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Boron
CHAPTER 12 Boron
In acidic rocks boron frequently occurs in the mineral tourmaline, and high-boron granites usually contain abundant quantities of this mineral. about 3 per cent boron, 10-11 per cent B p 0^·
Tourmaline contains
A procedure is given below for the
determination of boron in tourmaline, which can be adapted for other minerals high in boron, such as datolite (6% B, 20-22% B O ) . Much of the earlier data on the occurrence of boron is unreliable owing to the lack of accurate and suitable methods of analysis. However, renewed interest has led to a re-examination of older methods of separation involving distillation as methyl borate, and the devising of new methods based upon pyrohydrolysis, solvent extraction and ion-exchange.
A number of colour-forming reagents have been described for the
photometric determination which, except for those minerals high in boron, have displaced the earlier titrimetric procedures. Distillation as methyl borate is regarded as the classical procedure for the separation of boron, but conflicting reports exist in the literature regarding the adequacy of this method.
Difficulties can arise from the presence of boron in almost all
laboratory glassware, and the distillation apparatus itself should be made from fused silica.
Another difficulty that has been noted is the need for maintaining anhydrous
or near anhydrous conditions during the esterification and evolution of the borate. Ion-exchange separation procedures are generally more rapid and simpler than distillation methods. Silicate rocks and minerals are brought into solution by an alkaline fusion, and the aqueous extract acidified with hydrochloric acid.
The
passage of this solution through a column of cation exchange resin removes iron and other interfering i>ns to give a clear solution that can be used directly for the determination of boron by either titrimetry or spectrophotometry. In a version of (1) this procedure Fleet dispenses with the use of a column and adds the resin directly to the acid extract of the fused rock melt.
This procedure is described in
detail below for application to silicate rocks. Spectrophotometric Determination of Boron in Silicate Rocks Of the many reagents described for the photometric determination of boron only curcumin, dianthrimide (1,1'-iminodianthraquinone) and carmine (carminic acid) have
CMRA - I
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110
Chemical Methods of Rock Analysis
been extensively used. Curcumin, the active colour principle of the vegetable product turmeric, has long been used for the detection and determination of small amounts of boron.
Considerable
difficulties were initially experienced in obtaining repeatable quantitative results, (2) but conditions necessary for a reliable procedure have now been established. Alonso and Sanchez
described its application to the determination of boron in
geological materials after a preliminary separation using Dowex 50W-X8 cation exchange resin. The use of dianthrimide for the photometric determination of boron has been carefully (4) examined by Danielsson. This reagent is more sensitive than carmine, but less so than curcumin.
It gives a linear calibration in concentrated sulphuric acid solution.
The reagent itself has an absorption band with a maximum value below ^+00 nm, clearly separated from that of the boron complex which has a maximum value at 620 nm.
The
rate of reaction of boron with dianthrimide is strongly dependent upon temperature. Optical density values are also temperature dependent, although this effect is related to the acid concentration. Carmine, the name given to a naturally occurring dyestuff from cochineal, is a calcium-aluminium compound of carminic acid, which is a derivative of anthraquinone. Both carmine and carminic acid react with boron in concentrated sulphuric acid solution to give blue coloured complexes, but as carminic acid is deliquiescent, carmine is preferred.
In the absence of boron the colour of the dye at pH 6.2
is bright red, but in the presence of boron this changes to blue.
The wavelength of
maximum absorption changes from 520 nm for the reagent to 585 nm for the boron complex. boron. used.
The Beer-Lambert Law is obeyed over the concentration range 0-10 ppm of
The colour development characteristics may vary with the brand of reagent Hatcher and Wilcox
have reported that the coloured complex with boron
can be measured after ^5 minutes and then shows no appreciable change at the end of *f hours. Fleet,(1) however, noted that the absorption reached a maximum after ko minutes and thereafter decreased.
The procedure, described below by Fleet uses a
cation exchange resin to separate interfering ions from boron. An alternative collection procedure using pyrohydrolysis has been described by (7) The sample material was mixed with pure calcium
Farzaneh, Troll and Neubauer.
fluoride and the BF., liberated was collected in a sodium hydroxide-sodium carbonate solution prior to photometric determination with carminic acid.
111
Boron Method Reagents:
Mannitol solution, dissolve 1 g of reagent in 100 ml of water. Carmine solution, dissolve 50 mg of the reagent in 100 ml of concentrated sulphuric acid. Hydrochloric acid, 0.6 N. Sodium hydroxide solution, 0.1 N. Standard boron stock solution, dissolve 0.5716 g of recrystallised boric acid in water and dilute to 1 litre.
This solution contains
100 μg boron per ml. Standard boron working solutions, dilute aliquots of the stock solution with water to give three new solutions containing 5i 10 and 20 μg boron per ml respectively. Cation exchange resin, wash 50 ml of a strongly acid resin such as Amberlite IR 120(H) or Dowex 50W-X8 with 6 N hydrochloric acid and then water until the eluate is free from acid. Procedure.
Accurately weigh approximately 0.2 g of the finely powdered rock
sample (or a smaller amount if the sample material contains more than 200 ppm boron) into a 10-ml platinum crucible and add 1.25 g of potassium carbonate. over a Bunsen burner for 1 hour.
Mix, and fuse
Allow the crucible to cool, loosen the melt by
warming with a small amount of water, and transfer the solution and residue to a 50-ml polypropylene beaker.
Cover the beaker and add 2 ml of mannitol solution,
followed by 20 ml of the cation exchange resin and 2 ml of 0.6 N hydrochloric acid. Break up any lumps of residue, mix with the ion exchange resin and add enough water to give a slurry.
Allow to stand overnight.
Collect the ion-exchange resin and any precipitated silica on a small medium-textured filter paper, wash well with water and discard it.
Collect the filtrate and washings
in a 400-ml polypropylene beaker, add 23 ml of 0.1 N sodium hydroxide solution and carefully evaporate to dryness on a steam bath. 5 ml of 0.6 N hydrochloric acid.
Allow to cool and add by pipette
When the residue has dissolved, pour this solution
into a centrifuge tube and centrifuge. Pipette 2 ml of the clear solution into a 50-ml polypropylene beaker, add 2 drops of concentrated hydrochloric acid and with great care add 10 ml of concentrated sulphuric acid.
Allow the solution to cool, then add 10 ml of the carmine reagent solution.
Swirl gently to mix the contents of the beaker and allow to stand for *f0 minutes. Measure the optical density of the solution in 1-cm cells with the spectrophotometer set at a wavelength of 585 nm.
112
Chemical Methods of Rock Analysis
For the reference solution, transfer 2 ml of water to a 50-ml polypropylene beaker and add concentrated hydrochloric acid, sulphuric acid and carmine reagent as described for the sample solution.
A reagent blank should also be prepared from
1.25 g of potassium carbonate fused without rock material in a separate platinum crucible, and carried through the procedure as described.
A series of three
standards can be used for calibration by transferring 2 ml aliquots of the three working solutions, containing 10, 20 and kO μg boron respectively, to separate beakers and proceeding as described above. Titrimetric Determination of Boron in Tourmaline One of the earliest procedures to be devised for the determination of boron in silicate rocks and minerals was that of titrating liberated boric acid with standard alkali in the presence of a polyhydric alcohol - mannitol being now commonly employed. The boric acid-mannitol complex acts as a strong monobasic acid.
When combined with
an ion-exchange separation, this procedure can be simply and easily applied to the analysis of tourmaline and other silicate minerals containing boron as a major /ON
component.
The procedure described here has been adapted from that given by Kramer. Method
Reagents:
Sodium hydroxide solutions, approximately 5 M, and also 0.05 M standardised by titration against standard hydrochloric acid in the usual waye Mannitol Ion-exchange resin, strongly acid, cationic resin such as Amberlite IR 120(H) or Dowex 50W-X8, in the form of a bed 2 cm in diameter and 25 cm in length.
The column should be of borosil^cate glass
(attack of the glass is negligible) or polypropylene tubing. To prepare the column for use or to regenerate for further use wash the bed with 100 ml of 6 N hydrochloric acid followed by water until the eluate is free from acid. Procedure.
Accurately weigh approximately 0.5 g of the finely powdered tourmaline
or other boron mineral into a small platinum crucible, mix with 3 g of anhydrous sodium carbonate and fuse over a Bunsen burner for 30 minutes.
Transfer the crucible
to a Meker burner and continue the heating for a further 30 minutes.
Allow the melt
to cool, spreading it around the sides of the crucible in the usual way.
Boron
113
Place the crucible on its side in a 100-ml polythene or polypropylene beaker containing 20 ml of water and add concentrated hydrochloric acid down the sides of the beaker until there is an excess of about 1 ml above the amount required to neutralise the alkali carbonate used for the fusion.
Warm the solution and allow
to stand until the melt has completely decomposed and all soluble material has passed into solution.
Rinse and remove the platinum crucible and lid.
At this
stage no unattacked mineral grains should be present, and the only residue a few flakes of silica precipitated from the solution.
Filter the solution through a
9-cm open-textured paper supported in a polythene funnel into a polythene beaker, and wash the residue with hot water to give a solution volume of about 50 ml. Discard the residue.
Add 5 M sodium hydroxide solution drop by drop until the formation of a precipitate that only just fails to dissolve on warming.
Clear this precipitate with a few drops
of hydrochloric acid, and transfer the solution to the ion-exchange column, previously washed with water.
Allow the eluate to collect at a rate of between 50 and ^0 ml per
minute in a *f00-ml polypropylene beaker, and wash the resin with about 200 ml of water. Add 0.5 ml of concentrated hydrochloric acid and boil the solution for 1 minute but not longer (Note 1 ) , to expel any carbon dioxide, and cool the solution to room temperature. Using a magnetic stirrer and a pH meter, add sodium hydroxide solution, first concentrated then diluted, drop by drop, until the pH of the solution reaches 7· Now add 20 g of solid mannitol and titrate the boric acid with 0.05 M standard sodium hydroxide solution until the pH again reaches 7· from the titre before calculating the results.
Subtract a reagent blank value
The reaction that occurs may be
expressed by the equation Η,ΒΟ, + NaOH = NaHJBO, + H^O so that 1 ml of 0.05 M sodium hydroxide solution is equivalent to 1.7*f1 mg B O Notes;
1.
(Note 2 ) .
Prolonged boiling of hydrochloric acid solutions will result in (9)
substantial loss of boron. 2.
Where boron is included in the summation of the rock or mineral, it is
customary to express the results as per cent boric oxide, B O . only as a trace constituent, parts per million boron is used.
Where boron occurs
Chemical Methods of Rock Analysis
References FLEET M E., Analyt. Chem. (1967) 39, 253 HAYES M R and METCALFE J., Analyst O962) 87, 956 ALONSO S J and SANCHEZ G A., An. Quim. (1972) 68, 335 DANIELSSON L., Talanta (1959) 2» 138 DANIELSSON L., Organic Reagents for Metals, Hopkin & Williams Ltd, Chadwell Heath, Essex.
Ed. JOHNSON W C , Vol 2, p.32, 196^
HATCHER J T and WILGOX L V., Analyt. Chem. (1950) 22, 567 FARZANEH A, TROLL G and NEUBAUER W., Z. Anal. Chem. (1979) 296, 383 KRAMER H., Analyt. Chem. (1955) 27, iMf FELDMAN C , Analyt. Chem. (196D 33, 1916
In acidic rocks boron frequently occurs in the mineral tourmaline, and high-boron granites usually contain abundant quantities of this mineral. about 3 per cent boron, 10-11 per cent B p 0^·
Tourmaline contains
A procedure is given below for the
determination of boron in tourmaline, which can be adapted for other minerals high in boron, such as datolite (6% B, 20-22% B O ) . Much of the earlier data on the occurrence of boron is unreliable owing to the lack of accurate and suitable methods of analysis. However, renewed interest has led to a re-examination of older methods of separation involving distillation as methyl borate, and the devising of new methods based upon pyrohydrolysis, solvent extraction and ion-exchange.
A number of colour-forming reagents have been described for the
photometric determination which, except for those minerals high in boron, have displaced the earlier titrimetric procedures. Distillation as methyl borate is regarded as the classical procedure for the separation of boron, but conflicting reports exist in the literature regarding the adequacy of this method.
Difficulties can arise from the presence of boron in almost all
laboratory glassware, and the distillation apparatus itself should be made from fused silica.
Another difficulty that has been noted is the need for maintaining anhydrous
or near anhydrous conditions during the esterification and evolution of the borate. Ion-exchange separation procedures are generally more rapid and simpler than distillation methods. Silicate rocks and minerals are brought into solution by an alkaline fusion, and the aqueous extract acidified with hydrochloric acid.
The
passage of this solution through a column of cation exchange resin removes iron and other interfering i>ns to give a clear solution that can be used directly for the determination of boron by either titrimetry or spectrophotometry. In a version of (1) this procedure Fleet dispenses with the use of a column and adds the resin directly to the acid extract of the fused rock melt.
This procedure is described in
detail below for application to silicate rocks. Spectrophotometric Determination of Boron in Silicate Rocks Of the many reagents described for the photometric determination of boron only curcumin, dianthrimide (1,1'-iminodianthraquinone) and carmine (carminic acid) have
CMRA - I
-109-
110
Chemical Methods of Rock Analysis
been extensively used. Curcumin, the active colour principle of the vegetable product turmeric, has long been used for the detection and determination of small amounts of boron.
Considerable
difficulties were initially experienced in obtaining repeatable quantitative results, (2) but conditions necessary for a reliable procedure have now been established. Alonso and Sanchez
described its application to the determination of boron in
geological materials after a preliminary separation using Dowex 50W-X8 cation exchange resin. The use of dianthrimide for the photometric determination of boron has been carefully (4) examined by Danielsson. This reagent is more sensitive than carmine, but less so than curcumin.
It gives a linear calibration in concentrated sulphuric acid solution.
The reagent itself has an absorption band with a maximum value below ^+00 nm, clearly separated from that of the boron complex which has a maximum value at 620 nm.
The
rate of reaction of boron with dianthrimide is strongly dependent upon temperature. Optical density values are also temperature dependent, although this effect is related to the acid concentration. Carmine, the name given to a naturally occurring dyestuff from cochineal, is a calcium-aluminium compound of carminic acid, which is a derivative of anthraquinone. Both carmine and carminic acid react with boron in concentrated sulphuric acid solution to give blue coloured complexes, but as carminic acid is deliquiescent, carmine is preferred.
In the absence of boron the colour of the dye at pH 6.2
is bright red, but in the presence of boron this changes to blue.
The wavelength of
maximum absorption changes from 520 nm for the reagent to 585 nm for the boron complex. boron. used.
The Beer-Lambert Law is obeyed over the concentration range 0-10 ppm of
The colour development characteristics may vary with the brand of reagent Hatcher and Wilcox
have reported that the coloured complex with boron
can be measured after ^5 minutes and then shows no appreciable change at the end of *f hours. Fleet,(1) however, noted that the absorption reached a maximum after ko minutes and thereafter decreased.
The procedure, described below by Fleet uses a
cation exchange resin to separate interfering ions from boron. An alternative collection procedure using pyrohydrolysis has been described by (7) The sample material was mixed with pure calcium
Farzaneh, Troll and Neubauer.
fluoride and the BF., liberated was collected in a sodium hydroxide-sodium carbonate solution prior to photometric determination with carminic acid.
111
Boron Method Reagents:
Mannitol solution, dissolve 1 g of reagent in 100 ml of water. Carmine solution, dissolve 50 mg of the reagent in 100 ml of concentrated sulphuric acid. Hydrochloric acid, 0.6 N. Sodium hydroxide solution, 0.1 N. Standard boron stock solution, dissolve 0.5716 g of recrystallised boric acid in water and dilute to 1 litre.
This solution contains
100 μg boron per ml. Standard boron working solutions, dilute aliquots of the stock solution with water to give three new solutions containing 5i 10 and 20 μg boron per ml respectively. Cation exchange resin, wash 50 ml of a strongly acid resin such as Amberlite IR 120(H) or Dowex 50W-X8 with 6 N hydrochloric acid and then water until the eluate is free from acid. Procedure.
Accurately weigh approximately 0.2 g of the finely powdered rock
sample (or a smaller amount if the sample material contains more than 200 ppm boron) into a 10-ml platinum crucible and add 1.25 g of potassium carbonate. over a Bunsen burner for 1 hour.
Mix, and fuse
Allow the crucible to cool, loosen the melt by
warming with a small amount of water, and transfer the solution and residue to a 50-ml polypropylene beaker.
Cover the beaker and add 2 ml of mannitol solution,
followed by 20 ml of the cation exchange resin and 2 ml of 0.6 N hydrochloric acid. Break up any lumps of residue, mix with the ion exchange resin and add enough water to give a slurry.
Allow to stand overnight.
Collect the ion-exchange resin and any precipitated silica on a small medium-textured filter paper, wash well with water and discard it.
Collect the filtrate and washings
in a 400-ml polypropylene beaker, add 23 ml of 0.1 N sodium hydroxide solution and carefully evaporate to dryness on a steam bath. 5 ml of 0.6 N hydrochloric acid.
Allow to cool and add by pipette
When the residue has dissolved, pour this solution
into a centrifuge tube and centrifuge. Pipette 2 ml of the clear solution into a 50-ml polypropylene beaker, add 2 drops of concentrated hydrochloric acid and with great care add 10 ml of concentrated sulphuric acid.
Allow the solution to cool, then add 10 ml of the carmine reagent solution.
Swirl gently to mix the contents of the beaker and allow to stand for *f0 minutes. Measure the optical density of the solution in 1-cm cells with the spectrophotometer set at a wavelength of 585 nm.
112
Chemical Methods of Rock Analysis
For the reference solution, transfer 2 ml of water to a 50-ml polypropylene beaker and add concentrated hydrochloric acid, sulphuric acid and carmine reagent as described for the sample solution.
A reagent blank should also be prepared from
1.25 g of potassium carbonate fused without rock material in a separate platinum crucible, and carried through the procedure as described.
A series of three
standards can be used for calibration by transferring 2 ml aliquots of the three working solutions, containing 10, 20 and kO μg boron respectively, to separate beakers and proceeding as described above. Titrimetric Determination of Boron in Tourmaline One of the earliest procedures to be devised for the determination of boron in silicate rocks and minerals was that of titrating liberated boric acid with standard alkali in the presence of a polyhydric alcohol - mannitol being now commonly employed. The boric acid-mannitol complex acts as a strong monobasic acid.
When combined with
an ion-exchange separation, this procedure can be simply and easily applied to the analysis of tourmaline and other silicate minerals containing boron as a major /ON
component.
The procedure described here has been adapted from that given by Kramer. Method
Reagents:
Sodium hydroxide solutions, approximately 5 M, and also 0.05 M standardised by titration against standard hydrochloric acid in the usual waye Mannitol Ion-exchange resin, strongly acid, cationic resin such as Amberlite IR 120(H) or Dowex 50W-X8, in the form of a bed 2 cm in diameter and 25 cm in length.
The column should be of borosil^cate glass
(attack of the glass is negligible) or polypropylene tubing. To prepare the column for use or to regenerate for further use wash the bed with 100 ml of 6 N hydrochloric acid followed by water until the eluate is free from acid. Procedure.
Accurately weigh approximately 0.5 g of the finely powdered tourmaline
or other boron mineral into a small platinum crucible, mix with 3 g of anhydrous sodium carbonate and fuse over a Bunsen burner for 30 minutes.
Transfer the crucible
to a Meker burner and continue the heating for a further 30 minutes.
Allow the melt
to cool, spreading it around the sides of the crucible in the usual way.
Boron
113
Place the crucible on its side in a 100-ml polythene or polypropylene beaker containing 20 ml of water and add concentrated hydrochloric acid down the sides of the beaker until there is an excess of about 1 ml above the amount required to neutralise the alkali carbonate used for the fusion.
Warm the solution and allow
to stand until the melt has completely decomposed and all soluble material has passed into solution.
Rinse and remove the platinum crucible and lid.
At this
stage no unattacked mineral grains should be present, and the only residue a few flakes of silica precipitated from the solution.
Filter the solution through a
9-cm open-textured paper supported in a polythene funnel into a polythene beaker, and wash the residue with hot water to give a solution volume of about 50 ml. Discard the residue.
Add 5 M sodium hydroxide solution drop by drop until the formation of a precipitate that only just fails to dissolve on warming.
Clear this precipitate with a few drops
of hydrochloric acid, and transfer the solution to the ion-exchange column, previously washed with water.
Allow the eluate to collect at a rate of between 50 and ^0 ml per
minute in a *f00-ml polypropylene beaker, and wash the resin with about 200 ml of water. Add 0.5 ml of concentrated hydrochloric acid and boil the solution for 1 minute but not longer (Note 1 ) , to expel any carbon dioxide, and cool the solution to room temperature. Using a magnetic stirrer and a pH meter, add sodium hydroxide solution, first concentrated then diluted, drop by drop, until the pH of the solution reaches 7· Now add 20 g of solid mannitol and titrate the boric acid with 0.05 M standard sodium hydroxide solution until the pH again reaches 7· from the titre before calculating the results.
Subtract a reagent blank value
The reaction that occurs may be
expressed by the equation Η,ΒΟ, + NaOH = NaHJBO, + H^O so that 1 ml of 0.05 M sodium hydroxide solution is equivalent to 1.7*f1 mg B O Notes;
1.
(Note 2 ) .
Prolonged boiling of hydrochloric acid solutions will result in (9)
substantial loss of boron. 2.
Where boron is included in the summation of the rock or mineral, it is
customary to express the results as per cent boric oxide, B O . only as a trace constituent, parts per million boron is used.
Where boron occurs
Chemical Methods of Rock Analysis
References FLEET M E., Analyt. Chem. (1967) 39, 253 HAYES M R and METCALFE J., Analyst O962) 87, 956 ALONSO S J and SANCHEZ G A., An. Quim. (1972) 68, 335 DANIELSSON L., Talanta (1959) 2» 138 DANIELSSON L., Organic Reagents for Metals, Hopkin & Williams Ltd, Chadwell Heath, Essex.
Ed. JOHNSON W C , Vol 2, p.32, 196^
HATCHER J T and WILGOX L V., Analyt. Chem. (1950) 22, 567 FARZANEH A, TROLL G and NEUBAUER W., Z. Anal. Chem. (1979) 296, 383 KRAMER H., Analyt. Chem. (1955) 27, iMf FELDMAN C , Analyt. Chem. (196D 33, 1916