Table 1. Current and Transition Time for OH- Protonation in 02-free 3.96 x lO%i NaOH-0.5M Na~S04.Electrode area, 0.43 cm.2 iT1/2 ax.’
T,
i, pa. 477.6 287.1 239.1 205.0 143.4
sec.
(av.) 3.21 8.2 11.9 15.5 31.4
Range, amp. sec.1’2 sec. cm. mole-’ 0.12 0.3 0.3 0.7 0.3
502 483 484 474 472
With the same anodic current flowing as in Curve 1, the solution was stirred until the time marked with the arrow. The stabilization of the potential a t -0.1 volt during stirring and the normal chronopotentiogram without stirring indicate an electrode process for which the potential is controlled by the transport of a solute in the electrolyte to the electrode surface. Since there was considerably more hydrogen present in the electrode before Curve 2 was recorded, than before Curve 1, the inflection corresponding to its exhaustion a t the electrode surface was delayed beyond the end of the recording. The current-transition time relation for the first wave verifies its control by the diffusion of hydroxide ions. Table I lists values of 7 and i?/a.c. for five current densities.
Comparison of the observed values of i71’2/a.c. with a theoretical value presents a problem, since the diffusion coefficient of the hydroxide ion is a strong function of the concentration and identity of the salts present in solution (6). From the average value of iT1/2/a.c.J483, one may calculate a value for this diffusion coefficient, DOH,of 3.2 X 10-6 cm.2 set.-' By graphing the data of hIairanovsky and Polievktov (e), the author has found that DOHis a roughly linear function of the variable pl’z/(l p112), where p is the ionic strength, fitting within 3% the equation DoH(cm.2sec.-l) = 5.8 X 10-5-7.2 X 10-5 p112/(l p112) for KN03 solutions over a range of concentrations from 0.05 to 1.5M. None of Mairanovsky and Polievktov’s data were taken in sodium sulfate solutions; however, Cosijn (4) has published a value for DOHin 0.1-%! K2SO4of 4.188 X 10-5 cmS2sec.-I By arbitrarily assuming that DOHis a linear function of p112 (1 p112)in K2S04 solutions, and has the value 5.8 X 10-5 cm.2 sec-l a t infinite dilution, one may extrapolate linearly through Cosijn’s single value to arrive a t a rough guess for DOHin 0.5M KzS04. This value is 3.2 X cm.2 sec.-l That it agrees with the value calculated from the chronopotentiometric data for Na2S04is surely coincidence, but even a rough agreement is further evidence that the interpretation of the chronopotentiogram given here is correct.
The electrode system used in this study is not, in principle, dependent on the solvent,nor on any substance present in the electrolyte for its function. As such, it should find applications in the extension of voltammetric studies of acids and bases to hitherto inaccessible systems and solvents. Coulometric titration of a base in water and in anhydrous methanol has been accomplished in this laboratory with this type of electrode (9).
+
+
+
LITERATURE CITED
( 1 ) Blackburn, T. R., Ph.D.
Thesis, Harvard University, Cambridge, Mass.,
1962. (2) Blackburn, T. R., Greenberg, R. B.,
unpublished data, Wellesley College,
1965. (3) Blackburn, T. R., Lingane, J. J., J . Electroanal. Chem. 5, 216 (1963). (4) Cosijn, A. H. M., J . Electroanal. Chem. 2, 437 (1961). (5) Delahay, P., Vielstich, W., J . Am. Chem. SOC. 77, 4955 (1955). (6) Mairanovsky, S. G., Polievktov, M. K., Zh. Fiz. Khim. 37, 885 (1963). ( 7 ) Streuli, C. A., ANAL. CHEM. 28, 130 (1956). (8) Szebelledy, L., Somogyi, Z., 2. Anal. Chem. 112, 395 (1935).
THOMAS R. BLACKBURN Department of Chemistry Wellesley College W‘ellesley, Mass. 02181 RECEIVED for review December 23, 1965. Accepted January 27, 1966. Work supported by a National Science Foundation research grant (GP-3510).
Vapor Deposited Silver Bromide as an Ion Detector for Mass Spectrography SIR: In the field of mass spectrometric analysis of solids using a spark as the source of ions, the photographic plate is without a competitor. Such photographic plates are sensitive to integrated ion currents on the order of lo4ions per square millimeter but may be overloaded by a factor of 106 or more without seriously affecting their sensitivity. However, because of the nature of the emulsion used, several disadvantages are apparent: they are nonuniform in sensitivity, the emulsion rapidly deteriorates a t room temperature, the sensitivity is a function of ion mass and energy, and the plates are sensitive to damage by abrasion. Thompson (11),in 1921, realized that the penetration of ions into the emulsion was slight. He, therefore, used Schumann plates which have only sufficient gelatin to provide adhesion of the silver halide grains to the glass, as a detector 620
0
ANALYTICAL CHEMISTRY
for the ion beams from his mass spectrograph. With the advent and application of mass spectrographs employing hlattauch-Herzog geometry, and whose ion beams were produced by an RF spark source, it became necessary to have commercially available plates which were sensitive to ion energies between 5 and 25 k.e.v. Today, there are in use for this purpose five different plates: Eastman SWR, Word’s Q-I, Q-2, and Q-3, and the Agfa Schumann Plates (8). Rudloff (9) has investigated the absolute sensitivity of these plates and has also investigated an x-ray film, Agfa Fluorapid, sensitized with silver-activated ZnS. McCrea (6) has studied in detail the emulsion characteristics of SWR and Ilford Q plates. Owens (7) has reported on the response of Q-2 plates to ions of various masses whose energies varied between 1.88 and 15 k.e.v. It is the consensus of
these authors, as well as others in the field, that the variation of the sensitivity of the photoplate as a function of ion mass (-m-’/2) and ion energy ( N E ) is a consequence of the silver halide grains being embedded in a layer of gelatin. From this, we may conclude that a plate completely without gelatin (such as a continuous monolayer as in evaporated AgBr) would have a greater sensitivity to an ion beam; furthermore, this sensitivity should be independent, or nearly so, of ion mass and energy since energy exchange rather than penetration is necessary for exposure. EXPERIMENTAL
We have obtained (Technical Operations, Inc., South Ave., Burlington, Mass.) a small experimental batch of evaporated AgBr plates (1-4, 10) 2 X 10 x 0.040 inches. These plates were not uniform because they were pre-
385
"'}
387
0-2 EVAPORATED Ag Br
.9
10-l~
10-l~
10-l~ EXPOSURE, COULOMBS
Figure 1 is a plot of transmittance us. log exposure for the three plates. Figure 2 shows the same data plotted as
1 I I I I I
(f
log
I
I
,I
/
I 1 I
11,1
,
1 1 1
11111
1
, ,, I II
EXPOSURE, COULOMBS
Figure 2.
Seidel plot of Q-2 and evaporated AgBr plates Same data as Figure 1
Seidel functions; that is, the ordinate is
- 1). The parameter,
GI is
that 'introduded by McCrea to replace the conventional photographic y. It is the slope of the plate characteristic on the Seidel plot, defined as:
threshold sensitivity about one third that of the Q-2. The data shown in Figures 1 and 2 were obtained by the usual method of evaluating a spectrographic plate; that
Table 1.
Modified Mitchell Developer
Amt./ liter, grams -
Solution A
where T i s the transmittance E is the exposure From these two figures, it would appear that the Q-2 plate is easily ten times as sensitive as the evaporated AgBr plate for threshold exposures, but Figure 3 and Table I1 show the disparity between the two to be considerably less. The almost complete lack of background fog in the evaporated AgBr plate yields a very favorable signal-to-noise ratio which makes the
Table 11.
Plate RESULTS A N D DISCUSSION
1
10-'O
Figure 1 . Comparison of Q-2 and evaporated AgBr plates exposed to 20-k.e.v. Zn+ ions
pared in an apparatus designed for continuous coating of a flexible base. However, the initial results were encouraging and indicated the need for further investigation. The plates were loaded into an Associated Electrical Industries Model MS-7 mass spectrograph and exposures mere made after a short pumpdown. Pressures in the analyzer were 3 to 5 X 10-7 mm. Hg during a typical exposure. After exposure, the plates were removed from the mass spectrograph and developed in a modified Mitchell developer (f O), the composition of which is given in Table I. The plates were fixed with a 20% sodium thiosulfate solution, the pH of which was adjusted to 10 by the addition of sodium hydroxide. Two of the plates exposed, numbers 386 and 387, exhibited the highest sensitivity, differing slightly in latitude. Furthermore, there is clearly an increase in sensitivity from the edge to the center, probably due to the evaporation geometry. I t is the most sensitive portions of these two best plates which are compared with a typical Q-2 plate which was exposed during the same sequence of exposures. The Q-2 plate was developed for 3 minutes in Ilford ID-19 a t 20° C. and fixed for 10 seconds in Kodafix.
1
Q-2
Evaporated AgBr Clear glass
No plate T = 1.0
Component Elon 2 Hydroquinone 7.5 Sodium sulfite 78 ( anhyd. ) B Sodium sulfite 78 (anhyd.) 2 Potassium bromide C Purified calfskin 5 gelatin D Sodium thiosulfate 200 Mix equal proportions of solutions A, B, and C and add 1 part of solution D to 20 parts of the mixture.
Comparison of Plate Quality
Clear glass
T
= 1.0
Clearest area T = 1.0
0.75
0.83
0.86
0.91 0.915
0.995 1.0
0.995
...
Minimum Noise, r.m.s. transmittance 0.0075 0.03 0.0015
0.005
VOL 38, NO. 4, APRIL 1966
621
improved threshold sensitivity and uniformity. There is no doubt that every ion striking the evaporated film must react with an AgBr crystal. According to Mees (6), the latent image consists of as few as four atoms of free silver. Surely, a 20-k.e.v. ion can produce this many dissociations in the AgBr lattice. It must be emphasized, a t this time, that the data reported herein are of a very preliminary nature and that more definitive results will be forthcoming a t a later date when plates produced under more rigorous controls have been evaluated. LITERATURE CITED
Figure 3.
A.
Evaporated AgBr plate
Exposure ”%I+, 6.8
X 1 0-l4coulomb
E. Q-2 plate Exposure lZzSn+,3.1
X
lo-“
coulomb
(1) Goldberg, G. (To Technical Operations, Inc.), U. s. Patent 3,219,450 (Nov. 23, 1965). (2) Hartouni, E., Ibid., 3,219,452. (3) LuValle, J. E., Goldberg, G., Pack, J. G., Ibid., 3,219,448. (4) Ibid., 3,219,451. (5) McCrea, J. M., 12th Annual Conf.
on Mass Spectrometry and Related Topics, Montreal, Paper No. 92 (June
is, the value of transmittance equal to 1.0 was set on the clearest portion of the plate under evaluation. Other settings of the reference transmittance of the microphotometer reveal the differences between the two plates. The Q-2 characteristic is, therefore, asymptotic to 0.86 and 0.03, whereas the evaporated AgBr characteristic is asymptotic to 0.995 and 0.005. The close approximation to linearity of the latter characteristic (Figure 2) would tend to improve the analytical accuracy of these plates and to compensate, to some extent, their narrow latitude. It may be that the sensitivity of the evaporated AgBr plates is not energy nor mass dependent. Two plates were exposed to ions whose energy varied between 10 and 20 k.e.v., but, because of the apparent variation of the sen-
622
ANALYTICAL CHEMISTRY
sitivity of the film across the plate, the results were inconclusive. It did appear, however, that those exposures made near one edge a t 10 k.e.v., were just as effective as those near the opposite edge a t 20 k.e.v. On the basis of the exposure of this small batch of plates, it is apparent that this method of producing an ion sensitive plate deserves further consideration. Simply by specialization of the process for the mass spectrograph application, there would be made available a plate which is nearly as sensitive as presently available plates, has a much improved appearance, and because of its steeper characteristic, is capable of greater accuracy in analytical work. Furthermore, further research in this area of thin film technology should make possible plates of generally
7-12, 1964). (6) Mees, C. E. K., “The Theory of the Photographic Process,” p. 163, Macmillan, New York, 1952. (7) Owens, E. B., Appl. Spect. 16, 148 (1962). (8) Owens, E. B., 12th Annual Conf. on
Mass Spectrometry and Related Topics, Montreal, Paper No. 40 (June 7-12,
1964). (9) Rudloff, W., 2. Naturforsch 16a, 1263 (1961); 17a, 414 (1962). (10) Saxe, M. H., Hartouni, E. (To Tech-
nical Operations, Inc.), U. S. Patent 3,219,449 (Nov. 23, 1965). (11) Thompson, J. J., “Rays of Positive Electricity,” 2nd ed., p. 4, Longmans Green and Co., London, 1921. MAYNARD H. HUNT Air Force Cambridge Research Laboratories Office of Aerospace Research Bedford, Mass. RECEIVED for review December 1, 1965. Accepted February 23, 1966.
