in an effort to obtain uniform molecular geometry or constant geometric distribut ion.
and stimulating discussions during the course of this investigation.
ACKNOWLEDGMENT
(1) Alley, S. K., Jr., Scott, R. L., J . Chem. Eng. Data 8, 117 (1963). (2) Meyer, L. H., Gutowsky, H. S., J . Phys. Chem. 57, 481 (1953). (3) Pauling, L., “Sature of the Chemical Bond,” p. 58, Cornell University Press, Ithaca, S . Y., 1940.
(4) Pople, J. A., Schneider, W. G., Bernstein, H. J., “High-R&solution Nuclear Magnetic Resonance, pp. 87, 130, McGraw-Hill, New York, 1959. (5) Smith, T. S., Smith, E. A., J . Phys. Chem. 63, 1701 (1959).
LITERATURE CITED
The author thanks Verner Schomaker and E. €3. Whipple for helpful criticisms in the preliminary manuscript, and V. A. Yarborough for his continuing interest
RECEIVED for review January 28, 1964. Accepted March 25, 1964. Division of Analytical Chemistry, 144th Meeting, ACS, Los Angeles, Calif., April 1963.
Mass Spectra of Diary1 Sulfones SEYMOUR MEYERSON Research and Development Department, American Oil Co., Whiting, Ind. HARALD DREWS’ and ELLIS K. FIELDS Research Department, Amoco Chemicals Corp., Whiting, Ind.
b The mass spectra of 2 4 diaryl sulfones have been measured. These spectra are sensitive not only to differences in molecular weight but also to structural differences among isomers. Correlation with molecular structure furnishes a basis for deducing
A
EARLIER study of reactions of phenyl alkyl sulfones under electron impact (24) suggested that mass spectrometry might be applicable for determining diaryl sulfones. This tech’ Present address, Stauffer Chemical Co., Richmond. Calif.
structural features when pertinent reference spectra are not available, and also suggests decomposition paths underlying the spectra. Mass-spectral analysis of the product mixture obtained by sulfonating toluene has revealed all six possible isomeric sulfones.
N
Table 1.
Kone
2
2,5
218
232
246
100 0
100 0
100 0
100 0
100 0
48.6 24.5 56.8 10.1 45.4 143
2 56 2 21 6 97 6 45 67 4 55.6 1.48 112 124 4.81
2.34 33.0 63.0 7.03 31.6 98.4 35.6 37 5 53 4 1 33 3 75 10 5 23 4 27.4 9.60 85.7 166 4.44
40 5 75 7 2 97
1.49 13.2 46.4 36.4 57.2 119 168. 46 4 81 2 2 32
3.28 32.9 91.2 9.15 41.9 141 74.1 40 5 82 9 3 25
21 1
52 7
83 2
1.16
1.83
Ion
Mass
100 0 ArSOzAr ’ ArS02Ar’ less 0.04 15 CHI 0.17 17 OH 0.17 18 Hz0 6.49 64 SO? 14.5 SOZH 65 17.3 66 SO& 1 88 79 SOZCHJ 0 i2 80 SOtCH4 0 10 SOZCH~ 81 4 55 Ar ’ ArSOz Ar Ar ‘SO2 384 Ar ’0 ArSO Ar’SO ArO 3.10 Ar ’SO ArO Ar’O+ ArSO 190 Ar Ar’SO? Ar‘+ ArSO2 7.43 Sensitivity at mass Pb
P P less
Positions of methyl groups 2,2’ 2,3’ 2,4’ Molecular weight 246 246 2 46 Normal peaks
+
Partial Spectra of
3,3‘
3,4’
4,4’
246
246
246
100 0
100 0
100 0
+
+
+
+
+
+
+
0.97 14.7 52.4 15.8 56.9 97.7 12 1
22 S
Transition denoted ...
90 5
0.27 0.24 0.13 3.12 4.86 0.83 8.10 10 2 ‘ 15 7 1 55 210
11.7
19.0
305
0.17 0.15 0.28 3.19 3.90 0.83 5.67 6 73 11 7 2 57 323
la
Q
13.6
0.25 0.18 0.13 3.54 4.66 0.67 6.80 8 78 14 2 2 36
4.48
50.4
37.8
LI
212
199
209
126
131
128
a
4.06
4.06
3.79
8.62
9.38
9.35
Metastable peaks 1.03 1.42
1.98
...
...
...
0.56 0.82 0.95 0.64 1.05 ... ( P less 66) ( P less 66) -. ... ... ... 1.12 0.86 0.77 0.74 ... ( P less 81)+ 1.49 ... ... ... ... ... 0.94 0.70 P + .-,ArSO+ ... 0.23 P + -. Ar’SO+ ... 0.29 0.94 2.16 2.11 1.93 1.29 1.30 P -r ArO +d Ar ’-containing ion not distinguishable from corresponding Sr-containing ion. Expressed in scale divisions of peak height per 1% liquid volume, corrected to instrumental conditions at which sensitivity n-hexadecane at m/e 57 is 20.0. +
+
+
1294
ANALYTICAL CHEMISTRY
...
1.13 1.15 of
Diphenyl and 4,4‘-dimethyldiphenyl sulfones were Eastman products, recrystallized. Other sulfones were prepared by Friedel-Crafts reaction of a n arenesulfonyl chloride with a n arene (8, I S , 2 5 ) ; oxidation with 30% hydrogen peroxide in glacial acetic acid of the corresponding diaryl sulfide, which was prepared by the copper-catalyzed reaction of an arenemercaptide with a n iodoarene (5, l e g ) ; or reaction of a n arenesulfonic acid with a n arene (8, 60). All but four of the sulfones have been reported previously, and satisfactory agreement of melting points with literature values confirmed the assigned structures. The four new compounds are 2,5,2’trimethyl, 4-isopropy1, 4-isopropyl-4’methyl, and 2-isopropyldiphenyl sulfones. R,erpective melting points of the first three, prepared by Friedel-Crafts alkylation of an arenesulfonyl rhloride with an arene, are 86’, 96’) and 98’ C. The material taken as 2-isopropyldiphenyl sulfone was the by-product from the synthesis of 4-isopropyldiphenyl sulfone by reaction of cumenesulfonyl chloride with benzene in the presence of aluminum chloride (14, 16). Specifically, it comprised the mother liquor
nique has proved extre nely well suited for the analysis, by virtue of high sensitivity not only to differences in molecular weight but also to structural differences among isomers. Empirical correlations based on t i e spectra of an initially small group of model compounds effectively multiplied the available reference data, and were in turn tested and extended as new compounds were prepared. The correlittions are presented here in full. Ry revealing unexpected products from preparations of unsymmetrical diaryl sulfones, mass; spectrometry facilitated discovery of transsulfonation in the reaction of an arenesulfonic acid with another arene (7, 8 ) . l3y means of mass-spectral analysis, we have now detected and determined all six possible isomeric ditolyl sulfones in the product mixture obtaineli by sulfonating toluene. EXPERIMENTAL
Mass spectra were measured with 70-volt electrons on a modified (10) Consolidated Model 21- 103 instrument with the inlet system a t 250’ C.
residue after crystallization of the para isomer. The mass spectrum showed it to be isomeric with the crystalline product; intense peaks corresponding to loss of OH, Son, S02H, S02CH3, SOzCH4, and S02CH5,in the light of correlations for methylated diphenyl sulfones, suggested that it was the ortho isomer. Isomeric purity is uncertain.
RESULTS AND DISCUSSION
Spectra and Correlations. Table I shows partial spectra of diphenyl sulfone and 19 methylated derivatives; Table I1 shows partial spoctra of four diphenyl sulfones substituted with isopropyl or with. isopropyl and methyl groups. I n each spectrum the scale of relative intensities is defined b y assigning a value of 100.0 to the parent peak. N o corrections have been made for contributinns of naturally occurring heavy isotopeq. Correlations based on the data in Table I have proved useful analytically and suggest decomposition mechanisms that can, in turn, account for spectral features. The data in Table I1 help test
Methylated Diphenyl Sulfones, ArSOgAr’
2,496
2,4,4‘
2,5,2‘
2,5,4’
2,6,4’
260
260
260
260
100,o
100,o
100.0
2.44 43.0 212 22.0 90.2 145 105 139 109 7.56 16.2 13.2 36.2 16.2 65.4 247 2.40
1.75 21.7 78.1 6.78 42.0 161 51.1 31.6 70.8 0.90 2.41 17.4 45.9 39.0 5.39 52.4 181 2.86
4.51 2.56
3.38
2.93 0.38 1.13
...
.____
3,4,4’
2,4,2’,4’
2,5,2‘,5’
2,5,3’,4’
260
260 Nornial p e a k
2 74
274
274
274
302
100.0
100.0
100.0
100 0
100.0
100.0
100.0
100.0
46.2 22.2 30.9 9.43 31.9 108 84.8 27.3 59.9 1.60 2.49 11.7 11.1 20.5 4.75 66.9 105 4.28
2.93 39.7 60.9 5.15 28.0 109 45.9 25.6 55.4 0.77 2.14 14.0 52.5 18.9 6.17 50.6 136 3.73
6.52 0.20 41.9 95.6 0.14 17.8 432 0.30 45.6 31.8 3 .03 7.79 112 2.98 38.0 311. 0.75 132 287 6.37 99.8 153 4.32 25.7 214 4.07 69.9 3.79 0.66 2.16 8.35 7.89 28.5 132 22.6 70.8 136 a 48.6 57.3 33.0 LI 14.8 7.20 132 31.5 105 389 63.1 1.37 9.64 4.23 Metastable peaks
47.5 17.5 21.8 9.76 31.4 105 93.8 23.9 59.4 1.90
2.35 24.7 35.9 10.0 28.8 94. I 117 21.8 52.9 1.56
2.86
1.31
1.74
5.30
...
2.26
1.54
1.13
3.03
. .
1.80
1.43 ...
2.64 2.08
3.41 4.70
,..
3.46 ...
...
...
...
...
...
(I
...
...
1.29
...
0.21
n
s
20.1
52.9
0
(1
15.0
20.0
108
3,4,3’,4’ 2,4,6,2’,4’,6
0.23 0.46 1.53 3.87 4.44 4.89 10.1
5.20 4.80 1.12
125 18.7 31.0 67.9 81.8
67.9 570 67.9 155 12.9
D
300
0
38.2
n
41.3
(I
a
77.1
67.0
a
119 d
591 (I
4.38
5.02
8.81
1.54
1.47
1.07
1.49
...
0.89
1.10
0.95
1.18
...
1.96
1.25
0.19 1.12
...
1.27
... 0
...
1.19
... 0
...
... D
...
3.10
0
0.16
2.38
Not, resolved. KO metastable peaks detected for transition P* + Ar’O+ (Ar’ # Ar).
VOL. 36, NO. 7, JUNE 1964
0
1295
the correlations and throw some further light on decomposition mechanisms. SOMEGENERALFEATURES. All the diaryl sulfones studied give intense parent peaks. Determination of molecular weight is thus straightforward, and sulfones differing in molecular weight from the major component of a mixture are easily detected even at concentrations below 1%. Peaks a t masses one and two units greater than the molecular weight are all accounted for by ions containing heavy isotopes in natural abundance ( 3 ) . Thus, diaryl sulfone spectra measured a t the low pressures ordinarily used in analytical mass spectrometry, unlike those of the phenyl alkyl sulfones (24) and aliphatic sulfones (26) that have been reported, appear to arise solely from unimolecular processes. The masses of the aryl groups can be ascertained from those of the ArSO+ and ArO+ ions. Peaks due to these ions stand out prominently in the respective mass series 125 14n and 93 14n, where n is the number of side-chain carbon atoms. Scission of the hrSOz-hr bond is not an important reaction of diaryl sulfones under electron impact. Peaks due to ArSOZ+ions are of low intensity; peaks due to the analogous ArCO+ ions, in contrast, are among the most prominent in the spectra of benzophenones (17). The sulfones all give intense peaks at the masses of the original aryl groups. But scarcity of the complementaiy .4rS02+ suggests that even the aryl ions are not primary decomposition products. NUMBER AND D I S T R I B U T I O OF ~~ ORTHOSUBSTITUENTS. The spectra in Table I permit sharp differentiation among species with ortho substitution (,4)in neither ring, (B) in one ring only, and (C) in both rings. Further, those members of groups B and C in which one ring is substituted in both ortho positions can be distinguished. That such differentiation is possible implies that ortho substituents play an important role in ionic decomposition of diaryl sulfones. Prominent spectral features- both normal and metastable peaks (23)define three major reaction paths:
+
Table II.
2
... 260 Ma88 P P less 15 17 18 64 65 66 79 80 81 Ar Ar Ar ’0 ArO Ar ’SO ArSO Ar’S02 Arson Sensitivity
+
Ar+
- CiH4
ANALYTICAL CHEMISTRY
129 37.7 0.16 16.7 93.0 1.51 39.5 17.1 9.77 1.86 13.5 37.2 158 27.9 3.49 74.4 205 4.20
so2
SOzH S02H2 SOzCHa SOzCHI SOzCH6 ArS02 +
Ar’S02 ArSO Ar ’SO ArO + Ar’O Ar Art+ a t mam Pc +
+
+
+
+
0.44 ( P less l5)+ ArSO+ ... Ar‘SO+ ... (ArSO less 28)+ 0.70 2,5-dimethylphenyl > 2,6-dimethylphenyl > 3,4-dimethylphenyl.
ISOPROPYL-SUBSTITUTED DIARYL SULFONES.
+
VOL. 36, NO. 7 , JUNE 1964
0
1297
Table 111.
ArSO+:Ar’O+ Intensity Ratios in ArSOZAr’ Spectra
Positions of methyl groups in:
A. Ar
= Ar’
Ar
Ar ’
None 3
Xone
4 3,4 2
3 4 3,4 2
B. Ar constant, Ar‘ varied
None
2 275 4 2,5 324 2,6 2,4 2 4 2,s
None 374 2,4
4 3,4 4 2,4
2,4,6
Xone
None
None
2,4,6
C. Ar varied, Ar’ constant
2 2,5 2,4,6 2,5 2
n’one
374 4 294 215 2,6 4 2,5
2,s
None
2 3,4 4 4 2,4
3,4
None
2,496
2,4,6
2,4
Intensity ratio 124 47 8 7 1 1 0 0 124 1 0 0 4 1 0 8 2 2 1 1 2 2 1 1 18 7 3 0 1 0
5 3 6 3 68 32 2 85 36 4 6 54 5 8 4 5 2 5 3 3 1 3 2 68 0 32
124 4 4 1 1 1 0 2 5 1 6 1 2 18 8 5 3 2 2 3 1 9 2 8 1 3 0 85 0 54 7 3 2 4 1 2 0 68 0.36 0.32
aryl group better able to function as an electron donor should preferentially migrate and be incorporated in the aryloxy ion or radical. The data for each individual sulfone in Table IV are in accord with this expectation, and therefore tend to support the suggested interpretation. On the other hand, intercomparison of the data for sulfones containing one aryl group in common cannot be so correlated with electrondonating character of the second aryl group. The difficulty may arise from an oversimplified treatment of the data. In the correlation attempted here, relative intensities of selected ions are used as first approximations to relative probabilities of their formation (28). Actually, relative intensity measures the number of ions formed minus the number decomposed further in transit. Thus, differences in the extent of further decomposition of ions can invalidate the approximation and thereby blur underlying relationships. Possibly, steric effects are also involved. The ArSO+:A\r’O+intensity ratio is taken as a measure of the relative probabilities of the complementary processes leading to *irSO+ -1r’O Ar’O-. Such relative and hrSO probabilities presumably reflect the abilities of the ArSO and Ar’O radicals to tolerate a charge-that is, their ionization potentials (28, Sf). .A ratio far in excess of unity implies that the ionization potential of the arylsulfoxy radical is substantially lower than that of the aryloxy; a value far less than unity implies the reverse. Thus, the trend of intensity ratios for series in which Ar is held constant and Ar‘ varied implies that the effectiveness of methyl substituents for lowering the ionization potential of aryloxy radicals varies in the order meta < ortho < para. This order seems reasonable and can be rationalized in terms of resonance and inductive effects. The same order of effectiveness of meta and para substitution was inferred from appearance potentials of isomeric aryloxy ions derived from substituted anisoles (SO); such data are not available for orthosubstituted species. The anomalously low CsH9SO : C6H50+intensity ratio in the spectrum of 2,5-dimethyldiphenyl sulfone may well be caused by a contribution t o apparent C6H50 yield by loss of CO from (CH&CsH30+. Normal and metastable peaks in mass spectra of anisole and p-phenylanisole @ I ) , in which formation and subsequent decomposition of aryloxy ions comprise the predominant reaction sequence, furnish evidence for loss of CO following primary loss of CH,. Although relevant data are scant, this reaction, which resembles the highly favored loss of CO from quinones and aromatic cyclic ketones ( 4 ) , seems to be char-
+
+
+
+
Table IV. (ArSO+ Ar’O+): (Ar’SO+ f ArO+) Intensity Ratios in Spectra of Unsymmetrical Sulfones, ArSOsAr
’
Positions of methyl groups in: Ar 3, 4
2, 4, 6 2, 5 2, 5 2, 6 2, 5 2, 4 2
129 8 *
Art 4
None 2
None 4 4 4
None
Intensity ratio 0.72 0.66 0.52 0 40 0 36 0 28 0 27 0 06
ANALYTICAL CHEMISTRY
account also for prominent peaks in the spectra of methyl vinyl and methyl ethyl sulfones (26). This reaction sequence accounts fairly well for the effects shown in Tables I11 and IV in terms of charge distribution within the molecule and between complementary decomposition products. Specifically, migration of an aryl group from the sulfur to an oxygen atom suggests that the electron deficiency is largely localized in a nonbonding oxygen orbital. Rearrangement would result from attack of the electron-deficient center on the electron-rich aryl group. In an unsymmetrical sulfone, then, the
+
~~
acteristic of aryloxy iclns. I t parallels the loss of CO in the thc,rnial decomposition of the phenoxy radcal (12). Absence of a consistent trend of A\rSO+:Alr’O+ intensity ratio with number and position of substituents in series in which h r ’ is held constant and .Ir is varied suggests that the extent of further decomposition of arylsulfoxy ions varies more with structure than that of aryloxy ions. In symmetrical sulfones, ortho or para substitution is, again, more effective than meta in reducing the .lrSOC:A\rO+ intensity ratio, although the ortho-para order is here reversed. In so far as this ratio depends on t’he difference between the ionization potentials of the corresponding radicals, su xtitut’ion effects seem to be more prono inced in aryloxy than in arylsulfoxy rad: cals. Structural effects on further decomposition of arylsulfoxy ions may be responsible for the ortho-para reversal. PATH11. Loss OF OH AND H20, FOLLOWED BY Loss OF SO. Loss of SO following primary loss of HzO is established by metastable peaks in the spectra of all ortho-methylated sulfones studied. hIetastable peaks were not’ found for loss of SO following pri n a r y loss of OH. However, high intensities of normal peaks a t masses correiiponding to loss of SOzH strongly suggest such a sequence. A h y prcposed reaction scheme must specify t i e source of the OH and HzO hydrogen atoms, and must provide for the makin,I,, hleyerson, S., in "The Mass Spectrometry of Organic Ions," F. LV. llcT,afferty, ed., p . 153j Academic Press, S e w York, 1963. (12) Harrison, A . G., Honnen, L. R., Dauben, H. J., Lossing, F. P., J . A m . Chern. Soc. 82, 5,593 (1960). (13) Holt, G., Pagdin, B., J . Chern. Soc. 1960, 2508. (14) Huntress, E. H., Autenrieth, J. S., J . A m . Chem. Soc. 63, 3446 (1941).
(15) Huntress, E. H., Carten, F. H., / b i d . , 62, 511 (1940). (16) Lansbury, P. T., Colson, J. G., Chern. f n d . ( / , o n d o n ) , 1962, 821. ( 1 7 ) lIcI,afferty, F. W., in "Determina-
tion of Organic Structures hv Physical Methods," F. C. Sachod aiid LV. I>. Phillips, eds., Vol. 2 , p. 03, hcadeniic Press, S e w York, 1862. (18) llcl,affertv, F. W., Gohlke, R. S., A N A L .CHEM." 31, 2Oi6 (1959). (19) llauthner, F., Her. 39, 35!)5 (1'306). ( 2 0 ) Lleyer, H., .Inn. 433, 327 (1023). (21) hleyerson, S., unpublished spectra. ( 2 2 ) Rleyerson, S., Ihews, H. J., Fields, E. K., to be presented 'at the Twelfth Annual Conference on Alass Spectrometry, ASTJI Committee E-14 llontreal, Canada, June 1964. (23) Lleyerson, S., ?*IcColluni, J. I)., A d o a n . .-lnal.
Chenz. Instrittnentation
17'3 (1I. S., Shannon, T. PV., Harrison, A. G.j J . d m . Chern. Soc. 84, 4 (lo62). (31) Wallenstein, hl. B., Wahrhaftig, A . L., Rosenstock, H. >I., Eyring, H.,
in "Symposium on Radiobiology," J. J. Sickson, ed., p. 70, Riley, Kew York, 1952.
RECEIVED for review Sovember 29, 1963 ilccepted Februarl 21, I064 Presented in part before Eleventh Annual Conference on 3Iass Spectrometr) and .411ied Topics, ASTM Committee E-14, San Francisco, Calif , l l a y 1063
Design Parameters and Performance of a Miniaturized Co Iorimetric Recording Air Ana Iyze r BERNARD E. SALTZMAN and ALFRED L. MENDENHALL, Jr. Division of Air Pollution, Robert A. raft Sanitary Engineering Center and Division o f Occupational Health, Occupational Health Research and Training Facility; U . S. Public Health Service, Cincinnafi, Ohio
b Design parameters were studied in a prototype model of an improved recording air analyzer. Nitrogen dioxide was absorbed efficiently in a microcolumn packed with 20- to 60mesh crushed glass in an improved absorbing reagent, which flowed through a rugged spectrophotometer employing a stainless steel cell (with glass windows) and stainless steel tubing connections. The system was designed for minimal liquid holdup to achieve rapid response with small liquid reagent flows. The improved electronic circuit provided a very stable output with only infrequent checks of the 0 to 100% transmittance points. A 90% response time of 3 minutes was achieved. For fluctuating gas concentrations with a period as short as 2 minutes, 624/,of the full response amplitude was obtained. The results indicate the success of the rugged miniaturized design.
F
and improvement of available automatic instrumentation for air analysis are urgently needed, although great advances have been made ( I , 2, 5 ) . Continuous analysis is desirable to provide a complete picture of fluctuating levels of contaminants; manual methods for analysis at frequent intervals are very costly and are practicable only for brief periods. Good colorirnetric methods have certain advantages of URTHER DEVELOPMENT
1300
ANALYTICAL CHEMISTRY
specificity and sensitivity and are available for determination of several gases. Colorimetric recording air analyzers have been described for oxidants ( d ) , nitric oxide and nibrogen dioxide (9), and sulfur dioxide (3). These instruments are slow in response, however, and too bulky to be readily transported. The main objective of this study was to explore the design parameters for a small rugged instrument having a lox rate of liquid reagent' consumption but improved speed of response, that would be readily portable for bot'h industrial hygiene studies inside of industrial plants and air pollution st,udies outside. Most of the present work was conducted with nitrogen dioxide reagents, although a monochromator was incorporated in t,he prototype instrument to provide versat,ility for application to several gases. This report describes a practical design, and the responses to both steady and fluctuating low concentrations of nitrogen dioxide in air. Effects of construct,ion materials, column height, liquid and air flow rates, and the period of gaseous concentration fluctuat'ions are given. SELECTION OF MATERIALS OF CONSTRUCTION
Preliminary studies were made of various materials of construction because glass tubing, which is commonly used, is not well adapted for portable instrumentation. The effect of metals on the pink color produced from sam-
pling nitrogen dioxide in Griess-Saltzman reagent (6) was studied first. .I few centigrams of silver, hlonel, and stainless steel filings mere added, respectively, to three matched colorimeter tubes containing the colored solutions. The optical absorbances were read a t various time intervals, with the results shown by the solid lines in Figure 1. These data show that the addition of silver filings produced practically no effect. Stainless steel filings produced a small decrease in the absorbance of the color over a period of 20 hours; None1 metal filings produced a somewhat larger increase in the absorbance over the same period. Since the colored solution in a recording air analyzer would be exposed to metal for only a few minutes, any of theye materials could be used with nitrogen dioxide reagent. -1 similar study was made for the iodine colors produced by sampling ozone in neutral buffered 1% potassium iodide reagent ( 8 ) . Comparison with a blank is necessary, since stability of the iodine is relatively poor. In the first run, shown in Figure 1 by the dashed lines, absorbance changes produced by stainless steel and Monel filings were compared with those of Blank 1. The stainless steel accelerated the fading whereas the Monel metal produced an increase in the iodine color, both over an &hour period. In the second run, absorbance changes produced by silver filings were compared with those of Blank 2 as shown in Figure 1 by the dash-dot lines. The silver stabilized the color and produced a better result than that indicated by the blank.
