To illustrate further the accuracy of the method, a comparison of results from undepentanized FIA analyses and GLC analyses is presented in Table I1 for three gasolines of differing compositions. FIA data normally reported in per cent volume are converted to per cent weight to allow direct comparison with GLC data. The aromatics content of the gasoline samples was also determined by mass spectrometry and these results are included in Table 11. An inspection of these data shows that relatively good agreement is found among the different analytical procedures. Interestingly, the GLC method shows lower aromatics and higher olefin values than those measured by FIA for both olefinic gasolines. Since the boundary between aromatic and olefin fractions in the FIA analysis is the more difficult to determine accurately, the aromatics content measured by the GLC method is probably more reliable. In the case of the light catalytically cracked gasoline analysis, the high saturates value obtained by FIA may be a consequence of not depentanizing the sample. In general, spreading of the
saturate zone by the volatile C6-and-lighter hydrocarbons leads to erroneously high saturates content. On the basis of four replicate measurements of the intermediate catalytically cracked gasoline, the relative deviations from the mean values average about 1 %. Again, the largest variance appears in the olefin value. The results in Table I1 also show that a similar, if not better, precision is achieved for the triplicate analyses of the reformate and light catalytically cracked gasoline. ACKNOWLEDGMENT
The authors are indebted to J. B. Maynard, Wood River Research Laboratory, for his initial development work on hydrocarbon-type analysis by gas chromatography. RECEIVED for review September 28, 1970. Accepted December 14, 1970. Permission to publish granted by Shell Oil Company.
New Spectrophotometric Method for Determination of Submicrogram Quantities of Selenium Robert L. Osburn Department of Chemistry, Louisiana State University at Eunice, Eunice, La. 70535
A. D. Shendrikar and Philip W. West Coates Chemical Laboratories, Institute for the Environmental Sciences, Louisiana State University, Baton Rouge, La. 70803
SELENIUM DIOXIDE is an oxidizing agent for unsaturated hydrocarbons, aldehydes, ketones, heterocyclic nitrogen compounds, terpenes, sterols, fatty oils, and other natural products (I). Analytical methods which are based on its oxidizing properties usually lack sensitivity and hence cannot be used for the determination of selenium. However, Postowsky, Lugowkin, and Mandryk (2) investigated the oxidation of arylhydrazines by selenous acid, and they found that diazonium salts were produced which could be coupled with aromatic amines to produce intensely colored azo dyestuffs. Fiegl and Demant (3) employed this reaction for the detection of small amounts of arylhydrazines, and Kirkbright and Yoe ( 4 ) have developed a spectrophotometric method for the determination of selenium based on the spot test developed by Feigl. However, the useful analytical range of this procedure is 2 to 40 pg of selenium in a maximum sample volume of 2 ml. Feigl (5) has mentioned that hydroxylamine hydrochloride is oxidized to nitrous acid by selenous acid under strongly acidic conditions. It is commonly known that nitrites react with primary aromatic amines in acidic solution with the formation of diazonium salts which will couple with certain (1) G. R. Watkins and C. W. Clark, Chem. Rev., 36,235 (1945). (2) J. J. Postowsky, B. P. Lugowkin, and G. T. Mandryk, Ber., 69, 1913 (1936). (3) F. Feigl and V. Demant, Mikrochim. Acta, 1, 134 (1937). 35,808, (1963). (4) G. R. Kirkbright and J. H. Yoe, ANAL.CHEM., (5) F. Feigl, “Spot Tests in Organic Analysis,” 7th ed., Elsevier, New York, 1966, p 237. 594
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ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
compounds to form intensely colored azo dyes. Because of the extreme sensitivity of the diazotization-coupling reactions sequence with which nitrite determinations are routinely made in the parts per billion range, this reaction offers a unique and attractive approach for the determination of submicrogram quantities of selenium. Various combinations of reagents for the diazotizationcoupling reactions have been used by different workers including sulfanilic acid and 1-aminonaphthalene (6-8) sulfanilic acid and N-N-dimethyl-1-aminonaphthalene ( 9 ) , sulfanilic acid and N-( 1-naphthyl)-ethylenediamine hydrochloride (IO), and sulfanilamide and N-(1-naphthyl)-ethylenediamine dihydrochloride (11,12). Because the last combination of reagents is somewhat more sensitive than others, it was chosen as the basis for the development of a new spectrophotometric method for the determination of selenium which can be represented by the following
(6) “Standard Methods of Water Analysis,” American Public Health Association, New York, N. Y., 1936, p 46. (7) “Official and Tentative Methods of Analysis,” Association of Official Agricultural Chemists, Washington, D. C., 1940, pp 222, 527. (8) W. P. Mason and A. M. Bushwell, “Examination of Water,” John Wiley and Sons, New York, N. Y., 1931,p SO. (9) F. G. Germuth, IND.ENG.CHEM., ANAL.ED.,1,28 (1929). (10) B. E. Saltzman, ANAL.CHEM., 26,1949 (1954). ANAL,ED., 12,325 (1940). (11) M. B. Shinn, IND.ENG.CHEM., (12) N. F. Kershaw and N. S. Chamberlin, ibid.,14,312 (1942).
HSe0,-
+
Hf
4- ",OH
Se
+
0.61
HK02
+
2H,O
(1)
H
w
H
5 IO
EXPERIMENTAL
Apparatus. Beckman Spectrophotometer, Model DB, with 1-cm quartz cells; Thermostatic Bath ( + l "C); test tubes (2.5 X 20 cm), fitted with size No. 11 cork stoppers were used. Reagents. STOCKSELENIUM SOLUTION.A solution containing 0.50 mg of selenium per milliliter was prepared by dissolving 50 mg of pure selenium metal (Eimer and Amend, CP) in a few drops of concentrated nitric acid, boiling gently to expel brown fumes and to remove excess nitric acid and making up to 100 ml with distilled water. The standard stock solution was diluted as necessary for preparing standard working solutions. HYDROXYLAMINE HYDROCHLORIDE. Solutions of 0.1, 1, 5 , 10, 12, 16,20,30,40, and 50% (w/v) were prepared by dissolving the required amounts of hydroxylamine hydrochloride (Mallinckrodt Chemical Works or J. T. Baker Chemical Co.) in the appropriate volumes of distilled water. SULFANILAMIDE (~AMINOBENZENENESULFONAMIDE). Soluand 7 (w/v) were prepared tions of 0.05,0.1,0.5,1,2,3,4,5,6, by dissolving required amounts of sulfanilamide (Mallinckrodt Chemical Works) in the appropriate volumes of 1 :1 hydrochloric acid-distilled water (v/v). It was necessary to heat the higher concentrations of this reagent on a water bath in order to effect solution, Where there was a visible residue after all of the sulfanilamide had dissolved, the solution was filtered, while hot, through a fine porosity sintered glass funnel. The solution was stable for a month when kept tightly stoppered and in a refrigerator. N-( 1-NAPHTHYL)-ETHYLENEDIAMINE DIHYDROCHLORIDE. Solution of 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6% (w/v) were prepared by dissolving the appropriate amount of the reagent (Fisher Scientific Company) in a 1 % (v/v) hydrochloric acid solution. The solutions were kept in a refrigerator in a lightproof container and tightly stoppered. They were stable for 10-14 days. INTERFERENCE SOLUTIONS.Various solutions of diverse ions used for the interference studies were prepared by dissolving the calculated weight of each compound in distilled water in order to give 10 mg per ml of the respective interfering ions. Procedure. Five milliliters of sample which contains between 0.01 and 0.20 Hg per ml of selenium(1V) were introduced into a lightproof reactions vessel and 2 ml of 10% hydroxylamine solution is added. After the addition of 10 ml of concentrated hydrochloric acid, 2 ml of 6z sulfanilamide solution in 1 :1 hydrochloric acid-distilled water is added, the reaction tubes are tightly stoppered, and the reaction mixture is heated in a thermostatically controlled water bath set at 60 ==I 1 "C for 90 minutes. This is followed by the addition of 5 ml of 0.1 N-(1-naphthyl)-ethylenediamine dihydrochloride in 1 hydrochloric acid plus 1 ml of
20
40
50
Temperature
C
30
60
70
Figure 1. Variation of absorbance with temperature during the oxidation-diazotization reaction distilled water and a further heating period of 30 minutes in a constant temperature water bath set at 60 + 1 "C, again taking care to ensure that the reaction tubes are tightly stoppered. Absorbance measurements are made at 544 nm and the amount of selenium(1V) present in the sample is determined from a standard curve. The system obeys Beer's law in the range of 0.01 to 0.20 pg/ml. It is imperative that the reaction medium be mixed thoroughly after the addition of each reagent in order to obtain consistent and reliable results. DISCUSSION AND RESULTS
The Oxidation-Diazotization Reaction Sequence. EFFECT CONCENTRATION OF HYDROCHLORIC ACID. Because reactions 1 and 2 are both influenced by the acidity of the reaction medium, these reactions were studied by introducing 25 pg of selenium, 2 ml of 50 % hydroxylamine hydrochloride, and varying amounts of concentrated hydrochloric acid into lightproof reaction vessels followed by the addition of 2 ml of 0.5 sulfanilamide solution. The reaction mixtures were allowed to stand at room temperature for 45 minutes. One milliliter of 0.1 N-(1-naphthyl)-ethylenediamine dihydrochloride was added, and the reaction was continued for an additional 30 minutes. The absorbance was then measured at 544 nm against distilled water. The absorbance of the coupled azo product is a maximum at concentrations of hydrochloric acid above 6N during the oxidation-diazotization reaction sequence. The absorbance is not affected by further increases in the acid concentration. EFFECTOF TEMPERATURE. In order to establish the effect of temperature on the oxidation-diazotization reaction, 5 , 25, and 50 pg of selenium(1V) were reacted under conditions of optimum acidity with hydroxylamine hydrochloride and sulfanilamide at different temperatures between 5 and 70 "C for 60 minutes. N-(1-naphthyl)-ethylenediamine dihydrochloride was then added and the coupling reaction proceeded at room temperature for 30 minutes. Figure 1 shows that the coupled product has a maximum absorbance when the oxidation-diazotization reaction is carried out at 60 "C. At temperatures above 60 "C,there is a decrease in the amount of azo product formed. REACTION RATEAT 60 "C. Twenty-five-microgram portions of a standard selenium(1V) solution were allowed to OF
ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
e
595
t
a l 0 c
o.6
0
0.5
2ln n
ct
0.4
-0 0) c V 0)
0.3 0
0.2
1
I
I
I
0
5
IO
15
I
I
20 25 Percentage ( w / v )
I
30
Figure 2. Variation of absorbance with concentration of hydroxylamine hydrochloride
Table I.
Statistical Analysis of Absorbance a t Various Concentrations of Selenium (IV) Number of Concentration Precision, determina- of selenium, stand dev, mg/l. Re1 error, tions mg/l. 0.01 0.0016 12.0 10 0.02 0.0021 10 8.0 0.04 0 .0023 4.3 10 0.05 10 0,0027 4.8 0.10 10 0.0046 4.4 0,0057 2.2 0.20 10
react with excess hydroxylamine hydrochloride solution and 2 ml of 0.5% sulfanilamide solution at 60 "C for different periods of time, under conditions of optimum acidity. The coupling reaction was then carried out for 30 minutes and absorbance measurements were made at 544 nm. The optimum reaction time for the oxidation-diazotization reaction at 60 "C was found to be between 75 and 120 minutes. A 90minute period was adopted as most favorable. EFFECTOF SULFANILAMIDE CONCENTRATION. Five-microgram portions of a standard selenium(1V) solution were reacted with 2 ml of sulfanilamide solutions of varying concentrations under optimum conditions of acidity, temperature, and time. The diazonium salt was then coupled with N-(lnaphthyl)-ethylenediamine dihydrochloride as above and the absorbance of the dyestuff measured at 544 nm. Two milliliters of 6 % sulfanilamide solution was required for maximum absorbance. The use of a larger excess produced no further increase in the absorbance. EFFECTOF HYDROXYLAMINE HYDROCHLORIDE CONCENTRATION. Using 2 ml of each solution in a series of hydroxylamine hydrochloride solutions which varied in concentration between 0.1 and 50%, 5-pg portions of selenium(1V) were reacted with 2 ml of 6 % sulfanilamide solution under optimum conditions of acidity, temperature, and time. The coupling reagent was then added, and after a 30-minute reaction time, absorbance measurements were made at 544 nm. Figure 2 shows that the intensity of the color developed in the system is highly dependent upon the concentration of hydroxylamine hydrochloride and that a maximum absorbance is obtained when the concentration of this solution is 10 t o 12%. The Coupling Reaction. EFFECTOF TEMPERATURE AND TIME. Preliminary studies revealed that more of the highlycolored coupled product is formed when the coupling reac596
ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
tion is carried out at 60 O C than at room temperature. In view of this, a time study was made for the coupling reaction at 60 O C . Twenty-five micrograms of selenium(1V) were reacted under optimum conditions of acidity, temperature, time, and concentration of hydroxylamine hydrochloride and sulfanilamide. Coupling reaction times of 5, 15, 30, 45, and 60 minutes were used, and from the results obtained it was determined that the most favorable reaction time for the coupling reaction at 60 "C is 30 minutes. The rate of reaction for the coupling process should be higher as the acidity of the reaction medium is decreased. This can be readily seen from reaction 3, where the hydrogen ions are seen to appear on the right side of the equation. For this reason, most methods used for nitrite determinations decrease the acidity of the medium to about p H 2 by the addition of sodium acetate solution prior to the coupling reaction. However, when this was attempted for the present system, extremely high blanks resulted. Thus, it is more advantageous t o raise the temperature of the system during the coupling process in order to obtain a reasonable reaction time rather than to lower its acidity. EFFECTOF N-(~-NAPHTHYL)-ETHYLENEDIAMINE DIHYDROCHLORIDE CONCENTRATION. Five micrograms of standard selenium(1V) were reacted under optimum conditions of acidity, temperature, time, and concentrations of hydroxylamine hydrochloride and sulfanilamide. The coupling reaction was then carried out using different concentrations of the coupling reagent. A maximum in absorbance was reached at concentrations of N-( 1-naphthyl)-ethylenediamine dihydrochloride of 0.4 or greater. Interferences. To determine the effect of diverse ions on the determination of selenium, solutions were prepared containing 1 microgram of selenium and varying concentrations of each ion to be tested. The selenium was then determined under the optimum conditions as described in the recommended procedure. The presence of a 100-fold excess of the following ions caused no interference: Li+, K+, Na+, Ba2+, Ca2+, Sr2+, Mg2+, Be2+, Ni2+, Cd2+, Zn2+, Co2+, Mn2+, Sn2+, Hg2+, A13+, Cr3+, Zr4+, Sb5+, F-, S032-, Sod2-, SO3+, W042-, GeOs2-, H2P201e-,BO3-, HPOc2-, HAsOd2-,citrate, tartrate, malonate, and oxalate. Those ions which interfere when present in a thousand-fold excess are: Cu2+, Bi3+, La3+, Fe3+, CN-, I-, Br-, SCN-, Br03-, IO3-, NOS-, VOa-, T e 0 32--, SeOd2-, Cr042-, M004*-, U0d2-, and B40,2-. However, when the concentrations of BrOs-, and Te032- were reduced to a hundredfold excess; the concentration of Fe3f, Cr042-, Cr2012-, CN-, and c103reduced to a twenty-fivefold excess and that of M 0 0 4 ~ -reduced to a tenfold excess; no interference was observed. Of the remaining ions which interfere when present at a tenfold excess, only Cu2+and Se042-are likely t o be significant in air pollution studies. Selenates are easily reduced t o Se(IV) by boiling in a solution which is 4-5M with respect t o hydrochloric acid. The interference due to Cu2+can easily be obviated using an ion exchange procedure or by extracting the cupferrate ion with chloroform from a solution which has been acidified with hydrochloric acid. Statistical Evaluation. Table I presents the results of a statistical evaluation of selenium(1V) determinations at the respective concentrations using the hydroxylamine-oxidation procedure which has been developed. Analysis of Air Samples. The method thus developed was applied for the determination of selenium in smoke samples collected with Telematic 150 A air samplers (Unico, Fall
Table 11. Selenium Content of Smoke Samples SeOl present, F g Hydroxylamine-oxidaCatalytic Sample No. method5 tion method Recovery, Zb 82.0 2.10 1 2.57 81 .O 3.10 2 3.86 100.0 3 3.08 3.08 2.10 100.0 4 2.10 97.0 3.78 5 3.92 95.0 2.38 6 2.52 88 .O 2.94 7 3.36 a Reference (13). * Based on results of catalytic method. River, Mass.). The solution from the sampler was made 4-5M in hydrochloric acid and boiled gently to effect the reduction of selenium(V1) to selenium(1V). Selenium was then determined by the recommended procedure. The samples were also analyzed by the catalytic method of West and Ramakrishna (13). A comparison of the results obtained by the two procedures is shown in Table 11.
SUMMARY
A new spectrophotometric method for the determination of submicrogram quantities of selenous acid has been developed. The procedure is based upon the oxidation of hydroxylamine hydrochloride to nitrous acid by selenous acid followed by the diazotization of sulfanilamide by the nitrite produced and subsequent coupling of the diazonium salt with N-(1-naphthy1)-ethylenediamine dihydrochloride. Reaction parameters such as temperature, time, and reagent concentrations have been studied in detail and optimum conditions for the system have been established. The range of determination extends from 0.01 to 0.20 milligrams of selenium(1V) per liter. The method is simple, sensitive (e = 193,000), and reproducible. There are no common interferences which cannot be easily obviated. RECEIVED for review August 7, 1970. Accepted December 1, 1970. This work was supported by the U. S. Public Health Service Grant AP 00724 from the Division of Air Pollution, Bureau of State Services. (13) P. W. West and T. V. Ramakrishna, ANAL.CHEM., 40, 966, (1968).
Automatic Monitor for Surfactants in Aviation Turbine Fuel G . P. Pilz and J. L. Manis Shell Oil Company, Research Laboratory, P. 0. Box 262, Wood River, 111. 62095 SURFACE-ACTIVE AGENTS in aviation turbine fuel have long posed a problem in the delivery of high-quality fuel to airline terminals, Contaminants in parts per million and lower concentrations can cause serious operational difficulties in fuel distribution systems as well as in gas-turbine aircraft engines. Surfactants promote the entrainment of finely-dispersed water droplets and particulate matter in the fuel, which, if not removed, may lead to abrasion of fuel pumps and plugging of fuel screens in engines by particulate matter and ice. T o assure that satisfactory turbine fuel is delivered to aircraft, filtericoalescers are commonly employed as a final defense mechanism in fuel distribution systems. These filter/ coalescers are capable of functioning effectively over extended periods provided they encounter virtually surfactant-free fuel. In order to ensure their long-term effective performance, it has become common practice to install adsorbent beds ahead of filter/coalescers in fuel distribution systems to remove surface-active components. Eventually, however, adsorbent beds become deactivated and allow the passage of surfactants which ultimately results in the disarming of filter/ coalescers. Thus, a sensitive method for monitoring surfactant concentrations in turbine fuel is needed to determine when adsorbent life has been expended. The literature contains numerous references to chemical tests for surfactant detection. These tests, however, do not lend themselves to monitoring surfactant concentrations in turbine fuels for one or more of the following reasons : Many of the tests described in the literature are applicable only to aqueous systems. Test methods are generally limited to the detection of only a specific surfactant type (i.e., anionic, cationic, or nonionic species). Most of the tests lack the sensitivity for detection of surface-active agents in the very low parts-per-million range.
Several surfactant test methods, devised for specific application to aviation turbine fuels and responsive to various surfactant types, have been published. These methods are based on the responses to secondary effects induced in turbine fuel by surface-active agents. ASTM Method D1094-67 ( 1 ) determines the effect of water-miscible components in aviation turbine fuel on the appearance of the fuel-water interface. A gravimetric procedure, ASTM Method D2276-67T ( I ) , determines particulate contamination in turbine fuel, which relates to surfactant concentration. ASTM Method D255066T ( I ) , intended to simulate filter/coalescer performance on a small scale, determines the ease with which entrained or emulsified water is released from a fuel on passage through a coalescer-type water separator. Although the latter test provides a reliable measure of surfactant concentration, operation of the equipment is rather laborious and the sensitivity of the method is somewhat limited. The automatic Constant-Volume Drop Time (CVDT) monitor described in this paper measures the total contribution of all surfactant species which may be present in aviation turbine fuel and possesses the sensitivity required for detection of surface-active agents at extremely low concentration levels. The test is based on a modification of the drop-volume interfacial tension (IFT) method (2). In the latter method, water drops of varying size are dispensed to the tip of a capillary immersed in the nonaqueous phase and the IFT is calculated from the volume of the drop, the densities of the two phases, the radius of the capillary, an empirical correction factor, and known physical constants. In the CVDT test, constant-volume drops of aqueous 1N NaOH are delivered to the tip of (1) ASTM Standards, Part 17 (1968). (2) W. D. Harkins and E. C . Humphrey, J . Amer. Chem. SOC.,38, 228 (1916). ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
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