SUMMARY
Table 111.
Comparison of Direct Absorption and Colorimetric Methods on Polygard Analysis
PolyType of Polymer Naugapol 1503 Naugapol 1018 a b
Method of Analysis Direct Colorimetric Phosphorus" Direct Colorimetric Phosphorus"
gard ReAdded, Av., covery, % PolYgardFound, % % Av., %
l.18
1.25
l.l5, l.08,'.14 1'2211'171 1.17, 1.16 "18 1.22,1.10, 1.17 1.20, 1.24, 1.23 1.23, 1.24
'.12 1.17 ''19 1.16 1.22 1.22
g5 lo'99 93 98 98
Std. Dev., % 0.039 0'!27 0.061 0.021 b
(1).
Standard deviation by phosphorus method has been shown to be 0.015%.
taining Polygard mere analyzed with over 95% recovery of alkyl phenol. Both hot and cold SBR polymers, as well as cross-linked and noncrosslinked polymers, were tested. Synpol 1013 shortstopped with hydroquinone gave good results; the hydroquinone did not interfere under the described procedure. The method has not been applied to oil extended polymers. The results given on Synpols 1707 and 1708 mere made on latex prior to the oil addition. The precision of this method varied, with standard deviations of 0.008 to 0.025y0 (Table 11). It conforms to Beer's law over the concentration range of 0.010 to 0.030 gram per liter. Sources of Error for Both Methods. Peroxide-free Cellosolve should be used. T o test for solvent suitability, heat 50 ml. with 2 ml. of 2.5N sodium hydroxide for 20 minutes, and follow the directions for the blank solution under item I11 of the procedure f o r
the direct absorption method. The difference in absorbance betxeen the alkaline and acid solutions should not exceed 0.020 absorbance unit a t 295 mp. If the difference is greater than 0.020 unit the solvent should be discarded. In latex determinations, the film must be uniform and thin. This is achieved by rotating the tube while the water is being evaporated. With dry polymer, the thickness of the sheeted sample is important. It should be as thin as possible (0.005 inch gage) and preferably the determination should be made as soon as possible after removing the sheeted polymer from the mill. Otherwise, the rubber may thicken by relaxation. The final solutions should be free from turbidity before readings are taken on the spectrophotometer. If this is not the case the solutions should be filtered.
The t15.0 procedures for determining trisinonvlated Dhenvl) DhosDhite (Polygard) in" synthetic ribber are based on the measurement of the phenolic fragment produced by alkaline alcoholysis of the phosphite, using Cellosolve as the alcohol. These procedures are more rapid than previous methods based on phosphorus, and are not subject to inaccuracies due to the presence of inorganic phosphates occasionally found in SBR latices, The direct absorption method is more ratid but less accurate than the colorimetric method. The colorimetric method is recommended in preference to the direct method, as the latter is subject t o interference from the rubber extracted, and to incomplete extraction because of the size of the sample (Table
111)* The colorimetric procedure could well be applied to Wingstay S, C2X1000 (Winstay T), Agerite Superlite, and Nevastain A. ACKNOWLEDGMENT
The author wishes to thank J. E. Newel1 and B. A. Hunter for suggestions and aid in this work. LITERATURE CITED
(1) Xewell, J. E.,Mazaika, R. J., Naugatuck Chemical Div., United States Rubber Co., Tech. Bull. 204. ( 2 ) Wadelin, C. W., ANAL. CHEM.28, 1530 (1956). RECEIVED for review August 26, 1957. Accepted May 23, 1958.
Determination of Traces of Mixed Halides 'by Activation Analysis JAMES F. COSGROVE, ROBERT P. BASTIAN, and GEORGE H. MORRISON Research laboratories, Sylvania Electric Products Inc., Bayside, N. Y.
)In order to evaluate the luminescent effects of halides on zinc sulfide phosphors, a neutron activation method has been developed to determine trace amounts of mixed halides. After activation, solvent extraction was used to isolaie iodide and bromide. Chloride was isolated b y distillation. The activity of the respective halides was measured by standard beta counting techniques. The accuracy and precision of the method are within 3.5% and the limit of sensitivity is approximately 1 y for chloride, 1 y for iodide, and 0.00 1 y for bromide.
1872
ANALYTICAL CHEMISTRY
Z
sulfide phosphors are usually activated or coactivated by the introduction of small amounts of halides into the crystal lattice. It is of interest to evaluate the luminescent effects of the individual halides: chloride, bromide, or iodide. As chloride often appears as a difficultly removable impurity with bromide or iodide, it is necessary to determine the respective concentrations of the halides together. The present study is concerned with the development of a method for the determination of trace amounts of chloride, bromide, and iodide in zinc sulfide. IKC
An accurate, \vet chemical determination of trace amounts of the three halides in the same sample is difficult. There are a t present no accurate chemical methods for simultaneous halide determination in a solid sample. In a chemical analysis, volatile halides may be lost during dissolution of the sample, or the sample contaminated with halide impurities during the analysis, either from the reagents or from other sources. The proposed method takes advantage of the specificity and sensitivity of neutron activation analysis and possesses a sensitivity of 1 y for chloride,
1 y for iodide, and 0.001 y for bromide. Errors due to losses during chemical processing are eliminated by the use of stable carriers to establish chemical yields. Finally, there is no possibility of contamination of the sample. In the past, activation analysis has been used for the determination of individual halides. Thus, Bancie-Grillot and Grillot determined chloride in zinc sulfide in concentrations of 10 to 50 p.p.m. by irradiation of a &gram sample in the Chhtillon reactor ( 2 ) . Atchison and Beamer determined microgram quantities of chloride, bromide, and iodide in aqueous solutions using a Van de Graaff accelerator ( 1 ) . HOYever, chloride, bromide, and iodide all interfere in the determination of any one of the three elements. The method described is capable of determining all three halides in the same sample. Many elements, when irradiated with thermal neutrons, undergo an n, y reaction with the formation of the corresponding radioisotopes. The amount of the radioisotope formed is proportional t o the integrated neutron flux, the reaction cross section, and the weight of the particular element. TSThen zinc sulfide containing chloride, bromide, and iodide is bombarded with thermal neutrons, the reactions summarized in Table I occur. Thus, five radioisotopes are formed from the matrix alone-three of zinc and two of sulfur. The activity of the chlorine-38 isotope is used as a measure of the amount of chloride present, while the combined activity of the bromine-80m and bromine-82 isotopes and the activity of the iodine-128 isotope are used to measure the bromide and iodide concentrations, respectively. Gamma scintillation spectrometric measurement has been used in the past for determination of trace impurities in semiconductors and other materials ( 3 ) , but, unfortunately, this technique cannot be used in this instance. The intense gamma activity of the matrix necessitates a chemical separation before the analysis can be performed. Secondly, the large number of gamma raj s emitted by the bromine-82 would make it difficult t o resolve the three halides completely. Thirdly, separation of the individual elements and measurement of their total activity result in an increase in sensitivity over a scintillation spectrometric measurement. Interference may result from secondary n, y reactions during irradiation of the zinc sulfide. Thus, stable chlorine formed by the decay of sulfur gives rise to chlorine-38, n hich is indistinguishable from the chlorine-38 formed from the chloride impurity originally present in the sainple. The amount formed by this secondary reaction can be calculated ( 4 ) , and, under the irradiation condi-
-
Table 1.
Naturally Occurring Stable Isotope Zn64 Zns
Zn7Q
Abundance,
%
48.9 18.6
Br79
0.63 4.2 0.017 75.4 24.6 50.6
B91
49.4
534
s36
e 1 3 5 e137
I122
100
n, y Reactions of Zinc Sulfide and Halogen Isotopes Isotopic Cross Section, Barns 0.5 0.09 1.0 0.09
0.26 0.14 43 0.6 2.6 7.6 . 2.6 6.1
Isotope Formed Zn66 Zn6gm Zn69
Zn71 s35 s 3 7
c136
Cla8 B90" B9Q BF I128
tions in this study, amounted to an apparent concentration of less than 1 p,.p.m., which lyas the limit of sensitivity of the method. Likewise, any selenium impurity in the sample )Till result in the formation of bromine isotopes by a secondary reaction. The probable reaction n-ould be the eventual formation of bromine-82 from selenium-80. Here again, the amount formed will usually be small, depending on the amount of selenium in the sample and the irradiation conditions. But should the amount formed be of any consequence, the analysis can still be performed by resolving the complex decay curve of the bromide fraction and basing the analysis on the measurement of the bromine-80m activity only. The amount of this isotope formed will be unaffected by the selenium and will be directly proportional to the amount of bromide originally present in the sample.
Method
Half Life
250 days 14 hours 52 min. 2 . 2 min. 87 days 5 min.
4 X lo6 years
37.3 min. 4.6 hours 18 min.
35.9 hours 25 min.
Isotope Formed
of
Decay
IC, Y, P +
IT 88-Y Y 8-
P-, 4-
Y
Cue5 (stable) Zn6g GaBg (stable)
Ga71 (stable) Cl3: (stable) el3' (stable) A36 (stable) A38 (stable) Br8Q KI? (stable) K P (stable) Xe128 (stable)
Brookhaven reactor. During this time interval the chlorine and iodine isotopes have reached their saturation activity and the shorter-lived bromine isotope has approached saturation activity. The maximum flux of the pile was used to obtain the highest activity from the trace impurities.
EXPERIMENTAL
Chemical Separations. As the chlorine-38 and iodine-128 have very short half lives, the chemical separations, in addition to producing radiochemically pure fractions, must be rapid. The samples weie received in this laboratory approximately 2 hours after they were removed from the pile and the separations ere begun immediately. The t n o short-lived isotopes had already passed through a number of half lives, so that the chemical manipulations had t o be completed before many more half lives had elapsed. The analytical scheme TT hich follows can be completed in about 1 hour and a number of samples can be rwi simultaneously.
Samples and Comparative Standards. The method has been developed to determine trace amounts of halides in zinc sulfide and may be applied with some modification to other materials. Weighed amounts of the ammonium salts of the halides were irradiated vvith the samples for eventual comparison with the unknowns. Zinc Sulfide Standard Samples. Synthetic zinc sulfide standards were prepared t o test the accuracy and precision of the method. A sample of pure zinc sulfide, to which known amounts of the three halides had been added, was irradiated and carried through the analytical procedure. It had been previously analyzed chemically and by activation analysis, and was found to contain 6 p.p.m. of chloride and no detectable bromide or iodide. Irradiation. Samples of 0.5 t o 1.0 gram of zinc sulfide, together with the comparative standards, were sealed in quartz ampoules and irradiated for 24 hours a t a flux of 3.4 X 1012 neutrons per sq. cm. per second in the
The zinc sulfide vas dissolved, under reflux conditions, in a solution of 30 ml. of water and 5 ml. of concentrated nitric acid containing 20 mg. of each of the halides as carriers. The solution was heated very gently until the zinc sulfide was completely decomposed and the sulfur had accuniulated at the top of the solution. The bource of heat was removed and the solution cooled with ice m t e r . Twenty milliliters of carbon tetrachloride n ere poured down the condenser into the distilling flask to wash down the iodine n hich had crystallized on the walls. IODIDE SEPARATIOS.The solution n-as filtered into a separatory funnel and shaken for a fex minutes to extract the iodine into the carbon tetrachloride. The organic phase nas separated and a second extraction of the aqueous phase performed with a fresh 20-ml. portion of carbon tetrachloride. The aqueous phase was reserved for the subsequent separation of the chloride and bromide. The organic extracts were combined and shaken with 50 ml. of nater containing 1 to 2 ml. of 1% hydroxylamine VOL. 30, NO. 11, NOVEMBER 1958
1873
Table
II. Analysis of Zinc Sulfide Standard Sample Amount of Halide in 0.5 G.
c1
Average
52.5 52.1 51.0 49.8 51.5 53.9 53.7 52.1. f1.5’
Coefficient of variation, % 2.9 Amount of halide present 53.0 Error, % 1.7
ZnS, -( Br I 50.2 48.8 50.8 48.7 49.5 50.0 51.7 51.2 53.7 50.3 48.9 51.7 53.0 50.7 51.1. 50.5, f1.8’ f1.5’ 3.5
3.0
50.0 2.2
50.0 1.0
sulfate and 1 mg. of bromide as carrier. The organic layer Tws drawn off and the backwash of the organic phase repeated. This removed any bromine which might have been extracted along with the iodine. The carbon tetrachloride was filtered through dry filter paper into another separatory funnel. The iodine was stripped from the organic phase by the addition of 20 ml. of water followed by 1% sodium bisulfite, dropwise, with shaking, until the carbon tetrachloride layer had just become colorless. This aqueous solution was then transferred to a beaker. A few drops of nitric acid were added and the iodide was precipitated with 0.1N silver nitrate. The precipitate was centrifuged, washed, dried, weighed, and mounted on a planchet for measurement of activity. The average chemical yield was approximately 40%. BROMIDE SEPBRATIOK. The initial aqueous phase from the iodine extraction was extracted twice with 20-ml. portions of carbon tetrachloride t o remove any residual iodine. The organic phases were discarded. Twenty milliliters of carbon tetrachloride were added to the aqueous phase, follorred by the dropwise addition of 15 potassium permanganate until the purple color just persisted in the aqueous phase. The phases were shaken for a fern minutes, the layers separated, and the extraction with a fresh portion of carbon tetrachloride was repeated. This aqueous phase was reserved for the chloride separation. The extract? were combined and the bromine stripped from the carbon tetrachloride by extracting with 20 ml. of water containing 1 to 2 ml. of 1% hydroxyamine sulfate. The aqueous phase was transferred to a beaker and silver bromide precipitated as in the procedure for the iodide. The chemical yield averaged approximately 65%. CHLORIDE SEPARATION. The aqueous phase from the bromine extraction was placed in a distilling flask and an equal volume of concentrated nitric acid
1874
ANALYTICAL CHEMISTRY
added. Distillation was begun and the first few milliliters were discarded. Finally, approximately 20 ml. of the solution were distilled into water. Silver chloride was precipitated from the distillate as in the procedure for iodide. An average chemical yield of approximately 7 5 7 , was obtained. Comparative Standards. The coniparative standards were dissolved in water and diluted t o a known volume. A small aliquot of the standard solution was added to 20 mg. of the respective carriers and each was precipitated as the silver salt in the same manner as the unknowns. Because of some decomposition of the ammonium iodide during irradiation, the amount of iodide in the standard solution was determined colorimetrically after its activity had decayed. Measurement of Activity. All measurements were made with a standard Geiger-Muller counter. The respective halides were measured by comparing the activity of the unknown with the activity of the corresponding standard. A minimum of 10,000 counts was taken on each sample. Appropriate decay corrections were applied in calculating the amount of halide present. Radiochemical purity of the fractions was verified by taking decay measurements on each sample and by gamma spectrometric scanning. RESULTS A N D DISCUSSION
The sensitivity of the method as described here is 1 y for chloride, 1 y for iodide, and 0.001 y for bromide. This is based on an elapsed time of 3 hours required in transporting the sample to the laboratory and completing the analytical scheme. In the 3-hour interval from the time the sample is removed from the pile and the time of the first activity measurement, the shortlived chloride and iodide isotopes have decayed through a number of half lives. If the chemical separations could be started immediately after the sample was removed from the reactor, the sensitivities for the neutron flux employed would be 0.005 y for chloride and 0.0004 y for iodide, with no radical change in the bromide sensitivity. Thus, an increase in sensitivity of several orders of magnitude is possible, a t least for two of the elements, depending on the proximity of the reactor used for the irradiation. In all cases, an attempt was made to obtain radiochemically pure precipitates for activity measurements. However, this was not always possible. If bromide is present in high concentrations, it is difficult to remove the last traces of it from the silver iodide precipitates. To attempt further chemical processing to remove these last traces would be im-
practical because of the short half life of the iodide isotope. The amount of bromide contamination can be determined by gamma spectrometric measurement or by measurement of the bromide activity after the iodide activity has decayed. The activity of the iodide can be readily determined by applying the appropriate correction to the initial activity. I n those instances in which a correction had to be applied, the activity due to the bromide contaminant was small in comparison to that of the iodide. To test the accuracy and precision of the method, seven samples of pure zinc sulfide, to n-hich known amounts of the three halides had been added, were analyzed by this technique (Table 11). The method exhibits both good accuracy and good precision. The averages of the results for the three elements are all within 2.2% of the true amount present, and the coefficient of variation never exceeds 3.5%. The concentrations of the halides added were \vel1 above the sensitivity of the method; however, these were the concentration levels of immediate concern. The accuracy and precision should be approximately the same a t lower concentrations. The data given represent the accuracy and precision of the complete method, including chemical manipulations and counting techniques. At lower concentrations, longer counting times would be required to achieve the same precision. As this would be the only difference, the over-all accuracy and precision should not be too much different than at the higher concentrations reported here. The method described primarily can be used to determine trace amounts of mixed halides in zinc sulfide phosphors and other materials. Secondly, as the possibility of contamination of the sample in the activation method is remote and losses during chemical processing can be accounted for by the use of stable carriers, the method can be used to supply primary standards to aid in developing other chemical methods for the determination of traces of individual halides. LITERATURE CITED
(1) Atchison, G. J., Beamer, W.H., A N A L . CHEM.28,237 (1956). (2) Bancie-Grillot, M., Grillot, E., Compt. rend. 237, 171 (1953). (3) Morrison, G. H., Cosgrove, J. F., AKAL. CHE~I.27, 810 (1955); 28, 320 (1956); 29, l O l i (1957). (4) Rubinson, W..J . Chem. Phys. 17, 542 (1949). RECEIVED for review February 27, 1958. Accepted June 3, 1958.
