The potentiometric microdetermination of cobalt in organic compounds after combustion in a modified oxygen flask

MICROCHEMICAL

JOURNAL

46,

215-224

(1992)

The Potentiometric Microdetermination of Cobalt in Organic Compounds after Combustion in a Modified Oxygen Flask ANTONIO CAMPIGLIO Department

of Pharmaceutical Received

March

Chemistry, 27100-Pavia,

University Italy

12, 1991; accepted

of Pavia, October

Via Taramelli

12,

3, 1991

A reliable method for the microdetermination of cobalt in organic cobalt compounds is described. The sample is combusted in a modified oxygen flask and the products are absorbed in 6 M HCl. The Pt basket is then boiled in the absorption solution and the sparingly soluble residue of cobalt oxides is completely dissolved. After cooling, the solution is transferred to a titration cell and neutralized with NaOH in the presence of methyl red. After careful adjustment of the pH, the Co(H) is finally determined in a buffered 60% (v/v) dimethylformamide solution by potentiometric titration with 0.033 M sodium diethyldithiocarbamate solution. The results obtained are accurate within 20.08%; the recoveries of Co are in the range of 99.52 to 99.97%; the standard deviation is 0.06%. The potentiometric titration of Co(H) with sodium diethyldithiocarbamate and other reagents, as well as the conditions for the oxygen flask combustion of organic cobalt compounds, is discussed. 0 1992 Academic Press, Inc.

INTRODUCTION Organic cobalt compounds (OCoC) are important in pharmaceutical and biological areas. Several methods for the determination of cobalt in OCoC are described in the literature. After decomposition of the sample by ignition in an hydrogen stream (1, 2), wet digestion (2-7), oxygen flask combustion (8-12), or combustion in oxygen plasma (13, 14), cobalt has been determined by gravimetric (I, 2, 5, 6, 11, 13, Z4), titrimetric (5-8, 21, 14), spectrophotometric (3, 4), or polarographic (9, 10, 12) methods. Oxygen flask combustion was used in the analysis of OCoC by burning the sample in a mixture with 10 to 20 mg of sodium carbonate (8, l&12). After combustion, boiling the sample holder in hydrochloric acid solution was also recommended for a complete recovery of cobalt (9, IO). However, the results obtained are not always satisfactory because of a sparingly soluble residue of cobalt oxides formed on the Pt basket during the combustion. Therefore, the possibility of determining Co in OCoC by combustion in a modified oxygen flask described previously (15, 16) and potentiometric titration of Co(I1) was investigated. EXPERIMENTAL Reagents The following reagents were used: oxygen; hydrochloric acid at 6, 1, 0.1, and 0.01 M; sodium chloride, 4 M solution; dimethylformamide (DMF); methyl red, 0.1% solution in ethyl alcohol; sodium hydroxide, 5, 0.5, and 0.01 M solutions; 21.5 0026-265X/92

$4.00

Copyright Q 1992 by Academic Press, Inc. All rights of reproduction in any form reserved.

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potassium nitrate, 20 and 10% aqueous solutions; a 7.5% solution of potassium nitrate in 44% (v/v) DMF made by diluting 30 ml of 20% aqueous KNO, solution with 15 ml of double-distilled water and 35 ml of DMF; 8 M nitric acid; 2 M ammonia-2 M ammonium nitrate buffer solution at pH 9.65 (pH adjusted by adding 8 M nitric acid). All chemicals used are of analytical grade, and the solutions are prepared with double-distilled water. A 0.033 M solution of sodium diethyldithiocarbamate (NaDDTC) in 60% (v/v) DMF is made by dissolving 1.487 g of NaDDTC * 3 H,O in 60% (v/v) DMF (prepared 34 h before the use) and diluting to 200 ml. For a standard solution of Co(H) (700 ug of Co/6 ml), 116.67 mg of cobalt (Fluka, puriss. p.a., powder, ~99.8%) in a IOO-ml conical flask is treated with 10 ml of 6 M HCI and the reaction mixture is repeatedly heated gently to boiling with occasional swirling and then allowed to stand for a few minutes until the cobalt is completely dissolved. After cooling, the solution is diluted to 1 liter with doubledistilled water (Co as CoCl,). Alternatively, 306.8 mg of CoSO, [cobalt(II) sulfate hydrate, 99.999% (Aldrich), dried in a muffle to constant weight at SOO’C] is placed in a loo-ml beaker and dissolved in about 40 ml of double-distilled water under heating to 50-6O”C. The solution is then diluted to 1 liter with doubledistilled water (Co as CoSOJ. Equipment

Rectangles (ca. 23 x 29 mm) of ash-free paper (Schleicher & Schuell, No. 5892) with the usual fuse (total weight of the paper piece, 65 + 5 mg) are cut to wrap the samples. A modified oxygen flask, described previously (15, Z6), consists essentially of a 300-ml round-bottom flask, provided with a short test tube sealed to its bottom and equipped with a ground stopper to which an original detachable U-shape Pt basket is connected. A pH meter equipped with magnetic stirrer and glass electrode is used for pH measurement. The titration assembly consists of a 50-ml beaker with magnetic stirrer; sulfide ion-selective electrode [(Orion, 941600) the membrane of the electrode must be polished with a special polishing strip (Orion, 94-82-01) before each titration]; double junction reference electrode [(Orion, 900200) the inner and outer compartments are filled with the Orion filling solution (Orion, 90-00-02) and the 7.5% KN03 solution in 44% (v/v) DMF, respectively]; Titroprocessor Metrohm E 636 with Dosimat Metrohm E 635 (l-ml burette with l-liter dark glass reservoir), operating mode = monotonous titration at controlled drift according to the following program: potential drift = 3.75 mV/min (2 Mod = 3), constant addition volume = 0.02 ml (3 Mod = 3), start titrant addition = 0.2 ml (4 Mod = 0.2), Stop E/mV = 10,000, Stop V/ml = 2, number of endpoints = 1, potential range - 100 to - 500 mV. Alternatively, any pH meter with an expanded scale in connection with a l-ml burette can be used. Procedure

The sample, preferably containing

600-800 pg of Co, is dispersed in a platinum

MICRODETERMINATION

OF

COBALT

217

microboat, weighed, added to a filter paper rectangle, and finally wrapped in it. The packet is inserted into the Pt basket, and the adequate fixation of the Pt wire (welded to the gauze) into the glass capillary sealed to the base of the stopper is carefully controlled. As absorption solution 2 ml of 6 M HCl is introduced into the flask. Oxygen is blown in for 30 s and the sample combusted. As soon as the combustion is finished, the Pt basket with the incandescent residue is dropped into the underlying test tube by means of gentle push on the flask. The walls of the flask are then washed with the absorption solution by rotating the flask in horizontal position, and the flask is allowed to stand for 30 min while its inner wall is occasionally washed with the absorption solution. The stopper is then raised to equilibrate the pressure inside the flask. By holding the stopper 1-2 mm over the ground joint, the test tube is heated by means of a microflame and the solution is boiled gently for 10 min. At the 4th and 7th min and at the end of boiling, the flask is stoppered and the walls are washed carefully with the hot solution to dissolve completely the small residue particles dropped from the Pt gauze into the flask during combustion. The flask is allowed to cool and the reference solution for pH adjustment (Ref Sol) is prepared by mixing 12 ml of double-distilled water, 3 ml of 4 M NaCl, and 23 ml of DMF. After cooling, the stopper is removed from the flask, the Pt basket is extracted from the solution by using a glass rod with a hook and washed in the flask with 5 ml of double-distilled water, and the contents of the flask are transferred to a SO-ml beaker. The stopper is then washed in the flask with 3 ml of double-distilled water and 2 ml of DMF and then with 5 ml of DMF, and the wash liquid is poured into the beaker after the flask is rinsed. The flask is finally washed with 1.5 ml of double-distilled water + 4 ml of DMF, 1 ml of double-distilled water + 4 ml of DMF, and then two times with 4 ml of DMF. After addition of 3 drops of methyl red, the solution (35.5 ml) is stirred and neutralized with 5 M NaOH (about 2.4 ml) and a few drops of 1 M or 0.01 M HCl as required. One drop of 1 M HCl in excess is then added and the solution allowed to cool to room temperature for about 1 h. The pH of the Ref Sol (pH,,r) is measured by using a pH meter with a glass electrode. The pH of the solution from combustion is adjusted carefully to the pH,,r value (this value must not be exceeded in any case, although a value up to 0.05-0.06 pH unit lower than the pHa,, value is quite adequate) by adding a few drops of 0.5 or 0.01 M NaOH and of 0.01 M HCl if necessary. Co(H) is finally titrated with 0.033 M NaDDTC. Five drops of buffer solution at pH 9.65 are added to the solution (about 38 ml). After the membrane of the sulfide ion-selective electrode is polished with the special strip, the electrodes and the tip of the burette are rinsed with double-distilled water and immersed into the solution, which is stirred vigorously. The potentiometric titration is then carried out automatically using the Titroprocessor or manually using a pH meter with expanded scale. A soluble greenish complex is formed during the titration, which takes place approximately in the potential range - 200 to -400 mV. A potential break of 8&100 mV/20 ~1 of titrant is observed at the endpoint. The titration requires 9 to 12 min. No blank value is present.

218

ANTONIO

CAMPIGLIO

The titrant is standardized against 700 pg of Co(I1) as follows: 6 ml of standard Co(I1) solution (CoCl, or CoSO,) is placed in a 50-ml beaker, 4 ml of doubledistilled water, 2 ml of 6 M HCl, 23 ml of DMF, and 3 drops of methyl red are then added, and the solution is neutralized under stirring with 5 M NaOH (about 2.4 ml) and a few drops of 1 M or 0.01 M HCl as required. One drop of 1 M HCl in excess is then added, and the solution is allowed to cool about 1 h to room temperature. Finally, the pH of the solution is adjusted and the Co(I1) titrated as described. RESULTS Potentiometric

Titration

AND DISCUSSION

of Cobalt(ZZ)

Since no cobalt(I1) ion-selective electrode is commercially available, complexometric or precipitometric titrations of Co(H) are usually carried out potentiometrically by using an ion-selective electrode (ISE) sensitive to the titrant as indicator electrode. According to Kosturiak and Kalavska (17), l-20 mg of Co(I1) in 80% (v/v) DMF is determinable by direct potentiometric titration with 0.02 or 0.04 M NaDDTC in 80% DMF using an Ag,S ISE. The average error in the determination of 1 mg of Co(I1) is 4.9%. Pilipenko et al. (18) determined Cu, Cd, Pb, Bi, Ni, Ag, Hg, and Co by potentiometric titration with a solution of diethyldithiocarbamate, dithio-oxamide, &mercaptoquinoline, or 2,3-dimercaptoquinoxaline in 30% aqueous acetone or with unithiol in aqueous solution by using an Ag,S ISE. Hassan and Habib (19) reported that OS-5 mg of Ag, Cu, Cd, Ni, Pb, Th, Co, and Zn in 10 ml of 50% aqueous ethanol can be titrated potentiometrically at pH 4-7 with 0.01 M NaDDTC in 50% ethanol by using a graphite-AgDDTC indicator electrode (recovery = 96.90 for 0.640 and 1.290 mg of Co, standard deviation = +0.90%). As titrant for the potentiometric titration of 400-800 p,g of Co(II), a number of organic sulfur-containing reagents able to form complexes with Co(I1) and at the same time monitorable by means of an Ag,S ISE vs a double junction reference electrode (DJRE) was investigated; ammonium bis(2-hydroxyethyl)dithiocarbamate (NH,BHDTC), used in the determination of Co, Cu, Hg, and Ni by HPLC (20), was included in these reagents. Potentiometric titration of Co(I1) with K,[Fe(CN),], based on the precipitation of the sparingly soluble Co,[Fe(CN),] [Ksp = 1.8 X lo-l5 at 25°C (21)], was also investigated by using an Ag,[Fe(CN),]-graphite indicator electrode [prepared according to the procedure described for the AgDDTC-graphite electrode (19)] vs DJRE. The results obtained are shown in Table 1. In these experiments, the volume of the sample solution was always 30 ml, and buffering was accomplished by addition (a few drops) of concentrated buffers (to minimize the consequent dilution of the sample solution) such as 1 M AcOH-1 M AcONa buffer at pH 5.30 (10 drops), 1 M AcOH-1 M AcONH, buffer at pH 6.60 (10 drops), 2 M AcONH,-2 M NH40H buffer at pH 8.50 (5 drops), or 2 M NH,NO,-2 M NH,OH buffer at pH 9.60 (5 drops). As Table 1 shows, the most reproducible results were obtained with the titration with NaDDTC in 66% DMF, and this was chosen for the rest of the experiments. Because at least 15 ml of water was necessary for the microdetermination of Co in OCoC, the volume (30 ml) of the sample solutions should be modified and was fixed at 38 ml (15 ml of water + 23 ml of DMF). The titration of Co(I1) should be carried out not in 66% but in 60% DMF.

MICRODETERMINATION

OF

TABLE Results

of Potentiometric Co(H) added (Id

Titrant

Titration

219

COBALT

1 of Co(I1)

with

Various

Medium

Titrants

Remarks

Potassium ethyl xanthogenate, 0.01 M aq. solution

600

H,O or 50% EtOH without or with buffer at pH 5.30, 8.50 or 9.60

Continuous slow potential drift during the titration; poor potential break (AEl at the end point (EP)

S-Mercaptoquinoline hydrochloride, 0.01 M solution in 30% acetone

408-600

30% acetone with buffer at pH 5.30

AE at EP = 35-65 mV/50 ul of titrant; in the range 4OC&Xl pg of Co, reproducibility (Repr) = CO.55%, average recovery (Av Ret) = +0.52%, time required for the titration (Titr Time) = 25-30 min

Dithio-oxamide 0.005 M solution in 50% EtOH or in 30% or 50% acetone

600

50% EtOH or 30% or 50% acetone without or with buffer at pH 6.60 or 8.50

No endpoint

detectable

Thiomalic acid, 0.01 M neutral aq. solution

600

H,O or 50% EtOH

No endpoint

detectable

2,5-Dimercapto-1,3,4thiadiazole, 0.01 M solution in 25% EtOH

600

25% EtOH without or with buffer at pH 8.50 or 9.60

No endpoint

detectable

Meso-2,3-dimercaptosuccinic acid, 0.01 M neutral aq. solution

500-700

H,O or 50% EtOH without or with buffer at pH 5.30, 6.60 or 9.60

In 50% EtOH with buffer at pH 9.60: AE at EP = 7.5-11 mV/50 pl, Repr = t0.80%, Av Ret = ? 1.33% in the range 500-700 ug of Co, Titr Time 25-30 min

NaDDTC: 0.01 M solution in 50% EtOH; 0.02 or 0.03 M solution in 30, 50, 60, or 70% acetone

600-800

50% EtOH with different buffers 30, 50, 60, or 70% acetone without or with buffer at pH 9.60

Unsatisfactory Repr even under the best pH conditions (buffer at pH 9.60) In 30% acetone without or with buffer: unreproducible results associated to the precipitation of the Co(DDTC), complex during the titration. In 60% acetone with buffer at pH 9.60, the most reproducible results: in the range 45&750 fig of Co, Repr = + l.SO%, Av Ret = -t5%, Titr Time = 10-24 min

0.02 M solution 50% DMF

500

50, 66, 73, or 80% DMF

In 50% DMF, Repr reduced (-t 1%) due to precipitation of Co(DDTC)s complex; in 66% DMF, the best Repr (?O.SO%); in 80% DMF, Repr ?0.66%

600

H20, 66% EtOH or p-dioxane with various buffers

No endpoint detectable or erratic or unreproducible results

H,O with buffer at pH 5.30

Reproducible results (AE at EP = 20-26 mV/50 &I) despite some initial irregularities; in the presence of salts, however, diicult EP detection (e.g., NaNOs) or electrode response suppression (e.g., NaCI)

in

NH,BHDTC, 0.03 M aq. solution I&[Fe(CN)d, 0.01 N aq. solution

450-750

Potentiometric titration of 200-1000 kg of Co(I1) in 38 ml of 60% DMF with 0.02 in 60% DMF was then investigated to check its reliability and to identify the best experimental conditions, A study of the influence of pH on titration showed that the optimum pH conditions were obtained by buffering with buffer at pH 9.65. The use of a 7.5% KNO, solution in 44% DMF instead of 10%

M NaDDTC

220

ANTONIO

CAMPIGLIO

aqueous KN03 in the outer compartment of the DJRE was found to improve the reproducibility. No precipitation of the Co(DDTC), complex was observed during the titration of Co(I1) amounts up to 1000 pg. A 0.033 M NaDDTC solution to be added from a l-ml burette was the most convenient, because it allows titration of up to 900 pg of Co(H) without refilling the burette at or near the titration endpoint. The titration program was also studied and optimized. The influence of salts on potentiometric titration was also investigated. No interference was found to be due to the presence of NaNO, at 0.32 M or NaCl at 0.4 M in the solution to be titrated. All these experiments allowed the optimization of the potentiometric titration of Co(H) in 60% DMF with NaDDTC. The results obtained for 20&1000 pg of Co(I1) are shown in Table 2. The sample solutions (volume = 38 ml) contained NaCl at 0.32 M (as the samples after combustion). Five drops of buffer at pH 9.65 were added before the titration. As Table 2 shows, sharp potential breaks (66-106 mV/20 ~1 of titrant) are observed at the endpoint of the titration. If the titrant is standardized against 700 p,g of Co(II), results with an accuracy of t0.34% are obtained in the range X&1000 pg of Co. The reproducibility of the results is within 20.38% and the standard deviation from 20 titrations of 700 pg of Co is 0.8 pg (0.11%) of Co. Titration of Co(H) in amounts up to 3 mg, which occurs without precipitation of the Co(DDTC), complex, was found to be quite possible. Adequate standardization of the titrant was required. Titration of amounts of Co larger than 3 mg or smaller than 200 pg was not investigated. The titration is not specific for Co(I1). However, the OCoC do not usually contain other metals. Oxygen Flask Combustion

of OCoC

According to preliminary experiments, the oxygen flask combustion of OCoC gives rise to the formation of a sparingly soluble residue of cobalt oxides on the Pt TABLE

2

Results of the Potentiometric Titration of 200.2 to 1001 .O p,g of Cobalt(D) with Sodium Diethyldithiocarbamate (NaDDTC) Standardized against 700.7 pg of Co(I1) as Cobaltous Sulfate

Co(I1) taken (I%) 1001.0 900.9 800.8 700.7 600.6 500.5 400.4 300.3 200.2

Potential break at the endpoint (mv)

Titrant (PI) 1029.0 927.0 824.5 722.0 619.6 517.5 415.5 320.0 213.5

* k 2 + f f + k f

3.0 3.0 2.5 2.0 2.4 1.5 1.5 1.0 0.5

81 * 805 81 + 96 f 82? 902 94+92 + 87?

15 10 10 10 11 16 11 13 11

required for the titration (mitt) 13 f 2 13 f 1 13 f 1 12* 1 11 t 1 10 ?Z 1 lo? 1 821 7+1

Molar&y of the titrant 3.3009 3.2977 3.2957 3.2932 3.2892 3.2818 3.2700 3.1844 3.1819

x x x x x x x x x

1o-2 lo-* lo-* lo-* lo-* lo-* lo-* lo-* lo-’

Cobalt(I1) found % lJ4 998.7 899.7 800.2 700.7 601.3 502.2 403.2 310.6 207.2

rt 2.9 f 2.9 + 2.4 -c 1.9 f 2.3 + 1.5 k 1.5 f 1.0 + 0.5

99.77 99.87 99.93 100.00 100.17 100.34 100.70 103.43 103.50

+ 0.20 ‘- 0.32 rf: 0.30 + 0.27 + 0.38 2 0.33 + 0.37 + 0.33 f 0.25

Note. Molarity of the titrant = 3.2932 X lo-*. Average values of six determinations are given for each Co(I1) amount.

MICRODETERMINATION

OF

COBALT

221

basket. The initial investigation was therefore devoted to determine the best reagent for dissolving the cobalt oxides. For this purpose, samples of 0.8-l .2 mg of Co,O, in a U-ml round-bottom flask were boiled under reflux with 2 ml of 1,2,4, or 6 M HNO, or HCl for 3,5,10,20, or 30 min. The contents of the flask were then transferred to a 50-ml beaker, and the flask was washed carefully with 11 ml of double-distilled water and then with 23 ml of DMF. After neutralization with ca. 2 ml of 1, 2, 4, or 6 A4 NaOH and addition of 5 drops of buffer at pH 9.65, the Co(I1) was titrated potentiometrically with NaDDTC. It was found that Co30, could be dissolved more easily by boiling with HCl than with HNO, of the same molarity. Complete dissolution was achieved by boiling with 2 ml of 6 M HCl for 10 min. The microdetermination of Co in OCoC was then investigated. Samples of 0.8-0.9 mg of cobalt were combusted in the modified oxygen flask charged with 2 ml of 6 M HCl as absorption solution. After 30 min, the Pt basket was boiled for 10 min in the absorption solution, which was then transferred to a beaker with 10.5 ml of double-distilled water and 23 ml of DMF to wash the Pt basket, stopper, and flask. After neutralization with NaOH and addition of the buffer at pH 9.65, the Co(I1) was titrated with NaDDTC standardized against cobalt as CoCl,. Low recoveries (about 90%) were obtained for Co. Satisfacory recoveries of Co were found only after a careful study of the individual steps involved in the analytical procedure, i.e., (1) combustion, (2) complete dissolution of cobalt oxide residue, (3) quantitative transferring to the titration cell, and (4) neutralization. (I) Combustion. The combustion of cobalt samples wrapped in paper pieces of 60-70 mg was found to occur satisfactorily. However, at the end of the combustion, the Pt basket containing the incandescent residue of cobalt oxides must be allowed to fall into the underlying test tube by means of a gentle push on the flask. This operation avoids an undesirable prolonged ignition of the residue as well as fragmentation into a great number of small sparks which drop into the flask and make complete recovery of the cobalt difficult. (2) Complete dissolution of the cobalt oxide residue. This was obtainable by boiling the Pt basket in the absorption solution for 10 min. During the boiling, the walls of the flask should be washed carefully three times with the hot solution to dissolve any cobalt oxide particles deposited on the walls during combustion. The solution must be boiled by holding the stopper l-2 mm over the ground joint of the flask in order to avoid loss of Co due to aerosol formation. (3) Quantitative transferring of the solution to the titration cell. Conditions adequate for this operation were identified by introducing 2 ml of a standard Co(I1) solution (containing 400 pg of Co/ml) into 50-ml beakers used as titration cells and into modified oxygen flasks. The Co(I1) in the beaker was then titrated directly after dilution with 10.5 ml of double-distilled water and 23 ml of DMF. The Co(H) in the oxygen flasks was titrated only after boiling of the Pt basket in the solution for a few minutes (by holding the stopper l-2 mm over the flask) and subsequent transfer of the solution to the beaker with use of 10.5 ml of doubledistilled water and 23 ml of DMF to wash the extracted Pt basket, stopper, and flask under various conditions. The transfer of the solution was obtainable without loss of Co by following the steps described under “Procedure.”

222

ANTONIO

CAMPIGLIO

TABLE 3 Results of the Microdetermination of Cobalt in Organic Compounds Samvle (4

Titrant (I.4

Cobalt

0.811 0.743 0.724 0.693 0.676

Cobalt(B) oxalate dihydrate

Substance

Cobalt (%) Calcd

Found

A

Recovery

828 760 740 709 691

100.00

99.66 99.84 99.77 99.86 99.78

-0.34 -0.16 -0.23 -0.14 - 0.22

99.66 99.84 99.77 99.86 99.78

2.718 2.416 2.394 2.253 2.096

895 797 789 742 690

32.21

32.14 32.20 32.17 32.15 32.13

-0.07 -0.01 -0.04 -0.06 -0.08

99.78 99.97 99.88 99.80 99.75

Cobalt(B) acetilacetonate

3.191 3.173 3.125 2.785 2.590

748 743 732 652 606

22.92

22.88 22.86 22.86 22.85 22.84

-0.04 -0.06 -0.06 - 0.07 -0.08

99.83 99.74 99.74 99.69 99.65

Cobalt(I1) anthranilate

4.284 3.910 3.776 3.546 3.449

778 710 687 645 626

17.80

17.73 17.72 17.76 17.75 17.72

-0.07 -0.08 -0.04 -0.05 -0.08

99.61 99.55 99.78 99.72 99.55

Cobalt(H) cyclohexanebutyrate

5.288 5.284 5.011 4.885 4.620

800 800 758 739 700

14.83

14.77 14.78 14.77 14.77 14.79

-0.06 -0.05 -0.06 -0.06 -0.04

99.60 99.66 99.60 99.60 99.73

Cobalt(B) phthalocyanine

7.715 7.482 6.180 6.148 5.993

811 787 650 647 631

10.31

10.26 10.27 10.27 10.27 10.28

- 0.05 -0.04 -0.04 -0.04 -0.03

99.52 99.61 99.61 99.61 99.71

-

(4) Neutralization. When the HCl present in the 60% DMF solution was neutralized with NaOH in the presence of methyl red as indicator, the final adjustment of pH to a value corresponding approximately to neutral was not obtainable visually with the reproducibility required by the relevant pH sensitivity of the Co(H) titration with NaDDTC. The subsequent addition of buffer at pH 9.65 was often not adequate to avoid the differences in pH among the various solutions. Thus, low results for Co can be obtained if the solution to be titrated remains slightly acidic after the pH adjustment. High results must be expected if the solution is basic. Therefore, a more accurate adjustment of pH with the use of a pH meter is necessary before the titration. Because the solution consisting of 15 ml of a neutral aqueous Co(I1) solution and 23 ml of DMF has a pH,,, different from 7 which can vary from day to day even under system standardization against

MICRODETERMINATION

OF COBALT

223

the same two buffers before the measurement, adjustment of the pH,,, to a prefixed value had to be discarded. It was more reliable to measure the pH,,, of a reference solution of a given composition (3 ml of 4 M NaCl + 12 ml of double-distilled water + 23 ml of DMF as the sample solutions) and then adjust a the PH,, of the solutions to be titrated to this value. For the neutralization, two-step procedure was therefore devised: (1) the solution was neutralized with visual adjustment of the pH to a slightly acidic value (orange color), and (2) after cooling of the solution (heat of neutralization) to room temperature (i.e., after 1 h), the PH,, of the solution was measured by using a pH meter and adjusted to the value found for the reference solution. Finally, the buffer was added to the solution just before the titration to minimize the volatilization of ammonia. As a result of this study, a reliable method for the microdetermination of Co was developed. This was checked by analyzing a variety of OCoC, and satisfactory results were obtained. Combustion of samples of OCoC alone or in mixture with sodium carbonate as recommended in the literature (8, 10-12) was also investigated. Combustion of the sample mixed with 13 to 20 mg of Na,CO, was found to give no advantage. Typical results of cobalt microdetermination in OCoC are given in Table 3. All samples contained amounts of cobalt in the range of 591-874 kg. The titrant was standardized against about 700 pg of Co(I1) as CoS04 (molarity = 3.3122 x lo-*). The errors for the OCoC are within +0.08%; the recoveries for Co are in the range of 99.52-99.97%. The reproducibility of the results is good. The standard deviation from 20 microdeterminations of Co in cobalt cyclohexanebutyrate was 0.06%. No blank value is present in the Co determination in OCoC. Nevertheless, particular attention must be given to the purity of the reagents used. The NaDDTC solution can be standardized against metallic cobalt or CoSO, dried to constant weight at 500°C. In conclusion, this method can be regarded as particularly reliable compared with other methods for the microdetermination of cobalt in organic elemental analysis. ACKNOWLEDGMENTS The author is indebted to Professor Giorgio Traverso for his interest in this work. This work was supported by a financial contribution to scientific research (60%) assigned by the “Minister0 della Pubblica Istruzione” and the “Minister0 dell’Universit6 e della Ricerca Scientifica e Tecnologica” for 19811990.

REFERENCES 1.

2. 3. 4. 5. 6. 7. 8.

Pregl, F.; Roth, R. Quantitative Organische Mikrbanalyse, VII ed., pp. 192-193. Springer Vet-lag, Vienna, 1958. Ingram, G. Methods of Organic Elemental Microanalysis, pp. 317-319. Chapman & Hall, London, 1962. Belcher, R.; Crossland, B.; Fennell, T. R. F. W. Talanfa, 1970, 17, 112-115. Debal, E.; Chassin, R.; Peynot, S.; Poliakoff, 0. Talanta, 1977,24, 491-495. Sakla, A. B.; Helmy, A. A.,; Beyer, W.; Harhash, F. E. Talanta, 1979, 26, 51s522. Abou-Taleb, S. A. J. Indian Chem. Sot., 1980,57,56-59: through Chem. Abstr., 1980,93,60484r. Xie, Z.; Duan, J.; Wang, G. Huaxue Shijie, 1982, 23(7), 206-207. Macdonald, A. M. G.; Sirichanya, P. Microchem. J., 1969, 14, 199-206.

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