Determination of boron in different alloys and compounds

Journal

of the Less-Common

DETERMINATION COMPOUNDS* S. S. GRAZHULENE,

Metals,

OF BORON

117 (1986)

401

IN DIFFERENT

Y. I. POPANDOPULO

Institute of Problems of Microelectronics of Sciences of the U.S.S.R., Chernogolovka

401

- 405

ALLOYS

AND

and G. F. TELEGIN Technology (U.S.S.R.)

and Superpure

Materials,

Academy

Summary Methods of flame-spectrometric determination of boron in boroncontaining compounds of different composition have been developed from BO, molecular spectra in flames. The determination of boron can be made after separating large amounts of attributable elements (e.g. iron) by extraction chromatography as well as by selective methylation of boron irrespective of the content and composition of the boron-containing compound. The influence of the alloy components and the experimental conditions on the accuracy of the determination of boron was investigated. The validity of the determination was estimated by comparison with other methods. The relative standard deviation is 0.03 for boron-containing compound.

1. Introduction The problem of the determination of boron in various boron-containing compounds is still urgent, because of insufficient reliability in its determination owing to the interaction of the elements of the analysed material, and the labour content of the majority of the available methods is high [ 11. Several papers, related to the determination of boron in its molecular compounds by flame emission spectrometry, have been recently published [2 - 41. The boron spectrum is composed of wide bands within the wavelength region 460 - 640 nm attributed to B02 molecules [5]. The possibility of the determination of boron by the maxima of the molecular bands at 546 and 518 nm in an acetylene-air flame was studied in refs. 2 and 4, while in ref. 3 acetylene-air and propane-air flames were used to analyse organic samples and artificial compositions based on aluminium and magnesium. This method for the determination of boron is advantageous because of its selectiveness, simplicity and speed, but its applicability to the analysis of different boron compounds is as yet not fully understood. *Paper presented at the 8th International Symposium on Boron, Nitrides and Related Compounds, Tbilisi, October 8 12, 1984. @ Elsevier

Sequoia/Printed

Borides,

Carbides,

in The Netherlands

402

In the present paper the possibility of the dete~ination of boron in complex alloys with different components such as iron, cobalt, nickel, molybdenum, phosphorus and boron was investigated using the emission flame spectrometry method.

2. Experimental

details

An AAS-I atomic absorption spectrophotometer in the emission mode was used with air-acetylene and air-propane flames. The following analytical conditions were chosen: wavelength, 518 nm; slot width, 0.01 mm; amplification factor, 3; photomultiplier voltage, 4 kV. The flow rates employed throughout were air, 500 1 h-‘; acetylene, SO 1 h-l; propane, 25 1 h-r _ Standard solutions of boron were prepared by dissolving an exact weight of boric anhydride or boric acid of os.4 rating in doubly distilled water. The boron concentration was determined by potentiometric titration of boric acid in the presence of mannitol. The analysed samples were dissolved when heated in hydrochloric acid in the presence of hydrogen peroxide, transferred to gradua~d flasks and produced into the flames of the spectrophotometer in various ways as required by the determination procedure. The analytical signal for every boron content was recorded 4 or 5 times. A least-squares technique was applied to construct calibration charts.

3. Results and discussion When boron-containing compounds were analysed by emission flame spectrometry, direct spraying of the solutions of the analysed material was found to lead to wrong results for the determination of boron owing to the interaction of the containing elements: iron, cobalt, nickel, silidon and others. Besides, in this case spectral interferences were observed as a result of an increase in the continuous background from the flame emission. The change in the height of the luminous zone [ 21, the decrease in the acetylene flow rate and the change of the analytical wavelength enable us to reduce the continuous flame background but they do not exclude it completely. The elements were found to be arranged in the following order according to their capacity to increase the boron signal in the solution sprayed: Fe > Si > Co > Ni (Fig. 1) all other constraints being constant. In the case of an alloy of simple composition, e.g. ferroboron, the iron, which prevents the determination of the boron content, can be separated. For this purpose the potential of column extraction chromatography was considered. A 0.5 M solution of trioctyl~monium chloride in nitrobenzene was used as a stationary phase. Hydrochloric acid solutions of various concentrations were used as a mobile phase and fluoroplastic powder as a carrier( 4) were used as a mobile phase. The investigation of the relationship between the boron distribution coefficient and the concentration of HCl revealed that

403

the boron can be qu~titatively separated from iron by elution in 40 ml of SM HCI. Under these conditions the iron is extracted and left on the column. The completeness of the boron elution was checked on model compounds containing boron and iron in different ratios by potentiometric titration of the eluate. Following the iron separation, the boron-cont~ning solutions were sprayed into the flame of the spectrophotometer burner. The determination of boron was made from the calibration chart or by the method of additions. But the procedure of the element separation becomes more complicated when complex boron compounds with cobalt, nickel, silicon, phosphorus and iron as the basis are analysed. Therefore we tried to develop a procedure for the determination of the boron regardless of the composition of the alloys and element content. It is based on the selective formation of fugacious methyl borate in the treatment of boron-containing alloys with a methylated mixture [ 3) and its further flame-spectromet~ determination. The formulation of the methylated mixture (CH,0H:H2S04, 3:l) was found from the investigation of the dependence of the degree of methylation on the relationship of the reagents. It is an optimal formulation which makes it possible to obtain good reproducibility in the results. The volume of the methylated mixture in all the experiments was 1 - 1.5 ml with the boron content being up to 200 pg. The reaction time of the quantitative formation of methyl borate for all the alloy types was 20 min. A system was developed to introduce the methyl borate fumes into the burner flame (Fig. 2). It provides stability of the methyl borate feed and

Fig. 1. The influence of iron (curve l), silicon (curve Z), cobalt (curve 3) and nickel (curve 4) on the emission of 25.0 pg ml -I of boron. Ao, signal of a solution of an analytical quantity of pure boron; A, as A0 but in the presence of other elements; CE/CB reiationship between the element concentration and that of boron. Fig. 2. Scheme for the introduction of methyl borate fumes into the burner flame of the AASspectrophotometer: 1, burner; 2, spray chamber; 3, restrictor; 4, test tube; 5, cock; of methyl 6, rotameter. ----+, air; - - -+, air for burning; - . - . -+, air for the introduction borate fumes; -X----f, fuel gas.

404

the gas flow rate in burning which considerably affects the accuracy and reproducibility of the results. The influence of the alloy composition on the analytical signal was studied by considering a sample of artificial mixtures which are close in composition to Co,0Fe5Sin,Mo5B10 and Fe&osNi,sSisB,2 and the standard F-22 sample composed of 72.8% Fe, 8.98 f 0.04% B, 7.18% Al, 7.56% Si, 3.28% Cu, 0.021% P, 0.015% S, 0.163% C by mass. The influence under optimal conditions was found to be insignificant. Although artificial mixtures imitating the analysed compounds are preferable for the construction of calibration charts in the analysis of the alloy, the method of additions may also be used. Table 1 shows the results of the determination of boron in real samples of amorphous alloys and in the F-22 standard sample according to the procedure developed. The accuracy of the procedure was supported by comparison with other methods. The relative standard deviation of the distribution procedure is 0.5 f 0.6, while that of the methylation procedure in the concentration interval of 1% - 50% is 0.03. The procedures developed were applied to determine the bonded and free boron in boron nitride. The content of the free boron as Bz03 varies within 0.5% - 2%, while that of the bonded boron is 47 - 50 mass%. Preliminary investigations showed that the free boron in boron nitride can be accurately determined in the air-propane flame applying the flame spectrometry procedure with the methylation of boron nitride samples. Bonded boron can be determined by introducing water solutions into the air-acetylene flame after the solid boron nitride samples have been converted into a solution.

TABLE

1

Results tion

of the determination

Formulation alloy

of the

(at.%)

Fedho

of boron

in different

alloys;

correctness

Calculated boron

Found boron, I!?f S (mass%)

(mass%)

Flame spectrometry

Few&o

4.62 2.10

F-22 Fe4&40Bzo

8.98 4.50

Fe4&4d’14B6

1.27

Fe39Ni&&Mo4B12 Cos7NilOFe$illB17

2.52 3.90

With separation

With methylation

4.70 f 0.21 2.00 k 0.11

4.65 f 0.14 2.12 f 0.06 9.0 + 0.15 4.30 f 0.12 1.10 f 0.04 2.46 f 0.07 3.80 + 0.11

of the determina-

Potentiometric titration

Spectrophotometry

4.50 + 0.18 2.32 ?r 0.10 4.40 1.00 2.70 3.90

f + + f

0.04 0.01 0.03 0.04

405

4. Determination

of boron

(a) In the case of Fe,Bioo_X 0.1 g of the alloy sample is dissolved in 5 ml of concentrated HCl and 0.5 ml of 30% Hz02 and introduced into the chromatographic column. The boron is washed out with four portions of 10 ml of 8 M HCl. The eluate is transferred into a 100 ml measuring flask, brought up to the mark and sprayed into the air-acetylene flame. The boron content was determined from the calibration chart. (b) In the case of complex alloys 0.1 g of the alloy sample with a boron content of 1 - 10 mass% is dissolved by heating in 5 ml of HCI and 0.5 ml of 30% H202. The solution obtained is quantitatively transferred into a 25 ml measuring flask. 0.2 - 0.6 ml of the solution are placed into five or six test tubes. The solvent is removed under reduced pressure at 50 “C. The dry residue is added to 1 ml of the methylated mixture and held for 20 min. Then the test tubes are connected in turn to the introduction system of the spectrophotometer and the resultant methyl borate is blown out into the air-propane flame.

5. Conclusions 1. The influence of various factors on the determination of boron by flame spectrometry from BOz molecular spectra in complex alloys was investigated. 2. A procedure for the determination of boron in FeXBloo_, was developed after the separation of iron by extraction chromatography. 3. A procedure for the determination of boron in alloys containing iron, cobalt, nickel, silicon and molybdenum was developed by selective methylation with the formation of fugacious methyl borate.

References 1 D. Blazejak-Ditges, 2 K. A. Semenenko, MGU, Ser. 2 Khim., 3 N. V. Zuykova and 4 G. F. Telegin, Yu. 10. 5 V. K. Matveev, V. (1961) 51.

2. Anal. Chem., 247 (1969) 20 - 23. S. N. Slepnev and Yu. Ya. Kuzyakov, Vestn. N. V. Zuykova, 4 (1979) 369. K. A. Semenenko, Zh. Anal. Khim., 36 (1) (1981) 94. I. Popandopulo and S. S. Grazhulene, Zauod. Lab., 49 (6) (1983) A. Maltsev

and

V. M. Tatevskij,

Vestn.

MGU,

Ser.

2, Khim.,

1