Second, the sensitivity of the method in this particular example is greatest over the range 0-5% acetone, and it is possible to determine 1 acetone in chloroform to + 0.05% with the unresolved 8 p band. Third, whereas the peak ratio for pure chloroform is close to 1.00, the ratio for pure acetone is much lower due to the asymmetrical shape of the acetone peak. It should also be noted that, although a perfectly symmetrical peak will give a peak ratio of 1.00 (assuming negligible distortion due to instrumental factors), the reverse does not necessarily apply. A second example is shown in Figure 4. This is the analysis of adiponitrile in benzonitrile with only the unresolved 4 ji CN bands (separation 0 . 0 6 ~ ) Because . the derivatives are obtained by operating the instrument in the single-beam mode, some interference may occur in this particular case due to overlap of the CN and atmospheric carbon dioxide bands. Provided,
however, that the carbon dioxide concentration remains constant, its only effect will be to modify the shape of the peakratio us. concentration curve. (Alternatively, of course, the instrument could be purged to eliminate this interference.) Conversely, it is possible to modify the shape of these curves, and therefore the sensitivity of the method, by the addition of a known amount of interfering material. ACKNOWLEDGMENT
The author gratefully acknowledges the encouragement and assistance of Dr. J. H. Beynon and Mr. M. St.C. Flett, Research Department, Imperial Chemical Industries Ltd., Manchester, England, where this work was carried out.
RECEIVED for review November 18,1968. Accepted December 23, 1968. ~
Anion Determination with Ion Selective Electrodes Using Gran’s Plots Application to Fluoride Arnaldo Liberti and Marco Mascini Istituto Chimica Analitica, Unicersith di Roma, Rome, Italy WITHTHE RECENT introduction of ion selective electrodes, very sensitive analytical tools have become available for many ions. The construction and the operation of these electrodes is similar to glass pH electrodes and as they are both indicators of single ion activities in aqueous solution should be valuable for a large number of physico-chemical investigations. Since the evaluation of concentration is frequently of more interest than activity, a number of procedures have been suggested to express the millivolts response in terms of concentrations. Though the described procedure can be extended to most electrodes, the present experimental work and discussion refer to a fluoride ion electrode which has been extensively used by various authors for fluorine determinations in bone ( I ) , tungsten ( 2 ) , chromium plating baths (3), urine (4), enamel of teeth (3, saliva ( 6 ) , and others (7-8). The procedures which have been suggested can be grouped as follows: preparation of fluoride solution at an ionic strength close to the samples under investigation to build up a reference calibration curve for the indicator electrodes; dilution of the samples 1 : 1 with a solution of high ionic strength [total ion strength adjustment buffer, TISAB (7)] and use of a reference calibration curve; titration of the fluoride ion with thorium or lanthanum (8-9); the use of linear null-point potentiometry (IO).
The titration procedure has been recommended by Lingane (8) to obtain higher precision but a number of limitations have been noted: the equivalence point is not the point of maximal slope, but has to be determined in careful titrations of precisely known amounts of fluoride in the particular medium used; the titrations are quite time consuming as equilibria are rather slow to be obtained, namely, near the equivalence point; thorium and lanthanum salts are not primary standard and require standardization. The procedure described in this paper makes use of Gran’s plots (11-12) ; this method, permitting the weighing of several data points is more accurate than a single point direct potentiometry measurement and yields satisfactory results for a number of applications.
(1) L. Singer and W. D. Armstrong, ANAL.CHEM.,40, 613 (1968). (2) B. A. Raby and W. E. Sunderland, ibid., 39, 1304 (1967). (3) M. S. Frant, Plating, 54, 702 (1967). (4) L. Singer, W. D. Armstrong, and J. J. Vogel, Abstract, 45th General Meeting of Inter. Assoc. Dental Research, p 77, March 1967. (5) B. Richardson and H. G. McCann, ibid., p 77. (6) P. Gron, F. Brudevold, and H. G. McCann, ibid., p 79. (7) M. S. Frant and J. W. Ross Jr., ANALCHEM.,40, 1169 (1968). (8) J. J. Lingane, ibid., 39, 881 (1967); 40, 935 (1968). (9) T. S . Light, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1967. (10) R . Durst, ANAL.CHEM.,40, 931 (1968).
where E, E,, and E, are, respectively, the equilibrium potential, the normal potential, and the liquid junction potential. COVo and CV are, respectively, the concentration and volume of the sample solution, and of the standard solution; y i s the activity coefficient of the fluoride ion. From Equation 1 is obtained:
676
ANALYTICAL CHEMISTRY
THEORY
To a sample solution in which a fluoride and a reference electrode are immersed, known amounts of standard fluoride solutions are added and the potential difference is measured. The potential of the indicator electrode according to the Nernst equation is: 2.3RT (COP‘, C V ) 2.3RT E=E,-log Y 4 F log v, F
+ +v
+
(1)
( v O + v ) X 1 O - E & - ? = 1 0 - ( E O + *.3RT E ) L x y x (CJ,
+ cv) (2)
~
~~
(11) G. Gran, Analyst, 77, 661 (1952). (12) F. J. Rossotti and H. Rossotti, J. Chem. Educ., 42, 375 (1965).
Figure 1. Determination of fluoride of 20 ml of 5.5 X lO-5M fluoride and 1M NaN03 Fluoride standard solution added was 10-3M NaF and 1M NaNO,
V(ml) NaF
If the sample and the standard solutions are prepared in such a way that the ionic strength is kept quite high and almost constant, y and E, do not change appreciably, and by plotting
+
A
( V , V ) 10- E 6s. V a straight line is obtained which intercepts the abscissa for a V, value where C,V, = -We, (Figure 1). The initial fluoride concentrations is obtained from C, = CVJV,,. The determination of the unknown concentration of fluoride is thus carried out in a simple manner by adding a standard fluoride solution. EXPERIMENTAL
Apparatus and Reagents. A model 94-09 fluoride ion electrode (Orion Research Inc.) and a conventional fiber type calomel electrode Beckman 39170 were employed with the Beckman Research pH meter 1019. A polyethylene beaker was used as titration cell; solutions were magnetically stirred. The standard fluoride solutions were made from N a F reagent grade Carlo Erba. Procedure. A 20-ml sample 1 M in sodium nitrate with a fluoride concentration in the range 10-2-5.10-6M was taken. The standard solution 1M in sodium nitrate which had a fluoride concentration 10-100 greater than the sample under investigation, was added in 1 ml at a time up to 10 ml. The solution was stirred and the potential, which was attained rapidly, was recorded. The variation of potential from the beginning to the end of the addition was about 30-90 mV. In Table I some of the analytical results are collected; they are compared with the ones obtained by using the TISAB procedures. Figure 1 reports the Gran’s plot for the determination of 20-ml solution containing 20.7 pg of fluoride by addition of standard fluoride solution. Determination of Fluorides in Mineral and Waste Water. The determination of fluoride in mineral and waste water is of great importance. All procedures involve a preliminary distillation of fluorides and a colorimetric determination. The specificity of the fluoride electrode permits the satisfactory use of the above procedure. Table I1 shows some results obtained in the analysis of mineral waters where the fluoride concentration is usually in the microgram per milliliter range; the samples were 1M in NaN03 and a solution lO-3M in NaF and 1M in N a N 0 3 was added. This analysis has also been carried out with the TlSAB procedure. It is important to
Table I. Determination of Fluoride in Solution by the Gran’s Plot and the TISAB Procedure (Sample Solution, 20 ml) Sample taken Found, pg Error, % Gran’s Gran’s M !e plot TISAB plot TISAB 5.0 1.00 2.63 3.69 1.00 1.00 1.00
x
x 10-5 x 10-5 x
x 10-4 x 10-3
x
1.90 3.80 10.0 14.0 38.0 380 3800
1.97 3.88 9.9 13.7 38.0 382 3820
2.47 3.0 9.4 14.8 39.6 372 3720
-3.5 f2 -1 -2 0 +0.5 -0.5
+30 -21 -6 +6 $4 -2 -2
Table 11. Determination of Fluoride Content in Mineral Waters by Gran’s Plot and TISAB Procedure (20-mlSample Solution) Sample Gran’s plot, TISAB, M x 105 M x 105 A
B C
D
5.3 4.2 6.4 5.45
5.1 4.5 6.7 5.3
point out the additional information which can be obtained on the state of the fluoride ion in solution when ions which form strong complexes with fluoride such as Al3+ or FeS+ are present. This information may be quite useful when the fluoride ion is utilized for biological purposes. When all points of the Gran’s plot lie on a straight line, it means that the concentration of the binding ions is negligible or lower than the concentration of the fluoride: the intercept on the abscissa yields the concentration of free fluoride ion. If the concentration of the binding cations is larger than fluoride concentration, the first points, due to small additions of standard fluoride solution, d o not lie on a straight line. By drawing a line through the values corresponding to larger additions and by extrapolation on the volume axis, a negative fluoride concentration is obtained, which means the absence of free fluoride ions (Figure 2). To obtain the total fluoride concentration, it is necessary to add competing ions, as citrate ions, which may replace the VOL. 41, NO. 4, APRIL 1969
677
1.0 0.9 0.6 Figure 2. Gran’s plot relative to fluoride determination in the presence of aluminum ions All samples were 20-ml solutions, 1 ppm in fluoride and 1M NaNO,-. Standard fluoride solution was 10-3M NaF and 1M NaN03
0.7
:: 0.6
!-
LL
wIN 0.5
’
0.4 -3 + 043
(1) No A13+added (2) 0.10 ppm Al (3) 0.33 ppm Al (4) 1.0 ppm A1 (5) 10 ppm
0.2 0.1 5
0.0
0
1
2
3
4
5
6
7
8
9
10
V(m1) Na F 5 X lO-dM NaF, 0.1N in acetic acid, and sodium acetate are added and the emf is measured; 0.1 ppm fluoride can be measured with about 5 error. Titrations of Slightly Soluble Salts. Gran’s plot can be successfully applied to locate the end point in precipitation titrations, namely, when the equivalence point is affected by the solubility of the salt. In Figure 3 are reported the experimental graphs relative to the potentiometric titration of 20 ml of 10-2MCa2+ with NaF 0.1M. The usual plot, potential us. volume of fluoride added, does not permit accurate location of the equivalence point because of the high solubility of calcium fluoride; to obtain more reliable results, the addition of organic solvents to decrease the solubility of CaFz has been suggested (8). This limitation is overcome by Gran’s plot: the equivalence point is obtained by extrapolation through the points obtained when fluoride is present in excess, thus depressing the solubility effect.
fluoride from the complexes. In this case to 10 ml of sample are added 10 ml of a buffer complexing solution which is 2M in NaN03, 0.5M in CH3COOH, 0.5M in CH3COONa, and 0.1M in sodium citrate. The standard solution is 1M in NaN03, 0.25M in CH3COOH, 0.25M in CH3COONa, 0.05M in sodium citrate and lO+M in NaF; it is thus possible in a simple way to have information concerning the total and the free fluoride concentration. The former is determined by using Gran’s plot in the presence of the buffer complexing solution and the latter only with the neutral salt. Determination of Fluorides in Polluted Air. The procedure can be successfully applied for the determination of fluoride in a polluted air providing a sufficient fluoride amount is collected. Air is made to bubble into a standard impinger containing 0.1N NaOH. To 20 ml of the adsorbing solution are now added 2 ml of 2N acetic acid: the fluoride and reference electrode are inserted and the standard fluoride solution
- 70
- 60
->
Figure 3. Potentiometric titration of 20 ml of Ca2+ 0.01 with NaF 0.100M Upper curve is usual potentialvolume plot and the lower is Gran’s plot
50
5-40 W
2-30 VI
3-20 -1 0 0
10 20
30 40 3
4
5
6
7
V(mL) NaF 01 M
678
ANALYTICAL CHEMISTRY
8
9
10
In this case Equation 1 is modified to the following which is valid after the equivalence point: C(V - V,) 2.3RT E = E, - 2.3RT log logy E. (3) Vo F and thus :
+
+v
(Vo
+ v>10-"&
=
10-(Eo+Ej)&
x y x C ( V - V,) (4)
The extrapolation of the straight line described by the experimental points [ V,(V,+ V l O intercepts the abscissa for a value V = V,.
.&]
DISCUSSION
Fluoride concentration may be directly determined by adding a sodium fluoride solution and by using Gran's plot to locate the equivalence point. It is assumed that the liquid junction potential and activity coefficient of the fluoride ion do not change appreciably under the conditions of the experiment. If the solution contains only fluoride ions in the microgram per milliliter range, then the small additions of fluoride solution have little effect on these values. In more general cases, however, it is necessary to have the sample and the
standard fluoride solution at the same ionic strength; this goal is reached by making both solutions 1M in any salt such as NaC104 or N a N 0 3 or by using an acetate buffer. The procedure described is more accurate than the direct measurement of the electrode potential and of the other suggested procedures; it is carried out quite rapidly because no slow equilibria are involved when a fluoride solution is added. It can be used to measure fluoride concentration as low as lO-5-lO-6M which corresponds almost to the sensitivity of the fluoride electrode. In this range fluoride cannot be determined by titration with La3+ or Th4+, which are the most accurate and sensitive titration procedures. It is noteworthy to mention that the response of the membrane electrodes is more rapid when going from dilute to concentrated solutions than in the other direction. The discussions of this paper have been concerned with the fluoride electrode; they may be applied to any selective electrode providing a linear relationship exists between electrode response and logarithm of the concentration. RECEIVED for review October 9, 1968. Accepted December 16, 1968.
AIDS FOR ANALYTICAL CHEMISTS I
I
Qualitative Thin-Layer Chromatography of Some Irritants William D. Ludemann, Martin H. Stutz, and Samuel Sass Chemical Research Laboratory, Edgewood Arsenal, Md. 21010
THISREPORT describes a method developed for the qualitative analysis (and identification) of some irritant agents used at one time or another by military and law enforcement agencies. These agents include a-bromobenzyl cyanide (brombenzylcyanide, CA), o-chlorobenzalmalononitrile (CS), a-chloroacetophenone (CN), diphenylaminechloroarsine (DM), and diphenylcyanoarsine (DC). All are considered effective irritants. As there are no published methods for the identification of these compounds as a group of chemical agents, a need existed for such a method. This is especially true today with the increasing use of chemicals in combating crime, preventing personal attacks, and in controlling civil unrest, where one (or a combination) of these agents might be used for riot control. Also included here are qualitative thin-layer chromatographic procedures for the analysis of individual irritant samples which allow detection of the most common impurities associated with the compounds. EXPERIMENTAL Adsorbents and Equipment. Adsorbents used were silica gel-G (Merck-Brinkmann Instruments, Inc.) and acid alumina (Woelm-Alupharm Chemicals). The binder used with the acid alumina adsorbent was Ultracal30 (a modified calcium sulfate, United States Gypsum Corp.). Desaga thin-layer chromatographic apparatus, including a variable thickness applicator (Brinkmann Instruments, Inc.), was used to apply the adsorbents to standard (50 X 200 mm) glass plates in thicknesses of either 250 or 500 p . The chromatoplates were prepared in the usual manner (after Stahl). Slurry compositions used in the chromatoplate preparation are shown in Table I. Only the neutral silica gel-G plates were activated; these were heated at 105 to 110 "C for 30 minutes
and stored over silica gel (desiccant). All plates were allowed to equilibrate over silica gel overnight before use. Standard Irritants. Irritant samples were obtained from various internal agencies located at Edgewood Arsenal. Physical Properties of Irritants CA, brownish liquid, b.p. decomposes at 242 "C CS, white to cream solid, m.p. 93-95 "C CN, pale yellow crystals, m.p. 54-55 "C DM, yellow to green solid, m.p. 195 "C DC, white to pink solid, m.p. 30 "C Detection Reagents. o-Dianisidine (Eastman Organic Chemicals), Technical grade, 0.1 % solution in ethyl alcohol. 4-(4'-Nitrobenzy1)pyridine (K and K Laboratories), 5 % solution in acetone. Quinone (Eastman Organic Chemicals), White Label, 0.5 % solution in methyl alcohol. Sodium perchlorate, 5 solution in aqueous potassium acid phthalate (92.11 mg/100 ml of water), adjusted to pH 5.0 with 1N sodium hydroxide. Iodine, resublimed. m-Dinitrobenzene (Eastman Organic Chemicals), White Label, 1 % solution in ethyl alcohol. Sodium hydroxide (Baker), c.P.,20% aqueous solution. Cupric acetate (Baker), c.P.,0.1% solution in water containing one drop of acetic acid per 50 ml. PROCEDURES
Sample aliquots are applied to the plates by conventional means in amounts of 5 to 50 pg per spot on the 250-11 layers and 50 to 100 pg per spot on the 500-p layers. The chromatoplates are developed in solvent-saturated chambers (lined with filter paper) at a room temperature of 25 f 2 "C. On comVOL. 41, NO. 4, APRIL 1969
679
