- Email: [email protected]
Phosphorimetric determination of traces of boron
220
A~dyrica 0
SHORT
Elscvier
Publishing
Company.
Chimica
Amsterdam
Actn. 67 (1973)
- Printed
220-224
in The Netherlands
COMMUNICATION
Phosphorimetric
determination
M. MARCANTONATOS, Department
(Rcccived
Scientific
of traces of boron
G. GAMBA
oj’ Illorganic nnd Amdyticd
20th March.
and D. MONNIER C/w~r~istr)s Utziversitg
OJ Gerwvt~, Gcr~evo (Switzerlmd)
1973)
It has been shown’ that boric acid forms a highly luminescent complex with dibenzoylmethane, the phosphorescence emission of which in a 8% sulfuric acid (96%)-diethylether (v/v) glassy medium (77°K) permits a quantitative determination of nanogram amounts of boron. Benzoylacetone is also capable of forming phosphorescent complexes with boric acid, under similar conditions. Compositions (1 :l, 2:1), stability constants (pl = 1.4. lo’, pz = 2.4. 108) and probable structures of these chelates have been given from phosphorescence excitation-emission, spectral and mean life-time determinations2q3. In a recent extensive study of the “boric acid-dibenzoylmethane” system, it has been shown4 that the phosphorescence calibration curves previously obtained’ are based on the T-W&, emission of a 2:l dibenzoylmethane-boric acid chelate which most probably has the structure II, formed as follows: Ph
HYHpSO4)
(DBMH*)
l
DBMH**
?2(HS0414
(H20)
_
-
(B-DBM)
B-DBM
.DBMH+
_
A
0\,00 o/
‘0
It (B(D8M12)
HSO:
.
HJO*
SHORT
221
COMMUNICATION
This chelate has the following triplet; z = mean life-time): S,(rrn*):
39210
cm-
T,(nrr*):
19610 cm-‘;
spectroscopic
‘; Sz(nx*): Sr-T,:
31350
properties cm -I;
4780 cm-‘;
(S = singlets;
Sr(7rn*):
r=0.65
Tr = lowest
24390 cm-r;
s.
Until now,neither of these two highly sensitive luminescence reactions of boric acid has found analytical application and no other phosphorimetric method has been proposed for the determination of traces of boron. In the present study, the technique described previously’ is applied to the direct determination of micro-amounts of boron in sea water and in solutions containing an interfering metal. Experimental Apparatus. An Aminco-Keirs phosphorimeter was used for excitation and emission determinations. Spectra and phosphorescence decay curves were obtained with a Houston Omnigraph recorder, Model HR-96, and a Tektronix oscilloscope, Type 564. Calibrations can be made either with a boric acid-dibenzoylmethane solution of definite composition ([DBM] >,200[BOJHs]), prepared as described below, or with a fresh standard solution of solid 1: 1 boric acid-benzoylacetone complex (for its preparation, see ref. 2) in 8% sulfuric acid (96%) - diethylether (v/v). Reagents. Boric acid, sulfuric acid (d= 1.84) ether and ethanol were of analytical-grade quality (Merck). Dibenzoylmethane (Fluka) was purified by double recrystallization with water-ethanol (85 + 15). Preparation of solutions for phosphorescence determi,lations. In a silica test tube, graduated to 5 ml, heat 0.4 ml of 96% sulfuric acid containing nanogramamounts of boron as boric acid, in an oven at 70” for 60 min. After cooling, add small portions of ether, mix cautiously and complete the volume to 5 ml. Introduce a portion of this solution into a 2-mm diameter phosphorimetric cell, and measure the emission (77°K) at 508 nm, with an excitation wavelength of 402 nm. Results
and discussion Calibration curves and limits of determination. Phosphorescence calibration curves with benzoylacetone were constructed in the ng B ml-’ range and compared l. Relative errors with the benzoylacetone with those obtained with dibenzoylmethane reagent were approximately the same as with dibenzoylmethane, but the apparent phosphorescence efficiency was about three times lower. The limits of determination were found to be 0.4 ng B ml-’ with benzoylacetone and 0.1 ng B ml- 1 with dibenzoylmethane. Effects of diverse ions. The borondibenzoylmethane reaction was chosen for analytical purposes and the effects of the following ions were investigated for solutions containing 22 ng B ml- i of sulfuric acid-diethylether ([ion]/[boron] = 100): Cr6+,Ce4+, Ni’+, Cu2+, Mg2+, Mn2+, Pb2+, Cd2+, ZnZf, Sn2+, Mo6+, Li+, Na+, K+, A13+, V’+, Fe3+, Co2+, Zn2+, Ag+, Te6+, Hg2+, In3+, Tl+, NH:, W6+, F-, Br-, Cl-, I-, NO;, SCN-, PO:-, S,Og-, CH3COO-. All the ions mentioned gave errors of less than lo%, except for molybdenum
._
222
SHORT
COMMUNICATION
which caused positive interferences as follows : B-I-MO . B B+W 85 270 75 Phosphorescence intensity An investigation of molybdenumEffects of tungstert and molybdenum. dibenzoylmethaneand tungsten-dibenzoylmethane solutions in diethylether-sulfuric acid (the metal ions were added as (NH&Mo,O~~ and Na,WO,) prepared as described above for the boric acid-dibenzoylmethane solution, showed the formation of phosphorescent complexes. This was not unexpected, for complexation of molybdenum and tungsten by dibenzoylmethane in acidic media is well known5. As the excitation and emission bands of these two complexes were found to be situated very close to those of the boron-dibenzoylmethane (principal bands: excitation, 403 nm (W), 405 nm (MO); emission, 506 nm (W), 509 nm (MO), positive interferences result from emission contributions. This, however, was only part of the observed effects. External heavy-atom perturbations were also found to be responsible for the increase in phosphorescence intensity of the boron-dibenzoylmethane complex. and tungsten
t!!g z=16.7
1GlS
Fig. 1. MO-DBM
decay curve. [DBM]/[Mo]=3;
[Mo]=2*
10-4.
Molybdenum heavy-atom perturbation. The mean life-time 7Mo of the phosphorescent molybdenum-dibenzoylmethane species was found to be 16.7 ms (Fig. 1). This is sufftciently different from the rB value for the boron-clibenzoylmethane chelate (0.65 s), for it to be possible to determine boron in the presence of molybdenum (Fig. 2), with reference to a calibration curve, from curves of log P, against time, where U,, the total phosphorescence, is (P$Oe-“‘Mo+ Pie-f’rB). For t $ rMo, the function log P, reduces to log Pij = log PB”- (t/2.303
zf3)
(1)
It can be seen from the results in Table I and from Fig. 3 that the presence of molybdenum causes an increase of cu. 30% in the phosphorescence emission of the boron-dibenzoylmethane complex. Possible boron impurities in ammonium heptamolybdate, high enough to make a significant contribution to the boron-dibenzoylmethane phosphorescence, are improbable, as can be seen from the time and purely exponential form of the molybdenum-dibenzoylmethane emission decay (Fig. 1). However, for solutions containing molybdenum, the mean life-time ‘F~ of the boron-dibenzoylmethane complex, obtained from the slopes of eqn. (1) is 0.61 s, which corresponds to a 40 ms decrease in the z value of the boron-dibenzoylmethane complex in pure solution. These two variations -increase in phosphorescence intensity and decrease in the z value-are well known to be due to a heavy-atom perturbation.
SHORT TABLE
223
COMMUNICATION I OF BORON
IN THE PRESENCE OF MOLYBDENUM , (P,” and Pu”* arc the phosphorcsccnce intensities of boron-dibcnzoylmcthanc in the absence and in the presence of molybdenum. respectively. P,p* is obtained from log Pu =f(r) (see eqn. 1). Excitation, 402 nm; emission. 508 nm.) DETERMINATION
B taken (ng ml- ‘)
PP
Pi*”
B fowd (rt.g nrl- ‘)
(Pi’
10.8
35.8 35.8 73.5 73.5
47.3 * 0.4 44.8 + 0.1 91.4k2.4 99.7 + 2.6
13.3 12.5 25.9 27.6
+32.1 +25.1 + 24.4 + 35.7
10.8 21.6 21.6
a Values from four decay curves.
IogP
[Mo]/[B03H3]=
-PSI
m.vP,o
cw
100.
I
I
0.4
0.6
1.2
Fig. 2. Log of total phosphorescence (pt) uerst~s time; dibenzoylmethane in sulfuric acid-diethylcther.
Determination
1.6
t (5)
10S6 M H3B0,
+ 1. 10m4 M Mo+6.
10S4 M
of boron in sea water. This determination was carried out by taking 0.05-0.3 ml of a stock solution (0.5 ml of sea water sample n 10 ml of 96% surfuric acid), and adding it to 0.1 ml in 96% sulfuric acid. The solution was diluted to of 4. lo- 2 A4 dibenzoylmethane 0.4 ml with 96% sulfuric acid and heated at 70” for 1 h. After cooling, small portions of ether were added to a total volume of 5 ml and the emission intensity (77°K) was measured at 508 nm with an excitation wavelength of 402 nm. The only precaution one must take is that the final solution should not contain more than 3% of water, a limit above which transparent glasses could not be obtained at 77°K. The results obtained by means of a reference solution containing 10 ng of boron are given in Table II.
SHORT
224
COMMUNICATION
P 100.
60,
2 o-
Fig. 3. Phosphorescence presence of molybdenum, boron-dibcnzoylmethanc
TABLE
Pb
V”
0.10 0.10 0.10 Average
B(ng ml-‘) PE and
Pi*:
phosphorcscencc
in the absence and intensity is due
19.1 16.9 17.0 18.0 value: 4.35kO.15
DETERMINATION
OF
BORON
IN SEA WATER
(BORDEAUX)
uy B ,?I/- ’
p.p.tn. B
V
P
rtg B ttd- ’
p.p.,n. B
4.68 4.10 4.13 4.39 p.pm.
4.68 4.10 4.13 4.39
0.20 0.20 0.20 0.20
34.5 34.0 34.0 35.0
8.76 8.64 8.64 8.90
4.38 4.32 4.32 4.45
u V=ml of stock solution. s P = phosphorescence intensity
B.
rcfcrred
to a blank.
REFERENCES 1 2 3 4 5
in the to the
II
PHOSPHORIMETRIC
0.10
21.6 intensity uersus boron concentration. respectively. In the two cases, the complex alone.
M. Mnrcantonatos, G. Gamba and D. Monnier. Helo. C/lint. Acre, 52 (1969) M. Marcantonatos, G. Gamba and D. Monnier, Helv. C/lint. Acru, 52 (1969) G. Gamba and M. Marcantonatos, He/o. Chh. Acru, 54 (1971) 1509. M. Marcantonatos and G. Gamba. unpublished work. J. Stary and E. Hladky, &ml. Chinr. Acru. 28 (1963) 227.
538. 2183.
A~dyrica 0
SHORT
Elscvier
Publishing
Company.
Chimica
Amsterdam
Actn. 67 (1973)
- Printed
220-224
in The Netherlands
COMMUNICATION
Phosphorimetric
determination
M. MARCANTONATOS, Department
(Rcccived
Scientific
of traces of boron
G. GAMBA
oj’ Illorganic nnd Amdyticd
20th March.
and D. MONNIER C/w~r~istr)s Utziversitg
OJ Gerwvt~, Gcr~evo (Switzerlmd)
1973)
It has been shown’ that boric acid forms a highly luminescent complex with dibenzoylmethane, the phosphorescence emission of which in a 8% sulfuric acid (96%)-diethylether (v/v) glassy medium (77°K) permits a quantitative determination of nanogram amounts of boron. Benzoylacetone is also capable of forming phosphorescent complexes with boric acid, under similar conditions. Compositions (1 :l, 2:1), stability constants (pl = 1.4. lo’, pz = 2.4. 108) and probable structures of these chelates have been given from phosphorescence excitation-emission, spectral and mean life-time determinations2q3. In a recent extensive study of the “boric acid-dibenzoylmethane” system, it has been shown4 that the phosphorescence calibration curves previously obtained’ are based on the T-W&, emission of a 2:l dibenzoylmethane-boric acid chelate which most probably has the structure II, formed as follows: Ph
HYHpSO4)
(DBMH*)
l
DBMH**
?2(HS0414
(H20)
_
-
(B-DBM)
B-DBM
.DBMH+
_
A
0\,00 o/
‘0
It (B(D8M12)
HSO:
.
HJO*
SHORT
221
COMMUNICATION
This chelate has the following triplet; z = mean life-time): S,(rrn*):
39210
cm-
T,(nrr*):
19610 cm-‘;
spectroscopic
‘; Sz(nx*): Sr-T,:
31350
properties cm -I;
4780 cm-‘;
(S = singlets;
Sr(7rn*):
r=0.65
Tr = lowest
24390 cm-r;
s.
Until now,neither of these two highly sensitive luminescence reactions of boric acid has found analytical application and no other phosphorimetric method has been proposed for the determination of traces of boron. In the present study, the technique described previously’ is applied to the direct determination of micro-amounts of boron in sea water and in solutions containing an interfering metal. Experimental Apparatus. An Aminco-Keirs phosphorimeter was used for excitation and emission determinations. Spectra and phosphorescence decay curves were obtained with a Houston Omnigraph recorder, Model HR-96, and a Tektronix oscilloscope, Type 564. Calibrations can be made either with a boric acid-dibenzoylmethane solution of definite composition ([DBM] >,200[BOJHs]), prepared as described below, or with a fresh standard solution of solid 1: 1 boric acid-benzoylacetone complex (for its preparation, see ref. 2) in 8% sulfuric acid (96%) - diethylether (v/v). Reagents. Boric acid, sulfuric acid (d= 1.84) ether and ethanol were of analytical-grade quality (Merck). Dibenzoylmethane (Fluka) was purified by double recrystallization with water-ethanol (85 + 15). Preparation of solutions for phosphorescence determi,lations. In a silica test tube, graduated to 5 ml, heat 0.4 ml of 96% sulfuric acid containing nanogramamounts of boron as boric acid, in an oven at 70” for 60 min. After cooling, add small portions of ether, mix cautiously and complete the volume to 5 ml. Introduce a portion of this solution into a 2-mm diameter phosphorimetric cell, and measure the emission (77°K) at 508 nm, with an excitation wavelength of 402 nm. Results
and discussion Calibration curves and limits of determination. Phosphorescence calibration curves with benzoylacetone were constructed in the ng B ml-’ range and compared l. Relative errors with the benzoylacetone with those obtained with dibenzoylmethane reagent were approximately the same as with dibenzoylmethane, but the apparent phosphorescence efficiency was about three times lower. The limits of determination were found to be 0.4 ng B ml-’ with benzoylacetone and 0.1 ng B ml- 1 with dibenzoylmethane. Effects of diverse ions. The borondibenzoylmethane reaction was chosen for analytical purposes and the effects of the following ions were investigated for solutions containing 22 ng B ml- i of sulfuric acid-diethylether ([ion]/[boron] = 100): Cr6+,Ce4+, Ni’+, Cu2+, Mg2+, Mn2+, Pb2+, Cd2+, ZnZf, Sn2+, Mo6+, Li+, Na+, K+, A13+, V’+, Fe3+, Co2+, Zn2+, Ag+, Te6+, Hg2+, In3+, Tl+, NH:, W6+, F-, Br-, Cl-, I-, NO;, SCN-, PO:-, S,Og-, CH3COO-. All the ions mentioned gave errors of less than lo%, except for molybdenum
._
222
SHORT
COMMUNICATION
which caused positive interferences as follows : B-I-MO . B B+W 85 270 75 Phosphorescence intensity An investigation of molybdenumEffects of tungstert and molybdenum. dibenzoylmethaneand tungsten-dibenzoylmethane solutions in diethylether-sulfuric acid (the metal ions were added as (NH&Mo,O~~ and Na,WO,) prepared as described above for the boric acid-dibenzoylmethane solution, showed the formation of phosphorescent complexes. This was not unexpected, for complexation of molybdenum and tungsten by dibenzoylmethane in acidic media is well known5. As the excitation and emission bands of these two complexes were found to be situated very close to those of the boron-dibenzoylmethane (principal bands: excitation, 403 nm (W), 405 nm (MO); emission, 506 nm (W), 509 nm (MO), positive interferences result from emission contributions. This, however, was only part of the observed effects. External heavy-atom perturbations were also found to be responsible for the increase in phosphorescence intensity of the boron-dibenzoylmethane complex. and tungsten
t!!g z=16.7
1GlS
Fig. 1. MO-DBM
decay curve. [DBM]/[Mo]=3;
[Mo]=2*
10-4.
Molybdenum heavy-atom perturbation. The mean life-time 7Mo of the phosphorescent molybdenum-dibenzoylmethane species was found to be 16.7 ms (Fig. 1). This is sufftciently different from the rB value for the boron-clibenzoylmethane chelate (0.65 s), for it to be possible to determine boron in the presence of molybdenum (Fig. 2), with reference to a calibration curve, from curves of log P, against time, where U,, the total phosphorescence, is (P$Oe-“‘Mo+ Pie-f’rB). For t $ rMo, the function log P, reduces to log Pij = log PB”- (t/2.303
zf3)
(1)
It can be seen from the results in Table I and from Fig. 3 that the presence of molybdenum causes an increase of cu. 30% in the phosphorescence emission of the boron-dibenzoylmethane complex. Possible boron impurities in ammonium heptamolybdate, high enough to make a significant contribution to the boron-dibenzoylmethane phosphorescence, are improbable, as can be seen from the time and purely exponential form of the molybdenum-dibenzoylmethane emission decay (Fig. 1). However, for solutions containing molybdenum, the mean life-time ‘F~ of the boron-dibenzoylmethane complex, obtained from the slopes of eqn. (1) is 0.61 s, which corresponds to a 40 ms decrease in the z value of the boron-dibenzoylmethane complex in pure solution. These two variations -increase in phosphorescence intensity and decrease in the z value-are well known to be due to a heavy-atom perturbation.
SHORT TABLE
223
COMMUNICATION I OF BORON
IN THE PRESENCE OF MOLYBDENUM , (P,” and Pu”* arc the phosphorcsccnce intensities of boron-dibcnzoylmcthanc in the absence and in the presence of molybdenum. respectively. P,p* is obtained from log Pu =f(r) (see eqn. 1). Excitation, 402 nm; emission. 508 nm.) DETERMINATION
B taken (ng ml- ‘)
PP
Pi*”
B fowd (rt.g nrl- ‘)
(Pi’
10.8
35.8 35.8 73.5 73.5
47.3 * 0.4 44.8 + 0.1 91.4k2.4 99.7 + 2.6
13.3 12.5 25.9 27.6
+32.1 +25.1 + 24.4 + 35.7
10.8 21.6 21.6
a Values from four decay curves.
IogP
[Mo]/[B03H3]=
-PSI
m.vP,o
cw
100.
I
I
0.4
0.6
1.2
Fig. 2. Log of total phosphorescence (pt) uerst~s time; dibenzoylmethane in sulfuric acid-diethylcther.
Determination
1.6
t (5)
10S6 M H3B0,
+ 1. 10m4 M Mo+6.
10S4 M
of boron in sea water. This determination was carried out by taking 0.05-0.3 ml of a stock solution (0.5 ml of sea water sample n 10 ml of 96% surfuric acid), and adding it to 0.1 ml in 96% sulfuric acid. The solution was diluted to of 4. lo- 2 A4 dibenzoylmethane 0.4 ml with 96% sulfuric acid and heated at 70” for 1 h. After cooling, small portions of ether were added to a total volume of 5 ml and the emission intensity (77°K) was measured at 508 nm with an excitation wavelength of 402 nm. The only precaution one must take is that the final solution should not contain more than 3% of water, a limit above which transparent glasses could not be obtained at 77°K. The results obtained by means of a reference solution containing 10 ng of boron are given in Table II.
SHORT
224
COMMUNICATION
P 100.
60,
2 o-
Fig. 3. Phosphorescence presence of molybdenum, boron-dibcnzoylmethanc
TABLE
Pb
V”
0.10 0.10 0.10 Average
B(ng ml-‘) PE and
Pi*:
phosphorcscencc
in the absence and intensity is due
19.1 16.9 17.0 18.0 value: 4.35kO.15
DETERMINATION
OF
BORON
IN SEA WATER
(BORDEAUX)
uy B ,?I/- ’
p.p.tn. B
V
P
rtg B ttd- ’
p.p.,n. B
4.68 4.10 4.13 4.39 p.pm.
4.68 4.10 4.13 4.39
0.20 0.20 0.20 0.20
34.5 34.0 34.0 35.0
8.76 8.64 8.64 8.90
4.38 4.32 4.32 4.45
u V=ml of stock solution. s P = phosphorescence intensity
B.
rcfcrred
to a blank.
REFERENCES 1 2 3 4 5
in the to the
II
PHOSPHORIMETRIC
0.10
21.6 intensity uersus boron concentration. respectively. In the two cases, the complex alone.
M. Mnrcantonatos, G. Gamba and D. Monnier. Helo. C/lint. Acre, 52 (1969) M. Marcantonatos, G. Gamba and D. Monnier, Helv. C/lint. Acru, 52 (1969) G. Gamba and M. Marcantonatos, He/o. Chh. Acru, 54 (1971) 1509. M. Marcantonatos and G. Gamba. unpublished work. J. Stary and E. Hladky, &ml. Chinr. Acru. 28 (1963) 227.
538. 2183.