modification of the dye could result in a 100-fold increase in the sensitivity of this reaction.
5-aminofluorescein and other chemicals. John B. Pate has also contributed some helpful discussions on SO2 analysis.
ACKNOWLEDGMENT
RECEIVED for review December 15, 1969. Accepted January 16, 1970. The National Center for Atmospheric Research is sponsored by the National Science Foundation.
We acknowledge the invaluable assistance of C. I. Crowley and R. Webb of Eastman Kodak Company for furnishing the
Nonionic Surfactants as Polarographic Maximum Suppressors Wahid U. Malik and Puran Chand' Chemical Laboratories, Unieersity of Roorkee, Roorkee, India USE OF IONIC surfactants as polarographic maximum suppressors of difficultly suppressible maxima of simple and complex metal ions has been reported (I-.?). Comprehensive studies, however, have not been done in the case of nonionic surfactants ( 4 , 5 ) , although in view of their strong wetting and foaming properties they are expected to act as more effective maximum suppressors. The present communication desaibes the comparative results of our studies on the effect of Nonidet P40, P42, and Nonex 501 with Triton X-100 on the maxima of a number of simple and complex metal ions and on the electrocapillary curves. EXPERIMENTAL
Nonionic surfactants-namely Nonidet P40 (100 polyethylene oxide condensate), Nonidet P42 (condensation product of dioctyl phenol and ethylene oxide), and Nonex 501 (methoxy polyethylene glycol laurate) Triton X-100-were all B. D. H. products and were used without further purification. Biuret (6) was prepared in the laboratory. Analytical grade reagents and chemically pure reagents were used in all of the investigations. Doubly distilled water (all glass) was used for preparing the solutions. The procedure used in these investigations is described in detail elsewhere (5). All of the measurements were carried out at 25 i 0.1 "C in a thermostated water bath. A Beckman pH meter model H was used for pH measurements. For studying the effect of surfactants on the electrocapillary curves, 10.0 ml of 0.1NKC1 solution was deaerated by bubbling purified nitrogen in the polarographic cell. At least 20 drops were counted and droptime was measured by means of a stop watch. Each set of measurements at a constant potential was repeated three times. The electrocapillary data obtained from 0.0 to - 1.2 V (SCE) were repeated in the presence of different concentrations of the surfactant. RESULTS AND DISCUSSION
The data on the suppression of maxima of both positive and negative types, observed during the reduction of simple and complex metal ions, are summarized in Table 1. Present address, Chemistry Department, Jodhpur University, Jodhpur, India. (1) E. L. Colichman, J. Amer. Chem. SOC.,72, 4036 (1950). 32, 1528 (1960). (2) W. U. Malik and R. Haque, ANAL.CHEM., (3) W. U. Malik and R. Haque, J. Polarog. SOC.,8, 36 (1962). (4) R. Tamamushi and T. Yamanka, Bull. Chem. Soc. Japan, 28, 673 (1955). ( 5 ) W. U. Malik and P. Chand, ANAL.CHEM., 37, 1592 (1965). (6) R. C. Howorth and F. C. Mann, J. Chem. Soc. (London), 603 (1943).
Considering the above data, the nonionic surfactants investigated are superior to the anionic surfactants in that a very small amount of the former is required to suppress difficultly suppressible maxima-namely, the Ni2+-Co2+ mixture in pyridine and the KC1, Cd12-KI complex. However, the behavior of nonionic surfactants as maximum suppressors is not different from that of cationic surfactants, although in a few cases, e.g., Ni2+,Cozf, and Pb2+,a smaller quantity of the nonionic surfactant is required. Furthermore, the nonionic surfactants have the extra advantage of not undergoing chemical interaction with the depolarizer. For example, the Cd12-KI complex maximum is easily suppressed with the nonionic surfactants, whereas the precipitation of the depolarizer takes place on adding cationic surfactants (cetyl trimethyl ammonium bromide and cetyl pyridinium bromide). The nonionic surfactants differ among themselves in their suppressive action. For example, the amount of Nonidet P40 and Nonex 501 required to suppress the maximum is much lower (order 10-3 g/l.), The efficacy of nonionic surfactants as maximum suppressors follows the order: Nonex 501
> Nonidet P 40 > Nonidet P 42
The amount of nonionic surfactants required to suppress the maximum depends on how far the El,z is removed from the electrocapillary zero (e.c.2.). Thus for Cu-biuret complex, the amount of the surfactant required for suppressing the maximum is quite small because the Eliz of Cu-biuret complex, -0.50 V, is very close to the e.c.z. value, -0.55 V, in KOH. Likewise, the amount required for the suppression of Pb*+ maximum is larger because its El,*, -0.40 V is a little removed from the e.c.z.; U6+with Eli2-0.15 V requires the largest amount for the suppression of its maximum. Similar type of behavior was also found in the case of Triton X-100. U6+gives reduction waves with pronounced maximum in phosphoric acid and potassium chlorate. Gelatin, even in small quantity (0.001 %), distorts the polarographic wave and even changes its nature from the reversible to the irreversible one. Higher concentration of the supporting electrolyte (1M KC103) no doubt suppresses the maximum but in its place a second maximum appeais in the polarographic wave at a more negative potential, thereby making the polarographic analysis difficult. On the other hand the use of nonionic surfactants suppresses the maximum of U6+completely without any insidious effect and well defined waves are realized even with dilute solutions of the supporting electrolyte (0.05M KClOI). From the comparative data given in Table I, it is evident that although Triton X-100 is much better than ionic surANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
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Table I. Maximum Suppression Points (M.S.P.) of Ionic and Nonionic Surfactants in Presence of Different Supporting Electrolytes M.S.P. (order Nonionic Nonidet Nonidet P40 P42 4.64 5.20 2.60 22.60 8.25 22.60 6.10 22.60 15.50 72.00
Nonex 501 4.80 5.20 6.60 8.60
g/l)
Anionica SPSAc STSA 76.60 25.40 191.80 226.50 76.60 543.60 748.00 71.60 1597.00 94.70
Triton Cationicb x 10-3 SXSA CTMAB CPB gm 100 17.70 5.80 38.50 4.80 233.00 5.80 385.00 6.85 368.80 4.35 115.00 9.50 73.60 pptn pptn 9.82 97.80 6.50 not suppressed 48.00
Ion or complexes F/z -0.40 Pb2+(0.005M) 0.2MKNO3 Ni2+(0.01M) 0.1M KCI -1.08 C02+(0.1M) 0.1MKCI -1.25 CdIdO.0025M) 0.5MKI -0.68 Ni *+(0.0025M-Co2+ 8.60 (0.0025M) in 0.5M pyridine and 0.1M KCI C~*
[email protected]) 0.0225M -0.50 3.20 10.60 3.50 345.76 54.40 13.05 3.60 biuret at pH 12.0 Cu2+(0.0025M) 0.01M -0.30 8.60 30.00 6.00 73.00 23.10 23.76 5.90 potassium glycinate 0.01M KN03 U6+(0.004M)0.05M -0.15 5.20 19.00 2.32 ... ... ... 14.50 KCIOI+ 3M phosphoric acid Mn2+(2.0X 10-3M)in 1M -1.52 1.2 10.0 2.00 ... ... ... ... NH4C1and 1M NH40H Zn2+(2.0 X lO-3M) in 1M -1.34 0.75 4.00 1.00 ... ... ... ... NHaCl 1MNHiOH Co2+(2.5X 10-3M)in 1M -1.25 2.00 7.00 1.5 ... ... ... ... NH4Cl 1M NH40H Cd2+(2.0 X 10-3M) in 1M -0.83 3.50 12.00 3.0 ... ... ... ... 1M NH4C1 "4OH a M.S.P. values ( X 10-6M) given in ref. (3) were converted in (x 10-8 g/l,). * Data taken from the Ph.D thesis. SPSA, STSA, SXSA = sulfonated,tolyl, xylyl stearic acids. CTMAB = Cetyl trimethyl ammonium bromide. CPB bromide.
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POTENTIAL (VOLTS > Figure 1. Effect of Nonidet P42 on electrocapillary curve
factants, Nonex 501 and Nonidet P40 have several advantages over it. First, both of them are required in smaller quantities to suppress the maxima (except in two cases, viz., Cd2+ in 1 M NH40H and 1 M NHICl and Pb2+ in KNOI). Second, the elimination of maxima takes place with a very small decrease in id (e.g., 2.0 X A in the case of 0.004M 516
ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
=
Cetyl pyridinium
U6+, 2.5 X lo-* A in the case of 0.0025M Cu2+-biuret complex). On the other hand Triton X-100 brings about an appreciable decrease in id and also changes the E,,z (e.g., Co2+ and Cd2+in 1 M N H 4 0 H and 1 M NH4C1). Electrocapillary Curves. Figure 1 depicts the result of the variation of droptime with concentration of nonionic surfactant (Nonidet P42) from 0.0 to -1.2 V (SCE). Curve A shows the relationship between the electrode potential and the droptime of 0.1N KCI. In the absence of surfactant, the curve is almost symmetrical and parabolic (e.c.z. = -0.56 V) because of little adsorption of chloride ions. The gradual addition of Nonjdet P42 (and also with the other two surfactants) shifts the electrocapillary zero toward the more positive potential. Curves B to H show the effect of gradual addition of the surfactant on droptime within the potential range 0.0 to -1.2 V. The nonsymmetry of the electrocapillary curves and shift toward the more positive potential in the presence of nonionic surfactants point toward the desorption in the region where the curves merge with the normal one. Larger positive shift in e.c.2. and the smaller amount of surfactants under investigation required to suppress the maximum show their large adsorption at the DME. Similar type of behavior was found in the case of Nonex 501 and Nonidet P40. Similar curves have been reported for Triton X-100 (7). RECEIVED for review June 12, 1968. Resubmitted October 9, 1969. Accepted December 16, 1969. Thanks are due to C.S.J.R. (India) for the grant of a fellowship to one of us (P. Chand). (7) R. G. Barradas and F. M. Kimmerle, J . Elecrronal. Chem., 11, 123, (1966).
