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Electrothermal atomic absorption spectrometric determination of cobalt in steel after chloroform extraction of its 1-nitroso-2-naphthol complex
Artafytica Chimicn Acta, 117 (1980) 275-283 o Elsevier Scientific Publishing Company, Amsterdam
-Printed
in The
Netherlands
ELECTROTHERMAL ATOMIC ABSORPTION SPECTROMETRIC DETERMINATION OF COBALT IN STEEL AFTER CHLOROFORM EXTRACTION OF ITS 1-NITROSO-2-NAPHTHOL COMPLEX
C. J. ESKELL
and Xl. E. PICK*
Ccntrol Elcctricify Gemrating Board. Bcrkclcy Cloucesiershire GL 13 9PB (Gf. Britain) (Received
14th December
Nuclear
Laboratories.
Bcrkeky,
1979)
SUWMARY Direct determination of pg g-’ levcfs of cobaft in steels and corrosion products by ele~trotherma1 atomic absorption is difficult because of suppression of the cobalt signal by mineral acids and other metals. Extraction of cobalt with l-nitroso-2-naphthol into chloroform in the presence of citrate overcomes this problem. Samples (0.4-10 mg) may be dissolved in a variety of acids, and a detection limit of 0.005 /.~gg-’ in the chloroform extract is obtained.
A major problem in the operation of water-cooled nuclear reactors is confields to levels which permit maintenance and inspection without exposure of operating personnel to escessive radiation doses. The mdioactive nuclides which give the greatest contribution to radiation fields in the circuit are “Co, formed by a n, y reaction on 59Co, and “Co, formed by a n, p reaction on “i\Ti . Cobalt is present in stainless steels and nickelbased alfoys such as inconei and Incoloy which are used in steam generators. In addition, it is a major component of stellites which are used for certain hard facing applications, e.g. for vaIve seats. Corrosion of these construction materials may release both metal ions and particulate matter to the coolant. Deposition and activation of this material in the reactor core, followed by its re-release to the coolant can then Iead to the deposition of activated corrosion products containing “Co and “Cc on pipework, which may result in high radiation fieIds. in order to predict and control activity arising in the circuit, knowledge of the behaviour of both active and inactive cobalt in the circuit is very important. Consequently, it is necessary to know the amount of cobalt in steels and corrosion products and, in particular, the proportion of cobalt present as ionic or particulate matter in the coolant [ I]_ Moreover, knowledge of the proportion of cobalt in different size fractions of particulate matter can provide additional important information on thecobalt transport mechanism. In the latter case, only limited quantities of material may be available; hence, an analytical procedure for cobalt which can operate with small quantities of
trol of radiation
276
sample is required. Furthermore, because samples are radioactive it is important to control the amount of material used, in order to limit radiation doses and reduce contamination of instruments, etc. For these reasons, a method based on electrothermal atomic absorption spectrometry appeared to be ideally sui’ted to the determination of cobalt in small quantities of radioactive material_ initially, the direct determination of cobalt in acidic solution containing excess of iron, nickel and chromium was investigated_ Because of suppression of the cobalt signal, and the fact that steels and corrosion products from reactor circuits would contain variable amounts of these metals and might also require different combinations of acids to effect dissolution, it was concluded tha: it would be necessay to estract the cobalt [Z-4]. One of the most popular techniques involves formation of the comples between cobalt and 1-nitroso-Znaphthol and its extraction from aqueous solution by chloroform. It is first necessary to remove or mask interfering metals. Young f3] recommends an initial precipitation with zinc oside to remove iron, chromium and several other metals, whilst Sandell [2] reports that interference from iron is reduced by addition of sodium citrate before complesation of the cobalt. Both procedures were examined_ The sodium citrate met!lod was much less tedious and, after suitable modification, provided more reliable results. The I-nitroso-2-naphthol estraction of cobalt had previously been applied to electrothermal atomization [4], but to digests of biological materials [ 5.6 1 rather than steels. The paper describes the extraction procedure in detail and reports on the optimisation of furnace parameters. Results obtained with British Chemical Standard Steel samples are recorded and detection limits and sample require-
ments
for the method
are given.
EXPERlRfENTAL
Instntrnentation A Shandon Southern atomic absorption spectrophotometer, hlodel A3400, together with an electrothermal atomiser power unit, Model A3470 &lark 2, were employed. A Bryans Southern Instruments Autograph S chart recorder was connected to the power unit. The Mark 1 graphite rod head fitted with a 5-,ul capacity graphite rod was employed. Samples were placed in the cavity on the graphite rod from an Oxford Laboratories micropipette. The automated sequence programmed on the A3470 unit consists of an evaporation stage, two dry ashing stages and finally an atomisation stage. The absorption signal is passed +&rough the A3470 unit to the chart recorder. The graphite rod head was cooled by an air flow through the structure. A nitrogen purge (2 dm3 min-‘) WE passed over the graphite rod. In most of the work a pyrolysis gas (90% argon/lo% methane) at 80 cm3 min-’ was added to the purge gas flow in order to maintain a pyrolytic graphite coating on the rod.
277
Reagents and soiu tions ChemicaIs of the highest available purity were always used. A commercially available, 1000 ppm standard cobalt solution was employed. Solutions containing iron, nickel and chromium were prepared from the high-purity metals. A&tar grade acids (BDH Ltd.) were used; other reagents were analytical grade. Highly purified water was prepared by distillation, followed by passage through a Milli-Q deionisation system (Miilipore Corporation). Preparation of solutions for direct determination of cobalt. A 10 ppm cobalt standard solution was freshly prepared from the 1,000 ppm stock solution just before use. \Vorking standards containing up to 0.20 ppm cobalt in water, nitric acid 5% (v/v), sulphuric acid 5% (v/v) or hydrochloric 2% (v/v)/sulphuric 3% (v/v) acid were prepared from this solution. Standards in 5% (v/v) acid were also containing 500 ppm iron, nickel or chromium prepared by adding 5,000 ppm stock metal solution in 50% nitric or 50%. sulphuric acid. In order to investigate the effects of varying concentrations of iron, nickel and chromium in 5% (v/v) nitric or sulphuric acid, aliquots of the 5,000 ppm stock metal solution in 50% acid, together with appropriate amounts of extra acid were made up with cobalt to give 0.1 ppm cobalt solutions containing between 10 and 500 ppm of each metal. Preparation of a solution of the I-nitroso-2-naphthol complex of cobalt. The 1-nitroso-2-naphthol reagent was prepared by dissolving 1 g of the solid in 25 cm3 of glacial acetic acid to which 25 cm3 of hot water were then added [71_ It was stored in the dark and portions were filtered immediately before use. The reagent was added to 200 cm3 of water containing 0.01 g of cobalt and 5 cm3 of hydrochloric acid until precipitation was complete. The mixture was simmered for a few minutes, then left to stand for 3 11.The complex was extracted with chloroform, 1 X 25 cm3 followed by 2 X 15 cm3 aliquots. These extracts were combined and shaken with 20 cm3 of 30% hydrochloric acid. The chloroform layer was transferred to a lOOon flask and made up with chloroform to give a 100 ppm cobalt solution from which working standaxds were prepared. Extraction procedures After removal of interfering metals by zinc oxide precipitation. Samples of British Chemical Standard (BCS) steels (10-100 mg) were warmed with 5 cm3 of 50% sulphuric acid, 5 cm3 of 50% hydrochloric acid and 2 cm3 of concentrated nitric acid until they dissolved. If some material remained undissolved after 2-3 h, a few drops of hydrofluoric acid were carefully added to complete the dissolution. The solutions were brought to the boil, cooled, and diluted with about 30 cm3 of water. In order to oxidise the iron to iron(III), 0.5 cm3 of 30% hydrogen peroxide was added to each solution. Aftenvards the samples were boiled for 5 min to destroy the excess of peroxide. The zinc oxide precipitation procedure described by Young [31 was then followed. To the filtrate from this procedure, 10 cm3 of l-nitroso2-naphthol reagent were added and the solution was simmered for a few
275
minutes and left to stand for 3 h_ Samples were extracted with chloroform (1 X 15 cm’, 2 X 10 cm3) and the chloroform estracts shaken with 20 cm3 of 30% hydrochloric acid in order to break down other metal contpXexes which may have been formed. The chloroform layer was transferred to a 50-cm” volumetric fiask and made up to vcfume with chloroform, Mtx&inginterferingme&& iuithsodium citrate, Samples of stef4 (ca. 100 mg) were dissolved as described above and then transferred to ZOO-cm3 volumetric flasks; IO-cm’ aliquots conCaining 10 mg of steel were used in the prowdure, ~lt~toug~t with one steel sample (BCS 35’7), which contained a higher co~tce~~~tioit of cobalt, only a 4cm3 afiquot was taken in order to avoid dilution of the final chloroform estract. To each aliquot 10 cm3 of 40% (w/v) sodium citrate solution was added and the pH adjusted to 3-4 with either 10% (w/v) sodium hydroxide or 30% hydrochloric acid solution, t’est, 1 cm3 of 30% hydrogen peroxide was added followed, after reagent. The samples several minutes, by 2 cm3 of the l-~ti~roso-2-naplt~tol were left to stand for at leasst 3 h, before being estracted with chloroform and washed with h~r~iro~~tloric acid, Sufficient ifGl\ and nickel to cause interference remained if the above washing procedure was used; thus, it was modified. After cstrztion, the extract was transferred to a centrifuge tube and 10 cm3 of 30% hydrochloric acid added. The tubes were s~lppor~ed in an ultrasonic bath untit the two layers had become ~~toroug~tly mixed fca. 15 mitt), and then cen~~~ged to &ear the chloroform layer_ After removal of the acid the samples were made up to 50 cm3_ Standards were prepared by the same procedure front aliquots of 10 ppm cobalt solution to which the various acids used to dissolve the steel had also been added. The procedure was also tested with lcm3 aliquots of solutions containing I mg crnm3of steel. Reagent vofuntes were scaled down by a factor of ten and the sotutions were made up in &em3 volumetric flasks.
The optical parameters were set as recommended by the manufacturer. Graphite atomizer parameters were as shown in Table 1, TABLE Graphite
1 furnace settings
Cbannef
1 2 3 4
(Evaporation) (Dry _Ashing) (Dry Ashing) f~tornjs~t~o~)
Direct determination of cob& in aqueous sotution
Cobalt-I.nitroso-“,.naphthol compIex in chloroform
Voltage
Time (s)
Voltage
10 30 30 x.7b
2.5 4.0 6.0 9.0
4.0 5.0
(nominal)
(~orn~na~)
Time (s) 20 20 10 1.75
279
A comparison was made between the three read-out modes available on the instrument: direct, peak height and integrate. The responses with direct and peak height were similar but the latter was not accurate at less than 10% full scale deflection. Integration gave acurved calibration plot which gradually became less sensitive at higher concentrations. Therefore, the direct read-out mode was most suitable for these tests. The scale expansion was initially set to X3 in order to give a response of about 60% Poll scale deflection with the top standard_ RESW LTS
Direct
AND
DISCUSSION
determination
of cobalt
The effects on the cobalt signal of nitric, sulphuric and hydrochloric/ sulphuric acids alone and in the presence of varying amounts of iron, nickel and chromium are illustrated in Figs. 1 and 2. The results in Fig. 1 show that a 0.20 ppm cobalt solution in water has an absorbance equivalent to 63% full scale deflection when the scale expansion is X3 and there is a linear response to cobalt. Solutions prepared in 5% nitric acid gave virtually the same responses_ However, the addition of 500 ppm iron or nickel to 5% nitric acid suppressed the cobalt response by 30% and 5070, respectively, although a linear calibration plot was still obtained. The 5% sulphuric acid solution reduced the cobalt signal by 40%. \Vhen 500 ppm iron, nickel or chromium are also present, the cobalt response was decreased by 50-70% from the signal in 570 sulphuric acid alone and the sensitivity was only about 15% of that for cobalt in 5% nitric acid. Linear calibrations were again obtained, however. A similar reduction in sensitivity occurred with samples made up in 2% hydrochloric/3% sulphuric acid. Furthermore, this mixture gave a higher background signal and a non-linear response to cobalt. The cobalt response in hydrochloric acid alone was not esamined because it has been well documented that metals
Fig. 1. Calibration graphs for cobalt in the presence of (a) water, (b) (1 f 19) nitric acid, (c) (1 + 19) nitric acid and 500 ppm iron, (d) (1 + 19) nitric acid and 500 ppm nickel, (e) ( I + 19) sulphuric acid.
Fig. 2, The effects on the absorption signal for cobnit (0. I ppm) of increasing concen traCons of (a) iron, (b) nickel and (c) chromium: (A) in (1 + 19) nitric acid solution;(B) in (1 + 19) sulphuric acid soiution.
may be lost from the graphite rod as volatile chlorides during dry ashing [S-10] _ In Fig. 2 the responses to 0.10 ppm cobalt in dilute nitric acid and sulphuric acid with varying concentrations of iron, nickel and chromium in solution are illustrated. The presence of only a few ppm of these metals resuited in a marked drop in the cobalt signal but the decrease levelled out at about 40 ppm of interfering metal and remained more or less constant with greater amounts. The decrease in response may arise from incomplete atomisation of cobalt when it is in the presence of an excess of other metaJs with very similar boiling points. Nitric acid is therefore the most suitable acid for use with the graphite rod and a 5% soiution has no significant effect on the cobalt sensitivity. The detection limit at the full scale expansicn of X 10, expressed as twice the background signal, was found to be 0.005 ppm, It has been suggested fl0, 111 that oxidising acids, such as nitric and pcrchloric, are preferable for use with the graphite rod because on charring these soIutions the me&I oxides are formed, and these are readiIy reduced by the carbon rod resulting in a higher metal atom density during atomisation. However, nitric acid alone will not dissolve most samples of steel, Furthermore, even a few ppm of iron, nicke1 or chromium reduce the cobalt signal. Hence, in order to determine cobalt directly in such mixtures it would be
281
necessary to accept a lower sensitivity; in addition, matrix matching or the standard addition technique would be required_ A more satisfactory solution
is therefore
to isolate the cobalt from the interfering
metals.
Extraction of cobalt as its I-nitroso-2-nap11 thol complex A linear plot of absorbance versus concentration was obtained when cobalt was extracted with 1-nitroso-2-naphthol and measured on the graphite rod. A 0.20 ppm cobalt solution after extraction, gave an absorbance of 50% full scale deflection when the scale expansion was X 3, which was about 20% less than that obtained with a 0.20 ppm cobalt solution in 5% nitric acid. The detection limit (twice background) was 0.005 ppm which was the same as for cobalt in nitric acid. It was noted that if the dry ashing voltage was reduced below a scale setting of 6.0 a double peak was obtained during atomisation; this was found to be due to the excess of 1-nitroso-2naphthol. This was eliminated by keeping the voltage setting above 6.0 but below 7.5 in order to prevent loss oi’ cobalt at this stage. Zinc oxide separation_ The British Chemical Standard samples analysed by the procedure involving zinc oxide separation were BCS No. 387, a nimonic alloy, and Nos. 331 and 334, which were austenitic steels. The cobalt con-
centrations obtained for six samples are listed in Table 2. Analyses of the chloroform extract showed that the amount of iron extracted was <0.25 ppm and nickel <0.15 ppm whilst chromium was not detectable. Hence, the method was successful in decreasing the concentrations of these metals to levels at which they did not interfere. However, the procedure was laborious and although duplicate samples gave similar results the mean cobalt concentrations differed greatly from the certificate values. \Vhilst accuracy might well have improved with practice, it was decided to abandon this procedure in favour of the less tedious and more accurate sodium citrate method which was developed. Sodium citrate separation.
The samples analysed were BCS Nos. 387, 334, 331 and 335; cobalt concentrations ranged from 0.034 to 0.21%. A linear plot of absorbance versus concentration was obtained from standards taken
TXSLE Cobalt
2 in steels as determined
Steel BCS No.
Cobalt
%Iean
(I)
by the zinc oxide
method
Certificate value
Difference of mean from certificate value
(%I
(70)
38’i
0.168 0.163
0.1655
0.21
-21
331
0.049 0.056
0.0525
0.010
+ 31
334
0.046 0.048
0.04io
0.052
-10
282 TABLE
3
Cobalt concentration in steels determined by the sodium citrate method Steel BCS
Sample tvt.
NO.
(msl
Cobalt (701
Mean
Certificate
Sample
Cobalt
V~fW
wt.
P)
4%)
bw)
Mean
&ertifkxte value ml
367
J
0.212 0.203 0.x% 0.208
0.20~
0.21
O.-l
0.197 0.224 0.19-i
0.206
0.31
331
10
0.083 0.051 0.056 0.05 t
0.053
0.052
1
0.063 0.051 0.053 0.053
0.052,
0.052
33.5
10
0.036 0.031 0.03s 0.036
0.037
0.034
1
0.036 0.036 0.036
0.036
0.034
331
10
0.010 0.03s 0.042
0.010
0.040
1
0.044 0.04-i 0.04 1 0.039
0.042
0.040
thrcugh the whole procedure; this calibration graph was used for the steel sampk?s_ Results obtained for IO-mg samples of steei (4 mg for 357) are given in Table 3. Four independent anaiyses were performed with each steel (except 33X); each solution was measured two or three times and the average resuIt recorded. These rest&s show that good precision and accuracy were obtained. AU the resuIts lie within +7.X% of the mean, which is typicaI of ~lectrotI~erma1 atomic absorption spectrometric results. Apart irom sample 335, which gives a result 8% too high, the mean results are within 2% of the certificate value. Results obtained with I-mg sampies of steel {in the case of sample 387, 0.4 mg) are also given in Table 3. AII resuIts lie within ~8.5% of the mean. Apart from sample 335 which gives a resuIt 6% too high, the mean results are within 4% of the certificate value. Comparison with the data on the farger sampIes shows that the pm&ion and accuracy is very similar. In both instances the results for sample 335 are higher than the certificate value by an average of ‘i%, whereas those for the other sampIes are aI.I within _+I% of tfle certificate value, which suggests, perhaps, that the portions of 335 steel taken for analysis mat not have been representative of the bulk. The results given were aI.I obtained using the ultrasonic acid washing technique. This decreased the iron and nickel concentrations to less than 0.15 ppm and 0.2 ppm, respectively, and chromium was only just detectable. At these Ievefs no in terferenee was encountered, whereas pt-eviousls, when iron
283
and nickel were decreased only to 2--3 ppm, interference occurred. \Vith these higher concentrations, iron and nickel were not totally atomised at each firing and tended to accumulate on the rod leading to suppression of the cobalt signal and erratic results. This could be controlled by an additional firing of the rod after every 4 or 5 determinations, but the background signal remained almost double that obtained with the ultrasonically washed solutions. The cobalt detection limit of 25 pg in 5 ,~l of chloroform is equivalent to a detection limit for cobalt of 25 ppm in a 1-mg sample. If necessary this limit could be reduced by simply evaporating down the chloroform estract, e.g. from 5 cm3 to 1 cm3, or by evaporating multiple samples on the graphite rod followed by a single atomisation. Tests have shown that excess of l-nitroso-2-naphthol is removed during dry ashing and has no effect on the cobalt signal. The results demonstrate that by scaling down the volumes of solution the method can be successfully applied to samples of only 1 mg of steel. The ability to analyse small samples is of importance in the analysis of size fractions of particulate material carried by reactor coolant. Furthermore, because only small quantities of sample are required, the hazard from radioactive samples is reduced. This paper is published Board.
by permission
of the Central
Electricity
Generating
REFERENCES 1 P. J. Da&y and B. J. hlacfarlane, Proc. Int. Conf. Water Chemistry of Nuclear Reactor 1977, British Nuclear Energy Society, Systems. Boumemouth, U.K., October 2?--2i, London, 1978, p. 25’7. 2 E. B. Sandell. Calorimetric Metal Analysis, Interscience, New York, 1965. p_ -I?? 3 R. S. Young, The Analytical Chemistry of Cobalt. Pergamon Press, Osford, 1966. p. -16. -1 hl. S. Cresser, Solvent Extraction in Flame Spectroscopic Analysis. Butterworth, London. 197s. 5 P. Hocaucllet, Ann. F&if. Expert. Chim., Gi (197-A) -195. 6 L. Hagernan, L. Torma and B. E. Ginther. J. Assoc. Off. Anal. Chem., 56 (1975) 990. 7 -4. I. Vogel, A Test-Book of Quantitative Inorganic Analysis, Longmans, London, 196s. p. 529. S B. R. Culver and T. Surlcs. Anal. Chem., -17 (1975) 920. 9T. R. Dulski and R. R. Bixler. Anal. Chim. Acta, 91 (1977) 199. 10 F. Shaw and J. RI. Gttaway, Analyst, 100 (1975) 317. 11 W. B. Barnett and E. A. RIcLaughlin, Anal. Chim. Acta, SO (1975) 285.
-Printed
in The
Netherlands
ELECTROTHERMAL ATOMIC ABSORPTION SPECTROMETRIC DETERMINATION OF COBALT IN STEEL AFTER CHLOROFORM EXTRACTION OF ITS 1-NITROSO-2-NAPHTHOL COMPLEX
C. J. ESKELL
and Xl. E. PICK*
Ccntrol Elcctricify Gemrating Board. Bcrkclcy Cloucesiershire GL 13 9PB (Gf. Britain) (Received
14th December
Nuclear
Laboratories.
Bcrkeky,
1979)
SUWMARY Direct determination of pg g-’ levcfs of cobaft in steels and corrosion products by ele~trotherma1 atomic absorption is difficult because of suppression of the cobalt signal by mineral acids and other metals. Extraction of cobalt with l-nitroso-2-naphthol into chloroform in the presence of citrate overcomes this problem. Samples (0.4-10 mg) may be dissolved in a variety of acids, and a detection limit of 0.005 /.~gg-’ in the chloroform extract is obtained.
A major problem in the operation of water-cooled nuclear reactors is confields to levels which permit maintenance and inspection without exposure of operating personnel to escessive radiation doses. The mdioactive nuclides which give the greatest contribution to radiation fields in the circuit are “Co, formed by a n, y reaction on 59Co, and “Co, formed by a n, p reaction on “i\Ti . Cobalt is present in stainless steels and nickelbased alfoys such as inconei and Incoloy which are used in steam generators. In addition, it is a major component of stellites which are used for certain hard facing applications, e.g. for vaIve seats. Corrosion of these construction materials may release both metal ions and particulate matter to the coolant. Deposition and activation of this material in the reactor core, followed by its re-release to the coolant can then Iead to the deposition of activated corrosion products containing “Co and “Cc on pipework, which may result in high radiation fieIds. in order to predict and control activity arising in the circuit, knowledge of the behaviour of both active and inactive cobalt in the circuit is very important. Consequently, it is necessary to know the amount of cobalt in steels and corrosion products and, in particular, the proportion of cobalt present as ionic or particulate matter in the coolant [ I]_ Moreover, knowledge of the proportion of cobalt in different size fractions of particulate matter can provide additional important information on thecobalt transport mechanism. In the latter case, only limited quantities of material may be available; hence, an analytical procedure for cobalt which can operate with small quantities of
trol of radiation
276
sample is required. Furthermore, because samples are radioactive it is important to control the amount of material used, in order to limit radiation doses and reduce contamination of instruments, etc. For these reasons, a method based on electrothermal atomic absorption spectrometry appeared to be ideally sui’ted to the determination of cobalt in small quantities of radioactive material_ initially, the direct determination of cobalt in acidic solution containing excess of iron, nickel and chromium was investigated_ Because of suppression of the cobalt signal, and the fact that steels and corrosion products from reactor circuits would contain variable amounts of these metals and might also require different combinations of acids to effect dissolution, it was concluded tha: it would be necessay to estract the cobalt [Z-4]. One of the most popular techniques involves formation of the comples between cobalt and 1-nitroso-Znaphthol and its extraction from aqueous solution by chloroform. It is first necessary to remove or mask interfering metals. Young f3] recommends an initial precipitation with zinc oside to remove iron, chromium and several other metals, whilst Sandell [2] reports that interference from iron is reduced by addition of sodium citrate before complesation of the cobalt. Both procedures were examined_ The sodium citrate met!lod was much less tedious and, after suitable modification, provided more reliable results. The I-nitroso-2-naphthol estraction of cobalt had previously been applied to electrothermal atomization [4], but to digests of biological materials [ 5.6 1 rather than steels. The paper describes the extraction procedure in detail and reports on the optimisation of furnace parameters. Results obtained with British Chemical Standard Steel samples are recorded and detection limits and sample require-
ments
for the method
are given.
EXPERlRfENTAL
Instntrnentation A Shandon Southern atomic absorption spectrophotometer, hlodel A3400, together with an electrothermal atomiser power unit, Model A3470 &lark 2, were employed. A Bryans Southern Instruments Autograph S chart recorder was connected to the power unit. The Mark 1 graphite rod head fitted with a 5-,ul capacity graphite rod was employed. Samples were placed in the cavity on the graphite rod from an Oxford Laboratories micropipette. The automated sequence programmed on the A3470 unit consists of an evaporation stage, two dry ashing stages and finally an atomisation stage. The absorption signal is passed +&rough the A3470 unit to the chart recorder. The graphite rod head was cooled by an air flow through the structure. A nitrogen purge (2 dm3 min-‘) WE passed over the graphite rod. In most of the work a pyrolysis gas (90% argon/lo% methane) at 80 cm3 min-’ was added to the purge gas flow in order to maintain a pyrolytic graphite coating on the rod.
277
Reagents and soiu tions ChemicaIs of the highest available purity were always used. A commercially available, 1000 ppm standard cobalt solution was employed. Solutions containing iron, nickel and chromium were prepared from the high-purity metals. A&tar grade acids (BDH Ltd.) were used; other reagents were analytical grade. Highly purified water was prepared by distillation, followed by passage through a Milli-Q deionisation system (Miilipore Corporation). Preparation of solutions for direct determination of cobalt. A 10 ppm cobalt standard solution was freshly prepared from the 1,000 ppm stock solution just before use. \Vorking standards containing up to 0.20 ppm cobalt in water, nitric acid 5% (v/v), sulphuric acid 5% (v/v) or hydrochloric 2% (v/v)/sulphuric 3% (v/v) acid were prepared from this solution. Standards in 5% (v/v) acid were also containing 500 ppm iron, nickel or chromium prepared by adding 5,000 ppm stock metal solution in 50% nitric or 50%. sulphuric acid. In order to investigate the effects of varying concentrations of iron, nickel and chromium in 5% (v/v) nitric or sulphuric acid, aliquots of the 5,000 ppm stock metal solution in 50% acid, together with appropriate amounts of extra acid were made up with cobalt to give 0.1 ppm cobalt solutions containing between 10 and 500 ppm of each metal. Preparation of a solution of the I-nitroso-2-naphthol complex of cobalt. The 1-nitroso-2-naphthol reagent was prepared by dissolving 1 g of the solid in 25 cm3 of glacial acetic acid to which 25 cm3 of hot water were then added [71_ It was stored in the dark and portions were filtered immediately before use. The reagent was added to 200 cm3 of water containing 0.01 g of cobalt and 5 cm3 of hydrochloric acid until precipitation was complete. The mixture was simmered for a few minutes, then left to stand for 3 11.The complex was extracted with chloroform, 1 X 25 cm3 followed by 2 X 15 cm3 aliquots. These extracts were combined and shaken with 20 cm3 of 30% hydrochloric acid. The chloroform layer was transferred to a lOOon flask and made up with chloroform to give a 100 ppm cobalt solution from which working standaxds were prepared. Extraction procedures After removal of interfering metals by zinc oxide precipitation. Samples of British Chemical Standard (BCS) steels (10-100 mg) were warmed with 5 cm3 of 50% sulphuric acid, 5 cm3 of 50% hydrochloric acid and 2 cm3 of concentrated nitric acid until they dissolved. If some material remained undissolved after 2-3 h, a few drops of hydrofluoric acid were carefully added to complete the dissolution. The solutions were brought to the boil, cooled, and diluted with about 30 cm3 of water. In order to oxidise the iron to iron(III), 0.5 cm3 of 30% hydrogen peroxide was added to each solution. Aftenvards the samples were boiled for 5 min to destroy the excess of peroxide. The zinc oxide precipitation procedure described by Young [31 was then followed. To the filtrate from this procedure, 10 cm3 of l-nitroso2-naphthol reagent were added and the solution was simmered for a few
275
minutes and left to stand for 3 h_ Samples were extracted with chloroform (1 X 15 cm’, 2 X 10 cm3) and the chloroform estracts shaken with 20 cm3 of 30% hydrochloric acid in order to break down other metal contpXexes which may have been formed. The chloroform layer was transferred to a 50-cm” volumetric fiask and made up to vcfume with chloroform, Mtx&inginterferingme&& iuithsodium citrate, Samples of stef4 (ca. 100 mg) were dissolved as described above and then transferred to ZOO-cm3 volumetric flasks; IO-cm’ aliquots conCaining 10 mg of steel were used in the prowdure, ~lt~toug~t with one steel sample (BCS 35’7), which contained a higher co~tce~~~tioit of cobalt, only a 4cm3 afiquot was taken in order to avoid dilution of the final chloroform estract. To each aliquot 10 cm3 of 40% (w/v) sodium citrate solution was added and the pH adjusted to 3-4 with either 10% (w/v) sodium hydroxide or 30% hydrochloric acid solution, t’est, 1 cm3 of 30% hydrogen peroxide was added followed, after reagent. The samples several minutes, by 2 cm3 of the l-~ti~roso-2-naplt~tol were left to stand for at leasst 3 h, before being estracted with chloroform and washed with h~r~iro~~tloric acid, Sufficient ifGl\ and nickel to cause interference remained if the above washing procedure was used; thus, it was modified. After cstrztion, the extract was transferred to a centrifuge tube and 10 cm3 of 30% hydrochloric acid added. The tubes were s~lppor~ed in an ultrasonic bath untit the two layers had become ~~toroug~tly mixed fca. 15 mitt), and then cen~~~ged to &ear the chloroform layer_ After removal of the acid the samples were made up to 50 cm3_ Standards were prepared by the same procedure front aliquots of 10 ppm cobalt solution to which the various acids used to dissolve the steel had also been added. The procedure was also tested with lcm3 aliquots of solutions containing I mg crnm3of steel. Reagent vofuntes were scaled down by a factor of ten and the sotutions were made up in &em3 volumetric flasks.
The optical parameters were set as recommended by the manufacturer. Graphite atomizer parameters were as shown in Table 1, TABLE Graphite
1 furnace settings
Cbannef
1 2 3 4
(Evaporation) (Dry _Ashing) (Dry Ashing) f~tornjs~t~o~)
Direct determination of cob& in aqueous sotution
Cobalt-I.nitroso-“,.naphthol compIex in chloroform
Voltage
Time (s)
Voltage
10 30 30 x.7b
2.5 4.0 6.0 9.0
4.0 5.0
(nominal)
(~orn~na~)
Time (s) 20 20 10 1.75
279
A comparison was made between the three read-out modes available on the instrument: direct, peak height and integrate. The responses with direct and peak height were similar but the latter was not accurate at less than 10% full scale deflection. Integration gave acurved calibration plot which gradually became less sensitive at higher concentrations. Therefore, the direct read-out mode was most suitable for these tests. The scale expansion was initially set to X3 in order to give a response of about 60% Poll scale deflection with the top standard_ RESW LTS
Direct
AND
DISCUSSION
determination
of cobalt
The effects on the cobalt signal of nitric, sulphuric and hydrochloric/ sulphuric acids alone and in the presence of varying amounts of iron, nickel and chromium are illustrated in Figs. 1 and 2. The results in Fig. 1 show that a 0.20 ppm cobalt solution in water has an absorbance equivalent to 63% full scale deflection when the scale expansion is X3 and there is a linear response to cobalt. Solutions prepared in 5% nitric acid gave virtually the same responses_ However, the addition of 500 ppm iron or nickel to 5% nitric acid suppressed the cobalt response by 30% and 5070, respectively, although a linear calibration plot was still obtained. The 5% sulphuric acid solution reduced the cobalt signal by 40%. \Vhen 500 ppm iron, nickel or chromium are also present, the cobalt response was decreased by 50-70% from the signal in 570 sulphuric acid alone and the sensitivity was only about 15% of that for cobalt in 5% nitric acid. Linear calibrations were again obtained, however. A similar reduction in sensitivity occurred with samples made up in 2% hydrochloric/3% sulphuric acid. Furthermore, this mixture gave a higher background signal and a non-linear response to cobalt. The cobalt response in hydrochloric acid alone was not esamined because it has been well documented that metals
Fig. 1. Calibration graphs for cobalt in the presence of (a) water, (b) (1 f 19) nitric acid, (c) (1 + 19) nitric acid and 500 ppm iron, (d) (1 + 19) nitric acid and 500 ppm nickel, (e) ( I + 19) sulphuric acid.
Fig. 2, The effects on the absorption signal for cobnit (0. I ppm) of increasing concen traCons of (a) iron, (b) nickel and (c) chromium: (A) in (1 + 19) nitric acid solution;(B) in (1 + 19) sulphuric acid soiution.
may be lost from the graphite rod as volatile chlorides during dry ashing [S-10] _ In Fig. 2 the responses to 0.10 ppm cobalt in dilute nitric acid and sulphuric acid with varying concentrations of iron, nickel and chromium in solution are illustrated. The presence of only a few ppm of these metals resuited in a marked drop in the cobalt signal but the decrease levelled out at about 40 ppm of interfering metal and remained more or less constant with greater amounts. The decrease in response may arise from incomplete atomisation of cobalt when it is in the presence of an excess of other metaJs with very similar boiling points. Nitric acid is therefore the most suitable acid for use with the graphite rod and a 5% soiution has no significant effect on the cobalt sensitivity. The detection limit at the full scale expansicn of X 10, expressed as twice the background signal, was found to be 0.005 ppm, It has been suggested fl0, 111 that oxidising acids, such as nitric and pcrchloric, are preferable for use with the graphite rod because on charring these soIutions the me&I oxides are formed, and these are readiIy reduced by the carbon rod resulting in a higher metal atom density during atomisation. However, nitric acid alone will not dissolve most samples of steel, Furthermore, even a few ppm of iron, nicke1 or chromium reduce the cobalt signal. Hence, in order to determine cobalt directly in such mixtures it would be
281
necessary to accept a lower sensitivity; in addition, matrix matching or the standard addition technique would be required_ A more satisfactory solution
is therefore
to isolate the cobalt from the interfering
metals.
Extraction of cobalt as its I-nitroso-2-nap11 thol complex A linear plot of absorbance versus concentration was obtained when cobalt was extracted with 1-nitroso-2-naphthol and measured on the graphite rod. A 0.20 ppm cobalt solution after extraction, gave an absorbance of 50% full scale deflection when the scale expansion was X 3, which was about 20% less than that obtained with a 0.20 ppm cobalt solution in 5% nitric acid. The detection limit (twice background) was 0.005 ppm which was the same as for cobalt in nitric acid. It was noted that if the dry ashing voltage was reduced below a scale setting of 6.0 a double peak was obtained during atomisation; this was found to be due to the excess of 1-nitroso-2naphthol. This was eliminated by keeping the voltage setting above 6.0 but below 7.5 in order to prevent loss oi’ cobalt at this stage. Zinc oxide separation_ The British Chemical Standard samples analysed by the procedure involving zinc oxide separation were BCS No. 387, a nimonic alloy, and Nos. 331 and 334, which were austenitic steels. The cobalt con-
centrations obtained for six samples are listed in Table 2. Analyses of the chloroform extract showed that the amount of iron extracted was <0.25 ppm and nickel <0.15 ppm whilst chromium was not detectable. Hence, the method was successful in decreasing the concentrations of these metals to levels at which they did not interfere. However, the procedure was laborious and although duplicate samples gave similar results the mean cobalt concentrations differed greatly from the certificate values. \Vhilst accuracy might well have improved with practice, it was decided to abandon this procedure in favour of the less tedious and more accurate sodium citrate method which was developed. Sodium citrate separation.
The samples analysed were BCS Nos. 387, 334, 331 and 335; cobalt concentrations ranged from 0.034 to 0.21%. A linear plot of absorbance versus concentration was obtained from standards taken
TXSLE Cobalt
2 in steels as determined
Steel BCS No.
Cobalt
%Iean
(I)
by the zinc oxide
method
Certificate value
Difference of mean from certificate value
(%I
(70)
38’i
0.168 0.163
0.1655
0.21
-21
331
0.049 0.056
0.0525
0.010
+ 31
334
0.046 0.048
0.04io
0.052
-10
282 TABLE
3
Cobalt concentration in steels determined by the sodium citrate method Steel BCS
Sample tvt.
NO.
(msl
Cobalt (701
Mean
Certificate
Sample
Cobalt
V~fW
wt.
P)
4%)
bw)
Mean
&ertifkxte value ml
367
J
0.212 0.203 0.x% 0.208
0.20~
0.21
O.-l
0.197 0.224 0.19-i
0.206
0.31
331
10
0.083 0.051 0.056 0.05 t
0.053
0.052
1
0.063 0.051 0.053 0.053
0.052,
0.052
33.5
10
0.036 0.031 0.03s 0.036
0.037
0.034
1
0.036 0.036 0.036
0.036
0.034
331
10
0.010 0.03s 0.042
0.010
0.040
1
0.044 0.04-i 0.04 1 0.039
0.042
0.040
thrcugh the whole procedure; this calibration graph was used for the steel sampk?s_ Results obtained for IO-mg samples of steei (4 mg for 357) are given in Table 3. Four independent anaiyses were performed with each steel (except 33X); each solution was measured two or three times and the average resuIt recorded. These rest&s show that good precision and accuracy were obtained. AU the resuIts lie within +7.X% of the mean, which is typicaI of ~lectrotI~erma1 atomic absorption spectrometric results. Apart irom sample 335, which gives a result 8% too high, the mean results are within 2% of the certificate value. Results obtained with I-mg sampies of steel {in the case of sample 387, 0.4 mg) are also given in Table 3. AII resuIts lie within ~8.5% of the mean. Apart from sample 335 which gives a resuIt 6% too high, the mean results are within 4% of the certificate value. Comparison with the data on the farger sampIes shows that the pm&ion and accuracy is very similar. In both instances the results for sample 335 are higher than the certificate value by an average of ‘i%, whereas those for the other sampIes are aI.I within _+I% of tfle certificate value, which suggests, perhaps, that the portions of 335 steel taken for analysis mat not have been representative of the bulk. The results given were aI.I obtained using the ultrasonic acid washing technique. This decreased the iron and nickel concentrations to less than 0.15 ppm and 0.2 ppm, respectively, and chromium was only just detectable. At these Ievefs no in terferenee was encountered, whereas pt-eviousls, when iron
283
and nickel were decreased only to 2--3 ppm, interference occurred. \Vith these higher concentrations, iron and nickel were not totally atomised at each firing and tended to accumulate on the rod leading to suppression of the cobalt signal and erratic results. This could be controlled by an additional firing of the rod after every 4 or 5 determinations, but the background signal remained almost double that obtained with the ultrasonically washed solutions. The cobalt detection limit of 25 pg in 5 ,~l of chloroform is equivalent to a detection limit for cobalt of 25 ppm in a 1-mg sample. If necessary this limit could be reduced by simply evaporating down the chloroform estract, e.g. from 5 cm3 to 1 cm3, or by evaporating multiple samples on the graphite rod followed by a single atomisation. Tests have shown that excess of l-nitroso-2-naphthol is removed during dry ashing and has no effect on the cobalt signal. The results demonstrate that by scaling down the volumes of solution the method can be successfully applied to samples of only 1 mg of steel. The ability to analyse small samples is of importance in the analysis of size fractions of particulate material carried by reactor coolant. Furthermore, because only small quantities of sample are required, the hazard from radioactive samples is reduced. This paper is published Board.
by permission
of the Central
Electricity
Generating
REFERENCES 1 P. J. Da&y and B. J. hlacfarlane, Proc. Int. Conf. Water Chemistry of Nuclear Reactor 1977, British Nuclear Energy Society, Systems. Boumemouth, U.K., October 2?--2i, London, 1978, p. 25’7. 2 E. B. Sandell. Calorimetric Metal Analysis, Interscience, New York, 1965. p_ -I?? 3 R. S. Young, The Analytical Chemistry of Cobalt. Pergamon Press, Osford, 1966. p. -16. -1 hl. S. Cresser, Solvent Extraction in Flame Spectroscopic Analysis. Butterworth, London. 197s. 5 P. Hocaucllet, Ann. F&if. Expert. Chim., Gi (197-A) -195. 6 L. Hagernan, L. Torma and B. E. Ginther. J. Assoc. Off. Anal. Chem., 56 (1975) 990. 7 -4. I. Vogel, A Test-Book of Quantitative Inorganic Analysis, Longmans, London, 196s. p. 529. S B. R. Culver and T. Surlcs. Anal. Chem., -17 (1975) 920. 9T. R. Dulski and R. R. Bixler. Anal. Chim. Acta, 91 (1977) 199. 10 F. Shaw and J. RI. Gttaway, Analyst, 100 (1975) 317. 11 W. B. Barnett and E. A. RIcLaughlin, Anal. Chim. Acta, SO (1975) 285.