Anal. Chem. 1985, 57,783-784
previously mentioned or by lack of assurance that the coating material would be able to stand up to extended acid exposure. If the material were to eventually degrade, the coating should be easily removable prior to recoating. The final decision was to coat the aluminum surfaces with a thin film of polyethylene. There is, to our knowledge, no commercial product available to produce this coating, but one can be easily prepared in the laboratory. Low molecular weight branched polyethylenes are soluble in some organic solvents such as toluene or xylene at slightly elevated temperatures, usually just below the boiling points of those solvents. Parafilm M (a product and registered trademark of American Can Co.) is a commonly available laboratory film of proprietary composition. It may be speculated that Paraflim M is a composite of waxes, plasticizers, and polyethylene. While this speculation may be incorrect, the solubility properties are very similar to those of a low molecular weight polyethylene but the setting properties are more waxlike. The flexibility and elasticity suggest a plasticized polymer. The important property, however, is that the Parafilm M is soluble, and that on evaporation of the solvent, an adhesive film is produced that retains the properties of the original Parafilm M. (In the following discussion Parafilm M will be referred to as a "polymer" although that may be technically incorrect.) The polymer solution is prepared by heating, with continuous stirring, small pieces of Parafilm M with approximately 10 times their weight of toluene. When the solvent becomes sufficiently hot, but below its boiling point, the polymer dissolves to form a clear solution. The surface to be protected is coated with the hot solution by flowing it on with a brush or cotton swab. The solution must be applied while it is hot as the polymer precipitates on cooling. The solvent is allowed to evaporate at room temperature or with the assistance of an infrared heat lamp and leaves behind a uniform coating of the polymer on the material to be protected. The wet polymer solution is subject to abrasion, and care should be taken to allow it to dry completely before handling. The thickness of the coating can be controlled by varying the polymer concentration in the solvent, 10% by weight
783
appears to be the highest workable concentration. The thickness can also be increased by applying repeated coatings, allowing each to dry completely before applying the next. The effectiveness of the coating was tested by covering the flat surface of a disposable aluminum roasting pan with the polymer. A number of test solutions (approximately 0.5 mL of each) were placed on the coated surface and covered with petri dishes to prevent evaporation. The solutions used were as follows: 1:1,water:hydrochloric acid; 1:1,waternitric acid; l:l:l,water:nitric acid:30% hydrogen peroxide; 1:1, water: sulfuric acid; 1 M sodium hydroxide; 0.005 M mercuric chloride. After 96 h there was no indication of attack on the metal by any of the test solutions. While this film acts as a hydrophobic barrier to solutions, it may permit the diffusion of gases or covalent compounds through the polymer. The coating is intended only as protection against attack by dilute aqueous electrolytes. The polymer coating may be hardened by the addition of 1% to 5 % of conventional polyethylene (CPE) to the hot solvent. The resulting film appears to gain mechanical strength and hardness, but is less adherent to the protected surface. The Parafilm M coating is inert, noncontaminating, and flexible and has not been found to crack or peel from the protected surface. I t is easily prepared with common laboratory solvents. While it cannot tolerate mechanical abrasion or thermal stress, it can help to solve some simple corrosion problems encountered in the laboratory. Registry No. Polyethylene (homopolymer),9002-88-4;aluminum, 7429-90-5.
RECEIVED for review September 24,1984. Accepted November 5 , 1984. Although the research described in this article has been funded wholly or in part by the United States Environmental Protection Agency through Cooperative Agreement CR 809-706-0103 to the University of Nevada, Las Vegas, it has not been subjected to Agency review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
Two-Phase Titration of Poly(oxyethy1ene) Nonionic Surfactants with Tetrakis( 4-fluorophenyl) borate Masahiro Tsubouchi,* Nakamichi Yamasaki, a n d Kazumichi Yanagisawa Laboratory of Chemistry, Kochi Medical School, Nankoku, Kochi 781-51, Japan Anionic and poly(oxyethy1ene) nonionic surfactants are widely used in both industrial and domestic detergents. I t is known that the nonionic surfactant can form spirals which can trap metal ions such as potassium ( 1 ) and barium (2). The reaction between anions and the complex cation formed by the nonionic surfactant and the metal ions has been made the basis for spectrophotometric methods (1, 3, 4 ) and precipitation titration (5) for determination of the nonionic surfactant. However, it is generally difficult to determine total nonionic surfactants in the presence of anionic surfactants because the metal ion complexes of the nonionic surfactants form ion pairs with the anionic surfactants present. Two-phase titration is one of the most frequently used methods for the determination of ionic surfactants. This paper 0003-2700/85/0357-0783$01 SO/O
presents a simple method for the determination of nonionic surfactant in the presence of an anionic surfactant by twophase titration. EXPERIMENTAL SECTION Apparatus. A 25-mL buret was used. Materials. The nonionic surfactants used were Triton X-100, Triton X-405 (Rohm & Haas Co.), poly(ethy1ene glycol) monop-isooctylphenyl ether (Wako Pure Chemical Ind.), poly(oxyethylene) lauryl alcohol ether, and poly(oxyethy1ene) sorbitan monopalmitate (Nakarai Chemicals Ltd.). The number-average degree of polymerization of the oxyethylene chain of the surfactants used is 8-12, except for Triton X-405 (for which it is 40). The nonionic surfactants were dried at 80 "C and dissolved in water. The titrant was prepared by dissolving sodium tetrakis0 1985 American Chemical Society
784
ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
Table I. Effect of Concentration of Titrant [titrant], M 1 x 10-3 1 x 10-3 5 x 10-4 5 x 10-4
5 x 10-4 5 x 10-4 2 x 10-4 2 x 10-4 a
[Triton X-1001, mg
Table 11. Titration of 1 mg of Poly(ethy1ene glycol) with X M Titrant
mean titer," mL
std dev, mL
1.84 5.51
1.10 3.67
0.04 0.05 0.04 0.06
7.34
0.07
11.01 2.75 9.20
0.07
1
3 0.3 1 2 3 0.3 1
5
HO(CH2CH20)"H
mL of
mean mol wt
titer
ii-[PEG]/[titrant]"
200 400 600
1.3 4.8 5.1
31.5 9.0
5.1
0.06
1000 2000 3000 7500
0.08
20000
Results of 10 sample solutions.
(4-fluoropheny1)borate (C24H16F,BNa,Dojindo Laboratories, Kumamoto, Japan, purity is more than 98%) in water. Victoria was dissolved in ethanol Blue B (color index 44045, C33H32N3C1) to make a 0.04% solution; it was used as an indicator. Procedure. To 10 mL of a surfactant solution (nonionics,0.3-3 mg/lO mL; anionics,less than 5 X lo4 M) in a 300-mL Erlenmeyer flask, were added 5 mL of 6 M potassium hydroxide solution, 1 to 2 drops of the indicator, and 5 mL of 1,2-dichloroethane. The mixture was titrated with the titrant (5 X M) with vigorous shaking after each addition, until a color change from red (in= 505 nm) to blue (A, 615 nm) took place in the organic phase. One milligram of nonionic surfactant requires ca. 3.6 mL of 5 X loe4M titrant, irrespective of the anionic surfactant present.
RESULTS AND DISCUSSION It is reported that a 12-unit ethylene oxide polymer combines with one barium regardless of the kind of alkyl or alkylaryl groups in the nonionic surfactant (2). The nonionic surfactant can also trap potassium ions in its spirals and be extracted into dichloroethane from 1 to 2 M potassium hydroxide, as an ion pair with tetrakis(4-fluorophenyl)borate, but any anionic surfactant in the mixture is not extracted. The aqueous phase remains colorless throughout the titration, due to the insolubility of the Victoria Blue B in alkaline water. An indicator-tetrakis(4-fluoropheny1)borate ion pair is formed at the end point, but the indicator does not react with anionic surfactants in the presence of alkaline water (more than 0.2 M of potassium hydroxide). The best color change and most constant titer were obtained at a concentration of potassium hydroxide from 1to 2 M in the aqueous layer at the end point. The results of titration of Triton X-100 are shown in Table I. With a 2 X M titrant, a blank titration is necessary because the color change at the end point is not sharp. There is a slight tendency to decrease the sharpness of the end point with increasing amounts of nonionic surfactant because of its lather. The volume of titrant required is proportional to the amount of nonionic surfactant present in the sample solution, indicating that the nonionic surfactant is stoichiometrically M titrant needed for 2 mg extracted. The volumes 5 X of nonionic surfactant were found to be as follows: Triton X-100, 7.3-7.4 mL; Triton X-405, 6.7-6.8 mL; poly(ethy1ene glycol) mono-p-isooctylphenyl ether, 7.4-7.5 mL; poly(oxyethy1ene)sorbitan manopalmitate, 7.1-7.2 mL; poly(oxyethylene) lauryl alcohol ether, 6.8-6.9 mL. The volumes vary with the products and type of surfactant, but it seems that in general the volume needed for the same weight of nonionic surfactant is almost constant, about 3.6 mL of 5 X M titrant/mg of nonionic surfactant. It is reasonable to consider
4.8 4.7 4.7 4.6
8.6 8.7 9.5 9.5 9.7
10.0
' [PEG]/[titrant]: combination ratio of poly(ethy1ene glycol) to titrant. that the hydrophile-lipophile balance values of the surfactants used are similar, making the method usable for a range of surfactants. Chloroform cannot be used as the organic solvent, since it does not give a sharp end point. If sodium tetraphenylborate is used as the titrant, potassium hydroxide gives precipitates and the use of sodium hydroxide in place of potassium hydroxide does not give a sharp end point. The following species do not interfere a t the M level: Na+, NH4+, A13+, Cl-, NO3-, SO:-, and triethanolamine. Anionic surfactants (5 X lov4M) such as dodecyl sulfate, dodecyl benzenesulfonate, and bis(2-ethylhexy1)sulfosuccinate do not disturb the titration, but separation of the two layers is poor when the concentration of these anionic surfactants M, causing positive errors. Cationic is more than 1 X surfactants disturb the titration. The results of the titration of poly(ethy1ene glycol) appear in Table 11. It is seen that a 9-10 unit ethylene oxide polymer combines with one tetrakis(4-fluoropheny1)borate. The number-average degree of polymerization of commercial nonionic surfactants is mainly 8-13. A commercial detergent was diluted with water, and the anionic surfactant was first titrated with a solution of cationic surfactant (tetradecyldimethylbenzylammonium chloride) (6). A second aliquot of sample solution was diluted with water to reduce the concentration of anionic surfactant to 5 X M or less, and the nonionic surfactant was then determined by the procedure. The result found was 0.37 M anionic surfactant and 0.12 g/mL of nonionic surfactant in the detergent. A recovery test was done by adding 1.0 mg of Triton X-100 to the 1500-fold diluted detergent, and 1.03 mg was found. Registry No. Victoria Blue B, 2580-56-5; Triton X-100, 9002-93-1; poly(ethy1ene glyco1)mono-p-isooctylphenyl ether, 51651-58-2; poly(oxyethy1ene) lauryl alcohol ether, 9002-92-0; poly(oxyethy1ene)sorbitan monopalmitate, 9005-66-7; sodium tetrakis(4-fluorophenyl)borate, 25776-12-9.
LITERATURE CITED (1) Toei, K ; Motomizu, S.; Umano, T. Talanta 1082, 29, 103-106. (2) Uno, T.; Miyajima, K Chem. Pharm Bull 1063, 1 7 , 80-82 (3) Anderson, N. H ; Girling, J. Analyst (London) 1982, 707, 836-838 (4) Murai, S. Bunseki Kagaku 1084, 33, T18-T21 (5) Uno, T.; Miyajima, K. Chem, Pharm Bull 1063, 1 7 , 75-80 (6) Tsubouchi, M.: Yamasaki, N.; Matsuoka, K J Am. Oil Chem S O C . 1074, 56, 921-923
RECEIVED for review September 10,1984. Accepted November 7, 1984.
