAVE LENGTH AKD TRANS~TTANCE Wave Length Millimicrons
Transmittance
% 8.8
500 600 700 800 900 1000
0.3 17.0 13.0 21.5 16.0
Alkalies in wash solutions have little effect upon the test under the conditions observed-i. e., if the alkyl benzene sulfonate solution is alkaline, .the degree of dilution will be such that final pH will be adjusted by the o-tolidine solution. A color change is produced by as little as 0.00005 gram of pure sulfonate in 50 ml. of solution according to test. This is equivalent to 1 part per million. Alkyl benzene sodium sulfonates diluted with sodium sulfate are detected in proportionally small amounts, dependent upon the degree of dilution of the active ingredient. TABLE 111.
PHOTOMETRIC TRASSMITTAXCE
DATA
(Sample A 40% alkyl benzene sodium sulfonate; sample B, 100% a!kyi benzene sddium sulfonate: sample C, 100% n-butyl diphenyl sodium sulfonate) -Sample A-Sample B Sample C Parts PhotelSpectroPhoteiPhotelper ometer photomometer ometer MilB eter, B Spectrophotometer B lion filter 600 ma filter 525 mp 600 mp filter
0.25 0.6 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 6.0 7.5 10.0 15.0 20.0 25.0 30.0 40.0 50.0
..
9i:5 96.5 97 96 95 92 93 91
97 96 95 94 95 94 93 91 91
89 84.5 82.5 75.5 65 56 47 45 33 27
88
..
84.5 81 73.5 60 48 38.5 30 20.5 14.5
97 97 94.5 91.5 87.5 85 80.5 77.5 74.5 69 69 60.5 56.5 43.5 32 24 17 12
99 98 96 92 88 84 80.6 78 74.5 71.5 68
.. ..
44 28 19 13 8.5 4.5
97 95.5 92 87 84 78 73.5 70.5 66.5 63 59.5 52.5 42.5 32.6 18.0 11.5 6.0 3.5 1.7
95 94 J I
,
0,. 92
;.
,
