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Coulo-bipotentiometric titrations in the anlysis of chlorine in chlorine-doped cadmium chalcogenides
Electroanalytical Chemistry and Interfacial Electrochemistry, 57 (1974) 259-264
259
© Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
C O U L O - B I P O T E N T I O M E T R I C TITRATIONS IN THE ANALYSIS OF C H L O R I N E IN C H L O R I N E - D O P E D C A D M I U M C H A L C O G E N I D E S C. PELOSI, C. PAORICI, G. ATTOLINI and G. ZUCCALLI
Laboratorio MASPEC-CNR, 43100 Parma (Italy) (Received 29th July 1974)
INTRODUCTION
The determination of halogens, in the 0.1-2000 p.p.m, range, is of particular interest insemiconductor technology, in all those cases where vapor-phase chemicaltransport methods 1'2 are performed with halogens (or hydrohalogen acids) as transport agents. In fact the amount of halogen which remains incorporated in' the crystallized matrix plays an important role not only in the growth mechanism, but also in the electrical and optical properties of the semiconductor 3. In spite of that, many problems whose solution implies a previous determination of the halogen content in the solid matrix, are generally postponed since the analytical methods developed up to now to detect halogens, such as those based on mass spectrometry 4, neutron activation 5'6, radioactive tracers 7, are as a rule expensive, cumbersome, and do not comply with the requirements of simplicity and time saving necessary for a routine work. Spectrophotometric methods 8- lo, which in principle could met the above requirements, are impractical because of the intrinsic difficulty to separate and concentrate the halogen. The necessity to find a simple and time saving technique to reveal halogens in cadmium chalcogenides lead us to take into consideration the differential electrolytical potentiometric (d.e.p.) methods12. Such methods, amply studied both theoretically and experimentally, are reported 13 to allow detection of chlorine and bromine down to 10 - l ° mol of chloride or bromide at 2 x 10 -s M in a medium of 0.01 M nitric acid in 80:20 methanol-water. Applications of these techniques in practical cases have already been described x4, x5. In this paper we refer to the possibility of detecting chlorine, in the 4(~1800 p.p.m, range, in small-sized (20-100 mg) Cd-chalcogenide samples, by means of a d.e.p, version of argentometric titration, where the silver ions are coul.o-generated. An extension of the method to the determination of bromine and iodine will be the object of a forthcoming paper. DESCRIPTION OF THE APPARATUS
The titration cell (similar to that described in ref. 14) and the circuit diagram are schematically shown in Fig. 1.
260
C. PELOSI, C. PAORICI, G. ATTOLINI, G. ZUCCALLI
Indicator electrodes and d.e.p, circuit The indicator electrodes were prepared from 1 mm diam. silver wire, fixed in a perspex block by means of an epoxy resin. After hardening of the resin, the two wires were cut and ground-off square so as to have only the cross section exposed. The electrodes were then activated by dipping them in 40 ~ nitric acid for 30 s. When necessary, the contaminated electrodes were renewed by grinding and repeating the activation process. The indicating d.e.p, circuit follows conventional practice. The indicator electrodes were externally connected to a constant-current generator, where the current could be varied in the 20nA-100pA range, within about 0.2 ~. Potentiometric measurements (in mV) were made by a 7421-type Leeds-Northrup digital pH meter (input impedance: > 1012~; output current: 0-1mA) and recorded by a 7004BXY-type Hewlett-Packard instrument. Generator electrodes and coulometric circuit Silver ions are coulo-generated by a ring-shaped silver anode, made from the same silver wire used to prepare the indicator electrodes, and symmetrically placed around them. The cathode is made from a platinum wire spiral, immersed in a 20 ~o Cu(NO3) 2 solution, which is separated from the anolyte by an agar bridge. The cathodic half-cell is prepared with a 9mm inner diam. glass tube, filled up to a height of 7 cm by a boiling 10 ~ agar solution with contains 5 ~o KNO3. After hardening of the agar solution the upper free part of the tube is filled by the 20 ~ Cu(NO3) 2 solution. The electrodes were externally connected to a constant-current generator where the current could be varied in the 20nA-lmA range within 0.2%: The synchronization between titrant coulo-generation and recording of the d.e.p, curve is easily ensured by letting start the recorder simultaneously with the closure of the coulometric circuit, as shown in Fig. 1.
pH
e
e
Fig. 1. Titration cell and block circuit. (DG) d.e.p, current generator, (CG) coulometric current generator, (a) perspex block, (s) electrolytical solution, (m) magnetic stirrer, (e) silver indicator electrodes, (0 silver ring, (h) platinum spiral immersed in 20,% Cu(NO3) 2 solution, (g) agar bridge.
ANALYSIS O F CI IN C d - C H A L C O G E N I D E S
261
MATERIALS AND SOLUTIONS
1-10 x 10 -~ M standard chloride solutions, prepared immediately before use from Merck Suprapur NH4C1. No further purification of NH4C1 was attempted. Spectroscopic grade methanol (Koch-Light); 0.5-1.0 p.p.m, impurities detectable by blank titration. 65% nitric acid (R.P., A.C.S. Alpha Chimici; nominally containing 1 x 10 - 5 % chlorine). 32% ammonia (R.P., A.C.S. Alpha Chimici). 99.999% CdS, CdSe, CdTe (Koch-Light). Electrolytical solution: 0.01 M HNO3 in 80:20 (volume ratio) methanol-water. Deionized, bidistilled water was used in any case. PROCEDURE AND RESULTS
Detection of chlorine in CdS The proper amount of CdS, to which aliquots of standard chloride solutions are added, is dissolved, at room temperature, within a closed glass tube, by employing the minimum quantity of concentrated HNO3. When amorphous sulphur is seen floating on the liquid surface, the tube is cooled by evaporating ethyl ether on its outer walls, then the tube is opened and its content is accurately added to 10 ml of previously titrated electrolytic solution. Pre-titration is required to eliminate impurities (mainly due to methanol) that can be titrated by the coulo-generated silver ions. Ammonia is further added to the solution in order to reach a pH 3-4, and the titration is allowed to start. The equivalence time is given by summing up the time necessary to obtain the peak and the time elapsed from the pre-titration peak up to the end of the pretitration. It is to be noted that often the oxidation of the sulphide is not completed, because of a too short closed-tube treatment with HNO3, and some sulphide ions remain in the final solution to be titrated. The presence of such sulphide ions (as is shown in Fig. 2) can be accounted for since it gives rise to a "sulphide" peak in the titration S: sulphide C: chloride
peak peak
7( '~E/60
5O 4O 30,20
t 10 0
C
S
ii i ii I 100
310s
IO 2 0
3'00
I I 400 500 time/ s
I 600
I 700
Fig. 2. Typical titration curve when sulphide ions are present : 22.0 mg of CdS added with 507 p.p.m, of chlorine. Generating current 100 pA; d.e.p, current 20 nA; observed equivalence time 310 s; expected time 303.9 s; recovery 2.1%; pre-titration not necessary.
262
c. PELOSI, C. PAORICI, G. ATTOLINI. G. ZUCCALLI
TABLE 1 DETERMINATION OF CHLORIDE IN CdS SAMPLES
CdS /mg
Chloride Added/p.p.ra.
41.9 41.5 46.5 51.2 54.8 39.3 40.1 125.0 20.2 22.0 65.8 26.9 40.5
266.0 269.0 240.0 86.9 81.2 113.0 897.7 35.7 553.0 507.0 549.7 1344.6 1786.2
Found/p.p.ra.
Recovery/%
Generating current /pA
263.0 274.0 241.0 76.7 91.0 121.1 962.0 32.3 618.0 518.0 524.8 1474.9 1724.0
+ + + + + + + + + -
100 100 100 50 50 50 200 50 100 100 200 200 500
20 20 20 20 20 20 20 20 20 20 20 20 20
Generating current /pA
D.e.p. current /nA
200 50 50 200 50 50 50 200 200
20 20 20 20 20 20 20 20 20
1.1 1.8 0.4 11.7 12.0 7.0 7.0 9.5 11.8 2.1 4.5 9.6 3.4
D.e.p. current /nA
TABLE 2 DETERMINATION OF CHLORIDE IN CdSe SAMPLES
CdSe ~rag 38.0 34.4 33.0 28.6 170.7 71.6 55.7 55.0 34.7
Chloride Added/p.p.m.
Found/p.p.m.
951.0 194.4 201.0 1264.0 26.2 62.4 160.3 406.1 643.7
899.0 183.9 201.1 1233.1 26.6 61.0 158.2 361.8 679.7
Recovery~% + + +
5.5 5.4 0.08 2.5 1.8 2.3 1.2 11.0 5.5
c u r v e . T h e e q u i v a l e n c e is t h e n o b t a i n e d b y c o u n t i n g t h e t i t r a t i o n t i m e f r o m t h e s u l p h i d e p e a k u p t o t h e c h l o r i d e p e a k (Fig. 2). T h e r e s u l t s a r e s u m m a r i z e d i n T a b l e 1.
Detection o f chlorine in CdSe T h e p r o c e d u r e is t h e s a m e as t h a t d e s c r i b e d for C d S , w i t h t h e e x c e p t i o n t h a t t h e d i s s o l u t i o n o f c h l o r i n e - a d d e d C d S e s a m p l e s is p e r f o r m e d w i t h m o r e d i l u t e H N O 3 , t o l i m i t t h e o x i d a t i o n o f s e l e n i d e i o n s t o z e r o - v a l e n t s e l e n i u m . B y e m p l o y i n g 4 0 ~o H N O 3 , a t t h e e n d o f t h e d i s s o l u t i o n r e d s e l e n i u m is s e e n f l o a t i n g o n t h e s o l u t i o n . T h e r e s u l t s a r e s u m m a r i z e d i n T a b l e 2.
Detection o f chlorine in C d T e C d T e is d i s s o l v e d w i t h t h e s a m e p r o c e d u r e as d e s c r i b e d f o r C d S , b u t t h e final n i t r i c
ANALYSIS TABLE
263
O F C1 I N C d - C H A L C O G E N I D E S
3
DETERMINATION
Cd T e ~rag
OF CHLORIDE
IN CdTe SAMPLES
Chloride Added/p.p.m.
99.7 63.9 39.4 66.8 25.1 29.2 30.0 38.7 20.8 24.6 21.4 63.3 81.7
44.8 104.8 280.0 66.9 444.9 305.0 1200.0 290.0 540.0 450.0 521.0 570.0 442.0
Found/p.p.m.
Recovery~%
Generating current /#A
44.2 97.0 238.0 64.5 444.9 253.0 1078.0 325.0 530.0 369.8 438.0 542.8 460.0
- 1.4 - 7.4 - 15.0 - 3.5 0.0 - 17.0 - 10.0 + 12.0 - 1.8 - 17.8 - 16.0 - 4.7 + 3.9
50 50 100 50 100 50 200 I00 100 100 100 200 100
D.e.p. current /nA 20 20 20 20 20 20 20 20 20 20 20 20 20
solution is neutralized with the minimum amount of ammonia before being added to the pretitrated electrolytical solution. In this way, tellurium is precipitated as TeO 2 • xH20. After having adjusted the acidity at a pH of about 3~4, titration is started. The results are summarized in Table 3. CONCLUSION
The simple analytical method described proved suitable to reveal chlorine in small samples of Cd-chalcogenides. The amount of initial sample and of chlorine added to the samples were chosen such as to reproduce the average values of single crystals as grown by chemical transport, when HCI is employed as a transport agent a'2'16. The average recovery (within about 10-15~o) is generally satisfactory when growth mechanisms, Cl-doping profiles and gas-solid distribution constants are to be studied. The method should, however, be improved when the amount of chlorine in epitaxial layers is concerned, since the mean weight of the samples is generally much lower. ACKNOWLEDGEMENTS
This research was supported by a financial contribution from the CNR. The authors are indebted to E. Melioli and G. T~illi for their assistance in the preparation of the electronic apparatus. SUMMARY
The detection of chlorine in the 40-1800 p.p.m, range in Cd-chalcogenides has been proved feasible by a d.e.p, version of argentometric titration, where the titrant
264
C. PELOSI, C. PAORICI, G. ATTOLINI, G. ZUCCALLI
(silver ions) is coulo-generated. The analytical method described seems to meet the requirements of simplicity and time saving necessary when routine work is needed, and when small samples (as is the case for single crystals grown by chemical transport) are available. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
H. Sch~ifer, Chemical Transport Reactions, Academic Press, New York, London, 1964. R, Nitsche, Fortschr. Miner., 44 (1967) 231. F. A. Kr6ger, The Chemistry of Imperfect Crystals, North-Holland, Amsterdam. 1964. R. M. Elliott, R. D. Craig and G. A. Errock, Analysis of Solids by Mass Spectrometry, Associated Electrical Industries, Manchester, 1960. H. Sch~ifer and H. Odenbach, Z. Anorg. Allgem. Chem., 346 (1966) 127. C. Paorici, J. Crystal Growth, 5 (1969) 315. J. A. Beun, R. Nitsche and H. U. Boesterli, Physica, 28 (1962) 184. W. Kemula, A. Hulanicki and A. Janowski, Talanta, 7 (1960) 65. T. C. Ovenstone and W. T. Rees, Anal. Chim. Acta, 5 (1951) 123. Spectrophotometric Datafor Colorimetric Analysis, Butterworths, London, 1963. S. Ujiie and Y. Kotera, J. Crystal Growth, 10 (1971) 320. E. Bishop, Analyst, 83 (1958) 212. E. Bishop and R. Daneshwar, Anal. Chem., 36 (1964) 727. B. H. Priscott, T. G. Hand and E. J. Young, Analyst, 91 (1966) 48. M. Takahashi, Z. Yoshida, H. Aoyagi and K. Izawa, Mikrochim. Acta, 2 (1974) 329. C. Paorici, G. Attolini, C. Pelosi and G. Zuccalli, J. Crystal Growth, 18 (1973) 289.
259
© Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
C O U L O - B I P O T E N T I O M E T R I C TITRATIONS IN THE ANALYSIS OF C H L O R I N E IN C H L O R I N E - D O P E D C A D M I U M C H A L C O G E N I D E S C. PELOSI, C. PAORICI, G. ATTOLINI and G. ZUCCALLI
Laboratorio MASPEC-CNR, 43100 Parma (Italy) (Received 29th July 1974)
INTRODUCTION
The determination of halogens, in the 0.1-2000 p.p.m, range, is of particular interest insemiconductor technology, in all those cases where vapor-phase chemicaltransport methods 1'2 are performed with halogens (or hydrohalogen acids) as transport agents. In fact the amount of halogen which remains incorporated in' the crystallized matrix plays an important role not only in the growth mechanism, but also in the electrical and optical properties of the semiconductor 3. In spite of that, many problems whose solution implies a previous determination of the halogen content in the solid matrix, are generally postponed since the analytical methods developed up to now to detect halogens, such as those based on mass spectrometry 4, neutron activation 5'6, radioactive tracers 7, are as a rule expensive, cumbersome, and do not comply with the requirements of simplicity and time saving necessary for a routine work. Spectrophotometric methods 8- lo, which in principle could met the above requirements, are impractical because of the intrinsic difficulty to separate and concentrate the halogen. The necessity to find a simple and time saving technique to reveal halogens in cadmium chalcogenides lead us to take into consideration the differential electrolytical potentiometric (d.e.p.) methods12. Such methods, amply studied both theoretically and experimentally, are reported 13 to allow detection of chlorine and bromine down to 10 - l ° mol of chloride or bromide at 2 x 10 -s M in a medium of 0.01 M nitric acid in 80:20 methanol-water. Applications of these techniques in practical cases have already been described x4, x5. In this paper we refer to the possibility of detecting chlorine, in the 4(~1800 p.p.m, range, in small-sized (20-100 mg) Cd-chalcogenide samples, by means of a d.e.p, version of argentometric titration, where the silver ions are coul.o-generated. An extension of the method to the determination of bromine and iodine will be the object of a forthcoming paper. DESCRIPTION OF THE APPARATUS
The titration cell (similar to that described in ref. 14) and the circuit diagram are schematically shown in Fig. 1.
260
C. PELOSI, C. PAORICI, G. ATTOLINI, G. ZUCCALLI
Indicator electrodes and d.e.p, circuit The indicator electrodes were prepared from 1 mm diam. silver wire, fixed in a perspex block by means of an epoxy resin. After hardening of the resin, the two wires were cut and ground-off square so as to have only the cross section exposed. The electrodes were then activated by dipping them in 40 ~ nitric acid for 30 s. When necessary, the contaminated electrodes were renewed by grinding and repeating the activation process. The indicating d.e.p, circuit follows conventional practice. The indicator electrodes were externally connected to a constant-current generator, where the current could be varied in the 20nA-100pA range, within about 0.2 ~. Potentiometric measurements (in mV) were made by a 7421-type Leeds-Northrup digital pH meter (input impedance: > 1012~; output current: 0-1mA) and recorded by a 7004BXY-type Hewlett-Packard instrument. Generator electrodes and coulometric circuit Silver ions are coulo-generated by a ring-shaped silver anode, made from the same silver wire used to prepare the indicator electrodes, and symmetrically placed around them. The cathode is made from a platinum wire spiral, immersed in a 20 ~o Cu(NO3) 2 solution, which is separated from the anolyte by an agar bridge. The cathodic half-cell is prepared with a 9mm inner diam. glass tube, filled up to a height of 7 cm by a boiling 10 ~ agar solution with contains 5 ~o KNO3. After hardening of the agar solution the upper free part of the tube is filled by the 20 ~ Cu(NO3) 2 solution. The electrodes were externally connected to a constant-current generator where the current could be varied in the 20nA-lmA range within 0.2%: The synchronization between titrant coulo-generation and recording of the d.e.p, curve is easily ensured by letting start the recorder simultaneously with the closure of the coulometric circuit, as shown in Fig. 1.
pH
e
e
Fig. 1. Titration cell and block circuit. (DG) d.e.p, current generator, (CG) coulometric current generator, (a) perspex block, (s) electrolytical solution, (m) magnetic stirrer, (e) silver indicator electrodes, (0 silver ring, (h) platinum spiral immersed in 20,% Cu(NO3) 2 solution, (g) agar bridge.
ANALYSIS O F CI IN C d - C H A L C O G E N I D E S
261
MATERIALS AND SOLUTIONS
1-10 x 10 -~ M standard chloride solutions, prepared immediately before use from Merck Suprapur NH4C1. No further purification of NH4C1 was attempted. Spectroscopic grade methanol (Koch-Light); 0.5-1.0 p.p.m, impurities detectable by blank titration. 65% nitric acid (R.P., A.C.S. Alpha Chimici; nominally containing 1 x 10 - 5 % chlorine). 32% ammonia (R.P., A.C.S. Alpha Chimici). 99.999% CdS, CdSe, CdTe (Koch-Light). Electrolytical solution: 0.01 M HNO3 in 80:20 (volume ratio) methanol-water. Deionized, bidistilled water was used in any case. PROCEDURE AND RESULTS
Detection of chlorine in CdS The proper amount of CdS, to which aliquots of standard chloride solutions are added, is dissolved, at room temperature, within a closed glass tube, by employing the minimum quantity of concentrated HNO3. When amorphous sulphur is seen floating on the liquid surface, the tube is cooled by evaporating ethyl ether on its outer walls, then the tube is opened and its content is accurately added to 10 ml of previously titrated electrolytic solution. Pre-titration is required to eliminate impurities (mainly due to methanol) that can be titrated by the coulo-generated silver ions. Ammonia is further added to the solution in order to reach a pH 3-4, and the titration is allowed to start. The equivalence time is given by summing up the time necessary to obtain the peak and the time elapsed from the pre-titration peak up to the end of the pretitration. It is to be noted that often the oxidation of the sulphide is not completed, because of a too short closed-tube treatment with HNO3, and some sulphide ions remain in the final solution to be titrated. The presence of such sulphide ions (as is shown in Fig. 2) can be accounted for since it gives rise to a "sulphide" peak in the titration S: sulphide C: chloride
peak peak
7( '~E/60
5O 4O 30,20
t 10 0
C
S
ii i ii I 100
310s
IO 2 0
3'00
I I 400 500 time/ s
I 600
I 700
Fig. 2. Typical titration curve when sulphide ions are present : 22.0 mg of CdS added with 507 p.p.m, of chlorine. Generating current 100 pA; d.e.p, current 20 nA; observed equivalence time 310 s; expected time 303.9 s; recovery 2.1%; pre-titration not necessary.
262
c. PELOSI, C. PAORICI, G. ATTOLINI. G. ZUCCALLI
TABLE 1 DETERMINATION OF CHLORIDE IN CdS SAMPLES
CdS /mg
Chloride Added/p.p.ra.
41.9 41.5 46.5 51.2 54.8 39.3 40.1 125.0 20.2 22.0 65.8 26.9 40.5
266.0 269.0 240.0 86.9 81.2 113.0 897.7 35.7 553.0 507.0 549.7 1344.6 1786.2
Found/p.p.ra.
Recovery/%
Generating current /pA
263.0 274.0 241.0 76.7 91.0 121.1 962.0 32.3 618.0 518.0 524.8 1474.9 1724.0
+ + + + + + + + + -
100 100 100 50 50 50 200 50 100 100 200 200 500
20 20 20 20 20 20 20 20 20 20 20 20 20
Generating current /pA
D.e.p. current /nA
200 50 50 200 50 50 50 200 200
20 20 20 20 20 20 20 20 20
1.1 1.8 0.4 11.7 12.0 7.0 7.0 9.5 11.8 2.1 4.5 9.6 3.4
D.e.p. current /nA
TABLE 2 DETERMINATION OF CHLORIDE IN CdSe SAMPLES
CdSe ~rag 38.0 34.4 33.0 28.6 170.7 71.6 55.7 55.0 34.7
Chloride Added/p.p.m.
Found/p.p.m.
951.0 194.4 201.0 1264.0 26.2 62.4 160.3 406.1 643.7
899.0 183.9 201.1 1233.1 26.6 61.0 158.2 361.8 679.7
Recovery~% + + +
5.5 5.4 0.08 2.5 1.8 2.3 1.2 11.0 5.5
c u r v e . T h e e q u i v a l e n c e is t h e n o b t a i n e d b y c o u n t i n g t h e t i t r a t i o n t i m e f r o m t h e s u l p h i d e p e a k u p t o t h e c h l o r i d e p e a k (Fig. 2). T h e r e s u l t s a r e s u m m a r i z e d i n T a b l e 1.
Detection o f chlorine in CdSe T h e p r o c e d u r e is t h e s a m e as t h a t d e s c r i b e d for C d S , w i t h t h e e x c e p t i o n t h a t t h e d i s s o l u t i o n o f c h l o r i n e - a d d e d C d S e s a m p l e s is p e r f o r m e d w i t h m o r e d i l u t e H N O 3 , t o l i m i t t h e o x i d a t i o n o f s e l e n i d e i o n s t o z e r o - v a l e n t s e l e n i u m . B y e m p l o y i n g 4 0 ~o H N O 3 , a t t h e e n d o f t h e d i s s o l u t i o n r e d s e l e n i u m is s e e n f l o a t i n g o n t h e s o l u t i o n . T h e r e s u l t s a r e s u m m a r i z e d i n T a b l e 2.
Detection o f chlorine in C d T e C d T e is d i s s o l v e d w i t h t h e s a m e p r o c e d u r e as d e s c r i b e d f o r C d S , b u t t h e final n i t r i c
ANALYSIS TABLE
263
O F C1 I N C d - C H A L C O G E N I D E S
3
DETERMINATION
Cd T e ~rag
OF CHLORIDE
IN CdTe SAMPLES
Chloride Added/p.p.m.
99.7 63.9 39.4 66.8 25.1 29.2 30.0 38.7 20.8 24.6 21.4 63.3 81.7
44.8 104.8 280.0 66.9 444.9 305.0 1200.0 290.0 540.0 450.0 521.0 570.0 442.0
Found/p.p.m.
Recovery~%
Generating current /#A
44.2 97.0 238.0 64.5 444.9 253.0 1078.0 325.0 530.0 369.8 438.0 542.8 460.0
- 1.4 - 7.4 - 15.0 - 3.5 0.0 - 17.0 - 10.0 + 12.0 - 1.8 - 17.8 - 16.0 - 4.7 + 3.9
50 50 100 50 100 50 200 I00 100 100 100 200 100
D.e.p. current /nA 20 20 20 20 20 20 20 20 20 20 20 20 20
solution is neutralized with the minimum amount of ammonia before being added to the pretitrated electrolytical solution. In this way, tellurium is precipitated as TeO 2 • xH20. After having adjusted the acidity at a pH of about 3~4, titration is started. The results are summarized in Table 3. CONCLUSION
The simple analytical method described proved suitable to reveal chlorine in small samples of Cd-chalcogenides. The amount of initial sample and of chlorine added to the samples were chosen such as to reproduce the average values of single crystals as grown by chemical transport, when HCI is employed as a transport agent a'2'16. The average recovery (within about 10-15~o) is generally satisfactory when growth mechanisms, Cl-doping profiles and gas-solid distribution constants are to be studied. The method should, however, be improved when the amount of chlorine in epitaxial layers is concerned, since the mean weight of the samples is generally much lower. ACKNOWLEDGEMENTS
This research was supported by a financial contribution from the CNR. The authors are indebted to E. Melioli and G. T~illi for their assistance in the preparation of the electronic apparatus. SUMMARY
The detection of chlorine in the 40-1800 p.p.m, range in Cd-chalcogenides has been proved feasible by a d.e.p, version of argentometric titration, where the titrant
264
C. PELOSI, C. PAORICI, G. ATTOLINI, G. ZUCCALLI
(silver ions) is coulo-generated. The analytical method described seems to meet the requirements of simplicity and time saving necessary when routine work is needed, and when small samples (as is the case for single crystals grown by chemical transport) are available. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
H. Sch~ifer, Chemical Transport Reactions, Academic Press, New York, London, 1964. R, Nitsche, Fortschr. Miner., 44 (1967) 231. F. A. Kr6ger, The Chemistry of Imperfect Crystals, North-Holland, Amsterdam. 1964. R. M. Elliott, R. D. Craig and G. A. Errock, Analysis of Solids by Mass Spectrometry, Associated Electrical Industries, Manchester, 1960. H. Sch~ifer and H. Odenbach, Z. Anorg. Allgem. Chem., 346 (1966) 127. C. Paorici, J. Crystal Growth, 5 (1969) 315. J. A. Beun, R. Nitsche and H. U. Boesterli, Physica, 28 (1962) 184. W. Kemula, A. Hulanicki and A. Janowski, Talanta, 7 (1960) 65. T. C. Ovenstone and W. T. Rees, Anal. Chim. Acta, 5 (1951) 123. Spectrophotometric Datafor Colorimetric Analysis, Butterworths, London, 1963. S. Ujiie and Y. Kotera, J. Crystal Growth, 10 (1971) 320. E. Bishop, Analyst, 83 (1958) 212. E. Bishop and R. Daneshwar, Anal. Chem., 36 (1964) 727. B. H. Priscott, T. G. Hand and E. J. Young, Analyst, 91 (1966) 48. M. Takahashi, Z. Yoshida, H. Aoyagi and K. Izawa, Mikrochim. Acta, 2 (1974) 329. C. Paorici, G. Attolini, C. Pelosi and G. Zuccalli, J. Crystal Growth, 18 (1973) 289.