669
Anal. Chern. 1986, 58, 669-670
Titration of Nonionic Surfactants with Potassium Tetrakis(4-chlorophenyl)borate Arthur W. O’Connell J. L. Prescott Co., 27 Eighth Street, Passaic, New Jersey 07055 Nonionic surfactants are used in a variety of consumer products such as laundry detergents, dish detergents, and shampoos. Anionic and cationic surfactants may also be used in these products. These latter two classes are readily determined by either of two popular titration methods (1-31, which are rapid and accurate. Determination of nonionic surfactants, on the other hand, has relied on tedious separation procedures for the majority of those in use (4) or on somewhat faster and more reliable ultraviolet measurement for the few aromatic types used ( 5 ) . The titration method for the determination of poly(oxyethy1ene) containing nonionic surfactants published recently by Tsubouchi et al. (6) is a significant addition to the detergent analytical methodology and should become the method of choice in the quality control of detergents containing this class of surfactant. Obtaining the principal titration reagent, however, specified by Tsubouchi as sodium tetrakis(4-fluoropheny1)boratewas possible at this writing only by special order from Japan (CTC Organics, Atlanta, GA) at a very high price of $395 per 5 g. Perusal of a few chemical catalogs revealed that a similar compound, potassium tetrakis(4-~hlorophenyl)borate,is readily available a t $26 per 5 g, which makes it worthy of consideration as an alternative titrant. The following report will show that this reagent is a satisfactory and relatively inexpensive substitute for that used by Tsubouchi. A 0.025% aqueous solution ( 5 X M) of potassium tetrakis(4-chloropheny1)borate (Lancaster Synthesis 5609; Fluka 60591) is prepared by boiling vigorously with stirring. Titrations are carried out generally as directed in ref 6 with the exception of exact addition of indicator (50 pL) to a stoppered 50-mL tapered-bottom centrifuge tube used as the titration vessel. Precise reagent addition is necessary for unambiguous end point detection and centrifugation is useful in obtaining a transparent organic phase for the more difficult detergent formulations sometimes encountered. For the majority of cases, centrifugation is not required; however, the color shade and intensity are influenced by the presence of any insoluble matter left in the organic phase after shaking. A noncentrifuged organic layer that is hazy will appear to be under titrated (pink). Complicating end point detection further is the observation that the indicator changes color over a range of titrant addition that is dependent on the composition of the detergent matrix. Therefore, it is recommended for the quality control of detergent products that a standard detergent be made containing the target amount of nonionic surfactant in the same matrix as the detergent under test and titrating both the samples and standard to the same color under the same conditions. Indicator color changes from pink through purple to blue as titrant is added. The color perceived depends on the ratio of the absorbance maxima at 505 nm (pink) and 615 nm (blue). Table I and Figure 1 show the results of the titration of 1.52 M titrant. The largest visual mg of Nonoxynol-9 with 5 x change of color per 0.1 mL of titrant added occurs in the 6.0-6.2 mL range with the central 6.1-mL purple color chosen as the end point. This corresponds to approximately equal amounts of the absorbance maxima. Further addition of titrant past the blue color serves only to increase its intensity, which is difficult to discern visually but appears as an increase in the 615-nm peak. This increasing absorption finally stops at 20% past the purple end point, thereby indicating the end of the reaction.
3
300
400
500
600
WAVELENGTH,
700
800
nm
Figure 1. Spectra of the indicator at the pink (curve l), purple (curve 2), and blue (curve 3) points of the titration.
Table I. Titration of Nonoxynol-9 with Potassium Tetrakis(4-chloropheny1)borate
volume titrant, mL 0.0 5.9 6.0
6.1 6.2
6.3 6.4
indicator color pink pink
spectral curve % titrated 1
0
2
100
3
105
slight change purple blue no change more intense blue
Quaternary ammonium cationic surfactants titrate readily with tetrakis(4-chloropheny1)borate and were the only type found to interfere with this method. Although the end point is very sharp, the reaction is not quantitative at the purple color, a 5.0 x lo4 M Hyamine 1622 standard solution titrating M or 31% high. Nevertheless, detergents foras 6.5 X mulated with both cationic and nonionic surfactants are successfully analyzed by the combined use of the Epton titration for the quaternary determination with subtraction of this result from the tetrakis(4-chloropheny1)boratetitration to yield the nonionic amount. Three series of polyoxyethylenated alcohols were titrated with the results shown in Table 11. Increasing the moles of ethylene oxide (EO) requires increasing volumes of titrant for these surfactants as was the case for Tsubouchi’s study of
0003-2700/86/0358-0669$01.50/0 1986 American Chemical Society
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Anal. Chem. 1986, 58,670-671
Table 11. Titration of 1 mg of Surfactant with 5 Titrantn
X
M
vol of titrant, mL
mol of EO, n 2 3 4 5
Pareth 25
Pareth 15
Nonoxynol
1.7
LITERATURE CITED
1.7 3.5
6
7 9 12 15
Registry No. Potassium tetrakis(4-~hlorophenyl)borate, 14680-77-4; nonoxynol-9, 26027-38-3.
0.1 2.2
poly(ethy1ene glycols). An average value ( V = 4 . 1 mL, u = 0.4 mL) may be derived from the important detergent range covering n = 6-12 for use in determining unknown nonionics present in competitive products.
3.7 4.1 4.4
4.4 4.8 4.3 4.5
3.6 3.9 4.0 4.1 4.5
Pareth 25 consists of polyoxethylenated (POE) primary alcohols containing 12-15 carbons; Pareth 15 consists of POE secondarv alcohols containing 11-15 carbons and Nonoxvnol consists of POE nonylphenols ( T i e Cosmetic, Toiletry and “Fragrance Dictionary).
Epton, S . R. Nature (London) 1947, 160,795. Epton, S.R . Trans. Faraday SOC. 1948, 14, 226. Longman, G.F. “The Analysis of Detergents and Detergent Products”; Wiley: London, 1975; Chapter 10. Longman, G. F. “The Analysis of Detergents and Detergent Products”; Wiley: London, 1975; Chapter 5. O’Connell,A. W., unpublished work, J. L. Prescott Co., 1981. Tsubouchi, M.; Yamasaki, N.; Yanagisawa, K. Anal. Chem. 1985, 57, 783-784.
R~~~~~~~for review A~~~~~ 26, 1985. Accepted October 11, 1985.
Making Use of Information Contained in Folded-Back Peaks To Identify Low Mass Ions in Fourier Transform Mass Spectrometry Robert B. Cody* and James A. Kinsinger Nicolet Analytical Instruments, 5225 Verona Road, Madison, Wisconsin 5371 1 In order to characterize properly an analog signal that is to be digitized, the digitization rate must be at least twice the frequency of the highest frequency component. This is referred to as the Nyquist frequency. If a lower digitization rate is chosen, then high frequency components may appear as artifacts a t an incorrect lower frequency. This phenomenon is referred to as aliasing, and the artifact peaks are often called “folded-back’’ peaks. In Fourier transform mass spectrometry (FTMS) ( I - 4 ) , aliasing is generally avoided by exciting only the ion frequencies within the detection bandwidth, ejecting low mass ions before detection, or by using a filter to remove frequencies outside of the detection bandwidth. If a folded-back peak is suspected, the mass of the ion giving rise to that folded-back peak may be easily calculated. The true frequency, fo, of an ion giving rise to a folded-back peak is
f o = 2 x BW - f a p p
(1)
where BIV is the detection bandwidth and japp is the apparent frequency. The true mass of the ion may then be calculated from its true frequency and used to calculate an elemental composition. Calculation of the true frequency of folded-back peaks has been applied by Horlick et al. to Fourier transform visible spectroscopy ( 5 ) . In FTMS, this procedure may be useful if one wishes to detect low mass ions, while keeping a lower detection bandwidth to observe the signal for a longer time, and thus obtain the higher resolution associated with longer observation times. I t may also be useful to identify suspected aliased peaks. Since folded-back peaks often appear a t nonintegral masses, they are generally easy to identify. Calculating an elemental composition for a suspected folded-back peak provides more fragment ion information and assists in identifying folded-back peaks. We have applied this procedure for a mixture of perfluorotributylamine and isopentane. By selecting a narrow detection bandwidth and deliberately exciting ions with higher frequencies, we were able to determine the elemental com0003-2700/66/0358-0670$0 1.50/0
positions for nine isopentane fragment ions outside of the detection bandwidth.
EXPERIMENTAL SECTION All measurements were performed on a Nicolet FTMS-2000 Fourier transform mass spectrometer. Perfluorotributylamine and isopentane were leaked into the source region in approximately equal amounts, to give a total source pressure of about 1 X lo4 torr. Ion detection was performed in the analyzer region torr. at pressures less than 1 X The detection bandwidth was set to 800 kHz, which sets the low mass limit to 57.5 amu at a magnetic field strength of 3 T. The excitation bandwidth was deliberately set t o 2.667 MHz, which corresponds to a low mass limit of 17.5 amu at 3 T. A FORTRAN program was written to calculate the true mass for folded-back peaks, and the mass spectrometer software was used to calculate elemental compositions based on these masses. All measurements were taken in direct mode, with 64K data points acquired, and one zero fill employed to increase the number of points per peak. Perfluorotributylamine was used to calibrate the mass scale. Please see Table I.
RESULTS AND DISCUSSION Seven peaks in the mass spectrum of perfluorotributylamine were used to calibrate the instrument over the mass range 69-502 amu. Calibration was based on the equation derived by Ledford et al. ( 6 ) ,which allowed us to measure the lower masses with sufficient accuracy to readily determine elemental compositions. Folded-back peaks were identified by their “unusual” mass defects. True masses for folded-back peaks were calculated by moving the display cursor to each suspected peak and calling the FORTRAN program to calculate each mass. Table I shows the masses and elemental compositions calculated for each folded-back peak. Fragment ions from isopentane were easily identified by their calculated elemental compositions. 0 1986 American Chemical Society
