all three cases are those of a normal distribution. As it is accepted that the uranium particles are uniformly distributed (6, 7), it is concluded that the methanol-mixed samples are acceptably uniformly mixed. The analysis done when a higher background was present is not significantly different from the analysis done when the peak to background ratio had increased after another 7 days decay. This gives further confidence in the graphical obtainment of the background. Graph I in Figure 4 is a 45 O line on a log-log presentation of counts per minute per gram sample against the known Au content in various Witwatersrand composite samples. Triplicate sample values are given together with the average values for samples 6 and 11 (Table I). Specific Au reference activities were used for normalization. Graph I1 in Figure 4 is a similar graph of counts per 10 minutes per gram sample against the Au content of the various Witwatersrand composite ore samples as received before methanol mixing was applied. Samples of 1 to 2 grams were irradiated. The advantage of wet methanolmixing is clearly illustrated. Nonlinear corrections for difference in sample weights would be a necessity when higher accuracy is desired but in this paper, which serves as a demonstration of the principle of small ore sample activation, nonlinear weight corrections were ignored. The calibration curve above 10 dwt/ton concentrations of Au would not continue to follow a 45 O angle with the X-axis on log-log paper as suppression of the neutron flux can
occur at higher Au concentrations. With proper standards activation analysis of Au concentrations above 10 dwt per ton can be done quite easily. It has been shown by Lobanov et al. (3)that it can be advantageous to use cadmium sample containers. With greatly depressed S6Mn( t 1 / 2 = 2.6h) and 24Na( t l / * = 15.0h) activities, it is possible to do quantitative Au analysis after cooling times of 4 to 6 hours only. Figure 5 is part of a gamma spectrum obtained for 0.5 gram of sample No. 11 after an irradiation in a cadmium cover (0.025 inch thick) and a cooling time of 2 hours with the sample 4 cm away from the detector. Figure 6 is a similar measurement but the sample was not canned in cadmium. Leakproof containers could not be obtained in time so that epithermal neutron irradiation was not utilized further. Simultaneously, with the nondestructive activation analysis of gold ore, quantitative analysis of U, Mn, Th, As, and Na can also be accomplished when a Ge(Li) diode is used for gamma spectra analysis. Absolute activities can be determined when the detector system has been calibrated with standard sources available from the IAEA in Vienna. As a practical measure, it is preferable to use a calibration curve which can be obtained with activated ore samples, the contents of which have been chemically analyzed before irradiation. RECEIVED for review August 3,1967. Accepted September 18, 1967.
Adsorption of Silver on Borosilicate Glass Effect of pH and Time
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SIR: In a recent paper West et al. ( I ) reported a rather marked increase in the amount of silver absorbed on glass at a pH of 4 compared to that at pH's of 7 and 8. This is contrary to the accounts of Chambers and Proctor (2), Hensley et al. (3),and Hamester and Kahn (4) who observed a direct relationship between pH and the amount of silver absorbed on glass surfaces. When absorption experiments involving silver-1 10m and hydrous ferric oxides were initiated in this laboratory, it became apparent almost immediately that the containers used for handling the solutions adsorbed appreciable quantities of silver at low concentrations (-0.02 ppm Ag+). It thus became necessary to determine the surfaces most suitable for handling silver solutions and keep a close silver balance in all operations. The results of surface tests are listed in Table I. Borosilicate glass, treated with Beckman desicote (1) Foymae Kelso West, Philip W. West, and Frank A. Iddings, 38, 1566 (1966). ANAL.CHEM., (2) Cecil W. Chambers and Charles M. Proctor, Robert A. l a f t Sanitary Engineering Center, Technical Report W60-4, 18 pp (1960). (3) James W. Hensley, Arthur 0. Long, and John E. Willard, Ind. Eng. Chem., 41, 1415 (1949). (4) Hans L. Hamester and Milton Kahn, USAEC Report SCR-593, 46 pp (1963).
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ANALYTICAL CHEMISTRY
18772 (an organo-silicon product used to produce hydrophobic glass surfaces) gives lowest absorption values and is cleaned easily. However, untreated glass was found to be satisfactory as well. Although molded plastic surfaces adsorbed less strongly than glass surfaces during short intervals (one or two days) they were more difficult to clean, and over a period of several months adsorbed appreciably more than glass. The values obtained in the material tests are of the same magnitude as those reported by Sotnikov and Belanovskii ( 5 ) and Sotnikov et al. (6) who studied the adsorption of a large number of tracers, including Ag+, on germanium, silicon, and quartz surfaces in the presence of various etching agents. The effect of time and pH on the adsorption of silver on brown borosilicate glass is illustrated in Figure 1. The pH 3.8 solutions were obtained by dissolving AgN03 crystals in distilled water and the pH 1.5 solutions by adding "03 to the AgN03 solutions. By comparison Nalgene bottles adsorbed on half the silver from a 0.02-ppm solution at a pH of 3.8 in about 40 days. In one instance a Nalgene bottle (5) V. S. Sotnikov and A. S. Belanovskii, SoGier Radiochemistry (translation) 8, 159 (1966). (6) V . S . Sotnikov, A. S. Belanovskii, and M. I. Kuznetsova, Ibid. p. 238.
r
Figure 2. Room temperature adsorption isotherms showing the adsorption of Ag+ on clear borosilicate glass as a function of Ag + concentration and pH 0 pH 8 o pH7 0 PH6 0 PH 5 Q PH 4
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I
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TIME,
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40
20
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60
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Figure 1. Effect of pH and time on the loss of silver on brown borosilicate glass A . pH = 3.8; B. pH = 1.5. Lower graph 0.01, middle 0.1, upper 1 .Oppm Ag +
adsorbed nearly one third of the silver from a 10-ppm solution during the course of a year. The inverse pH effect observed by West et al. (I) was not observed in the experiments carried out in borosilicate glass beakers in this laboratory. Although the room temperature adsorption isotherms shown in Figure 2 overlap in some instances, a direct correspondence between pH and amounts of Ag+ adsorbed is clearly evident. The overlapping is probably due to the difficulty of cleaning surfaces reproducibly. Of interest is the apparent kink at log x / M = 3. However, the results are not accurate enough to permit speculation in detail on the cause. It may reflect on the nature of the glass surface or layers of A& on the surface. The silver-iron hydroxide adsorption data obtained in the very same beakers and solutions did not exhibit this kink. The values shown in Figure 2 were obtained in the presence of 1% N a N 0 3 and 200 mg of hydrated FezOa. Separate tests showed that the presence of N a N 0 3 suppressed the adsorption on glass by about the same amount as a pH change
Table I. Absorption o f Ag+ on Various Surfaces Contact time = 17 hours; pH = 4; Ag+ concn = 10-6 N Amount of Ag+ ions/cmz adsorbed Material Water rinse" Acid rinseb Borosilicate glass, with desicote 111 x 10'2 14 X 10l2 Borosilicate glass, untreated 358 X 10I2 32 X 10l2 Paraffin 206 X lo1* 267 X 10le Nalgene 174 X 272 X 1Ol2 161 X 10I2 Teflon 448 x 10'2 290 x 10'2 Plexiglas 1080 X 10l2 763 X 10l2 Silicone rubber 1420 X 10l2 1060 X 1012 Lava fired 1460 X 10l2 Specimen washed 3 times in distilled H2O. b In addition to water rinse, specimen was washed 10 minutes with 0.1N HN08in ultrasonic cleaner.
from 6 to 8. The presence of hydrous ferric oxide had no effect on the adsorption of Ag+ on glass. Neither did a 50" C temperature change (from 0" to 50" C). Because of the apparent competition between H+, A@, and Na+ for the adsorption sites on the glass surface, the pH effect evident in Figure 2 must in reality be larger, because NaOH was used to raise the pH-i.e., lower the H+ concentration. WILLYDYCK Geological Survey of Canada Ottawa, Canada
RECEIVED for review July 31, 1967. Accepted October 4, 1967.
VOL. 40, NO. 2, FEBRUARY 1968
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