The values of E in Table I range from -30 t o -1800 liter/ mole-cm. Pulse deposition is not 100% efficient. Rather, the fraction of molecules trapped at the cold surface remains constant over a range of pulse sizes. The thermal effects which occur at the substrate surface when too large a pulse (in excess of 0.5 millimole) impinges results not only in loss of a larger than normal fraction of the molecules but also in reduced matrix isolation, Both effects degrade the quantitation; the former for obvious reasons and the latter because it induces a change in band shape (broadening) which is not compensated for by reading peak absorbance. Conventional matrix isolation requires that dilute gas mixtures be deposited very slowly (over a span of many hours) t o ensure effective isolation of the reagent gas in the host lattice. Pseudo matrix isolation accepts a sharp decrease in molecular isolation in order t o provide a rapid method of sample preparation. Nonetheless, control of pulse size and dilution ratio provide sufficient dispersion of the reagent molecules in the condensed host to yield spectra which show intermolecular interactions only as high order effects. These bands are relatively sharp and
frequencies are reproducible generally irrespective of mixture components. The method described evidently provides a direct means of quantitative analysis for a11 infrared absorbing gases and volatile liquids. Applications of infrared pseudo matrix isolation t o molecular isotope analysis seem most appealing in the area of gas phase photochemistry where kinetic and mechanism studies so often require the use of isotope tracers. The technique is almost tailored to handle analysis of the complex reaction mixtures resulting from photochemical reactions, and because the sampling procedure can be very rapid, analytical studies of nonequilibrium mixtures seem possible. Applications of infrared pseudo matrix isolation in the field of atmospheric chemistry are also promising. ACKNOWLEDGMENT
The author is indebted to R. V. Albarino for assistance in preparing samples and recording spectral data.
RECEIVED for review December 18, 1967. Accepted January 12,1968.
Spectrophotometric Determination of Cationic Surfactalnts with Orange II George V. Scott Cotgate-Patmo five Research Center, Piscataway, N . J.
08854
A method described by Few and Ottewill for determining quaternary ammonium surfactants has been investigated. Dye salt is spectrophotometrically determined after chloroform extraction from aqueous solution of surfactant and excess Orange II dye. The method has been broadened by applying pH control, which permits use for other cationics or cationic precursors such as amine, amine oxide, and amphoteric surfactants and for mixtures of amine and quaternary ammonium surfactants. Results indicate that Orange II reacts in 1:l stoichiometry with cationic surfactants, and that the molar absorptivity and the wavelength of maximum absorbance for the dye salts in chloroform are independent of the reacting surfactant. Dependent on this information, a standardization factor, related to molar absorptivity, is obtained with purified surfactants and is applied in the analysis of commercial surfactants. A number of variables in the procedure have been examined in an effort to accelerate the extraction rate for certain surfactants. Isolation of dye salt in chloroform is suggested as a means to estimate average equivalent weights of commercial cationic surfactants.
IN
to well-recognized sanitizer and emulsifier activity, cationic surfactants are important for handle, antifriction and antistatic effects o n wool, cotton, human hair, and synthetic materials. Attesting to their importance, numerous publications (1-8) have appeared describing methods for quantitative determination at macro and micro concentration levels. A planned study of the sorption of quaternary ammonium surfactants by human hair required a method, empirical a t least, for determining concentrations in small withdrawals, ADDITION
768
ANALYTICAL CHEMISTRY
periodically taken from contact solutions. The amounts of cationic agent available for analysis per withdrawal were anticipated as 2 mg or less. Additionally, the method should be applicable to higher molecular weight surfactants bearing a broad variety of secondary substituent groups. Guided partly by these criteria, a spectrophotometric method described by Few and Ottewill(1) was selected for investigation. The method depends o n chloroform extraction of dye salt formed from quaternary ammonium surfactant and excess Orange I1 in aqueous solution. An attractive feature, confirmed by Zografi, Patel, and Weiner (9), is that the monosulfonate dye forms 1 :1 stoichiometric salts over a wide p H range with cationics bearing one quaternary group. Additionally attractive, the wavelength of maximum absorbance for dye salts appears independent of surfactant structure. The present investigation of the Orange I1 method contributes supportive and new information relative to the influence of parameters-such as the nature of the cationic functional group, pH, solvents, temperature, and salt concentration-on (1) A. V. Few and R. H. Ottewill, Colloid Science, 11, 34 (1946). (2) M. E. Auerbach, IND. ENG.CHEM.,ANAL.ED., 15, 492 (1943). (3) E. L. Colichman, ANAL.CHEM., 19,430 (1947). (4) N. D. Weiner and A. Felmeister, Zbid., 38, 515 (1966). (5) L. D. Metcalf, R. J. Martin, and A. A. Schmitz, J . Am. Oil Chem. Soc., 43, 355 (1966). 28, 870 (1956). (6) P. Mukerjee, ANAL.CHEM., (7) D. Hummel, “Identification and Analysis of Surface-Active Agents by Infrared and Chemical Methods,” Vol. I, Interscience, New York, 1964, p 238. (8) A. S. DuBois, Am. Dyestuff Reptr., 34, 245 (1945). (9) G. Zografi, P. Patel, and N. Weiner, J . Plzarma. Sci., 53, 544 (1 964).
Cationic type present B. Color in extract Basic buffer Acidic buffer C. Buffer for calibration(s) D.
Quaternary in solids
Table I. Information for Test Samples Quaternary Amine Appreciable Same Basic 100 S M
..
the transfer of Orange I1 color to the nonaqueous phase. Procedural changes were occasioned by more difficult extractability of higher molecular weight surfactants having lower water solubility. With provision for p H adjustment, the determination is extended t o nonquaternary cationic surfactants or cationic precursors such as amine oxide, primary and tertiary amine, and amphoteric surfactants and to mixtures of quaternary ammonium and nonquaternary amine surfactants. EXPERIMENTAL Apparatus. A Beckman D U spectrophotometer with matched 1-cm cuvettes was used. A motor-driven disc, 14 inches in diameter, with 20 rpm, was used for end-over-end motion of extraction cylinders. The cylinders were oriented spokewise with glass stoppers pressed into slots in a soft rubber hub and bodies held by clamps mounted near the periphery of the disc. Reagents, The reagents used were as follows: Orange 11, Sodium p-(2-hydroxy-l-naphthylazo)benzenesulfonate,Eastman Organic Chemicals, certified for use in histology; (DDA) N-Dodecyldimethylamine, Lachat Chemicals, Inc., Purity 99.5 %; ( D A . HCI) Dodecylammonium Chloride, ColgatePalmolive Co., M.P. 183”-184” C., neut. eq. found 222, calculated 222; (DTAB) Dodecyltrimethylammonium bromide, Colgate-Palmolive Co., Br found 26.01 Z, calculated 25.95 ; (CTAB) Hexadecyltrimethylammonium bromide, Eastman Organic Chemicals, technical grade; Tetramethylammonium chloride, Eastman Organic Chemicals, “Eastman” grade; Tetraheptylammonium bromide, Eastman Organic Chemicals, “Eastman” grade; Commercial Surfactants, identified as to major component in text. 11 STOCK SOLUTION. Preparation of Solutions. ORANGE Dilute 0.50 gram of “as received” Orange 11 to 1 liter with distilled water and store, protected from light. BUFFER SOLUTIONS (0.2M). Use sodium acetate a i p H 3.6, sodium citrate at p H 3.6, and sodium bicarbonate at p H 9.2. SURFACTANT STOCKSOLUTIONS.Determine the solids content of the surfactant sample. With exact weighings, prepare a solution to contain approximately 2.4 mg of solids per gram. Addition of acid may be needed t o solubilize free bases. Recommended Procedures. PROCEDURE FOR SAMPLE CALIBRATION.Take five withdrawals of 0.1 t o 0.6 gram of surfactant solution into small tared flasks and weigh the flasks exactly. Transfer each withdrawal to a 125-ml extraction cylinder, using 15 ml of water in portions and finally 1 ml of ethanol. Added 5 ml of buffer solution, 3 ml of Orange I1 solution, 12 ml of chloroform and adjust the aqueous volume to approximately 35 ml with water. Mount the cylinders on the disc and rotate. Adjust the time (and number) of extractions so that final extracts are visually color-free. Separate each successive 12 ml of extract and pass each through a funnel containing a small amount of glass wool and a sprinkling of anhydrous sodium sulfate at its throat. Collect the extracts in 50-ml volumetric flasks and dilute to volume.
Appreciable More Both 100 S M XB
XB
...
Amine in solids
Negligible Appreciable Acidic
Quaternary plus amine
100 SM‘ XA
100 SM’
(i; k) -
Determine the absorbance a t 485 mp using chloroform as a reference liquid. DETERMINATION OF SURFACTANTS. If a reference sample of known composition is available corresponding t o the sample under investigation, calibration data are obtained and employed in the usual way for surfactant determination. A buffer a t basic pH is suggested for the extraction of quaternary surfactants and an acidic buffer for amine surfactants. The following sample examination assumes that the cationic component of the test sample is known with respect to molecular weight and that no reference sample is available. The several steps are: A. Determine the calibration data for a solution containing a cationic surfactant--e.g., CTAB-of known molecular weight and concentration. Estimate the milligrams of cationic surfactant at an absorbance of 1.OO by extrapolating these data. Divide this weight by the milli-equivalent weight of the surfactant to provide a factor, S. B. Take two withdrawals, weighed exactly, from a stock solution of the test sample. Extract one withdrawal using basic buffer and the other using acidic buffer. Classify the test sample in one of the categories in Table I according to the amount of color extracted (ratio of absorbance to weight of solids in the withdrawal). C. Determine the calibration data, or two sets of data if both types of cationic are present, for the test sample using the buffer(s) indicated in Table I. Express the weight data as “mg solids” taken from the stock solution. D. Estimate the ‘‘mg solids’’ a t an absorbance of 1.00 by extrapolating the data for the basic extraction (X,) and/or for the acidic extraction ( X A ) . Assign values for the milliequivalent weight of quaternary ( M ) and/or of amine ( M ’ ) surfactant. Calculate the per cent of surfactant(s) in the solids according t o the test sample classification in Table I. The per cent noncationic solids may be calculated by difference and all percentages expressed on the basis of “as received” sample. CALIBRATION DATA TREATMENT. The data for a test sample may be extrapolated graphically to an absorbance of 1.00 by fitting a “best” straight line to the plotted data. In the present work, however, linear equations, x = by + a , are derived directly from the data, using regression of x (mg solids) on y (absorbance). Each set of data is processed by computer to provide an “averaging ratio” ( a = 0) equation, a “least squares” equation, and appropriate statistical information (10). A choice between the two equations is decided o n the basis of tests, applied a t a 0.05 probability level, for intercept significance (“t” test) and intercept importance for reducing error variance (‘‘F”ratio test). O n this basis, all but one of the equations selected to represent the samples in Table I1 are “averaging ratio,” indicating the absence of a significant “blank” o r (10) W. J. Youden, “Statistical Methods for Chemists,” Wiley, New York, 1951, Chapter 5. VOL. 40, NO. 4, APRIL 1968
769
I
Table 11. Calibration of Surfactants. Surfactants Reference DDA DA.HC1 DTAB CTAB
pH
8.
9. 10.
x=by+a b a
A A B B
5 4 5
5
0.523 0.545 0.745 0.874
... ... ... ...
0.0052 0.0061 0,0028
0,0028
80
W
c u
E 4
k
W
60
m
2 u A A A B B B A B B A B A
6 7 6 6 6 5 5 5 9 7 5
7
0.640 0.804 0.713 0.999 1.239 1.522 1.243 1.510 2.909 2.074 3.022 1.848
... ...
... ...
+0:016
...
...
...
0.0099 0.0172 0.0029 0.0068 0.0106 0.0032 0.0039 0.0037 0.0433 0.0173 0.0520 0.0088
a Raw data requested by one reviewer for the following acidic extractions :
Sample 6. rng solids: 0.261, 0.517, 0.676, 0.772, 1.048. Absorbance: 0.195,0.406, 0.534, 0.606, 0.830 Sample 8. mg solids: 0.581, 0.778, 0.851, 0.940, 0.950, 1.044, 1.184. Absorbance: 0.261, 0.381, 0.407, 0.457, 0.458, 0.505, 0.574.
constant error. For approximately half of the samples, standard deviations of measurements would have been lower if “least squares” equations were accepted. As a test of linearity of determined points, correlation coefficients deviated from unity by only 0.1 %, o n the average. RESULTS AND DISCUSSION
Variation of Orange I1 Concentration. F o r calibration purposes, the amount of cationic is varied while maintaining sufficient excess of dye. This produces straight line relationships of absorbance and sample solids. When cationic content is greater than anticipated, Orange I1 may not be in sufficient excess and a bending from the straight line is observed a t higher absorbances. To test the influence of varying excesses of Orange I1 upon extraction results, hexadecyltrimethylammonium bromide (CTAB) concentration was fixed and the Orange 11 concentration was varied. A curve is fitted to the plotted data in Figure 1. Corresponding to each volume of Orange I1 stock solution, the amount extracted is calculated from absorbance by means of a previously determined calibration equation and is expressed relative to amount of CTAB added. I t appears that maximum extraction of CTAB dye salt occurs with 50% or more molar excess of Orange 11. Few and Ottewill ( I ) obtained qualitatively similar curves by varying the quaternary concentration and holding the Orange I1 concentration fixed. The inflexion region, shaded in Figure 1, is attributed to CTAB cations and Orange I1 anions which remain in the aqueous phase. Dye salt is considered to be completely removed from the aqueous phase during the multiple extractions with chloroform. As an indication of completeness of extraction, the products of unreacted ion concentrations in the chloroform-saturated aqueous phases after multiple extraction were estimated at the four experimental points forming the inflexion region. The average value (0.11 x 10-lO) is similar t o 770
/
Std dev a
Commercial 1. 2. 3. 4. 5. 6. 7.
n
I
IO0
ANALYTICAL CHEMISTRY
c z 0 W
40
W Q
20
0
2
4
6
CC ORANGE II
8
IO
12
SOLUTION
Figure 1. Effect of Orange I1 concentration on extraction of hexadecyltrimethylammonium bromide (CTAB) solubility constants reported ( 9 ) for Orange 11 and several quaternary surfactants (not CTAB) in water at 25 O C. In a study of analogous interactions, Biles and coworkers (11-13) calculated the ratio of the product of aqueous ion concentrations to the adduct concentration in the chloroform. For the conditions tested, the ratio remained approximately constant whereas the product of aqueous ion concentrations did not. This suggests that multiple extractions are needed t o transfer more quantitatively the dye salt to the organic phase. Under fixed extraction conditions, it is expected that the bend region area or, more generally, the discrepancy between theoretically maximum and determined curves will increase for dye salts of surfactants having greater water solubility than CTAB-e.g. octyltrimethylammonium bromide (I). I n qualitative tests, highly soluble tetramethylammonium chloride in excess produced no color transfer to chloroform. O n the other hand, tetraheptylammonium chloride in excess caused visually complete transfer of color from the aqueous layer. The latter compound was added in chloroform solution because solubility in water is extremely low. Absorption Maximum for Orange I1 and Dye Salts. The wavelength of maximum absorbance for Orange I1 in water or its surfactant salts in chloroform remains invariant over a wide range of conditions. I n aqueous solution, the dye has been shown to have an unchanging spectrum from below p H 1.0 to above p H 8.0, at ionic strengths up t o 1.0. Absorbance starts to decrease near p H 10 and the wavelength maximum shifts above pH 12. Zografi, et al. (9) suggest that this effect at higher values of p H is due to increasing dissociation of the auxochromic phenolic group. The absorption maximum for cationic reaction products in chloroform occurs at 485-490 mp, the same as for the dye in water. Evidently the cationic portion of the dye salt provides little or no auxochromic effect. This feature was checked in several instances during the course of measurements with the DU spectrophotometer, and by curves obtained with a General Electric recording spectrophotometer. Figure 2 represents the visible spectra for dye salts of three widely different cationic materials and Orange 11, the dye salts of two being extracted at both (11) G. Divatia and J. Biles, J. Phn,ma. Sci., 50, 916 (1961). (12) R. Hull and J. Biles, Ibid., 53, 869 (1964). (13) J. Biles, F. M. Plakogiannis, B. J. Wong, and P. M. Biles, Zbid., 55,909 (1966).
100
I
90
80
70 0 W
3
60
t
2 z
a
50
QH
tIY
v
Figure 3. Effect of pH on extractability of nonquaternary surfactants
40
a LL W
DDA, Dodecyldimethylamine ODA, Commercial Sample 3 LDAO, Commercial Sample 1
30
20
10
0
1
,
,
1
1
1
1
500 600 WAVELENGTH I N MIUIMICRONS
/
,
,
1
700
Figure 2. Transmittance spectra for dye salts in chloroform obtained by extraction of commercial surfactants A . No. 5 at basic pH B . KO.8 at basic pH C. No. 8 at acidic pH D. N o . 1 at acidic pH E. No. 1 at neutral pH
acidic and higher pH. In all cases, the wavelength of maximum absorbance appears unaltered. Extractability of Orange I1 and Dye Salts. The inconveniently slow extraction of some commercial surfactants prompted a further investigation of the extraction procedure. Addition of 0.1M sodium chloride (1) had no effect o n the rate of color development in the chloroform and detectable transfer of free dye occurred. Except for buffer at approximately 0.03M, it is recommended that inorganic salts not be added. Larger amounts of ethanol than those used to aid cationic transfer also caused free dye transfer. A three-fold dilution of the aqueous phase gave no improvement in extraction rate and final absorbance was slightly less than for a control. Elevated temperature (35 ’ C) accelerated the extraction, but a third extraction was needed at either temperature. Choice of buffer anion appears to be important for the difficultly-extractable surfactants. As an example, Surfactant 6 (Table 11) extracted more completely in fewer extractions with citrate buffer than with acetate buffer at the same pH. Halogenated solvents, including three recommended ( I ) , have been compared for extraction effects. Six were visually ranked for color after a single fixed-time extraction of Surfactant 7. The order for decreasing color was: chloroform, trichloroethylene, I-bromobutane, ethylene chloride, tetrachloroethylene, ethylene bromide, trichlorobenzene. Six other chlorinated solvents were quantitatively compared after single fixed-time extractions of Surfactant 6. The results were: 1,1,1-trichloroethane, 63.0% extracted; chloroform, 6 2 . 3 z ; 1,2,3-trichloropropane, 56.6% ; 1,l-dichloroethane, 54.8% ; 1,2-dichIoropropane, 47.6%, tetrachloroethane, 44.8% ; 1,1,2-trichloroethane, 34.2%.
Only one solvent appears to be comparable to chloroform. Note that the best and the poorest solvents in the latter list are position isomers and that the two best solvents possess three chlorines on one carbon. The hydrogen bonding capacity of l,l,l-trichloroethane appears by one criterion (14) to be much less than that for chloroform. On this basis, pentachloroethane (14) is an interesting solvent for trial. Selection of solvents for similar extraction purposes on the basis of dielectric constant has not been successful (6, 11, 12). The slow extractability of some commercial surfactants persists as a possible criticism of the Orange I1 method, affecting precision and convenience. Similar difficulties apparently influence the precision of other methods for cationic determination. Errors are reported (7, 8) to be two to five times larger for some materials than for others. The standard deviations in Table 11 show that the Orange I1 method has a similar dependence on the test material. Determination of Nonquaternary Surfactants. With Orange I1 at acidic pH, tertiary amines produce straight line relationships of absorbance and concentration. Similarly, other materials which protonate at acidic p H are determinable with this minor provision in procedure. Investigation of extractability as a function of p H indicated that substantially complete extraction of nonquaternary cationics occurs with p H 3.6 buffer. At lower p H (2.2), results were the same. Data are plotted in Figure 3 for dodecyldimethylamine and a commercial sample of lauryldimethylamine oxide. “Per cent extracted” is based on calibration equations previously obtained at p H 3.6. Data for a commercial sample of octadecyldimethylamine, also shown in Figure 3, coincide with the data for dodecyldimethylamine, except for somewhat lower extractability from p H 6 to 9. The amines are stronger bases than the amine oxide and the “per cent extracted” values are greater through much of the p H range. Dye salt formation and transfer to chloroform consumes surfactant cation and leads to additional protonation of amine and amine oxide (15) at the expense of a slight rise in p H of the buffered aqueous phase. Determination of Mixed Quaternary-Nonquaternary Cationics. Determination of quaternary or nonquaternary cationic surfactants in the absence of the other is readily (14) A. Allerhand and P. Schleyer, J . A m . Chern. Soc., 85, 1715 (1963). (15) D. G. Kolp, R. G. Laughlin, F. P. Krauze, and R. E. Zimmerer, J . Phys. Chem., 67, 51 (1963). VOL. 40, NO. 4, APRIL 1968
771
Cationic DDA DA.HC1 DTAB CTAB Average
Table 111. Orange I1 Calibration Mol wt Cationic, mg Micromoles 213 0.523 2.45 222 0.545 2.44 308 0.745 2.42 364 0.874 2.40 2.43
Table IV. Analysis of Commercial Surfactants Sur-
factant 1. 2. 3. 4.
5. 6. 7.
8. 9.
M
...
.. . . .. 0.399 0.422 0.424 0.571 0.720 1.203
M’ 0.229 0.274 0.297 . .. ... 0.334 ... 0.594 . ..
V
70.0 49.1 4.4 2.4 43.6 73.7 2.0 19.9 22.5
Q ... ...
...
94.8 46.7 17.9 90.1 48.2 75.0
NQ 28.3 42.2 97.0 ,..
... 2.9 , , ,
16.0 , ,,
U 1.7 8.7 (1.4) 2.8 9.7 5.5 7.9 15.9 2.5
accomplished but mixtures presented an analytical problem, Previous workers ( I ) who studied quaternary surfactants showed extraction results t o be the same at p H 2.2 and 8.3, and present work confirms this. Extractability of some commercial quaternary surfactants is not independent of pH, however, because of the presence of unquaternized amine. Two characteristics of Orange I1 became more useful a t this point. The dye forms stoichiometric (1 :1) reaction products with surfactants and the absorption maximum remains unchanged with different cationic reagents. Assuming a molecular weight value for each cationic component, per cents of quaternary and nonquaternary cationics in a mixture can be determined from acidic and basic calibration equations. Initially a factor is derived to represent the micromoles of Orange I1 as dye salt in a chloroform extract (50 ml) having a specified absorbance. For this purpose, calibration equations for four relatively pure cationics were solved for milligrams of cationic at an absorbance of 1.OO, giving the data in Table 111. The average micromoles is the desired factor, S. Molar concentration of dye salt may be found from the micromoles in 50 ml of chloroform, and because absorbance and cuvette path length have values of 1.00, molar absorptivity is equivalent to the reciprocal of molar concentration. The factor S above represents a molar absorptivity of 2.06 X l o 4for Orange I1 dye salt in chloroform. This approximately agrees with 2.097 X lo4, reported (9) for Orange I1 dye in distilled water, but is lower than 2.29 X lo4,reported (16) for the dye acid and found in our Laboratories for recrystallized Orange 11. It is presumed that the low value is less accurate because of insufficient purification of dye and that dye in water and dye salt in chloroform differ with respect t o molar absorptivity. As a demonstration of analytical procedure for mixed cationics, a known solution of CTAB and D D A was prepared and six withdrawals were extracted, two at p H 3.6 and four at p H 9.4. Results are given below to illustrate the calculations using the equations of Table I. S M M‘
= =
=
2.43 (Orange I1 factor) 0.364 (meq wt CTAB) 0.213 (meq wt DDA)
(16) J. A. Maclaren, Arch. Bioc/zem. Biophys., 86, 175 (1960). 772
ANALYTICAL CHEMISTRY
X, X,
+ +
1.664 (mg cationic, slope intercept at basic pH) 0.652 (mg cationic, slope intercept at acidic pH) SM %quat. in solids = -~ 100 = 53.2 (50.0 added) = =
XB
% nonquat. in solids SM’
--
iJ
- - 100 =48.3 (50.0 added)
Agreement between “found” and “added” is considered satisfactory although improvement may, of course, be possible with determination of more extraction data a t each p H and with further investigation for optimum extraction conditions, The discrepancy between “added” and “found” may result in part from residual extractability of nonquaternary amine at p H 9.4 (Figure 3) which might be lessened at higher pH. Average molecular weights to represent quaternary or nonquaternary surfactant in a commercial sample are often difficult t o estimate reliably and in such cases accuracy of analysis is affected adversely. Evidence was obtained in the following way that reaction with Orange I1 may provide a basis for resolving this problem. CTAB dye salt was obtained by evaporation of a large chloroform extract and dried. Molecular weight of the dye salt was determined from the absorbance of a known weight in chloroform and the molar absorptivity (2.06 X lo4). The molecular weight of CTAB, calculated from the molecular weight of dye salt less the ionic weight of Orange 11, was 357 in practical agreement with the theoretical value of 364. Calibration of Surfactants. Equations are given in Table I for all materials examined in order to avoid bias in selection of “representative” examples. The number (n) of extractions a t basic (B) or at acidic ( A ) pH are shown for each sample, Standard deviations refer to the scatter of “mg solids” ( x ) values at given values of absorbance ( y ) . Commercial Sample 2, an amphoteric surfactant, was calibrated a t p H 2.1 near the lower limit of its isoelectric region. The calibration data permit determination of “mg solids” only. This is inadequate for comparing sorption (e.g.) of commercial surfactants whose cationic content is not equivalent to solids content. Analysis of Commercial Surfactants. Results of the Orange I1 analyses are shown in Table IV for the commercial surfactants. Molecular weights generally are obtained from the formula representing the principal component of a sample and, if minor cationic component is detected, a reasonably expected precursor. Values for “mg solids” at unit absorbance are obtained from the calibration equations. These quantities and an Orange I1 factor (2.43) comprise the information needed for the equations of Table I. Shown in Table IV are milli-equivalent weights estimated for quaternary ( M ) and nonquaternary cationic surfactants ( M ’ ) and the calculated per cents volatile ( V ) , quaternary cationic (Q), nonquaternary cationic ( N Q ) , and “unaccounted for” ( U ) . The latter, obtained by difference, is enclosed in parentheses when negative and represents a summation of noncationic solids content and errors involved in estimation of other components. The suppliers’ specifications for major component and composition are as follows: 1. Lauryl Dimethylamine Oxide, 30% active. 2. N-Lauryl-3-Aminopropionic Acid, minimum 5 0 x active. 3. N-Octadecyldimethylamine,tertiary amine 95% minimum, primary and secondary amine 2% maximum. 4. N-(Lauroyl Colamino Formyl Methyl) Pyridinium Chloride, minimum 90% active.
5. N-Oleyl-N-BenzyldimethylammoniumChloride, 50% quaternary and related cationics. 6. N-Stearyl-N-Benzyldimethylammonium Chloride, 1618% quaternary plus amine and fatty alcohol. 7. Di-Hydrogenated Tallow Dimethylammonium Chloride, 9 5 z active. 8. 2-Heptadecyl-1-Methyl-1 [(2-Stearoyl Amido) Ethyl] Imidazolinium Methylsulfate, 77-81 % total solids, 74-76Z active. 9. Polyoxyethylene (15 moles) di-Hydrogenated Tallow Methylammonium Chloride, 74-77Z quaternary, 2% maximum amine, 0.5% NaCI. 10. Cationic Fluoropolymer, no other information available. Some additional support for the Orange I1 method is inferred on the basis of reasonable agreement with suppliers’ specifications, Excluding Samples 2 and 8, the average deviation of Orange I1 results from product specifications is roughly 4%. For amphoteric Sample 2, p H 2.1 was satisfactory for calibration but perhaps not low enough for analysis. Sample 8 was most difficult to extract. Analysis for amine oxide in Sample 1 does not distinguish
~~~~
tertiary amine components which are commonly present. However, quaternization may enable distinction, as in other methods (17). Nonquaternized amine in Samples 4, 5 , 7, and 9 was found absent or at less than 1% of solids. Extraction of polymeric Sample 10 was rather easy and indicated amine substituents with minor amounts of quaternary. In short, the Orange I1 procedures have proven useful when applied for a study of cationic sorption by hair (18). Because of its characteristics, the dye has interesting potential and is recommended for analytical phases of similar studies. ACKNOWLEDGMENT
Appreciation is expressed t o J. D. Barnhurst and C. R . Robbins for valuable advice and to Mrs. W. Bartok for care in obtaining most of the experimental data. RECEIVED for review October 16,1967. Accepted February 9, 1968. (17) D. B. Lake and G. L. Hoh, J. Amer. Oil Chem. Soc.., 40.. 628 . (1963). (18) G. Scott, C. Robbins, and J. Barnhurst, J . SOC.Cosm. Chem.,
in press.
~
Spectrophotometric Determination of Copper R . L. Heller, Jr., and J. C. Guyon Department of Chemistry, Unifiersity of Missouri, Columbia, Mo. A sensitive spectrophotometric method for the determination of trace amounts of copper has been developed. The method is based on the catalytic effect of copper on the ascorbic acid reduction of the isopoly molybdate species at p H 1.85. Based upon copper concentration the molar absorptivity can be calculated to be 3.3 X 105. The method obeys Beer’s law and is sensitive to copper concentrations in the range 50 to 300 ppb. The method is subject to several interferences; however the copper can be separated utilizing a solvent extraction procedure. The method was applied to the analysis of a sulfide ore.
AN EXTREMELY sensitive spectrophotometric method for the determination of copper has been developed. The method is based on the apparent catalytic reduction of the isopoly molybdate by ascorbic acid, in the presence of Cu(I1). Many of the widely used photometric methods for the determination of copper utilize extraction into a nonaqueous phase coupled with complexation by an organic reagent. These methods include the carbamate method ( I ) , the cupron method (2), and the neocuproine method (3). These methods are not applicable at the ultratrace region and necessitate working in nonaqueous media. Other current methods operating in aqueous media utilize ammonia or tetraethylenepentamine (4), rubeanic acid (3, (1) C. A. No11 and L. D. Betz, ANAL.CHEM., 24, 1894 (1952). (2) R. A. Dunleavy, et al., Ibid.,22, 170 (1950). (3) J. D. Brown and J. Connell, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 3, 1953. (4) L. H. Williams, A m l y s t , 75, 425 (1950). 21, 628 (1949). (5) P. W. West and M. Campere, ANAL.CHEM.,
and cuprethol (6). Of these methods, only the cuprethol compares in sensitivity with the technique proposed here. EXPERIMENTAL
Apparatus. Absorbance measurements were made on a Cary Model 15 recording spectrophotometer with matched 1-cm quartz cells against deionized water. All p H measurements were made on a Beckman Zeromatic meter utilizing a Beckman No. 39183 long thin probe combination electrode. Appropriate Fisher standard buffers were used to standardize the meter. Deionized water was provided by passing steam condensate through Barnstead No. 8902 mixed bed and No. 8904 organic removal ion-exchange columns. A Blue M Magni-Whirl constant temperature water bath was used to maintain a constant temperature of 25” f 0.1 O C. Reagents. A stock solution of Na2MoOl.2Hz0, 10% wjv was prepared by dissolving 100 grams Mallinckrodt reagent grade N a 2 M 0 0 4 . 2 H 2 0in a liter volumetric flask and diluting t o the mark with deionized H 2 0 . The resulting solution was stored in a polyethylene bottle. A reagent mixture consisting of N a 2 M o 0 4 2. H 2 0 and HCl was prepared by pipetting 40.0 ml of the stock molybdate solution and 5.60 ml of concentrated (38%) HCl into a 2-liter volumetric flask and diluting to the mark with deionized HzO. This solution was stored in a polyethylene bottle and allowed to stand for at least 36 hours before use. Ascorbic acid, 5 % w/v, was prepared daily by dissolving 5.000 grams of Fisher reagent grade ascorbic acid in a 100-ml volumetric flask and diluting t o the mark with deionized (6) American Public Health Assoc. Inc., “Standard Methods for the Examination of Water, Sewage, and Industrial Wastes,” New York, N. Y.,1955, pp. 92-95. VOL. 40, NO. 4, APRIL 1968
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