V O L U M E 26, NO. 9, S E P T E M B E R 1 9 5 4
1465
(13) Lamneck, J. €I., Jr., and Wise, P. H., “Synthesis and Purifica-
tion of Alkyldiphenylmethane Hydrocarbons,” Xational hdvisory Committee for Aeronautics, Washington, D. C., December 1950. (14) Lipkin, 11. R., Hoffecker, W. A., Martin, C. C., and Ledley, R. E.. Jr.. A N A L . CHEM..20. 130 (1948). (15) Alair, B. J., Ind. Eng. Chem., 42, 1355-60 (1950). (16) Schalla, R . L., and McDonald, G. E., Ind. Eng. Chem., 45, 1497-500 (1953).
(17) Serijan, K. T., Goodman, I. A, ahd Yankauskas, W. J., “Infrared Spectra of 59 Dicyclic Hydrocarbons,” National Advisory
Committee for Aeronautics, Washington, D. C., November 1951. (18) U. S. Air Force, Military Specification JIIL-F-5624A, 1951.
RECEIVED for review March 12, 1954. Accepted June 12, 1954. Presented at the Regional Conclave, . ~ M E R I C A N CHEMICALSOCIETY, Xew Orleans, L a , December 12, 1953.
Colorimetric Determination of Boron with Tetrabromoehrysazin JOHN H. YOE
and
ROBERT
L. G R O B
Pratt Trace Analysis Laboratory, Department
o f Chemistry,
Eighty selected organic compounds were investigated qualitatively as possible reagents for boron. Nine of these showed distinct color changes when reacted with boron; four w-ere selected for further study. The purpose of this investigation was to make a critical study of the reaction of tetrabromochrysazin with boron and to develop a sensitive colorimetric method for its determination. The rose colored boron complex reaches maximum intensity after 1 hour and is measured at 540 m p . The reaction is carried out in 96% sulfuric acid and the mole ratio of boron to reagent is 1 to 1. The color reaction conforms to Beer’s law and has a practical sensitiiity of 0.020 y of boron when absorbance measurements are made in 1.00-cm. Corex cells-i.e., 1 part of boron in 50,000,000 parts of solution. The optimum concentration range is 2.5 to 8.5 y of boron per 10 ml. The tolerance of the colored complex to many diverse ions has been established. A separation of boron from interfering ions is accomplished by distilling off the trimethyl borate. The procedure is useful for the determination of trace quantities of boron.
R
C C O G S I T I O S of the role of boron in agriculture was an important contribution of science. Failure to realize the full importance of boron earlier was due in part to the lack of a method of analysis which was sensitive enough to determine the small amounts ( 1 to 10 p.p,m.) usually present in soils and plants. Boron is present, in trace quantities, in all plants and animals. The amounts in plants are higher than those found in animals (100 to 200 times greater) ( I ) . Part of the boron present in soils is fixed and part remains in the soil solution ( 3 ) . The mechanisms of fixation are ion exchange, molecular adsorption, and
i
1.0-
W
0.8
-
1 2 0.4G
I
Figure 1.
chemical precipitation. It has been postulated that boron enters the plants as boric acid and combines with polyhydroxy compounds in the cell ( I d ) . After entering the tissues of the plants, the boron tends to remain there instead of being moved about freely with the sugars and other compounds required for tissue growth ( 2 ) . One of the roles of boron in plants is to improve the oxygen supply of the tissues, particularly of the root systems ( I O ) . This action could possibly account for the formation of organic peroxides in plants. I n order to determine these minute quantities of boron, a new colorimetric method has been developed based upon the stable rose colored boron complex of tetrabromochrysazin in concentrated sulfuric acid solution. The results of this investigation show that the method is useful for the quantitative determination of boron and that it has a high sensitivity (1 part of boron in 50,000,000 parts of solution). APPAR4TUS AVD REAGENTS
Spectrophotometer. Absorbance measurements were made with a Beckman spectrophotometer, Model DU, using 1.00-cm. matched Corex cells. Distillation Apparatus. The distillation apparatus \vas constructed n-ith Corning alkali resistant glass KO. 7280. This type of glassware n as used becauqe it is essentially free of boron (maximum boron oxide content, 0.2%). Heat Lamp. -1250-watt G.E infrared heat lamp was employed for all evaporations. Standard Boron Solution. 11 standard solution containing 1000 y of boron per milliliter was prepared by dissolving 0.5736 gram of orthoboric acid in 96% sulfuric acid t o a volume of 100 ml. Solutions of greater dilution were made from the stock solution as required. Reagent Solution. A 1 X 10-3AlIsolution nas prepared by dissolving 0.0556 gram of tetrabromochrysazin in 100 ml. of 96% sulfuric acid. Diverse Ion Solutions. Reagent grade compounds n ere employed to prepare solutions of various test ions. The stock solutions were made up in 96% sulfuric acid and contained 1 mg. of the desired ion per milliliter, except in the case of Fe++, Fe+-+, Co-+, Cu-+, >In-+, Cr’+, and > I g T +ions; because of their low solubility (in 96g/, sulfuric acid), saturated solutions were employed. Other Reagents. All other reagents nere analytical grade and were used oithout further purification. ABSORPTION CURVES
0.2 -
300
University o f Virginia, Charlottesville, V a .
400 500 WAVE LENGTH, Mp
600
Absorbance of Reagent and Boron Complex
Tetrabromochrysazin Solution. A blank reagent solution was prepared by dissolving 0.0300 gram of tetrabromochrysazin in sulfuric acid to a volume of 100 ml. Figure 1 shows that the reagent absorbs throughout the entire visible region. It has an appreciable absorbance a t the wave
ANALYTICAL CHEMISTRY
1466 lengths of the two maxima for the boron complex. For this reason, an excess of reagent has a great influence on the precision and accuracy of the method. The absorbance of the reagent was measured over the range of 320 to 700 mr, the spectrophotometer being set on 96% sulfuric acid as a blank. Boron Tetrabromochrysazin. The rose colored complex of boron tetrabromochrysazin was formed by adding 6 y of boron and 1 ml. of reagent solution (5.4 X 10-41M) to a 10-ml. volumetric flask. Color was developed for 1 hour, then diluted to the mark with 96% sulfuric acid, and mixed. The resulting solution contained 0.6 p.p.m. of boron. Figure 1 shows the absorption peaks for boron tetrabromochrysazin to be 540 and 570 mp, when measured against 96Oj, sulfuric acid as a blank. EFFECT O F SULFURIC ACID COIVCEYTRATION
The effect of sulfuric acid concentration was determined by measuring the absorbance of solutions containing 10 y of boron, 1.0 ml. of reagent, and varying percentages of sulfuric acid. The boron complex showed considerable decrease in absorbance as the sulfuric acid concentration was decreased. Below 90% sulfuric acid (by weight) the reagent begins to precipitate. For consistent results the sulfuric acid must be a t least 96% by weight. The results suggested that if higher acid strengths were used the sensitivity n70uld increase. This, however, would necessitate employing fuming sulfuric acid, which is not only difficult to maintain in an analytical condition but also is very awkward to use. NATURE O F BORON COMPLEX IN SOLUTION
Three methods were employed to establish the empirical formula of the boron complex in solution. These were the mole ratio method of Yoe and Jones (IS), the method of continuous variations introduced by Job (8) and modified by Vosburgh and Cooper ( I I ) , and the slope ratio method recently proposed by Harvey and Manning ( 5 , 6).
0.11
0.09
vided the constituent in excess has no absorbance at the wave length used; if it does, there will be a gradual increase in absorbance which should form a straight line. I n applying this method to the boron complex, solutions were prepared so that the reagent concentration was maintained constant at 1 x l O - S M and the ratio of moles of boron to moles of reagent was varied from 1.0 to 5.0. A sulfuric acid blank was used for all measurements. The results in Figure 2 indicate a 1 to 1 ratio. The method of continuous variations involves varying the mole per cent of the two substances forming a compound, and then measuring a physical property of the solut'ions-e.g., absorbance of light a t a given wave length. .4 plot of the difference between the observed value and the calculated value, assuming no reaction, should show a maximum if the property measured has a larger value for the complex than either substance, or a minimum if smaller at the mole per cent corresponding to the compound formed. Solutions were prepared containing x ml. of a 1 x 10-4.11 reagent solution and (5 - x) ml. of a 1 X 10-(M boron solution. The absorbances of these solutions versus a sulfuric acid blank were then measured a t 540 mp. If one represents the formntion of the boron complex as:
B
+ nR = B R ,
and the function I' such that
Y
= A,(obsd.) - A,(calcd.)
against x should show a masinium then a plot of this function (Y) or minimum a t the point where: X ~-
5-x
- n
The results are shown in Figure 3. A maximum occurs at z = 2.5, so that n = 1.0, indicating a boron-to-reagent ratio of 1 to 1. The slope ratio method of Harvey and Manning makes use of the ends of the curve where a large excess of one or the other constituent is present. I n this method two plots of absorbance versus concentration are obtained; one in which the concentration of the reagent is kept constant and in excess, while the concentration of the boron is varied; in the other the boron concentration is kept constant and in excess, while the concentration of reagent is varied. In the reaction:
Y
nlR
y 0.07 4 m
+ nB % R,B,
if the concentration of B is constant and in sufficient excess to make dissociation negligible, the equilibrium concentration of the complex R,B, will be essentially proportional to the analyticd concentration of R added in the reaction; so
8
$ 0.05
(R,nB,L) = CR/m 0.03
I
0.08
1
'
7
0.01 I
I
I
I
0.06
MOLES OF BORON PER MOLE OF REAGENT
Figure 2.
Absorbance by Varying Ratio of Boron to Reagent
In the method of Yoe and Jones the absorbances of a series of solutions ivhich contain varying ratios of the tTvo constitueiits are measured. -4plot of absorbance against concentration should be a straight line t o the point Tvhere equivalent amounts of both constituents are present (if Beer's law is obeyed). The curve will then become horizontal and parallel to the z-axis because all of one constituent has been used up and increasing the amount of the other constituent should not increase the absorbance, pro-
0.04
0.OP
0 1 P 3 4 Figure 3. Absorbance of Boron and Reagent Solutions us. Sulfuric Acid Blank at 540 Mr
.
V O L U M E 2 6 , NO. 9, S E P T E M B E R 1 9 5 4 here C is total concentration. From Beer's law there is the relation:
M
E = ed(R,B,,) vhcw E = absorbance measured e = molecular extinction coefficient d = cell thickness in cm. Theirloie: E = edCR/na
If E is plotted against different concentrations of R, keeping B constmt and in excess, then the straight line portion of the curve is valid and will have a slope given by: Slopel
=
edjn
Sirnilitily with the reverse condition ( R constant and in excess, and R varied):
(Ran) = CB/n E = ed(R,B,) E = edCB/n Slope, = e d / n Then the ratio of n t o m in the complex may be determined t)j the ratio of the two slopes: Slopel - - n Slopel ~ I
I
In applying this method to the horon complex a series of solutions was prepared containing 20 y of boron per 10 ml. (an excess) and varying the concentration of the reagent. The absorbances were measured against a sulfuric acid blank. A srcond series was prepared 1 X l O - ~ . l I with respect to the reagent (an excess) and varying the amounts of boron. The absorbances of this series were measured against a reagent blank. The results are shown in Figure 4. From Figure 4,slopel = 0.1526 (boron in excess) and sloper = 0.1516 (reagent in excess) and the reagent-to-boron ratio is:
o.L!ls '.ls2'
=
1467
meter. However, the practical sensitivity (0.020 unit), determined by measuring the absorbance of decreasing amounts of boron, was found t o be 0.02 y per square centimeter--i.e., 1 part of boron in 50,000,000 parts of solution. Six solutions containing 0.020 p.p.m. of boron gave absorbance values of 0.021, 0.020, 0.020, 0.019, 0.020, and 0.021. Below a concentration of 0.02 p.p.m. the results could not be reproduced within an experimental error of *5%. EFFECT O F DIVERSE IORS
To determine the interferences by diverse ions individual solutions were prepared containing 5 y of boron, the reagent, and 1000 y of each ion t o be tested. An increase or decrease of 0.010 in absorbance (3.6%) was arbitrarily chosen as an interference. If interferences were noted, then more tests were made, using lower concentrations of the diverse ion, until a change of less than &0.010 absorbance unit was noted. All diverse ion solutionP were made up in 96% sulfuric acid. The following procedure was used: One milliliter of 5 y boron solution was placed in a 1O-in1. volumetric flask, and the diverse ion solution added. Then 1 ml. of a 1 X 10-3.11 reagent solution \vas added and the color developed for 1 hour. The colored solution was then diluted to the mark with 96% sulfuric acid, and mixed. Absorbance readings u ere measui ed a t 540 mp using a reagent blank. This study showed that boron, especially when present in trace amounts, must be separated from many of the common ions 1
~
-Excess
I
1
I
Boron
0.993-i.e., a ratio of 1 to I (within the limits of error), in agreement with the other two methods
The results obtained by the three methods strongly indicate that t h r comples exists as one atom of boron to one molecule of the reagent. The following formula is therefore proposed for the horon complex in solution: B=O ,'\
h h 0
I 9. 3 4 CONCENTRATION, MOLES X 104
Figure 4. Absorbance of Varying Boron to Reagent Ratio in Excess to Each Other
0 Rate of Reaction and Stability of Complex. The color formation of the boron tetrabromochrysazin was found t o be dependent upon the reaction time. The intensity increased up to an hour, after which there was no further increase noted. Upon standing for 27 days, the comples showed only 2.3% decrease in absorbance; a color stability quite adequate for analytical applications. Beer's Law. The boron tetrabromochrysazin complex obeys Beer's law over the concentration range of 1 to 10 y of boron per 10 ml. of solution, with a practical range of 2.5 to 8.5 y,where the absorbances occur between 0.2 to 0.7 unit; almost the same range as in the quinalizarin and curcumin methods. SENSITIVITY OF RE4CTIOh
T'sing Sandell's expression for thP sensitivity of a colorimetric reaction (9) and the absorbance value of 0.1 p.p.m. of boron (0.0821, the sensitivity is found to be 0.001 y per square centi-
Table I. Ion A-+As04
~
BatCd+Ca+' Cr;t_+ Co
cu+-
GO&-Fe+Fe++- + K
Mg+-
Mn*Pia
+
NifNO,PO&--SbzOi - - - -
Tolerances to Diverse Ions Added As .ilzOs NaaAsOd BaClo CdBrz CaClz Cre(SOa)a CoC12 CUSOI NazCzOa FeSO&(NHa)zSOc Fez(S0n)a KC1 MgO hInSO4 NaCl NiSOr KNOs Cas(P0a)z KzHzSbzOi SrCOs
Ti02
ZnCh
Limiting Concn., P.P.M. 10 120 107 100 100 10 280 100 n
105 0 500
50
70
500
10 0 0
100 100 0 0
1468
ANALYTICAL CHEMISTRY
before it can be determined accurately by this method; hence, the distillation as methyl borate. Table I lists the tolerances of the diverse ions and the form in which they were added. SEPARATION O F BORON
Gooch ( 4 ) found that alkali- and alkaline-earth borates, on being distilled with absolute methyl alcohol (acetone free), give up their boron in the form of methyl borate, a liquid which boils a t 65’ C. If the methyl borate is brought into contact with a weighed amount of base in the presmce of water, it is completely hydrolyzed.
B(0CHa)a
+ 2H2O = 3CHaOH + B(OH)3
NaOH A
Na3B03
borate and boron will be lost when the distillate is evaporated. The formation of the ester takes place in an acid medium and is very rapid, even in the absence of a catalyst. Acetic acid was used by Gooch ( 4 ) but in this investigation 96y0 sulfuric acid was employed. I n order to ensure as nearly complete separation as possible and to account for the sulfuric acid reacting with the alcohol, a large excess of methyl alcohol must be used. To determine the optimum conditions for the separation, a standard solution of boron ( 5 y per milliliter) in 96% sulfuric acid was used for all preliminary work on the efficiency of the distillation. The volume of alcohol, effect of other dehydrating agents, temperature, heating time, amount of base, and volume of water were each varied in order to determine which ratio gives the highest recovery and precision. The distilling apparatus was designed, so as to minimize extraneous glassware. The separation of boron is made in the following manner: To the solution containing the boron, add 25 ml. of methyl alcohol (acetone free). The receiving flask contains 5 ml. of O.lAV sodium hydroxide and 20 ml. of distilled water and is immersed in an ice bath. Heat the distilling flask in a water bath until no more liquid comes over, then treat two 50-ml. aliquots of alcohol in the same manner. Evaporate the distillate t o dryness and add the reagent. Allow the solution to stand for 1 hour, add 96% sulfuric acid to the mark, mix thoroughly, and measure the absorbance a t 540 mp using a reagent blank.
Standard Deviation. In order to determine the precision and standard deviation of the separation of boron by distillation, a set of experimental values were interpreted statistically. Three sets of distillations were run a t different times; in each case 15 y of boron was distilled. The first set contained 3 determinations, the second 4, and the third 5 determinations. Calculation of the variance, by dividing the sum of the squares by the degrees of freedom (one less than the number of determinations per sub set) gives a good estimation of the precision. The square root of the variance gives the standard deviation.
Separation of Boron by Distillation Recovery,
Solution Std. 1-1 1-2 1-3 2-1 2-2 2-3 2-4 3-1 3-2 3-3 3-4 3-5
Absorbance 0,589 0,575 0.575 0.580 0.585 0.575 0.585 0.578 0.683 0.577 0.580 0.575 0.585
Concn., 7.50 7.32 7.32 7.38 7.45 7.32 7.45 7.36 7.42 7.35 7.38 7.32 7.46 Av. 7.37
y
RECOMMENDED PROCEDURE
The sample weight should be chosen so that an aliquot of the dissolved sample will yield a solution containing 2.5 to 8.5 y of boron per 10 ml., the optimum concentration range. The boron is first removed from interfering ions in the form of the trimethyl ester. The distillate is treated in the same manner as given in separation of boron.
+ 3H20
If a small excess of base is not present, the ester will be converted to the unstable sodium metaborate ( 7 )instead of the stable sodium
Table 11.
All distillations were performed in the manner described above. Table I1 lists the results of the determinations in per cent recovery. The standard deviation was found to be 0.04% for a single determination.
%
100.00 97.60 97.60 98.40 99.33 97.60 99.33 98.13 98.93 98.00 98.40 97.60 99.33 98.35
APPLICATION OF PROCEDURE
A synthetic “blood ash” solution (B.S.S.) was prepared in 96% sulfuric acid. Table I11 lists the ions, the form in which they were added, and their concentrations to make up the solution. Results of these analyses are recorded in Table IV.
Table 111. Synthetic “Blood Ash” Solution (B.A.S.)“ Added .Is
Ion
;‘+ti’++
A1103
HaB03 CaCh (Sods CU?Cl? Crz
Ca++ Cr’++ cu Fe-++ K*
Concn., P.P.M. 0 1 1.0 50, I
1.
+
FeCla 400 KCI 2000 M g 50 \Ig0 SaCi 2000 SiSOa 0.1 p++c++ 100 SaHZP04 Ti++it 0.05 Ti02 Zn++ 15 ZnCln a All concentrations are based on representative values obtained with large number of samples of human blood, determined spectrographically in Pratt Trace Analysis Laboratory, University of Virginia, Charlottesville, Va. +
+
+*;;
Table IV. Source B. A . S. B AS. B.A.6. B . A .S.
.
Results of Analyses
Boron Present. y 5.60 5,60 4.20 4.20
Boron Recovered, y 5.20 5.10 3.95 3.90
Recovery, % 92.9 91.1 94.0 92.9
LITERATURE CITED (1) Bertrand, G., Rec. trau. chim., 57, 569-74 (1938); Bid. A h . 14, 147. (2) Eaton, F. M., J . Agr. Research, 69, 237-77 (1944). (3) Eaton, F. M.,and Wilcox, L. V., U. S. Dept. Agr., Tech. BuU. 696 (1939). (4) Gooch, F. A,, Am. Chem. J . , 9, 23 (1887). (5) Harvey. A. E., and Manning. D. L., J . Am. Chem. SOC.,72,4488 (1950). (6) Ibid., 74, 4744 (1952). (7) Hillebrand, W. F., and Lundell, G. E. F., “Applied Inorganic Analysis,” 2nd ed., p. 757, Kew York. John Wiley & Sons, Inc., 1953. (8) Job, Paul. Ann. chim. (IO), 9, 113 (1928). (9) Sandell, E. B., “Colorimetric Determination of Traces of hIetals,” 2nd ed., R . 50, Kew York, Interscience Publishers, 1950.
(10) Shkol’nik, 31. Ya., and Steklova, 11. M.,Doklady Akad. .Vauk
S.S.S.R.. 77, 137-40 (1951). (11) Vosburgh, W.C.. and Cooper, G. R., J . Am. Chem. Soc., 63, 437 (1941). (12) Winfield, AI. E., Aus!raZzan J . ExptZ. B1o2 ,Wed. Sci., 23, 11117 (1945). L., IND.ENG.CHEM.,AXLL.ED., 16, (13) Yoe, J. H., and Jones, -4. 111 (1944). RECEIVED for review January 14, 1954. Accepted June 4, 1954.
