UDK 669.715:669.781:543.42 Strokovni članek ISSN 1580-2949 MTAEC9, 37(6)385(2003) A SPECTROPHOTOMETRIC DETERMINATION OF BORON IN ALUMINIUM AND ALUMINIUM ALLOYS SPEKTROFOTOMETRIČNO DOLOČEVANJE BORA V ALUMINIJU IN ALUMINIJEVIH ZLITINAH Tatjana Drglin, Iztok Naglič Institute of Metals andTechnology, Lepi pot 11, 1000 Ljubljana, Slovenija tatjana.drglinŽimt.si Prejem rokopisa – received: 2003-10-09; sprejem za objavo – accepted for publication: 2003-12-28 A spectrophotometric methodfor the determination of boron in aluminium andaluminium alloys in the 5–100 µgg–1 range was developed. A colouredcomplex between orthoboric acidandcurcumin was formedin a bufferedacetic medium. The calibration was performed by means of "spiked" samples. The validation was used to provide documented evidence that the selected methodfulfils the requirements andthat the methodis "fit for purpose". The accuracy andtraceability of the purposedmethod were testedby an analysis of closely matchedmatrix certifiedreference materials (CRMs). The limit of detection (LOD) andthe limit of quantification (LOQ) were estimatedto be 1.7 µgg–1 and3.4 µgg–1, respectively. Key words: aluminium, aluminium alloys, boron, spectrophotometric method V prispevku je opisana spektrofotometrična metoda za določevanje bora v aluminiju in aluminijevih zlitinah za koncentracijsko območje 5-100 µg g–1. Pri kontroliranih pogojih se bor nahaja v raztopini kot ortoborna kislina, ki tvori s kurkuminom obarvan kompleks. Umeritveno krivuljo smo izdelali z dodatki standardne raztopine B aluminiju čistoče >99,98 %. Z validacijo smo dokazali, da izbrana metoda izpolnjuje zahteve in služi namenu. Pravilnost in sledljivost metode smo preverili z ustreznimi certificiranimi referenčnimi materiali. Določili smo mejo detekcije (LOD) in mejo določevanja (LOQ), ki sta 1,7 µg g–1 in 3,4 µg g–1. Ključne besede: aluminij, aluminijeve zlitine, bor, spektrofotometrična metoda 1 INTRODUCTION Although the main reason for alloying aluminium is to increase its strength, alloying also has important effects on other characteristics of aluminium alloys. Most alloys produced as "fabricating ingots" for fabricating wrought products, and those in the form of foundry ingots, have small additions of titanium or boron, or combinations of these two elements in controlledpropor-tions. Boron is usedin aluminium andits alloys as a grain refiner andto improve the conductivity by precipitating vanadium, titanium, chromium, and molybdenum. All of these are detrimental to the electrical conductivity at their usual impurity levels in commercial grade aluminium. Boron can be usedalone (at levels of 0.005 to 0.1 %) as a grain refiner during solidification, but it becomes more effective when usedwith an excess of titanium. Commercial grain refiners commonly contain titanium andboron in a 5-to-1 ratio. Boron has a high neutron-capture cross-section andis usedin aluminium alloys for certain atomic energy applications, but its content has to be restrictedto very low levels in alloys used in reactor areas where this property is undesirable 1. Atomic absorption spectrometry is not suitable for determining the levels of boron because of this technique’s low sensitivity to boron. The detection limit is 1 mg/L, andthere are refractory substances that interfere in the analysis 2,3. Extraction procedures for enrichment andfor separating the interfering matrix have, however, been proposed 4. Boron levels can only be determined with the graphite furnace technique using pyrollitically coatedtubes. A fast heating rate for the atomisation and the addition of barium hydroxide increases the sensitivity 5. Various authors have suggestedthe ICP-AES methodfor boron determination in steel andother metals 6,7. High concentrations of aluminium can cause spectral interference andincrease the detection limit. The separation of boron prior to spectrophotometric or ICP-AES determination has also been reported 8,9, either by the distillation of boric acidmethyl ester or by liquidextrac-tion of the 2-ethyl-1, 3-hexandiol complex. The curcumin methodis consideredthe most sensitive spectrophotometric methodfor the determination of boron, andthe most sensitive of all the known direct spectrophotometric methods for the determination of any element. The sensitivity of the methodandthe reproducibility of the results obtained depend on the quality of the curcumin reagent andon rigorous control of the reaction conditions (temperature, time, reagent quantities) 10. Numerous elements (e. g., Fe, Mo, Ti, W, Ge, Be, andTa) form colouredcomplexes with curcumin, andinterfere with the determination of boron. Oxidants (e. g., HNO3), andsubstances forming stable complexes with boron (e. g., HF), also interfere. In this study a procedure for determining boron levels in aluminium basedon a well-establishedmethodfor determining boron levels in steel 11 is described. The reliabil- MATERIALI IN TEHNOLOGIJE 37 (2003) 6 385 T. DRGLIN, I. NAGLIČ: SPEKTROFOTOMETRIČNO DOLOČEVANJE BORA V ALUMINIJU ity of the analytical results was provedby validation experiments that were performedin accordance with "A laboratory Guide to Method Validation and Related Topics " 12, "Guidelines for Calibration in Analytical Chemistry" 13 and"HarmonisedGuidelines for the Use of Recovery Information in Analytical Measurement" 14. 2 EXPERIMENTAL 2.1 Apparatus An OPTON PM 6 spectrophotometer was usedfor the measurements. The absorbance of the colouredsolution was measuredat a wavelength of 543 nm with a 1-cm optical cell. The glassware was rinsedwith acetic acid(p = 1.05 g/ml), then with water, and finally dried. 2.2 Reagents 2.2.1 Hydrochloric acid (p = 1.19 g/ml), p. a. (Merck) 2.2.2 Nitric acid(p = 1.40 g/ml), p. a. (Merck) 2.2.3 Sulphuric acid(p = 1.84 g/ml), p. a. (Merck) 2.2.4 Orthophosphoric acid(p = 1.71 g/ml, p. a. (Merck) 2.2.5 Acetic acid(p = 1.05 g/ml), p. a. (Merck) 2.2.6 Hydrogen peroxide (30 %), p. a. (Merck) 2.2.7 Sodium fluoride (40 g/L), p. a. quality (Kemika) 2.2.8 Mixture of acetic andsulphuric acid(1+1) 2.2.9 Acetic buffer solution: 225 g of ammonium acetate is dissolved in 400 mL of water, 300 mL of acetic acid is added. Solution is transferred in a 1000-mL volumetric flask anddilutedto the mark with water. 2.2.10 Curcumin, acetic acidsolution: 0.125 g of curcumin p. a. quality (Fluka), ŠCH3O(OH)C6H3CH: CHCO2CH2], is dissolvedin 60 mL of acetic acid(p = 1.05 g/mL). The solution is transferredin a 100-mL volumetric flask anddilutedto the mark with acetic acid. 2.2.11 Boron, standard solution, corresponding to 1000 mg/L, CertiPUR® Reference material (Merck). Boron, standard solution, corresponding 10 mg/L, shouldbe preparedimmediately before use by dilution. 2.2.12 CRM materials: HA1, HA3, HA4 andHA5 produced by ALUTERV-FKI, HUNGALU ENGINEERING AND DEVELOPMENT CENTRE 2.3 Procedure 2.3.1 Preparation of test solution: Carefully dissolve 1.00 g of sample in 10 mL of hydrochloric acid (1+1) and oxidise with 5 mL hydrogen peroxide. As soon as the attack is complete, boil for 10 min, transfer the solution quantitatively to a 100-mL volumetric flask, cool anddilute to the mark with water. The chemicals usedfor the sample preparation without 386 the addition of aluminium and boron standards should be preparedfor a blank test with each analytical run. 2.3.2 Formation of the colouredcomplex: Place 1.0 mL of the test solution in a 100-mL tall beaker. Add 3.0 mL of the mixture of acetic and sulphuric acid(1+1) and3.0 mL of the curcumin acetic acidsolution. Stir the solutions andleave to standfor 2 h 30 min to complete the development of the colour. Add 0.5 mL of orthophosphoric acidto stabilize the colour. Shake andallow to standfor another 30 min. Add15.0 mL of acetic buffer solution. The solution becomes orange. Allow to standfor exactly 15 min. 2.3.3 Preparation of the compensating solution: Take a 1.0-mL aliquot of the test solution andtrans-fer it to a 100-mL tall beaker. Add 0.2 mL of sodium fluoride solution. Carefully swirl the small volume of solution andwait for 1 h. The reaction of the colour development proceeds as described in 2.3.2., including the addition of 3.0 mL of the mixture of acetic and sulphuric acid(1+1). 2.3.4 Preparation of calibration solution: Weigh 1.00 g samples of Al (99.99 %) in four 250-mL beakers andadd0.5 mL, 1 mL, 3 mL, 5 mL of boron standardsolution (10 mg/L). Proceedas specified in 2.3.1., 2.3.2. and2.3.3. Carry out the measurement with a 1.00 g sample of Al (99.99 %) without the addition of the boron standard for a blank test with each analytical run. 2.3.5 Validating solutions The same procedures (2.3.1., 2.3.2. and 2.3.3.) are carriedout for the 1.00 g test samples of the relevant cer-tifiedreference materials (2.2.12.). The solutions were usedfor the validation of a calibration curve. 2.3.6 Spectrophotometric measurements After adjusting the absorbance to zero with water, carry out the spectrophotometric measurements on the calibration solutions, the test solutions, the validating solution, andthe corresponding compensating solutions at a wavelength of 543 nm using 1-cm cells. 3 RESULTS AND DISCUSSION The dissolution procedure was different to the standard test method. A few dissolution procedures were testedfor the aluminium samples. The dissolution in 25 % NaOH was successful, but the neutralization with di-lutedsulphuric acidwas not repeatable. The dissolution in hydrochloric acidandoxidising with nitric acidwas also sufficient, but after evaporation a trace of nitric acid remainedin the sample solution. The small quantity of oxidising agent interfered with the colour development. According to the described procedure (dissolution in hydrochloric acid and oxidising with hydrogen perox-MATERIALI IN TEHNOLOGIJE 37 (2003) 6 T. DRGLIN, I. NAGLIČ: SPEKTROFOTOMETRIČNO DOLOČEVANJE BORA V ALUMINIJU Table 1: Uncertainty assessment in spectrophotometric analysis Tabela 1: Ocena negotovosti spektrofotometrične analize CRM Certified value %B Standard uncertainty u(CRM) Precission study u(Precission) Bias t-test t(crit) = 2,57 Bias study u(Diff) Overall bias u(Bias) Combined std.uncert. u c Expanded uncertainty U,k=2 HA1 0.0007 0.000077 0.00004 -0.42 0.000017 0.000078 0.0000873 0.0002 HA3 0.0023 0.000189 0.00007 1.87 0.000071 0.000202 0.000214 0.0004 HA4 0.0065 0.000302 0.00033 0.87 0.0002 0.000363 0.000488 0.001 HA5 0.014 0.000756 0.0005 -0.57 0.0003 0.000813 0.000954 0.002 ide), the calibration graph was linear in the concentration range 5–100 µg g–1. The extension of the measurement range for higher boron contents is possible when the 0.5-g test sample is used. The validation of the calibration procedure was basedon the validation function (recovery function) Xestm = f(Xtrue), which was estimatedusing normal LS regression Y(Xestm) = b + a Xtrue, where a and b are validation coefficients with the analytical meaning of a constant bias (b) anda proportional bias (a). The observed values (as a dependent variable) for the CRMs were correlatedwith the theoretical values (as an independent variable) by linear regression. The coefficient of correlation (R2) was usedas an index that indicates how well the regression line represents the actual data (Figure 1). The linear range of the spectrophotometric method was more than one decade, i.e., between three and thirty times the detection limit. The calibration curve passes through the origin. The trueness was determined by means of four certifiedreference materials (2.2.12.). The bias t-test values were comparedwith the tcritical value (t(6) = 2,57). As long as the t-test value is smaller than tcritical we concluded that there is no statisticaly significant bias (Table 1). The limit of detection (LOD) was determined as three times the random variation in the blank. It was 1.7 µg g–1, and the limit of quantification (LOQ) for B was 3.4 µg g–1. 0.016 0.014 0.012 0.01 0.008 0.006 0.004 0.002 0 0.004 0.006 0.008 0.01 CCRM /% Figure 1: Validation of calibration curve by means of certified reference materials (CCRM – certifiedvalue, Cdet – determined value) Slika 1: Validacija umeritvene krivulje s certificiranimi referenčnimi materiali The uncertainty estimation of the methodwas basedon the best available estimation of the overall precision andthe best available estimation of the overall bias andits uncertainty. The uncertainty of the instrument operation was estimatedfrom a series of repeated observations by calculating the standard deviation as the overall precision of the method. The calibration uncertainty was contained in an overall bias study. The data for the measurement uncertainty evaluation is given in Table 1. The overall precision was estimatedunder a repeatability condition that gives a fair indication of the in-house variability normally encountered. It was performedwith four different CRMs (2.2.12.), markedas HA1, HA3, HA4 andHA5. The standarduncertainty u(precision) for a single determination was estimated by dividing standard deviation by 6. The overall bias was estimatedby an analysis of the relevant CRMs (Table 1) that cover the analysed concentration range using the complete measurement procedure. The bias estimated in this way combines the bias in the laboratory performance with any bias intrinsic to the methodin use. The mean difference (Diff), defined as the average difference of the certified value (CCRM) andthe determinedvalue (Cdet), andthe standard deviation of these differences s(Diff) for all four CRMs were determined. The standard uncertainty was calculated as a standard deviation of the mean difference, using the equation: u(Diff) = s(Diff) /n, where n is the number of repeatedanalyses (n = 6). A significance test was usedto establish whether the measurements differ significantly from the assumedcorrect values (Table 1). The test statistic t was calculatedusing equation (1). This value was comparedwith the two-tailedcritical value tcrit, for n-1 degrees of freedom at 95 % confidence (n = 6, t crit = 2,57). Diff u(Diff ) (1) The overall bias was insignificant (t < tcrit), the uncertainty associatedwith the bias couldbe calculated as a combination of independent variables (Equation 2): the standard uncertainty of the CRM value (type B) with the standarduncertainty associatedwith the bias (type A). t 0 0.002 0.012 0.014 0.016 MATERIALI IN TEHNOLOGIJE 37 (2003) 6 387 T. DRGLIN, I. NAGLIČ: SPEKTROFOTOMETRIČNO DOLOČEVANJE BORA V ALUMINIJU u(Bias) = u(CRM)2 +u(Diff)2 The combinedstandarduncertainty uc andexpanded uncertainty U were also calculated. In this study, all the important uncertainty sources were includedin the calculation of the combinedmeasurement uncertainty. The magnitudes of the uncertainty components vary with the concentration level of the measurandandthe combinedstandarduncertainties were similar to the standarduncertainties on the certifiedreference materials. It was provedfor the concentration range that boron was added as grain refiner. 4 CONCLUSIONS Boron can be determinedin aluminium andalu-minium alloys without preconcentration andseparation when the sensitive curcumin spectrophotometric method is used. The absence of bias demonstrates that the analytical methodis capable of measuring the investigated component accurately in CRMs. An extrapolation of this observation to real test samples is only warrantedas long as the test sample resembles the CRM very closely in terms of both matrix andhomogeneity. The uncertainty components were included in the calculation of the combinedstandarduncertainty uc. The contributions of individual uncertainty sources were comparable. The expanded uncertainty U was calculated by multiplying the combinedstandarduncertainty with a coverage factor of 2. Laboratories shouldknow the uncertainty associatedwith a test result, whether it is reportedor not. ISO/IEC 17025 requires that uncertainty estimates must be reportedwhen the client requires them, when it is relevant to the validity or application of the test result, or when the uncertainty affects the compliance with a specification limit. 5 REFERENCES 1 J. R. Davis, ASM Specialty Handbook®, Aluminum andAluminum Alloys, ASM International, Materials Park, 1994 2 H. Bader, H. Brandenberg, At. Absorption Newslett 7 (1968) 1 3 D. C. Manninig, At. Absorption Newslett 6 (1967) 35 4 E. J. Agazzi, Anal Chem, 39 (1967) 233 5 F. J. Szydlowski, Anal Chim Acta 106 (1979) 121 6 R. M. Hamner, L. A. De’Aeth, Talanta 27 (1980) 535-536 7 G. F. Wallace, At.Spectrosc 2 (1981) 61-64 8 G. Mazger, E. Grallath, U. Stix, G. Tolg, Fresenius Z. Anal.Chem. 317 (1984) 765-773 9 Z. Marczenko, Separation andSpectrophotometric Determination of Elements 2nd ed. Ellis Horwood Limited 10 P. Quint, F. Umland, Z.Anal.Chem. 295 (1979) 269 11 International Standard ISO 10153, First edition 1991-01-15 12 EURACHEM Guide, The Fitness for Purpose of Analytical Methods, a Laboratory Guide to Method Validation and Related Topics, 1998 13 IUPAC, Pure andAppl Chem 70 (1998) 993-1014 14 IUPAC, Pure andAppl Chem 71/2 (1999) 337-348 388 MATERIALI IN TEHNOLOGIJE 37 (2003) 6