RENDlCONTI DELL.... SOCIETA IT.... L1 .... N.... Dl MINER.... LOGI .... E PETROLOGI..... 1988, Vol. 43, pp. 16.H70

Spectrophotometric determination of boron in igneous rocks.Comparison between azomethine-H and 1-1' dianthrimide methods

NARA CORAOOSSI, ORLANDO V ASElll

Dipartimrcnto di Scircnzrc ddla Trcrra, Uni\"Crsit.l. Via La Pira 4, '012\ Fi~nzrc

ABSTltAcr. - Boron contrcnu of order of few ppm havebeen determined by spectrophotometry in silicate rocksamples, using different ~tS (azornethine·H and \.\'dianthrimide). to chel:k whether these reagenu can beutilized in analysis of complex chemical solulions. Theanalylical solution was obtained by melting 20·40 mgof the sample with NaGH in crucibles of Pd·Au. Thismethod was used to analyse six internalional ~fe~ne;emndards (G.2, GA. MA·N and GS-N gunites. BRba!alt and W·! diabase) and a sel of effusive rocks fromVesuvius Gtaly) and Fajal and San Miguel (Awresl. Theboron contenl of both the international standards andthe volcanitC$ obu.ined with azomahine-H were in allcases higher (by about 2'%) than the valUoe$ obtainedwith 1·1' dianthrimide. 1be values obtained for lheinternational slllndards with 1·\' diamhrimide we~ inexcdlent agrttmenl wilh data reported in the- literature.1be higher values obtained with a:romethine·H areprdumably the result of !he prese:nt"e of imerfering ions.J.\' dianthrimide is recommended as the most suitable~agem in spectrophotometric analysis of boron insolutions obtained from geological materials.

Kt)' w<Jrds: Geochemical method. spectrophotometry,boron, comparison between azomethine·H and J.\'diamhrimide, igneous rocks.

-.............._.6.... --".._......__...(>-I • <lA, ........ (ISo'" , _.. "'-11.' __.,.__ Coo lA_ _1___ • t<oOH ~ I __ __ •_________..__""'._".1·'_.__._----~.t.,_ - ._Il._--- _---_ .._-".,- _--.~,---_.-----._--_..... ; _._"_._____.>.1_._. .1-1--=----_.....__. ....--_..--_. ._~I-

.....-_.--_.__._­.....-.-.

RJASSUNTO. - Tenori di boro dell'ordine delle ppm sonoSlati determinati in campioni di rocce silicatiche perspettrOfolOmetria, utilizzando relUtivi distinti(azometina·H e J.\' diantrimidel a1lo scope di verificareI'applicllbilitli degli ueui all'analisi di soluzionichimicamente complesse. La soluzione analilies eSlataO{{enutll per fusione di 20-40 mg di camp.ione ron NaOHin crogKili di Pd-Au. 11 metodo estaloapplicato all'ana1isidi sei mndards internnionali (graniti G·2, GA, MA­N c GS-N; ba!altO BR e diabase W-l) e di una serie diI"OCU effusive del Vesuvio e ddIe isok Fajal c San Migud(Azzorre). I tenol"i in bom sia negli slandardsintCTtlnionali cbe ndIe roctt effusive. ouenuti cona:zometina·H risulta10 sempce superiori (di circa il2'9b)rispeno a qudIi OItO'lUli usando la 1·1' diantrimide. Ndcas<> degIi Slandards imernazionali questi ultimi sono ineccdknte accordo 00ll i dad dd1a !etterarura. Si ipotizuche i valori in eccesso onenUli ron a:zometina·H sianolegati alia presenza di ioni interfercnti. Si raccomandal'uso della 1·1' diantrimidc quale reat!ivo piu idoneonell'a.nalisi spettrofotometrica del boro nelle soluzioniO{lcnute a. partire dl mlteriali geologid.

Parole chiQlJc: Metodologia geochimica,spettrofotometria. boro, ronfromoazometina·H e \-1'diantrimidc, racee ignee.

Introduction

The determination of boron in igneousrocks is difficult. because methods which aresensitive. reproducible, and precise enough toreveal traces of it are few. The amounts ofboron present in most geological materials areless then 200 ppm.

In particular. igneous rocks have only a fewppm of boron. Only a few extremdydifferentiated acid rocks and somemetamorphic rocks reach concentration of

164 N CORAOOSSI. O. v.-\SElL!

TABLE 1

Analysed samples from Vesuvius (114/y) andAzores

VlSUVIl,lS

......I'LI: DEPOSIT "PE PLUIl..... EltUPTIQN

VSI' 02-0' PuIIICE-'ALL POIlI'EI

VSI' 05-\2 PUMICE-PALL AVELLUIO

VSI' 13-15 PUMICE-FALL GREEHISH

VSI' U-21 PllMICE-PI,1,l. KERCll'rO

VSI' H-14 PUMICE-PALL ~~

VES J9 PuIIICE·'At.L ~~

"'IO~

1NlI'Ll DDOSIT TYn ER1JPTI~ o:rnu:

~ 01-03 ,- CoU'ELIII_ ,.,~ " PUltICE-PALL CALDEIaA ,.,~ " \..\VII-P1.OW 1.\ OOA DE 1'000 '"~ .. ,~. CAUlEI .... ,.,~ " P\.IKlc!:-'AU. """"<'" '"c·. rAYAL ISu.IlD, III S ..... "ICUEt. ISL1\IfD.

1000 ppm or, rarely, a few percent.In the latter case, boron often forms

specific minerals such as tourmaline (TONANI,1957).

Boron is also relatively concemrated in S(:awater (4.6 ppm B), while in marine sedimentsthe concentration reaches values slightlyhi~r than lOOppm. (WALSH, 1985). In f~water and marine sediments boron is animportant indicator of paleosalinity (CURTIS,1964; WALKER, 1968; COUGH, 1971;CORADOSSI and CORAZZA, 1979).

Understanding the distribution of boron isImportant in many other fields, such asgeothermal systems, plant physiology,agriculture, metallurgy, nuclear power, etc.This justifies the interest that analyticalchemists have in finding techniques fordetermining boron in very different materials.

This paper deals with igneous rocks. Thedetermination of their boron content can bt:achieved by following two completelydifferent paths: direct analysis of a powderedsample or analysis of a sample in solution. Ifthe physical methods are excluded, thedetermination of boron in silicate rocksrequires several operations which can bt:divided into three steps: melting, removal of

interfering ions, and determination in the finalsolution.

The first step is certainly the most critical,because the loss of boron must be avoided.It is well known that boron compounds underhot acid conditions are volatile (FELDMAN,1961), so that mineral acids and acid solventscannot generally be used. They can be usedwhen working with special materials such asmetals, steels, alloys, and, in geology, withsoils and volcanic glasses.

Alkaline fusion is frequently used in thedetermination of boron in igneous rocks. Themost commonly usecl fluxes are sodium andpotassium carbonate (KRAMER. 1955; HAYESet al., 1957; FLEET, 1967; MAURlCE, 1968;SCHUCKER et al., 1975; YOSHIMURA et al.,1979). The use of sodium hydroxide (BE>'lNETet al., 1961; Smr..1A, 1963) or sodium peroxide(RA-FiER, 1950; BELCHER, 1963; Al-IAMADKHAN et al., 1980) is quite rare.

The second step of analysis, in theseparation of interfering elements, is equallycritical, especially when the samples containonly traces of boron. Here too many differenttechniques can be used, some of which aretraditional; like distillation (LUKE, 1955 and1958; LUKE and Fu..SCHEN, 1958; SPica andSTRlCKLAND, 1958; EBERLE and LERNER,1960; QUIJANO-RICO, 1968) andcentrifugation or filtration (MAURlCE, 1968;WOU", 1971); others are basecl on extraction(UNZA and BULDlNI, 1974; KORENAGA et al.,1980; AzNAREZ et al., 1983 and 1985). Lastly,there are ion-exchange separation procedure(KRAMER, 1955; HAYES et aI., 1957; PINON

et al., 1968; KOCKEN, 1974; NIAMAD KHANet al., 1980; Hlu and LASH, 1980).

The third step is determination of boronin analytical solution. The best known andmost used reagents, capable to give colouredcomplexes with it, are: curcumin (LUKE, 1955;COURSIER et al., 1955; DUCRET, 1957; LUKE,1958; LUKE and FLASCHEN. 1958; SPICER andSTRlCKLAND, 1958; EBERLE and LERNER,1960; HAYES and METCALFE, 1962;GREENHALGH and RILEY, 1962); 1-1'dianthrimide (Ews et al., 1949; BREWSTER,1951; CODELL et al., 1953; DANlELSSON, 1959;MAURICE, 19689; GUP'TA and BOL'L, 1971);carminic acid (RosENFELD and SELMER-OLSEN,

SPECTROPUOIOMETRIC DETERMINATION OF BORON IN IGNEOUS ROCKS, En::. 165

1979; AHAMAD KHAN et al., 1980; AzNAREZet al., 1983 and 1985; TROLL and SAUERER,1985); and azom~thine-H (CAPEllE, 1961;SIIANINA et al., 1967; BASSON et al., 1969;WOLF, 1971 ;JOHN et al., 1975; SCHUCKER et·al., 1975; YOSHIMURA et al., 1979; KRUG etal., 1981; AZNAREZ and MlR, 1985; BENCINI,1985).

The purpose of the present study is toseek a simple, and rapid colorimetric methodof boron analysis, applicable to igneous rocksand capable of providing results as preciseand reproducible as those of usually usedmethods. The analytical method devised wastested either on a set of internationalreference standards and on a set of effusiverocks containing boron in relatively wideconcentration range.

The following rocks, which have diHerentchemical and petrographical characters werechosen: G-2, GA, MA-N, and GS-N granites,BR basalt, and W-l diabase. The method wasthen applied to a group of volcanic rocks fromVesuvius (haly) and volcanoes on Fajal andSan Miguel (Azores). These rocks arecomposed of pumice-falls from Vesuviusbelonging to various Plinian-type eruptions(LIRER et al., 1973; DEUBRAlS et al., 1979;SANTACROCE, 1983; ROSI and SANTACROCE,1983) and of pumice-falls, surges, and lava·flow from various eruptive centres on Fajaland San Miguel; for the description of thelatter rocks, see VASELU (1985) andCAPACaONl et al. (1986). 10 Table 1 arereported the analyzed samples from Vesuviousand Azores.

Experimental techniques

1. Generalities on fusion procedureand organic reagents

The use of sodium hydroxide, which isavailable in boron-free pellets, turns out tobe essential for three important reasons: smallquantities of sample can be used (20-40 mg);sample dissolution is rapid (10-15 minutes);and dissolution does not effervesce whilebeing measured with the spectrophotometer.

The choice of the organic reagent withwhich to complex the .Daron present insolution is also important. In the course of

this project we tested two reagents:azomethine·H and I-I' dianthrimide.Azomethine·H is a sensitive reagent whoseresults are highly reproducible but it isinsuitable because it is affected by interferenceeHects of many other elements.

Instead, 1-1' dianthrimide is sensitive,highly reproducibile, and also very selectivewith respect to boron. In other words, thecentrifugation which the sample undergoesafter it is melted is sufficient to removeinterfering ions.

2. Material used3-ml Pd-Au crucibles; Schott Ceran

protection plate for heating samples overBunsen burner; 50-m! transparent fused silicadishes; steam bath; 50-ml polyethylene testtubes; centrifuge; lO-ml volumetric flasks;20-ml polyethylene containers; '-mltransparent fused silica test tubes; Socorex811-821 micropipettes (50-200 pI, and200-1000 p1); Socorex bottle top withdispenser with adapters; PhiIips PYEUNICAM SP6-350 spectrophotometer; l-emglass cells.

3. ReagentsSodium hydroxide 10 pellets (BDH, art.

30167)Azomethine-H solution: 0.9 azomethine­

H (SIGMA) and 2 g L( +) ascorbic acid (C.ERBA), diluted in 100 mJ of twice-distilledwater.

Masking-buffer solution: 250 g ammoniumacetate (MERCK, art. 1116), 17.5 g EDTAtetrasodium salt (SIGMA) and 10 g NTA-adddisodium salt (SIGMA, art. N-D128) dilutedin 400 ml glacial acetic acid (MERCK, art.63), and 150 m! of twice-distilled water.

1-1' dianthrimide solution: 25 mg 1-1'dianthrimide (MERCK) in 50 mI sulfuric acidd = 1.84. Sulfuric add (MERCK, art. 731),d = 1.84. Water distilled twice by fused silicadevice.

S14ndard boron solution (lOO pg m/ -18):0.449§ g sodium tetraborate (RIEDEL deHAEN) in 0.5 It of twice-distilled water. Thiswas the stock solution from which a solutioncontaining 10 pg ml -1B was prepared beforeeach determination. From this latter wereptepared solutions with concentrations

166 N. CORAOOSSI, O. VASELl.I

varying betweem 0 and I Itg ml -lB.

Analytical methodology

1. Solution preparation20·40 mg of the powdered sample, dri~

at 1100 e, was weighed directly in a Pd~Aucrucible. Two to three pellets of sodIUmhydroxide were added (total weight 200·300mg), the crucible was then placed on theSchon Ceran plate and the mixture meltedover a Bunsen burner. Six to seven samplescould be mehed simultaneously by using thissystem. After 10-15 minutes the reactionreacho::l. completion; the crucible was broughtto red heat for IQ.15 seconds and then allowedto cool. The fusion cake was dissolved in afew mJ of twice-distilled water over a steambath. The solution and precipitate (made upessentially of Fe and AI hydroxides) weretransferred to a polyethilene test andcentrifuged for 30 minutes. The solutions thusobtained were ttanferred to fused silica dishesand evaporated over the steam bath. Thehighly alkaline environment keeps boron frombeing lost during this step, as confirmed byexperimental evidence (BRAMAN, 1978). Theevaporation was continued until the saltsbegan to precipitate, and the mixture was thenput into a lO-m! volumetric flask and dilutedto volume with twice-distilled water. Thesolution was transferred to a polyethylenecontainer.

2. Determination 0/boron with azomethine-H

Although azomethine-H is the most widelyused reagent for the determination of boronin water, biological materials, and soils, it israrely used on geological materials, becauseof the interfering effects by the complexsilicate matrix. The interfering ions can beremoved; for example, by passing thesolutions under examination through ion·exchange columns (YOSHlMURA et al., 1979)or adding a masking·buffer solution, made upof the misture of EDTA-NTA acid (WOl..F,1971; ScHUKER et al., 1975). We carried outthe determination of boron following thesecond method.

The solutions with known concentrationsof boron and the samples were treated as

follows: 2-mJ of the masking buffer solutionand 1-mJ of the azomethine-H solution wereadded to 2-m! samples of the solutions to bemeasured and the mixtures were swirled fora few minutes. After one hour, the solutionswere transferred to 1-cm glass cells and theabsorption spectra were measured with aPhilips PYE UNICAM SP6-350spectrophotometer (415 mn).

3. Determination 0/ boron with1-1' dianthrimide

For the determination of boron by meansof 1-1' dianthrimide, as made in previousgeochemical studies (CORADOSSI and MARTINI,1965; CORAOOSSI, 1975; CORAOOSSI et al.,1984), we followed the ~thods developed byELUS et al., (949).

The standards and sample solutions wereprepared as follows: 2-m! of sulfuric acid(d = 1.84) and O.5-m! of 1-1' dianthrimidesolution were added to O.3-mJ samples of thetest solutions in trasparent fused silica testtubes. The solutions were then left to changecalor over a steam bath at 90°C for one hour.After they had cooled, the solutions weretransferred to I-cm glass cdls and colourintensities were measured with a Philips PYEUNICAM SP6-350 spectrophotometer (625nm).

Di~ussion of results

Table 2 gives the boron concentrations ofthe international reference standards G-2,

. GA, MA-N, GS-N, BR and W-1 obtainedwith azomethine-H and I-I' danthrimide. Forcomparison, Table 2 also gives the valuesreported by GoVINDARAJU (1984) and VlTIOZ

et al., (1987), using different techniques .. Asmay be seen, 1-1' dianthrimide method givesresults in good agreement with literature data.The deterrninations carried out, for example,on granite GA (recommended value 20 p~mBl give an average value of 21.7 ppm B, w~tha standard deviation of 1.2 ppm and a relativeerror of 5.6%. The mean value of 37 ppm ofthe GA granite using azo~thine-H (2.2alkaline fusions and 48 spectrophotomettlcmeasurements) is off by 85% with respect tothe average value (20 ppm) reported byGOVlNDARAjU (1984). The standard deviation

SPECTROPHaroMETRIC DETERMINATION OF BORON IN IGNEOUS ROCKS. ETC. 167

TABLE 2

Boron content (in ppm) in 6 inlonational refm:nce rock slandonh determined by a:zomethine·Hand diathrimid methods

UPI'.J:IICI .- ,.,'" '" i_ ,., '" '" ,,~ '" .- ,.,

n""CA..,1 UOMftHIHI-H I-I 'CIA!<T>IKIKIC (U'~I (191'1

,., (,nnn.) " ,. " l.1"1.3

~ 1'.......1 3' •~ " .. " ., " ..""'... 1'.......1 ".~ • " "<:1 ...4'......., JI.~ 12.l.. n..... I<I n.l I••S ".., 4<11.......1 "

," • " n.S·'.3

,., TllII ..., '"~ c. Iol&ALIn PVIII... 111.... """I'" nDMlnDl, '"~~I'~~C .oa:.u1lUXD!TS, '" --~ ll"~l , ,., VITnl: ,... AL. lllnl.

TABLE 3

Boron content (in ppm) In l.Iokanic rock samples from Vesuvius (Ilaly) and kortS

- '- '" ." ,- ." '"-_. ,-, ..,-...,.-n n, 0 .. .._..o. n

w... •• n

w... ,.. ...w. " n, .. ...w. ,. '" ,,'., " '" 'n

•• " ... on... " n' ..,-n 0" • ••-" on 0' ..-.. 0" oo-.. ... ."-.. ,.. .. ,..•• " ... 0"•• .. '" '"w... '" n ,..w. " '" ,.w... '" '"..... on ..... " .. ..• oo ..• oo ..• oo ..... .. ..• .. ..· .. n n

." " "on _,a or ....,..."., "'""....,"' _,"" IIYDIICIJIY", m ..-•• DO"....'_.,,_,"'c _...-n.

168 N CORAOOSSI, O. VASELLI

TABU: 4

Comparison between the azomethine-H method and the dialhrimid method/or all ana/yzed samples(S.D., Cv% and I-te!t), For /urlM explanation see text

COICZlftVlTIOM MlJ'XI!tl or • 00'. t ".1U... l y.a! ....~~ (_'I

S.D. ~. ,. .. ..~~ .. -'• · " " n.' It.6 Il.l S.U l.O' l.IO

" · ", • ".1 n.6 ~ 1.4 l.ts 2.14 2.98

", · '" " '" 56.0 14.1 loll L06 2080

is 6.3 ppm and the relative error 16.5%.These results indicate that azomethine-H isunsuitable for the method proposed, becausethe centrifugation and addidon of a masking­buffer solution are insufficient to eliminatethe interference of the ions present in thesolution.

Table 3 gives the concent,rations of boronin 21 pumice falls from various Plinian-typeeruptions of Vesuvius and in 7 volcanic rocksfrom Fajal and San MigueI (Azores), asdetermined with 3zomethine-H and 1-1'dianthrimide. As observed for the standardrocks, the concentrations determined withazomethine-H are always at least 25% higherthan those obtained with 1-1' diamhrimide.

To sumarize. the tests we carried out withthe two reagents on the internationalreference rock standards and the volcanicrocks indicate that I-I' dianthrimide is a moresuitable reagent for determination of boronin solution obtained by fusion with sodiumhydroxide. The high sensitivities which canbe reached with this procedure mean thatdeterminations may be extended to rock witha few ppm of boron.

Acknowkdgments. - The present research was partiallysupported by Italian C.N.R. (Centro Studi per laMinera.logia e la Gcochimica dei Stdimentil. We thankProfessor M. Marlini and Doclor B. Capaccioni forkindly giving us Ihc pumice-fall samples from Vesuvius.

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