not listed in the wave length or multiplet tables. For most elements the spectral lines of maximum sensitivity are used. The most sensitive line for hafnium proved to be 2820.2 A., and, fortunately, sufficient wave length coverage is available to detect simultaneously the sensitive tungsten line a t 4008 A. Background and separate internal standard lines must be read for both elements; however, for most routine determinations, visual estimations are sufficient. The selection of an analytical line for the determination of boron is limited to two possibilities: boron 2497.7 A. and boron 2496.8 A, The higher wave length line is used because of its greater sensitivity. Although some difficulty has been experienced in separating boron 2497.73 A. from iron 2497.82 A., sufficient resolution has been obtained with the Hilger Large Littrow spectrograph to perform this determination. For routine analyses, visual comparisons usually are satisfactory, but for rigorous determination the line and adjacent spectral regions are recorded. To date, cadmium has not been detected in any sample or standard, but it is felt that because of the high sensitivity of cadmium 2288 A., concentrations of 0.5 p.p.m. should be detected. The transmittances of the analytical lines and the internal standards lines are converted to log intensity ratios by means of the emulsion calibration curves ( 8 ) . Then the analytical curves (Figure 3) are prepared by plotting, on log scales, concentration us. average relative intensity ratio for the standard. The relative intensity ratios for the samples are referred t o the analytical curves to determine the concentration. Triplicate spectrograms are averaged for Method A, whereas duplicates are
averaged for Methods B and C. As the slope of the analytical curves remains reasonably constant, working curves may be set up on a Dunn Lowry calculator, and analytical results may be obtained more rapidly by using only one or two standards as reference points. In Figure 4 the effects of background correction are demonstrated, but to increase the speed of analysis, this practice is normally ignored for most of the impurities. This procedure may adversely effect the precision of the method, but the results are still adequate for control purposes. PRECISION AND ACCURACY
To determine the reproducibility and accuracy of the method, samples were analyzed on 11 different plates. The coefficient of variation for two representative samples is listed in Table VIII, together with the maximum. minimum, and average values for all elements found in these samples. The values obtained by the ignited alternating current arc techniques have been validated by comparison with chemical and independent spectrochemical direct current arc results (Table IX) . For some elements the reproducibility is markedly better in the samples than in the standards; for other elements the reproducibility seems better in the standards. This may indicate segregation of different elements in zirconium ingots, depending on the type of sponge used and the melting processes. Reproducibilities for boron, tungsten, and hafnium were not determined, because in the samples, these elements were less than the lower limits of detection. (Boron was less than 0.5 p.p.m., tungsten less than 50 p.p.m., and hafnium less than 100 p.p.m.)
CONCLUSION
This paper describes a rapid point-toplane procedure for the determination of impurities in zirconium metal, which with only slight modification can be extended to include Zircaloy 2. The expected coefficient of variation is When most approximately +5%. routine materials are tested, elements other than aluminum, chromium, copper, iron, silicon, and sometimes hafnium can be reported by visual inspection. When an optimum number of 12 samples is analyzed as one group the analytical time required per sample is less than 1 hour. The procedure is currently being studied for application to direct reader analysis, which should further reduce analytical time. ACKNOWLEDGMENT
The authors express appreciation to the Bettis chemical laboratory, the Bettis Zirconium Technology Section, and the Bureau of Mines facilities at Albany, Ore., for assistance in this program. REFERENCES
( 1 ) Churchill, J. F., IND.ENQ. CHEM., ANAL.ED. 16.668 (1944). (2) Del Grossd, D.' A., 'Landis, F. P.,
Knolls Atomic Power Laboratory,
KAPL 1539 (1956). (3) Fassel, V. H., Howard, A. M., Anderson, D., ANAL. CHEM.25,760-3 (1953). (4) Gordon, N. E., Jr., Jacoba, R. M., Ibid., 25, 1605-8 (1953). (5) Norria, J. A., U. S. Atomic Energy Comm.. AECD MIT 1049 (1950). - -,\ - -
(6) Roddkn, C. J., U. S . Atomic Energy Comm., AECD NYOD 2005 (1950). (7) Spitzer, E. J., Smith, D. D., AppZ. . Sp&roscopy 6, No. 5, 6 1 1 (1952). (8) Weisberger, S., U. S. Dept. Commerce, Office of Technical Services, PB 11842 (1955).
RECEIVED for review February 4, 1959. Accepted May 19, 1959. Ninth Annual Symposium on Spectrosco y, American Association of Spectrograpfers, Chicago, Ill., June 1958.
Analysis by Phosphor Poisoning ARTHUR BRADLEY and NORMA V. SUTTON Associated Nucleonics, Inc., Garden City, N. Y.
b The luminescence efficiency of a type of cadmium sulfide phosphor was reduced by reaction with traces of copper, silver, mercury, and platinum salts in solution. There is evidence that reproducible light attenuation resulted from the accumulation of an opaque surface layer of poison element by substitution. The data may b e presented as a calibration curve 1554
ANALYTICAL CHEMISTRY
for a new method of quantitative ana lysis.
T
HE acute sensitivity of many phosphorescent and fluorescent materials to poisoning by traces of impurities has been a handicap in their manufacture and use. Leverenz has reviewed some quantitative studies of
poisoning by metals added to phosphor preparations before firing, and has discussed some possible mechanisms (4). It was found, for example, that nickel and certain other atoms were poisons for zinc sulfide, and this was attributed to changes in the crystal structure that made it more difficult to store incident energy or convert it to light. I n some cases, elements could act as either ac-
tivators or poisons, depending on the composition of the phosphor and other conditions. Explanations for these effects (internal or mechanistic poisoning) must be consistent with more general luminescence theory, and the study is appropriate to the fields of atomic and solid state physics. Phosphor preparations may also lose their luminescent properties after compounding by chemical reactions a t or near the surface of the particles or physical damage to the crystals. This may be termed external poisoning, and could involve attack by water, acids or other reagents, or even atmospheric gases a t elevated temperatures. Certain types of radiation damage (such as ultraviolet light in the presence of moisture) and physical stress (such as grinding) may also come under this heading. The important factors in this type of poisoning would be the composition and structure of the luminescent compounds, the reactivity of n hich involves problems in inorganic chemistry. An example of external poisoning is described. In this laboratory certain phosphors have been damaged simply by exposing them briefly to aqueous solutions of metallic salts a t room temperature. The extent of the effect depended on the total mass of the poison element present, not the concentration, and the poison was consumed in the process. Thus a 200-mg. portion of cadmium sulfide phosphor scavenged 50 y of copper from a dilute solution after stirring for 5 minutes, and lost 95 to 1 0 0 ~ oof its light output under @-ray excitation. Cadmium was then the only cation identified in the solution which had lost all its poisoning power because another similar portion of the same phosphor was unharmed by long immersion. This type of poisoning evidently occurred by substitution on the surface of the cadmium sulfide crystals, the outer layer of copper sulfide being opaque to the emission of the phosphor. It was reproducible, depending on the proportions of poison to phosphor, and occurred not only with copper, but with silver, mercury, and platinum under the mme conditions. The experimental results recorded in this paper were accumulated from the point of view of demonstrating the analytical possibilities of this phenomenon, using the property of luminescence efficiency to characterize the extent of chemical substitution. EXPERIMENTAL
A sealed, long-lived, beta-cmitting radioisotope was selected as the simplest and most reproducible form of phosphor excitation. Gamma-emitters aere unsatisfactory because their energy was
Table 1.
Poison cu
Ag
Hg
Pt
Pb
Results of Cadmium Sulfide Phosphor Poisoning Experiments Y 1
4/23
2 5 10 20 50 2 5 10 20 50 100 5
43 32 22 54 46 33
56 46 37 27
46 42 32 23 16
16 11
32 14 12
59 54 57
10
20 50 100 10 20 50 100 100 100 100 100 100
Galvanometer Deflections, Mm. 7/2 9/19 Average Poisoningo
5/19 53 47 33 26
60
55 47 36
50 44 32 24 16 11 55 46 34 27 14 12 59
10 16 28 36 44 49 5 14 26 33 46 48
56 47 36
4 13 24 4 14 22 26
.SA -_
56 ._
46 38 34 220
46 38 34
18b 21b Xi 21b 24" Sn 19b 2gC Zn 20b 25c a ilverage subtracted from 60 (deflection for untreated phosphor). * Plus 10 y of c u . c Plus 5 y of c u .
Fe
57 54 59 55
not adsorbed efficiently in the small samples, and the large sources required were difficult to shield. Soft x-rays and ultraviolet light could be used effectively (the latter, particularly, would involve no external radiation exposure problem), although there are few advantages to justify the more elaborate instrumentation involved. The equipment consisted of a 50-mc. sealed source of strontium (yttrium)-gO, a silicon photocell, and an adapter which held a small sample cup close to both the beta source and the photocell (Figure 1). Under reproducible betaexcitation geometry, the luminescence efficiency of different phosphor samples was proportional to the light current generated in the photocell and measured by a galvanometer. The data were recorded in units of millimeters of galvanometer deflection. The difference between the luminescence (light current) of each sample and that of an untreated blank represents the extent of poisoning, and is of interest in connection with analysis. This number has been reported for the average of all individual determinations with each set of poisoning conditions (Table I). Apparatus. The radioactive source was a metal cylinder, 0.50 inch long and 0.654 inch in diameter, with the active area a t one end (0.50 inch in diameter, 0.002-inch Monel window) and a larger circular base a t the other. It was supplied by the United States Radium Corp., and is described in detail by drawing LAB 370. The sealed contents consisted of 50 mc. of Sr(Y)-N (as strontium sulfsjte) in December 1955. The data presented here were taken over a 5-month interval (April to September 1958), during which
time the source would have decayed about 1%. This change was considered negligible compared to the other experimental errors. The sample containers were aluminum rings, 0.50-inch inside diameter, 0.63inch outside diameter, 0.080 inch high. An 0.63-inch diameter circle of 0.00035inch aluminum foil (2.2 mg. per sq. cm.) was cemented to one end of each to make a little cup. This cup fitted into a holder constructed so that it could be placed over the strontium-90 source with reproducible geometry. The holder was handled by a 10-inch extension provided to safeguard the worker , The sample cup was lowered into place with tweezers, after which it was covered by a larger black plug, which rested on the shoulder around it and kept room light from reaching the photocell. The latter, a small silicon solar type obtained from Texas Instruments, Inc., was centered a t the bottom of the plug directly over the phosphor sample. Parts of the edges of the photocell were masked off to leave only a circle of about 1 sq. cm. exposed. The leads extended through the plug, and then along the holder arm and out to the k e d s & Northrup 2430-C galvanometer, which had a fullscale (100-mm.) sensitivity of about 0.3 pa. The radioactive source had a beta radiation intensity of 5 roentgens per b u r a t a point 16 inches directly in front of the window. It was handled in accordance with normal radiological safety procedures (6). The manual technique for placing the phosphors over the source was designed to minimize personnel exposure: the sample VOL. 31, NO. 9, SEPTEMBER 1959
1555
50
L I G H T - S E A L PLUG WITH PHOTO CELL INSERT
30
20
S A M P L E HOLDER WITH
P H O S P H O R CUP
C R O S S SECTION OF ASSEMBLY
Sr (Yl-90
SOURCE
;,/ ,,/
EXPLODED
Figure 1. Sample holder for phosphor poisoning measurements
X-SILVER 0 - COPPER
Zi
holder was always removed and the source covered before changing the phosphor cup or making any inspections or adjustments. Phosphor. The luminescent material used in all these experiments was a type of cadmium sulfide:silver, designated by the supplier, the United States Radium Corp., as No. 1527. The average particle size was about 10 microns. Preliminary tests indicated that the sensitivity of this material t o external poisoning was greater than that of a similar composition (United States Radium 1601) which had an average particle size of 40 microns. Measurements. With 200 mg. of untreated 1527 phosphor in the sample cup, activation with the strontium-90 source brought about a light current in the photocell of about 0.1 pa. This led to a double deflection on the galvanometer of 60 mm., reproducible to within *2 mm., if the sample were shaken, or emptied and refilled with the same material. The same order of reproducibility was found for poisoned samples as well, and the average of three successive readings was always taken. Because the significant data in each test were the difference between the actual reading for the sample and 60, the unpoisoned blank, the precision was Doorest in the cases of little or no pdisoning. There was also a d s c u l t y a t the other extreme, however, because some beta radiation from the strontium-90 source reached the photocell and contributed t o the galvanometer reading. Even with a completely poisoned or nonluminous material in the cup, about 10 mm. of double deflection were obtained. Thus 50 mm. (60 - 10) represented the maximum poisoning or light current loss that could be recorded. This explains why the 50-7 point on the copper curve in Figure 2 appears t o be inconsistent with the rest: Actually the practical limit of the experimental method (where residual light output blended into strontium-90 background) was about 35 y of Cu. Preparation of Samples. After 1556
a
ANALYTICAL CHEMISTRY
1 [,
2p
I
"I"
4f
s/o
R E L A T I V E POISONING E F F E C T
Figure 2. Plot of relative poisoning vs. log of copper and silver concentrations Calibration curve for quantitative analysis
certain preliminary experiments, which are outlined below, a standard poisoning procedure was adopted. The desired quantity of metal to be tested was pipetted from a stock solution into a 25-ml. vial or Erlenmeyer flask and diluted to 10 t o 15 ml. with distilled water. The 200mg. ( + 5 mg.) sample of fine cadmium sulfide phosphor was added and mixed by swirling for 30 to 60 seconds three times over a period of 10 minutes. Most of the liquid was then removed by decanting and the phosphor transferred in a slurry with a medicine dropper to a moist 4.25-cm. filter paper held by suction to an upright
Table il.
Stock Solutions
Amount Dissolved in 100 M1. Element, of Water, 100 Solution Gram P.P.M. cu cuso4 0.025 0.0157 AgNOa Ag 0.0136 HgCh Hg PtC1,a Fe 0,069 Fe(N03)3,9HzO Ni Ni(NOs)*.6Hz0 0.0495 Pb 0.0134 PhC1, SnCli. 2H20 0,019 Sn ZnSOl, 7H10 0.040 Zn a 1.00 ml. of Fisher 10% PtCla . 2HC1 . 6Hz0 solution diluted to 1000 ml. with H20. 38 p.p.m. Pt.
sintered-glass filter stick. The sample was washed with a little more water, then acetone, a n i dried for an hour in an oven a t 100 . It was then carefully tapped from the filter paper into one of the aluminum sample cups. No damage was observed if the phosphor remained several days on the filter paper at room temperature, nor was there any time factor in making the measurements. Samples showed the same light-output deflections months after they were initially measured. Simple precautions were taken against accidental contamination of the phosphor samples by copper in tap water. Otherwise, the only special treatment they received was that conditions that generally cause damage, such as prolonged heating, exposure to bright sunlight (especially when moist), or strong mineral acids were avoided. The stock solutions used in obtaining the data given in Table I are given in Table 11. Preliminary Experiments. The original screening was with approximately 50 y each of aluminum, barium, calcium, cadmium, cobalt, copper, iron, mercury, magnesium, manganese, nickel. lead, and zinc, Of these only copper and mercury had a significant poisoning effect on 200 mg. of cadmium sulfide. Other tests showed that the damage from a certain copper solution was the same a t pH 2, 4, or 6. In addition, it was found that after separating the poisoned phosphor by filtra-
tion, the solution had no further poisoning power. Additional cadmium sulfide samples were undamaged by exposure to the filtrates. A dithiaone spot test on one of these filtrates gave a raspberry color characteristic of cadmium rather than the intense yellowbrown of copper ( 2 ) . When the early work indicated that the phosphor was not adversely affected by immersion in acetone, a number of other organic solvents were tested, also without significant damage. These included ether, ethyl alcohol, carbon tetrachloride, chloroform, toluene, ethyl acetate, and petroleum ether. Analytical Results. The metals silver, lead, iron, nickel, tin, and zinc were selected for more quantitative tests because they are associated with copper in some cornm0.n alloys. hlercury was included as another example of a poison element, and platinum was added later when silver was also found t o be in this category. The data in Table I were accumulated in four sets over an interval of 5 months. Results obtained independently (7/2 and 9/19/58) were in all respects consistent with those obtained by one of the authors (4/23 and 5/19/58), which indicates the reproducibility of the method. The poisoning power of copper was approximately twice that of silver. As part of one series, 5 y of copper and 10 y of silver were tested together. The resulting double deflection was 23 mm., equivalent to 10 y of copper (or 20 y of silver) alone. The nonpoison metals were each tested ivith 10 y of copper added, and again with 5 y of copper added. The results showed a slight but consistent enhancement of the copper poisoning. At the 10-7 level, the average of five tests was 20 mm. as compared with 24 mm. for copper alone. I n the other set, the average of four was 25 mm. compared to 32 mm. for 5 y of copper. It appeared then that an unknown with 100 y of either iron, nickel, lead, tin or zinc, and 5 y of copper would give poisoning that could be interpreted as due to 10 y of the latter. Trial Analyses. A set of eight assorted aliquots of the standard copper solution plus two distilled water blanks, each identified only by number, were made up t o 20 ml. each and submitted to one of the authors (A.B.) for analysis by phosphor poisoning. The results are given in Table 111. The right-hand columns show the absolute error in micrograms and the per cent error, based on the known content of the initial aliquot. The analyses a t the low end ( 2 and 4 y ) were less accurate, as expected. In this range the experimental variation is relatively large compared to the magnitude of the poisoning factor. It may be recommended as part of this analytical technique that when low results are obtained the poisoning test should be repeated on a doubled sample. Copper assays in the range of 5 to 20 y can be carried out with an accuracy of +lo%.
It was evident first from the data
Table 111.
Actual Cu Content,
Mg.
Poisoning.
4
27
16 0 8 2 0
41
2 4
17 23 43
16
8
Analyses of Unknowns
Mg.b 4.3
Mg. 0.3 1
15
4 33
1 7.5
12 1
1.2 1 1.9
34
Error
Cu Assay,
3.0 18 8
0.5 0.8
W
/G
8 6 6 40
0.1
5
1.0 2
25 12 0
0
Average of 3 galvanometer deflection measurements subtracted from blank (60 mm.). From copper calibration curve, Figure 2.
(Table I) that four metals were unmistakably poisons, having a damaging effect on the luminescence of the phosphor in the order copper > silver > platinum > mercury. On the other hand, no significant effect was found using iron, nickel, tin, lead, or zinc. The poision metals as a group are more noble or electronegative than cadmium and the nonpoisons, and, which is probably more significant, have less soluble sulfides (3). It is suggested that the poisons in this test replaced cadmium from the phosphor crystal in a manner analogous to ion exchange, in which affinity for the sulfide linkage was the determining factor. The sulfides of copper, silver, mercury, and platinum are black, or brownish black ( I ) , and would be expected to be opaque to the red emission of cadmium sulfide. It also became apparent that the decrease in relative luminous efficiency was not directly proportional to the quantity of the interfering metal, but tended to be much less pronounced a t higher levels of poisoning. This behavior would be consistent with a mechanism in which layers of foreign materials were accumulated on the outer surface of each phosphor crystal, because light absorption falls off as an exponential function of thickness. In Figure 2, plots of the logs of copper and silver concentration us. the relative poisoning effects are fairly straight lines. These curves may be used to calibrate a new method of quantitative analysis for these metals. Successful application of the method requires previous identification of the metal, and a limitation of the quantity to below 20 y of copper or 40 y of silver (for 200 mg. of phosphor). The range for mercury and platinum analyses would appear to be about 10 to 100 y. For qualitative screening of an unknown, the method would serve chiefly to eliminate the group of less electropositive metals. Individual members of this group cannot be distinguished from each other. Their effects are
additive, as shown by one experiment in which 5 y of copper and 10 y of silver were tested together. The resulting poisoning was equivalent to that expected for twice the quantity of either metal taken by itself. The presence of large excesses of nonpoisons (100 y of iron, nickel, tin, lead, or zinc) slightly enhanced the specific damage due to 5- or 10-7 portions of copper. Nevertheless, it appears likely that the method will be applicable in cases where other standard procedures fail because of interference by extraneous metals. On the other hand, for analyses in which the copper-silver group would interfere, they may be quantitatively removed by scavenging with sufficient cadmium sulfide phosphor, as long as equivalent amounts of cadmium may be tolerated. This method has an advantage in that the volume of solution has little or no bearing on the results. Thus concentrating or exact diluting steps can be omitted from the preparation of samples. In addition, it is not necessary to adjust the pH except to avoid acidities that will attack the phosphor. The range 2 to 6 is probably the most suitable. No temperature control is required. The phosphor efficiency is unaffected by immersion in most common organic solvents (Experimental), so there is no necessary limitation to strictly aqueous systems. It has not yet been demonstrated, however, that poisons will react the same when in organic solution. In conclusion, this paper describes a new analytical method based on the external poisoning effects of certain heavy metals on a type of silver-activated cadmium sulfide phosphor. Many of the observations support a mechanism in which displacement of cadmium by the poison metal results in an opaque layer a t the surface of each phosphor particle. The method requires some unusual equipment for an analytical laboratory, but it is simple and rapid and for some special applications it may have unique advantages. VOL. 31, NO. 9, SEPTEMBER 1959
1557
ACKNOWLEDGMENT
LITERATURE CITED
Some preliminary work cited in the experimental part was carried out by Robert D. Goldberg. The design of the apparatus was suggested by Julius Chupak. The authors are especially indebted to Carol Thoren for checking the procedure and contributing some of the data given in Table I.
(1) Ephraim, F., “Inorganic Chemistry,”
5thed., p. 540, Throne, P. C. L., Roberts, E. R., eds., Gurney and Jackson, London, 1948. (2) Feigl, F.,,,“Qualitative Anal sis .by Spot Tests, 3rd ed., p. 72, &ewer, New York, 1947. (3) Handbook of Chemistry and Physics, 40th ed., p. 1740, Chemical Rubber Publ., Cleveland, Ohio, 1958.
(4)Leverenz, H. W., “Introduction to Luminescence of Solids,” p. 333, Wiley, New York, 1950. (5) National Bureau of Standards Handbook 66, “Safe Design and of Industrial Beta-Ray Sources, May 1958.
,p
RECEIVED for review November 20, 1958. Accepted April 28, 1959.
CoIo rimetr ic Dete rmina tion of 0 rga nic
Peroxides
MERLE I. ElSS and PAUL GIESECKE Central Research Division, American Cyanamid Co., Stamford, Conn.
b Benzoyl leuco methylene blue i s a new reagent for the quantitative spectrophotometric determination of organic peroxides. In a benzenetrichloroacetic acid solution, it reacted with peroxides and hydroperoxides to form the characteristic methylene blue color. The reaction was sensitive to ultraviolet light and to a lesser degree to artificial light and heat, but the color i s stable for several days if kept in the dark at 24” C. Zirconium naphthenate was used to accelerate the peroxide decomposition and thereby increase the leuco dye reaction rate. O f the five peroxides tested, benzoyl peroxide, lauroyl peroxide, p-methane hydroperoxide, and cumene hydroperoxide followed Beer’s law. tert-Butyl hydroperoxide deviated somewhat, and a calibration curve of concentration vs. absorbance was necessary for the compound. The method was found to be simple and sensitive. As little as 0.5 mg. of active oxygen could be detected. An estimate of precision of the method was obtained by running replicate samples for each compound. The 95% confidence limits ranged from h l . 7 to
=t2.6%.
R
interest in polymers required a reliable and simple colorimetric method for the determination of traces of organic peroxides and active sites on oxidized monomers and polymers. A method using leuco methylene blue as a reagent for peroxides was proposed by Ueberreiter and Sorge in 1956 (9, 10). However, the leuco base was not available in a usable form because of its extreme instability. It was difficult to synthesize and troublesome to store. Benzoyl leuco methylene blue n as found to be st new sensitive colorimetric reagent for organic peroxides. I n a bPnzene-trirrill~~r,a.eTic acid s o h ESEARCH
1558
a
ANALYTICAL CHEMISTRY
li,C2h a
:
a
N
l
%
b
+
[Ol-
WYr
3 Benzoyl leuco methylene blue
Methylene blue cation
:i 70I
Table 1. Effects of Metallic Naphthenates on Benzoyl Methylene Blue System
Metals5 Zirconium Lead Zinc Manganese Cerium Iron
Effects Excellent acceleration Some acceleration Some acceleration Excessive reagent blank Excessive reagent blank Highly colored, green Copper Highly colored, green Cobalt Yo acceleration No acceleration Calcium a One drop commercial naphthenate in 52 ml. of benzene-trichloroacetic acid solution. tion, it reacted with peroxides and hydroperoxides to form the characteristic methylene blue color. The reagent was stable in its crystalline form and storable in a refrigerator under normal conditions. It was not greatly affected by air, and in benzene solution could be stored in a brown bottle at room temperature. Benzoyl leuco methylene blue was affected by ultraviolet light (it turned blue rapidly and therefore had to be kept out of sunlight) and, to a lesser degree, by heat and artificial light. However, the benzoyl leuco dye-peroxide reaction was fairly slow. A hydroperoxide of the tert-butyl type required 36 hours to react at room temperature, and benzoyl peroxide took over 120 hours. An attempt was made to find a method of accelerating the color development by speeding up the decoiiiposition of the peroxide and, at the same time, maintaining a good level oi accuracy. Several heating variat’ions
Yy
/
c
B
20
, / , I , l , I
l , I , ! , ! , , , I
$W 20 40 €0 80 So0 20 40 60 BO 6W 20
1,l 40 50 BO
7W
WAVE LENGTH IN MILLIMICRONS
Figure 1. Spectrophotometric curves of methylene blue system a. N o active oxygen
b. 3 y active oxygen
were attempted, but even though the reaction was accelerated, the results were not quantitative. Light was also found to increase the reaction rate, but it caused a marked decrease in reproducibility (Figure 1). It was thought that amines and cobalt naphthenate would increase the rate of color development by quickly decomposing the peroxides. Several amines and amides were tried ( 8 ) but did not have the required speed Cobalt naphthenate was reported to be a useful compound for decomposing peroxides (1-4). However. it did not increase the rate of this reaction Ar; investigation of all available metallit naphthenates was undertaken Table 1 lists the metals and theii effects 011 the reaction. Zirconium w i t founa T O $.we the fastest reaction time and still h a w
