Radioactive inert gases. Tool for analysis of gases, liquids, and solids

RADIOACTVE NERT GAS Applications of radioactive inert gases in analytical chemistry include their use in volumetric titrations and various thermal methods. Ease of production and measurement should ensure a bright future for analytical techniques which use these materials

N

THE

USUAL

tracer methods,

I radioactive 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, however, 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 gaseous or liquid states and for its own thermal analysis in the solid state. Radioactive inert gas incorporated into a solid will be released by any process, chemical, physical, or mechanical that disturbs the crystalline lattice or only the surface of the solid. It is this fundamental 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 Integrated Circuits in Chemical Instrumentation,” page 30 A, middle column, second equation under 4. De. Morgan’s laws should read AB =

a

+E 16A

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 radioactive gases in the analysis of gases, liquids, and solids and to discuss related problems.

react with the substance. 228Th

&

224Ra

216Po

ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970

5 (1)

Other methods of inert gas incorporation are based on the direct introduction of the gas into solids (without its parent isotope). The following techniques are used. Recoil Energy of Nuclear Reactions: (n,4,

hP>

(n,T),h

Incorporation of Inert Gases into Solids

Different inert gases can be incorporated into various solids by a number of methods. One of them is the classical emanation method developed by Wahl ( 1 ) and Zimens and coworkers (2) which employs natural radioactive gas (radon isotopesemanation). I n this method, incorporation of the inert gas is usually carried out by coprecipitation of trace amounts (0.01 pCi/l g of substance) of the mother isotope of the gas (228Thor 224Ra)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 radioactive disintegration and does not

& 220Rn 1

f)

Solids are irradiated by neutrons in a nuclear reactor and the inert gases Ar, Kr, and Xe are produced (3) as shown in Table I. This method can be used for labeling of alkaline or alkaline earth substances. It is also possible to use neutron irradiation of halides for Table I. Formation of Radioactive Inert Gas Atoms by Nuclear Reactions (n,p) and @,a) Types n, P Li

n, (Y Be

Na Mg K a@Ar260 .41Ar~8 hr Ca a7Ar34 3 d4’Ah 8 d Rb 85mKr44hr87Kr73min Sr ssmKr4.chr Cs Ia3 *Xe2 8 d1=Xe5 3 d Ba 136Xes 2 h,Ia3Xe5 ad

REPORT FOR ANALYTICAL CHEMISTB production of inert gas atoms ( 4 ) (Table 11). The nuclear fission process ( n , f ) ( 5 ) has been used for incorporation of nuclear fission products 133Xeand 85Krinto solids. The energy of 222Rnrecoiled as a product of 22sRa disintegration can also be employed (6) to incorporate R n into powdered substances. Table II. Formation of Radioactive Inert Gas Atoms from Halogens -B

xh,y)

Y --f z a t o b

-6

xw,y)

Y

Y+ +

krypton435 is shown in Figure 1. All of these methods for labeling solids by inert radioactive gases produce more or less stable substances. Inert gas atoms are situated in substituting or interstitial positions of the crystalline lattice. In the substances which build clathrates with inert gases, t,he latter is bound by nonvalent forces regardless of the structure of the solid. Choice of the method for labeling is determined by the character of solid arid the planned application of the labeled substance. Inert Gas Release from Solids

Inert gas incorporated into a solid can be liberated as a result of chemical react’ions, physical trans-

3f

the method of its incorporation. If the inert gas atom is formed by radioactive decay of its parent isotope within a solid by the classical emanation method, the gas atom may escape from the solid in one of the following ways: When the parent atom lies close to the surface of the solid, the recoil energy of the inert gas atom 85 keV may be sufficient to carry it from the solid by recoil, or it may still escape by diffusion before it decays. The theories of both the recoil and diffusion processes have been developed by Flugge and Zimens (12)*

For large grains of solids, lo4 cm or greater, in which the diffusion coefficientof the inert gas is small, the

Gases, Liquids, and Soids

Accelerated Inert Gas I o n Introduction (7, 8 ) . The result is a volume- or surf ace-labeling of the solids concerned, which is dependent on the energy of ion bombardment of the inert radioactive gas used. Diffusion Process at Higher Temperature and Pressure in Inert Gas Atmosphere. Krypton435 is mostly used for this purpose and is incorporated into the crystalline lattice : ( a ) During phase transformation, e.g., of p-quartz into P-cristobalite (930 -C 15°C) or during surface changes of A1203in the temperature region of 600-1000°C (9). ( b ) Over 150 different solidselements, alloys, inorganic, and organic compounds-have been “kryptonated” a t elevated temperature and s5Kr pressure as proposed by Chleck e t al. (10) and Tolgyessy e t al. (11). Crystallization of Solids from M e l t or Sublimation of Solids in Atmosphere of Radioactive Gas ( 1 1 ) . Both these methods have been used mainly for preparation of labeled solids, which build clathrates with inert gases-i.e., quinone, hydroquinone, p-naphthol, benzoic acid, etc. A model of the hydroquinone molecule labeled with

release rate of inert gas emanation formation, or different damages of its crystalline lattice. We will first analyze inert gas release from solids when no chemical or physical transformations take place within the solid. The gas release is dependent on

is given by Equation 2

E

=

yo S E,+Ed= - - ‘ - - . p + 4 M

where

E, = part of emanation re-

.

.

.

,

. . . . .

.

.

.

Figure 1. Scheme of hydroquinone clathrate


17A

Report for Analytical Chemists

leased due to the recoiled emanation atoms, E d = diffusion part of the released emanation atoms, yo = range of recoiling atoms, S = specific surface, M = grain mass, D = diffusion coefficient, p = density, and X = decay constant of emanation. The term E,. is temperature independent and a t room temperature is usually greater than E d for the solids in which the diffusion coefficient of the inert gas is small. At elevated temperatures E d rises because:

where D o = pre-exponential term, Q = activation energy of diffusion of the emanation in the solid, R = gas constant, and T = absolute temperature. Therefore Ed could be evaluated from E total and E , which is obtained as a value E measured a t room temperature: Ed = E - E , When inert gas has been incorporated into a solid directly without its parent isotope, the inert gas release is caused by diffusion processes of various types. A number of equations for this process have been proposed (7,13-15). The inert gas release rate curves, under conditions of linear increase of temperature for a single-jump diffusion mechanism with a discrete activation energy Q, generally involve peaks of which the maxima T , are governed mainly by the activation energy, Q. To obtain the parameter of T , for different depths of inert gas incorporation and single-jump diffusion type under conditions of linearly increasing temperature, Kelly and Matzke (14) derived:

Q = 69.5 T,

+

4.6 loglo T m / R m 2 P m i n

* 4.6

(4)

where Q is in cal/mol; Pminis in "C/min, D o is the pre-exponential factor of the diffusion rate constant in cm2/sec and R, is the median range of the inert gas atoms involved in units 1, which is the mean atomic spacing. Assumption: D o = 3,10-1*1cm2/sec. All these theoretical considera18 A

tions are valid assuming that no chemical or physical transformation takes place in the solid during heating. When the process of recrystallization or free evaporation is considered, other theories apply (15). Measurement of Inert Gas Release

Generally it is possible to measure either the activity of the gas remaining in the solid or the activity of the gas released. Both the quantity of the gas collected in the measuring cell and the release rate of the gas can be measured. The radioactive gas released from the solid substance is carried by a carrier gas (air or nitrogen) from the reaction vessel into the cells for the radioactivity measurement. To measure the a-activity, a scintillation counter, ionization chamber, or semiconductor detectors can be used. All p-activity measurements (Kr, Xe, ,4r) are done by Geiger-Muller (G-11) tubes. Gamma-active gases can be measured by a gamma-spectronieter. Applications

The radioactive isotope of 85Kris most often used as an indicator because of its advantageous nuclear properties (relative long half-life = 10.76 years, and suitable energy of p-radiation = 0.7 MeV). As mentioned above, the basic presumption for the use of solids labeled with krypton (kryptonates) in analytical chemistry is that in entering into the chemical reaction, the crystalline lattice of the kryptonates will be destroyed and the radioactive krypton released. I n the course of a chemical reaction, the release rate of s5Kr is proportional to the reaction rate and this reaction rate is a measure of the concentration of the gas. The only requirement is that a solid exists that will react with the species of interest. The quantitative determination can be done by: Using a calibration curve Comparing with a standard or using a device calibrated by a previous standard Using titration methods The rate of ssKr release is 60 closely related to the rate of the


chemical reaction that this release may be used to determine the kinetics and shed light on the mechanism of particular reactions. Inasmuch as the kryptonates can be stabilized for use a t elevated temperatures up t o the melting point of the host solid, this technique is being employed a t elevated temperatures, for example, to study high-temperature oxidation of refracting materials. Such analyses can be done only rarely using specific radioisotopic tracers of the solids. I n addition to the need for a specific isotope, which only fortuitously may have a satisfactory type and energy of reaction and a suitable half-life, the reaction product must be gaseous. This latter restriction is particularly limiting. On the other hand, the use of kryptonates allows this type of analysis for any substance provided only that a solid exists which reacts with this substance. Through this approach 85Kr has become a universal tracer. The krypton-% incorporated by ion bombardment or diffusion methods into a solid to yield a kryptonate is concentrated near the surface of that solid (in the first l e 1 0 6 A ) . Bemuse of this near-surface location of the 85Kr,the kryptonates of this mode of preparation are particularly sensitive to surface phenomena. This near-surface concentration of the B5Krprovides all the sensitivity of a high specific activity in that portion of the solid involved in the reaction. At the same time, all the safety and handling advantages of low total activity are present (specific activities average -lo3 pCi/cm2 and 100 pCi/l g of the kryptonated solid). As to the biological risk of using radioactive kryptonates, krypton-85 is an isotope of an inert gas and under normal conditions does not enter metabolic systems and therefore is very safe. Some examples of analyses via kryptonates are given below. Determination of Gases

Methods have been proposed for the determination of gaseous components and traces of impurities in air--e.g., ozone, oxygen, SOn, F, C1,

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C1F3, NO, NO2, amines, and gaseous hydrogen. Some of these methods are intended for the determination of these components in the atmosphere of other planets e.g., Mars and Venus. They are of great importance also in the determination of components mentioned in the atmosphere of hazardous work places. As an example of gas analysis via the kryptonates, let us consider the measurement of hydrogen by means of Pt02-kryptonate. The apparatus proposed by Chleck (16) is shown in Figure 2. The reaction vessel, 2, contains kryptonated PtOz, 3. A GM-tube is joined to it. Decrease of activity of the kryptonated sample is measured by a count-rate meter, 5. Figure 3 demonstrates the dependence of the rate of activity loss of Pt02-kryptonate in per cent per minute a t various concentrations of hydrogen. When 85Krrelease from a solid is measured a t different temperatures and the results are plotted logarithmically against reciprocal values of absolute temperature, the order of the reaction is determined. This order is given directly by the slope of logarithmic “Arrhenius plot.” The following equation for the kinetics of the process a t various

Exhaust Air

3

Figure 2. Hydrogen detection system, after Chleck (16)

1. Metering valve 4. Geiger-Muller tube 2. Reaction vessel 5. Counts-rate meter 3. Kryptonated solid 6. Flow-meter

temperatures T and concentrations X was proposed by Chleck (16)

+ + 2 H20 + 4 OH- + 85Kr(,)

4 T1[85Kr] 0, 4 T1+ I n the Trace Lab Research Institute, two types of apparatus for the determination of oxidizing and reducing impurities in the atmosphere in concentrations of the ppm order were developed. Gases that can react directly or indirectly (by chemical, catalytic, or thermal reaction) with the radioactive kryptonated solids are measurable by these instruments.

Figure 3. Response of Pt0,-kryptonate to hydrogen at room temperature and nitrogen atmosphere 20A

*

Determinations of Liquids

I n addition to gas analysis via the kryptonates, determinations in s o h 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 CaCz has been used as the kryptonated reaction solid, and the activity of 85Krreleased has been measured. For 0.25-2.0% HzO in methyl alcohol, the activity of *SKr released and the amount of water are directly proportional. The determination of oxygen dissolved in water or other liquids is a difficult analytical problem. However, radioactive thallium kryptonate can be used. The determination is based on the reaction :


+

The radioactivity decrease of the thallium kryptonate in distilled water is linear with the concentration 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 kryptonates in radiometric titration methods (11). The application of radioactive kryptonates for end-point indication 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 85Kr 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 interfere with the end-point indication. If this interfering reaction on the surface of the kryptonate is slow, i t can increase the value of the background. However, the krypton release caused by the addition of an excess of the titrant will be detectable. If the interfering reaction is so quick that it is impossible to determine the onset of the krypton release, either the interfering ion must be removed or there must be an-


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Figure 4. Two typesof apparatusfortitration utilizing kryptonate as end-point indicator: (a) after Tolgyessy et al. (ll), (b) after Chleck (17)

1. Nitrogen supply 2. Regulator and valve 3. Geiger-Muller tube 4. I-ieaction vessel 5. Buret

6. 7.

Counting cell Proper countsrate meter 8. Flowmeter 9. Magnetic stirrer

other kryptonate chosen. Radioactive kryptonates commonly used in volumetric analysis are listed in Table I11

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1 Detect, Analyze and Measure

I

WATER POLLUTION WITH POLAROGRAPHY

~

Table Ill. Radioactive Kryptonated I Indicators I Radioactive kryptonate Substance, of detd.

M k< Zn

NaOH NaOH;

F-

Titrant soln.

! I

HCI HCI; Th(N0s)r K2Cr207 KCN EDTA

Ag Ad

Ba2+ Ni*+

AglOJ

Ca2-, Mg2+, Sr2+

Y3(C20&

Fe3+

EDTA

Glass

Ca2+, CdZC Th4’, H2SOa HCI, HNOa

NaF NaOH

SAMPLE

specimen groups to determine presence or absence of anions such as nitrite, nitrates, sulphate and others. ~ o ~ a r o e r aot ~ nriver water s n o w \ n g c o p ~ e r n. i c k e l ana i r o n C o n c e n t r a t i o n a t PPM ie“el5.

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H M EASU R E 1

Two types of experiinental apparatus for radiometric titrations with radioactive krypronates as 1 end-point indicators are shown in 1 Figure 4. 1

g r a p ? s h o w i n g zinc. l e a d , an!imony and copper a t cOnCentrdtiOnS O f

lesi

[ha”

1 0 0 PPB

quantities to parts-per-million, or even partsper-billion, w i t h anodic stripping.

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ANALYTICAL CHEMISTRY, VOL. 42, NO, 9, AUGUST 1970

21 A


The first type proposed by T61gyessy and others (If) is based on the measurement of 85Kr loss; the second one, proposed by Chleck ( 1 7 ) ,on the measurement of a rate of the 86Kr released from solids. The solution, the concentration of which is sought, is placed in the titration vessel (Figure 4a), 4, in the first tvue of annaratus directlv (1igure uul ,a w v i m u e is connected with counting cell, 6, and a kryptonated indicator is added into the solution tested. The titrant is added with a buret, 5. During titration, nitrogen from a tank is bubbled through the solution. Figure 5 shows the titration vessel of the first apparatus in detail. It is also possible to use radioactive kryptonates as end-point indicators in automatic analyzers providing continuous titration.

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In the titration of cation M with a complex-forming titrant C:

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M+C$MC (7) the end-point is determined by the kryptonated solid, designed AB[85Kr]. The excess of titrant C reacts with the kryptonated solid and forms a soluble compound according to the reaction:

+C

~~~~.

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AC B 86Krcs) (8) whereby gaseous radioactive krypton is released. The equilibrium of Vladimir Balek was born in 1940 in Reaction 7 is characterized by the Bohemia. He received his M.Sc. stability constant of the MC com( I S S l J from ths Technical Univerplex and by the solubility product of the kryptonated solid, AB [85Kr]. sity of Prague, Czechoslovakia, and his Ph.D. (1967) from Lomonsow Titration can be carried out if State University in Moscow, USSR. the relationship between the staDr. Balek is an assistant professor bility constants of the complexes at Charles University, Department MC and AC is: PKYG> PKAC and if the ratio between the stability constant of the AC complex and the solubility product of the oLIr ,I KrypTionaTie nn rL-nrj permi-.. Tine dissolution of the kryptonate by the titrant according; t o Reaction 8. During titraticIn to the end point, Reaction 7 takes place. The radioactivity of the :kryptonate is con3

I STA

22A

stant. Afte point, the exotm VI U K U U ~ Uu reacts with A and simultaneously dissolves the radioactive kryptonate AB[86Kr3 and releases s6Kr. The radioactivity of the kryptonate proportionally decreases (and activity of 86Kr in carrier gas increases) with the amount of the agent C added in excess. The titration 2 is shown in Figure 6. titrations, precipitation reacw ~ n 8are also often used as endpoint indicators. The excess of the titrant after the end point causes either the dissolution of the surface of kryptonated metal, disturbance of the surface of the kryptonated glass, or redox reactions on the surface of the radioactive kryptonate used. Kryptonated glass surface is most easilv destroyed bv HF or bases. Therefore t i e ra"dioactive glass kryptonate can be used with advantage as an indicator in the precipitation titration, where the cations determined form a barely dissolving precipitate with fluoride

.- ..-

I

.

. .. .

1-

- . .._

7

ANALYTICAL CHEMISIKY, VUL. 42, NU. 9, AUtiUSl 1970

of Radiochemistry, Prague. His research work concerns primarily the application of radioactive inert gases in various fields, such as analytical 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.


b

U

a

Figure 5. Titrating vessel, after Tolgyessy et al. (11); (a), mica bottom; (b), tube nitrogen supply

ions or in the titration of acids with bases. Cd2+, Ca2+, and Th4+ were determined by th'IS 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 Z n 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. I n 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). I n 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

Figure 6. Theoretical curves of radio-complexometric titration Curve 1. I n case when loss of the indicator radioactivity is measured Curve 2. I n case when radioactivity of krypton45 is measured

[emanation thermal analysis (ETA) ] under identical conditions, have recently been developed (1820).

A schematic of the reaction vessel is shown in Figure 7. Apart from the labeled sample (roughly 100 mg), a thermographic standard (A1203) 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 a t a constant flow rate into cells for gas radioactivity measurement. We have deyeloped a device allowing simultaneous recording of the a-activity of radon and the p-activity of xenon or krynton used in various

Figure 7. Reaction vessel of the emanation thermal apparatus (21) 1. Activated sample 2. Thermographic standard 3. Sample for dilatometric measurement 5. Thermocouples 6. Quartz rod of dilatometer 7. Quartz crucibles for samples 8. Holdertube 9. Metal block 10. Quartz reaction vessel proper 11. Ground glass joint 12. Cooler 13. Rubber seal


23A

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types of labeling methods. Temperature rise curves, ETA, DTA, and dilatometric curves are automatically recorded. At least three curves should be run for each of the measurements provided, and the thermal analysis conditions should be kept constant. More recently we have proposed another apparatus allowing simultaneous measurement of emanation release, DTA, dilatometry, and electrical conductivity. A general scheme of this apparatus is shown in Figure 8. Figure 9 shows the central part, which is placed in a

furnace. In Figure 10a and b, ETA curves (called also emanograms) and DTA I:urves (thermograms) of two com12ounds which suffer dehydration tnd decomposition on beating are rhown (21). CaS04 . 2H20 (Figure loa) exiibits in both types of curves two 3eaks revealing two steps of dehyIration and one peak a t 300-4Oo0C ihowing the transformation of the netastahle soluble form of the aniydrous calcium sulfate into the inLioluble one. Emanogram and thermogram of Fiaure 9. Central Dart of the emanation thsrmal iappararus permit1 ng 51mL taneous miBasurement of ETA, DTA. dila. ".. y o, electrical condLctiv ly ~ # t s ~ rd n

...--.

F i g u r e 8. G e n e r a l scheme of the emanation thermal apparatus, ETA (21)

~

FeS04 7Hz0 (Figure lob) show three-stage dehydration of ferrous sulfate, basic ferric sulfate formation and dehydration, and finally a decomposition of the salt to ferric oxide. An amorphous phase of ferric oxide is initially formed which changes to the crystalline one. The maxima of the emanograms coincide almost exactly with the onset temperature of the endothermic effect on thermograms due

Figure 10. Results of a thermal study (ETA and OTA) of (a) CaS04.2Hz0and (b) FeSO4.7H10

1. Emanogram; 2. Emanograms of a sample preheated to 210 and 11OO"C,respectively; 3. Thermograms


25 A

Figure 11. ETA and DTA results of a thermal study of KNOS(21) 1. Emanogram during heating 5. Thermogram during heating 2. Emanogram during cooling 6. Thermogram during cooling 3. Emanoaram during reheating

to the dehydration, decomposition, or other phase transformations. The characteristic analogy between emanograms and thermograms has also been demonstrated in a study of the phase changes of potassium nitrate (21). At a temperature of 130°C (Figure 11) transition of the orthorhombic to rhombohedral form occurs and forms a maximum on both DTA and ETA curves. Before the occurrence of any structural or chemical transformation (below 130°C in this case), the rate of release of emanation is due exclusively to diffusion and rises continuously with increasing temperature according to the exponential law (Equation 2 ) . If there is a polymorphic transformation or chemical reaction in the solid, this is marked by a definite change. At the temperature of the transformation a peak appears. The bottom of the peak of the emanogram is the temperature of the first stay of the transformation when a new crystalline lattice begins to form. On the other hand, the DTA peak corresponds to the maximal rate of the respective process in whole volume. The second peaks, both on the ETA and DTA curves in Figure 11, correspond to the melting point of

KN03. Mixtures of different substances were also studied by the emanation method. One of the components is labeled with the inert radioactive gas, and the release curve of the mixture is run. Thus changes in the surface and the state of the crystal lattice of individual components of the mixture and also the formation of the reaction product, e.g., spinel type, can be investigated. As an example, let us show results Fe203mixof the study of ZnO ture (21). ZnO was labeled with 228Th,and the stoichiometric mixture was prepared. Heating gives rise to zinc ferrite demonstrated in Figure 12. The emanogram (curve 3) has a peak a t 790°C corresponding to the termination of the ferrite formation. The emanogram of zinc ferrite (curve 4) has no maximum. The position of the characteristic maxima on the emanograms or other gas-release curves (similarly

+

Figure 12. Thermoemanation analysis of a mixture ZnO-Fe208(21) Curves: 1, dilatogram; 2, thermogram; 3, emanogram; 4, emanogram during reheating. The numbers indicate the percentage of ZnO which has undergone the reaction i n samples I-VI

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as the DTA curves) can be used for the detection of the presence of certain substances in the mixture analyzed. ETA of inorganic polymers and organic macromolecular compounds has been successfully shown (21). Zaborenko and coworkers (22,23) also used ETA for the construction of the equilibrium diagrams of multicomponent mixtures. Systems of KC1-Ca Clz, CaO-Fez03, NaBeF3-NaCl2PO3 urea-phosphoric acids were studied. The emanation method seems to be very useful in the study of glass-forming or bad crystallizing mixtures where DTA or X-ray methods do not give satisfactory results. Utilization of this method for the thermal analysis of minerals (e.g., spodumen, eleolite, pollucite) is possible. Unfortunately the emanation method has not yet been widely used in the quantitative determination of solids. A method of absolutely repeatable incorporation of the gas into the solid is essential for a comparison of results. Peak area could then be taken for the quantitative evaluation of the substance amount. Various crystalline modifications and different lattice dis-

order forms of the same substance could be analyzable by ETA. Substances which normally undergo no chemical or phase changes can be characterized by an inert gas-release curve. As has been shown by Jech (16, 24), inert gases previously incorporated into a sample by ion bombardment are released from the solid mostly in a characteristic way. Inert gas diffusion proceeds differently compared to a perfect crystal being influenced in a characteristic way in the course of tempering by processes of lattice reordering or damage annealing. Characteristic maxima appear on the release curve, which offers certain possibilities for identification of crystalline form and nature of the substance studied. I n Figure 13, examples of the characteristic release curves of and Si are shown. 85Kr and 133Xehad been incorporated by ion bombardment a t 10 keV. The inert gas release methods have been successfully used for solution of a number of problems related to analytical chemistry. As follows from Equation 2, when no chemical or physical con-

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Figure 13. Krypton-85 release curves of (a) Al2O3 and (b) Si; after Jech (24),B6Kr being incorporated by ion bombardment of 10 keV

.2,NO. 9, AUGUST 1970


versions take place within the solid, its release rate of emanation measured a t room temperature is directly proportional t o specific surface area E = IC * S. This is the principle of the emanation method employment to the study of aging of precipitates ( 1 ) or study of powder materials sintering a t elevated temperatures (66). The utilization of the emanation method is advantageous for the continuous qualitative observation of specific surface changes under

experimental dynamic conditions. The reactivity and sintering activity of ferric oxide obtained by thermal decomposition of various iron salts a t different temperatures were studied by means of the emanation method in our laboratory (26). As shown in Figures 14a and b, emanation release curves of FezOS prepared from different iron salt a t 1100°C differ. This is more clearly demonstrated in the graph plotted in semilogarithmic coordinates -

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Figure 14. Temperature dependence of the emanation release of P = FezOI prepared from different iron salts during heating to 1100°C (26) (a) Coordinates: a) x = T,y = E = f(T); b) x = 1/T, y = log Ed = f (1/T) (b) As initial iron salts were taken: iron carbonate (curve 1and ferrous sulfate heptahydrate (curve 2 and o), Mohr's salt (curve 3 and O), and ferrous oxalate dihydrate (curve 4 and +)

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n),

log (E - E,) = f (l/Z’). Activation energy of diffusion of emanation evaluated from the slope of this “Arrhenius curves” can be used as a characteristic parameter of the reactivity or sintering activity. The smaller its value is in the lowtemperature range, the greater the reactivity, due t o the particle size and nonequilibrium defects of crystalline lattice, the ferric oxide exhibits. Also the emanation method is more objective and more suitable than DTA, surface area, and catalytic activity measurements for choice of starting materials in important industrial reactions of solids (e.g., ferritization) (27).

renaissance. Radioactive inert gases of artificial production and improved measuring apparatus have all contributed to the renewed interest and usefulness. A radioisotopic method of analysis that retains all the precision and accuracy inherent in a volumetric titration while providing an objective end point has been developed by using the kryptonates ; the possibilities in the thermal analysis of solids have been improved with these techniques. Easy automation of the radioactive inert gas measurement promises this method a bright future. Acknowledgment

Applications of inert radioactive gases offer some new possibilities in analytical chemistry. The old emanation method with natural radon isotopes, which lost favor in the postwar years, has experienced a

The author is particularly grateful to Professor J. Tolgyessy, Slovak Technical University, Bratislava, and Professor 6. Jech, Institut of Physical Chemistry, Czechoslovak Academy of Sciences, Prague, for offering of manuscripts in advance of publication.

Literature Cited (1) .A. C. .Urahl and N.A. Bonner, “,a,; dioactivity Applied to Chemistry, John Wiley, New York, N. Y., 1951, p 284. (2) K . E. Zimens, 2. Phys. Chem., A 191, 1 (1942); A 191, 95 (1942); A 192, 1 (1943). (3) F. W. Felix and €1. Seelig, Nukleonik, 8, 389 (1967). (4) T. S. Elleman, L. S. Mears, and R. P. Christman, J. Amer. Ceram. SOC., 51, 560 (1968). (5) S. Yajima, S. Ichiba, Y. Kamemoto, and K. Shiba, Bull. Chem. SOC. Jap., 33, 426 (1960) ; 34 133 (1961). (6) R . Lindner and Hj. Matzke, 2. Naturforsch., 15a, 1082 (1960). (7) G. Carter and J. S. Colligon, “Ion Bombardment of Solids,” Heinemann, London, 1968. (8) 6 , Jech, I n t . J . Appl. Radiat. Isotop., 8, 179 (1960). (9) R. Sizmann and W. Rupp, 2. Naturforsch., 16a, 861 (1961) ; P. J. Hidalgo, C. Merin, and R. Sizmann, 2. Phys. Chem. Neue Folge, 54, 277 (1967). (10) D. I. Chleck, R . Maehl, and 0. Cucchiara, I n t . J . Appl. Radiat. Isotop., 14, 581 (1963); 14, 593 (1963); 14, 599 (1963). (11) J..Tolgyessy and S . Varga, Talanta, in print. (12) S. Flugge and K. E. Zimens, 2. Phus. Chem.. B 42. 179 (1939). (13) W . Inthoff and K. E. Zimens, Trans. Chalmers Univ. Technol., Goteborg, No. 176, 1956. (14) R. Kelly and Hj. Matzke, J. Nucl. Mater., 20, 171 (1966).

(15) Jech and R. Kelly, J. Phys. Chem. Solzds, 30, 465 (1969). (16) D. Chleck, “Symposium on Radiochemical Methods of Analysis,” IAEA, Paper No. SM-55/41, 1969. (17) D. I. Chleck, “Radiochemical Methods of Analysis,” Vienna, 1965, p 273. (18) P. BussiBre, B. Claudel, J. P. Renouf, Y . Trambouze, and M. Prettre, J . Chim. Phys., 58, 668 (1961). (19) K. B. Zaborenko, L. L. Melikhov, and V. A. Portyanoy, Radiokhim., 7, 319 (1965). (20) V. Balek, J . Mater. Sci., 4, 919 (1969); J. Vachugka, 0. VojtGch, V. Balek, M . Voboiil, and L. SchejbalovL, Report 2339, Nuclear Research Institute, R e i near Prague, 1970. (21) V. Balek, J.. Mater. Sci., 5, 166 (1970); J . Radzoanal. Chem., 2, 315 (1969). (22) K. B. Zaborenko, V. .P. Polyakov, and I. G Shoroshev, Radaokhzm. 7, 324 (1965) ; h i d . , 7, 329 (1965), (23) A. I. Czekhovskikh, D. Nitzold,.K. B. Zaborenko, and S. I. Volfkovich, Z h . Neorg. Khim., 11, 1948 (1966). (24) 6. Jech and R. Kelly,!. Phys. Chem. Solids, 30, 465 (1969); C. Jech, Paper presented at I11 Radioanalytical Conference, Star$ Smokovec, Czechoslovakia, 1967. (25) J. F. Gourdier, P. BussiBre, and B. Imelik, C o m p t . Rend., 264, 1625 (1967). (26).V. Balek, 2, Anorg. A&. Chem., in print. (27) V. Balek, J . Appl. Chem. (London), 20, 73 (1970).

Conclusion

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