inder was used to pass nitrogen over or through the solution. Prepurified nitrogen scrubbed with chromous chloride and then with water was used for deaeration and also for protection of the titrant in the reservoir. Magnetic stirring was employed during deaeration and titration. A coiled platinum wire indicator electrode and a commercial saturated calomel reference electrode were used. Stock solutions of osmium(VII1) were prepared by dissolving weighed amounts of the reagent grade tetroxide in cold water and diluting to known volume. All other chemicals were reagent grade. Titration Procedure. The aliquots of osmium solution titrated contained 0.05 to 0.5 mmoles of osmium(VII1). Each aliquot was added to a mixture of 5 ml of ethanol with 25 ml of 2.5F NaOH in a 250-ml beaker and allowed to stand at room temperature for 20 minutes. HCI and water were then added to give a final total of 100 ml of solution containing the desired concentration of acid. The beaker was then stoppered, the solution deaerated, and the sample titrated.
Table I. Analyses of Os04 Stock Solutions OsO,, pmoles Relative HCI, F Taken Found Mean error, 5.0 5.0 5.0 5.0 5.0 5.0 0.5 0.5 9.0 5.0 5.0 5.0 2.0 5.0 5.0 a
RESULTS AND DISCUSSION Three inflection points are obtained. The first corresponds to the reduction of osmium(V1) to osmium(1V); the second is ill-defined ; the third corresponds to the complete reduction of the osmium to the +3 state and occurs at -0.11 =t0.01 V us. SCE in 5.OF HC1. The occurrence of two inflection points during the reduction of osmium(1V) to the +3 state is probably due to the presence of two osmium(1V) species in solution which are in slow equilibrium (11). The third end point was
86.34 86.34 86.34 92.10 92.10 92.10 160.6 160.6 163.8 163.8 163.8 163.8 163.8 244.1 328.6
86.44 85.92 85.92 92.30 91.38 91.38 159.8 162.3 162.7 164.1 164.6 164.6 164.6 245.6 326.3
Relative standard deviation
=
86.09
-0.29
91.69
-0.45
161.1
+0.3
164.1"
+0.2
...
...
... ...
0.50
used to obtain the results shown in Table I. The method is accurate and precise to about * O S % and is reliable over a relatively wide range of HCl concentrations. RECEIVED for review August 7,1967. Accepted September 22, 1967.
Mechanism of the Determilnation of Phosphorus with a Flame Ionization Detector F. M. Page and D. E. Woolley The University of Aston in Birmingham, Birmingham 4 , England KARMENAND GIUFFRIDA (1-3) have shown that a conventional flame ionization detector shows a specific enhancement of response to phosphorus- or halogen-containing compounds in the presence of the vapor of a sodium salt. Originally this was provided by introducing a gauze covered with sodium hydroxide into the flame, but later developments showed that an equal response resulted if a sodium salt was fused onto the electrode system (3) or if the burner tip was embedded in a ceramic tube ( 4 ) containing the sodium salt. It has been claimed that the conductivity of the flame produced by the sodium salt as it volatilizes is increased when a phosphorus-containing compound is eluted from a gas chromatograph column, and that this increase is a function of the amount of phosphorus (or halogen) in the eluate (2). It is also claimed that quantities of the order of lO-*O gram of phosphorus (corresponding to a partial pressure of lo-* atm in the flame gases) may be detected by this technique, and commercial detectors are now on the market, using cesium bromide instead of the sodium hydroxide or sulfate. The original discoverers ( I ) attributed the effect to an increased amount of sodium entering the flame because of the effect of A. Karmen, ANAL.CHEM.,36, 1416 (1964). A. Karmen and L. Giuffrida,Nature, 201, 1204 (1964). L. Giuffrida,J . Assoc. Offic. Anal. Chemists, 47,293 (1964). D. R. Coahran, Western ACS Conference, Covallis, Ore., June 1965.
(1) (2) (3) (4)
210
ANALYTICAL CHEMISTRY
phosphorus on the volatility of the sodium salt, but it has been shown by flame photometry ( 5 ) that there was no increase in the neutral alkali atom population in the flame; a further explanation had to be sought. We ourselves have been able to show that this effect can be demonstrated in a completely homogenous system, so that it is of a fundamental nature. EXPERIMENTAL
A series of premixed flames, of known Hz/N2/O2 ratios, were selected and their temperatures measured by the sodium Dline reversal technique. They were burned on a multitubular burner based on that described by Padley and Sugden (6). All gas supplies were metered by rotameter tube and float meters and the preburning composition of both inner and outer flames was the same. The electron concentration was measured by a microwave cavity originally described by Horsfield (7) and by Padley and Sugden (6). Cesium was added to the inner flame by diverting a portion of the nitrogen supply through a scent-spray type of atomizer (5) W. A. Ave, G . W. Gehrke, R. C. Tindle, C. D. Rayle, D L. Stalling, and S. R. Koirtyohaan, 5th National Meeting, Applied Spectroscopy, Chicago, Ill., June 1966. (6) P. J. Padlev and T. M. Sugden, . 8th Combustion Symposium, p. 164, 1960.(7) A. Horsfield, Ph.D. Thesis, Cambridge, 1957. .
I
L 0
I
2.0
I
4.0
1
6.0
I
8.0
I
10.0X I O - 4
partial pressure of phosphorus [ a h ]
Figure 1. Effects of added phosphorus on the relative electron concentrations in a flame containing cesium 0, flame Hz/O2 = 5/1, temp. 1800" K e, flame H2/02= 5/1, temp. 2000" K A, flame H2/O2= 4/1, temp. 2000" K H, flame H2/02 = 3/1,temp. 2000" K
Observations made in burnt gases, 2 cm from reaction zone. which delivered enough cesium nitrate solution to give approximately 3 x 10-6 atm of cesium atoms in the flame gases. Phosphorus was added by passing a further portion of the nitrogen supply through a thermostated saturator containing dimethyl phosphite. The pipe connecting the saturator to the burner was heated to prevent condensation of the saturated vapor. RESULTS
The results are shown in self-explanatory form in Figures 1 and 2. The electron concentration, relative to the flame without added phosphorus, at first increases and subsequently falls off until, at relatively high atm) partial pressures it has fallen to about half the original value. The greatest atm partial pressure increase is found at approximately of phosphorus, and the relative increase at maximum increases as the temperature of the flame is lowered and also as the measurements are made lower in the flame-Le., nearer to the reaction zone. There was no measurable response to the addition of dimethyl phosphite in the absence of the cesium nitrate spray. DISCUSSION
If a mechanism is to be acceptable as an explanation of the observed increase in ionization, it must be shown that previous explanations are not. The suggestion that there is an increase in neutral alkali metal in the flame implies that there is some reservoir which can be tapped by the addition of phosphorus or halogen. This reservoir must be independent of the presence of solid phase, as the effect is observable in the present experiments.
0
2 .o
4.0
6.0
8,O
10.0~10-~
partial pressure of phosphorus [atml
Figure 2. Effect of height in flame H2/Oz = 5/1, temp. 1800' K. Height of observation above the reaction zone; 0, 1 cm; e, 2 cm; A, 3 cm; W, 4.5 cm
There is a very considerable body of evidence, too large to review succinctly, based on photometric and ionization studies of premixed hydrogen flames doped with alkali salts which indicates that virtually all the cesium added to such a flame is present either as neutral atoms or as gaseous cesium hydroxide and that these are in local equilibrium with the flame gases, with each other, and with excited cesium atoms and cesium ions. This general assertion does not preclude small departure from equilibrium values, but nowhere is there a reservoir of uncommitted cesium sufficient to produce the twofold increase in ionization which is observed. The only possible source would be the gaseous cesium hydroxide but as this is in equilibrium with cesium atoms, it could be effective only if a disequilibration were induced. The possible actions of phosphorus or halogen under the conditions of experiment are induction of disequilibrium between Cs and CsOH and alteration of the local conditionsi.e., in the known disequilibrium in the flame gases. This paper suggests that the latter is the cause of the increased ionization, as the former could only operate if a reaction capable of converting CsOH to Cs at a faster rate than CSOH
+H
+
CS
+ HzO
could be induced, which is most improbable as the reaction itself can produce equilibrium rapidly. The consequences of alteration of the flame gases will be examined. Under the conditions of experiment, it is known that the inner flame constitutes a high temperature reaction vessel, without turbulence, where the temperature and major constituents are sensibly constant over ten centimeters height in the flame, but where the concentrations of the minor constituents, notably hydrogen atoms, vary markedly but regularly. The radical and atom concentrations are balanced VOL 40, NO. 1, JANUARY 1968
21 1
at the equilibrium ratios-Le., at the ratios defined by the equilibrium which would obtain if the flame were an isolated system-through fast, reversible reactions such as H
+ H2O
Hz
+ OH
Their absolute level is, however, governed by the slow threebody recombination process H+H+X+Hz+
Y
This recombination is proceeding throughout the observable flame, and a disequilibrium parameter, y, may be defined such that (H)_ _ (Hed The fact that the increased level of ionization produced by phosphorus can be demonstrated in a system which is essentially free from solid phases, from concentration or temperature gradients, from turbulence, and from indrawn air rules out any explanation based on these factors. A similar effect has been reported before, in an equally uniform system. Page (8) described an increase in ionization which occurred when certain halogens were added to flames doped with certain alkali metals. The phenomenon appeared to be rather specific. Later, Padley, Page, and Sugden (9) gave a detailed explanation of the phenomenon for the particular limiting case of sodium and chlorine, and outlined the slight modifications appropriate to alkali metals with more stable hydroxides or halogens with less stable hydrides. Their theory is as follows. An alkali metal may ionize by either of two processes (A or B) X
+A
-+
A+
+e+X
A+2H-,A++Hz+e
(A) (B)
Reaction (A) involves a higher activation energy than (B) but is a two-body rather than a three-body process and in fact is the dominant process under ordinary conditions. At equilibrium, both processes must lead to the same degree of ionization, but in a flame where there is hydrogen atom disequilibrium, reaction B will lead to a steady state where the degree of ionization is higher by a factor of y z (y being the disequilibrium parameter defined above) than for reaction A. If, in the presence of an additive, the rate of reaction B can be steadily increased, in an amount proportional to the additive, the degree of ionization will steadily increase from 1 to y2. Such steady increase can be effected through the mediation of the reaction A B + A+ B(C)
+
+
which, combined with the fast balanced reactions
+H H +B - S
HB
yields A
+2H+
+B HB + e A+ + H2 + e Hz
The detailed analysis of the problem is made more complex, even for the halogens, by the reduction in electron concentration caused by the removal of alkali metal as undissociated salt and the removal of electrons as halide ions, which effects are superimposed on the disequilibrium in(8) F. M. Page, Ph.D. Thesis, Cambridge, 1955. (9) P. J. Padley, F. M. Page, and T. M. Sugden. Trans. Faraday Soc., 57, 1552 (1961).
212
ANALYTICAL CHEMISTRY
crease in ionization, and in the presence of massive quantities of halogen will inevitably cause an overall reduction in the electron concentration. Phosphorus. The effects of added phosphorus are even harder to determine quantitatively. Phosphorus, in a flame, can exist in many forms. Flame bands corresponding to PH, PO, and PH2, together with continuous emission corresponding to PzOj and carbon particles have been observed when massive quantities of dimethyl phosphite are added to a flame similar to those described here (10, 11). It would appear that PH2 might have a high enough electron affinity (12) to facilitate reaction C, but by analogy with NOz- (13), POz- would be expected to be very stzble, possibly stable enough to be effective, despite the low equilibrium concentration of POz which would be expected. In the absence of detailed knowledge of the distribution of phosphorus over the various species, and of the stabilities of the various species concerned, it is pointless to speculate as to the exact entity involved. There are several possible species, and none are likely to involve more than one phosphorus atom, SO that the effect can occur, and will be linear in added phosphorus. CONCLUSION Even though it is not possible to identify exactly the species responsible, it is abundantly apparent that this mechanism fits all the observed features of the increased ionization found in homogenous systems. It is possible, knowing the general levels of hydrogen atom concentrations found by Bulewicz, James, and Sugden (14) to predict certain features of the dependence of the effect on various flame parameters: The effect will be small at temperatures above 2200" K, but can be very large indeed around 1600" K. The disequilibrium parameter can amount to over 30 at these temperatures (14) so that y 2 might be 103. At a given temperature, the effect will increase somewhat in more hydrogen rich flames. The effect will increase markedly at earlier points in the flame, being a maximum near the reaction zone. It is not possible, of course, to translate these predictions into terms immediately applicable to a standard flame detector, which is essentially a turbulent diffusion flame, supported solely by indrawn air, and with the reaction zone dispersed throughout the flame. Any engineering feature which favors an increase in the hydrogen atom disequilibrium will help to increase the effect. One such feature is the use of a larger burner port. This, by reducing the linear velocity of the gas, will reduce turbulence and hence spread the reaction zone further through the flame. An increased ratio of hydrogen to nitrogen would also help, and blanketing the flame with diluted air (air with added nitrogen) would tend to reduce the flame temperature and again increase the hydrogen atom disequilibrium. ACKNOWLEDGMENT
Our thanks are due to the research staff of Varian Aerograph for calling our attention to this detector and its associated problems. RECEIVED for review March 27,1967. Accepted September 11, 1967. (10) J. Guest, Dip. Tech. Thesis, Aston, 1961. (11) R. Miller, Dip. Tech. Thesis, Aston, 1963. (12) A. F. Gaines and F. M. Page, Trans. Faraday Soc., 62, 3086 (1966). (I 3) A. L. Farragher, F. M. Page, and R. C . Wheeler, Discussions Faraday Soc., 37, 203 (1964). (14) E. M. Bulewicz, C. G. James, and T. M. Sugden, Proc. Royal Soc., (London) 235, 89 (1956).
