RADIOACTIVE INERT GASES Tool for Analysis Applications of radioactive inert gases in analytical chemistry include their use in volumetric titrations and various thermal methods. Ease of production and measure ment should ensure a bright future for ana lytical techniques which use these materials
THE USUAL tracer methods, INradioactive atoms easily traceable
in minimum concentrations are used as indicators for the behavior of chemical elements which are isotopic with radioactive atoms. The radioactive gaseous elements, how ever, can be used for other purposes too. If we have a solid compound which contains a radioactive inert gas, we may use this compound for analysis of other elements in gas eous or liquid states and for its own thermal analysis in the solid state. Radioactive inert gas incorpo rated into a solid will be released by any process, chemical, physical, or mechanical that disturbs the crystalline lattice or only the sur face of the solid. I t is this funda mental property which underlies the use of solids labeled by inert radioactive gases in analytical chemistry. Moreover, we can investigate the Correction: In the July Report for Analytical Chemists, "Using Inte grated Circuits in Chemical Instru mentation," page 3 0 A, middle col umn, second equation under 4 . DeMorgan's laws should read AB = Ά
+B
16 A ·
molecular state of the solid, its inner surface, or its deformations due to aging to molecular and chemical conversions. We can also follow the processes of reactions in the solid state from the amount of the radioactive gas escaping from the compound. The purpose of this report is to show applications of inert radio active gases in the analysis of gases, liquids, and solids and to discuss re lated problems.
react with the substance. 228 T h 4 . 2 2 4 R a Λ 2 2 0 R n _ ^ 21βρ 0 ^
Other methods of inert gas in corporation are based on the direct introduction of the gas into solids (without its parent isotope). The following techniques are used. Recoil Energy of Nuclear Reac tions : (η,α),
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
(η,ρ)
Κ τ), KO
Incorporation of Inert Gases into Solids
Different inert gases can be in corporated into various solids by a number of methods. One of them is the classical emanation method developed by Wahl (J) and Zimens and co workers (2) which employs natural radioactive gas (radon isotopesemanation). In this method, in corporation of the inert gas is usually carried out by coprecipitation of trace amounts (0.01 μ,Οϊ/Ι g of substance) of the mother isotope of the gas (228Th or 224 Ra) from a solution in the course of preparation of the substance studied. The inert radioactive gas is formed in the substance as a consequence of radio active disintegration and does not
(1)
Solids are irradiated by neutrons in a nuclear reactor and the inert gases Ar, Kr, and Xe are produced (S) as shown in Table I. This method can be used for labeling of alkaline or alkaline earth sub stances. It is also possible to use neutron irradiation of halides for Table I. Formation of Radioactive Inert Gas Atoms by Nuclear Reac tions (η,ρ) and ( η , α ) Types η,ρ Li Na Κ
η, a Be Mg
"Ar260.UAri.shr
Ca
Rb ""KruhrO'Krnmin Sr
Cs
133m
Xe2.s^Xes.3d
87
Ar M .,,,«Ari.,
Ο ,
ϋ 001
0.01
01 H-LoncntMtion, "4
10
10.0
Figure 3. Response of FtCvkryptonate to hydrogen at room temperature and nitrogen atmosphere 20 A ·
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
Determinations of Liquids
In addition to gas analysis via the kryptonates, determinations in solu tion can also be accomplished by direct reaction between the species of interest and a kryptonated solid (17). To determine trace water in organic liquids, kryptonated CaC 2 has been used as the kryptonated reaction solid, and the activity of 85 Kr released has been measured. For 0.25-2.0% H 2 0 in methyl alcohol, the activity of 85Kr re leased and the amount of water are directly proportional. The determination of oxygen dis solved in water or other liquids is a difficult analytical problem. How ever, radioactive thallium krypto nate can be used. The determina tion is based on the reaction : 4 Tl[85Kr] + 0 2 + 2 H 2 0 -* 4 T1+ + 4 O H - +
86
Kr(g)
The radioactivity decrease of the thallium kryptonate in distilled water is linear with the concentra tion of oxygen dissolved down to the lowest concentration 0.3 ppm measured. A special application of solids labeled with radioactive inert gas is the utilization of radioactive kryp tonates in radiometric titration methods (11). The application of radioactive kryptonates for end-point indica tion requires that a kryptonated solid does not react with a solution, the concentration of which is sought by titration, but does react with the titrant. The onset of the release of 85 Kr by the kryptonate marks the appearance of excess titrant and hence the passing of the equivalence point. This concept provides a new completely objective type of endpoint indicator. Substances reacting with the kryptonated indicator could inter fere with the end-point indication. If this interfering reaction on the surface of the kryptonate is slow, it can increase the value of the back ground. However, the krypton re lease caused by the addition of an excess of the titrant will be detect able. If the interfering reaction is so quick that it is impossible to de termine the onset of the krypton re lease, either the interfering ion must be removed or there must be an-
Report for Analytical Chemists
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Figure 4. Two types of apparatus for titration utilizing kryptonate as end-point indicator: (a) after Tôlgyessy et al. ( I l ) , (b) after Chleck (17) 1. 2. 3. 4. 5.
Nitrogen supply Regulator and valve Geiger-Muller tube Reaction vessel Buret
6. 7. 8. 9.
Counting cell Proper countsrate meter Flowmeter Magnetic stirrer
other kryptonate chosen. Radioactive kryptonates commonly used in volumetric analysis are listed in Table III.
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WATER POLLUTION WITH P0LAR0GRAPHY
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Table III. Radioactive Kryptonated Indicators Radioactive kryptonafe of
Substance, detd.
Titrant soin.
Mg
NaOH
HCI
Zn
NaOH;F"
HCI; Th(N03)4
Ag
Ba2+
K 2 Cr 2 0 7
Agi
Nis+
KCN
Agl03
C a 2 + , Mg 2 +, Sr2+
EDTA
Y2(CA)3
Fe 3 +
EDTA
Glass
C a 2 + , Cd 2 + T h 4 + , H 2 SOi HCI, H N 0 3
NaF NaOH
Two types of experimental apparatus for radiometric titrations with radioactive kryptonates as end-point indicators are shown in Figure 4.
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ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970 ·
21 A
Report for Analytical Chemists
'I B l MH matt
T h e first type proposed b y Tolgyessy and others (11) is based on the measurement of 8 5 Kr loss; the second one, proposed by Chleck {17), on t h e measurement of a r a t e of the 8B Kr released from solids. T h e solution, the concentration of which is sought, is placed in t h e titration vessel (Figure 4 a ) , 4, in the first t y p e of apparatus directly connected with Geiger-Mûller tube, 3, and count-rate meter, 7. I n the second type (Figure 4 b ) , a G M tube is connected with counting cell, 6, and a kryptonated indicator is added into t h e solution tested. T h e t i t r a n t is added with a buret, 5. During titration, nitrogen from a t a n k is bubbled through t h e solution. Figure 5 shows the titration vessel of the first apparatus in detail. I t is also possible to use radioactive kryptonates as end-point indicators in automatic analyzers providing continuous titration.
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I n the titration of cation M with a complex-forming t i t r a n t C :
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22 A ·
C^± AC + Β +
85
Kr fe)
(8)
whereby gaseous radioactive k r y p ton is released. T h e equilibrium of Reaction 7 is characterized by t h e stability constant of the M C com plex and by the solubility product of the kryptonated solid, AB [ 8 5 K r ] . Titration can be carried out if the relationship between t h e sta bility constants of the complexes M C and AC is: pKmc > pK A o
j
CITY
(7)
MC
the end-point is determined b y the kryptonated solid, designed A B [ 8 5 Kr]. T h e excess of t i t r a n t C reacts with the kryptonated solid and forms a soluble compound according to the reaction :
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stant. After exceeding the end point, t h e excess of the t i t r a n t C reacts with A and simultaneously dissolves t h e radioactive kryptonate A B [ 8 B K r ] and releases 8B Kr. T h e radioactivity of the kryptonate pro portionally decreases (and activity of 8 8 Kr in carrier gas increases) with the amount of t h e agent C added in excess. T h e titration curve is shown in Figure 6. I n titrations, precipitation reac tions are also often used as endpoint indicators. T h e excess of the t i t r a n t after the end point causes either the dis solution of the surface of k r y p t o nated metal, disturbance of the sur face of the kryptonated glass, or redox reactions on t h e surface of the radioactive kryptonate used. Kryptonated glass surface is most easily destroyed b y H F or bases. Therefore t h e radioactive glass kryptonate can be used with ad vantage as an indicator in the pre cipitation titration, where t h e cat ions determined form a barely dis solving precipitate with fluoride
and if the ratio between the sta bility constant of the AC complex and the solubility product of the kryptonate A B [ 8 5 K r ] permits the dissolution of t h e kryptonate b y t h e t i t r a n t according to Reaction 8. During titration to the end point, Reaction 7 takes place. T h e radio activity of the kryptonate is con
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
Vladimir Balek was born in 1940 in Bohemia. He received his M.Sc. (1961) from the Technical Univer sity of Prague, Czechoslovakia, and his Ph.D. (1967) from Lomonsow State University in Moscow, USSR. Dr. Balek is an assistant professor at Charles University, Department of Radiochemistry, Prague. His re search work concerns primarily the application of radioactive inert gases in various fields, such as an alytical chemistry, solid-state chemistry, and materials science. He has made several lecture tours to different European countries and is author or coauthor of numerous papers in scientific journals. He is also the author of two patents.
Report for Analytical Chemists
I I a Figure 5. Titrating vessel, afterTôlgyessy et al. ( I l ) ; (a), mica bottom; (b), tube nitrogen supply
ions or in the titration of acids with bases. Cd 2 +, Ca 2 +, and Th 4 + were determined by this manner. Radioactive kryptonate of glass is also used as an end-point indicator of the titration of acids with strong bases; radioactive krytonates of Mg and Zn were then used as indicators in the titration of strong bases with strong acids. Study of Solids
Apart from physical methods of analysis such as DTA, TGA, and
dilatometry used in the study of solids, the emanation methods are of interest. In these methods the radioactive inert gas release makes it possible to follow continuously various types of changes taking place during processes in a solid at the temperature of the experiment. These include such solid chemical reactions (dehydration, thermal decomposition, synthetic reactions) as polymorphic transformations, melting, conversion of metastable amorphous structures into crystalline ones, and changes in the concentrations of defects in the crystalline lattice. The emanation method has a number of advantages over the others: Under dynamic experimental conditions, it makes possible the study of structural changes of substances even when these changes are not related to a thermal effect (e.g., phase transformations of the second order). In other cases, when finely crystalline or amorphous phases are formed, the emanation method is more sensitive than X-ray analysis. For fuller interpretation of the experimental results, however, physiochemical methods mentioned above are frequently employed. Devices, which permit simultaneous measurements of DTA, TGA, dilatometry, and emanation
[emanation thermal analysis (ETA) ] under identical conditions, have recently been developed (18— 20). A schematic of the reaction vessel is shown in Figure 7. Apart from the labeled sample (roughly 100 mg), a thermographic standard (AI2O3) and a sample for dilatometric measurement are placed in the heated metallic block. A heating rate of 8 to 10°C/min is usually used, corresponding to the optimum for emanation as well as DTA measurement. The radioactive gas released from the solid substance is carried by a carrier gas stream at a constant flow rate into cells for gas radioactivity measurement. We have developed a device allowing simultaneous recording of the α-activity of radon and the /?-activity of xenon or krypton used in various
-13
Al 12
ABi85Krl + C • AC + Β +
A2
1
s5
Kr
