2nd Annuul Summer Sgmposiunc
- Organic Reagents
Colorimetric Determination of Boron Using 1,l'-Dianthrimide GORDON H. ELLIS, ELIZABETH GATES ZOOK', AND OSKAR BAUDISCH U. S. Plant, Soil, and Nutrition Laboratory, Ithaca, N. Y., and Saratoga Springs Authority, Saratoga Springs, N . Y .
Sixty organic compounds were tested qualitatively in concentrated sulfuric acid for their suitability as either colorimetric or fluorometric reagents for boron. Twenty of these gave color and seventeen fluorescence changes. Further testing in a quantitative way led to a choice of 1-amino-4-hydroxyanthraquinone as a fluorescent and 1,l'-dianthrimide as a colorimetric reagent. The more satisfactory colorimetric method is described in detail and compared with other methods. In the usual procedures for determining boron in plant material, losses of boron during ashing of the sample may occur. A method for wet ashinp snrh samples. by which losses of baron can he minimized, is also given.
T
HE iriipurtance of t r w e s of boron in such diverse fields a
-a>hed samples are low because of losses of borou during ahing. Austin and 1lcHargue ( 1 ) reconiniend the addition of t haturated solution of barium hydroxide to the sample in a platinum crucible prior to ashing a t 450" C. t o prevent losses of boron Their procedure wraz uzed in obtaining the values shown in Table IV. These values are essentially the same as those obtained by ashing in porcelain crucibles without the addition of a fixative, whereas the wet-ashing values are considerably highw. The hitter values are more in line with those expected from a consideration of the boron content of the nutrient solutions in which these plants were gronn. Individual values in this table give sonip idea as to the reproducibility of results. REACTION BETWEEY REAGENT AND BORON
A few observations may throw some light on the nature of the
reaction between boric acid and 1,l'danthrimide in concentrated iulfuric acid. The reaction producing the blue color is dependent upon the nature of the vessel in which the reaction occurs, the temperature and duration of heating, and the concentration of reagent and
Table 111. Dry us. Wet Ashing in Preparation of Plant Samples for Boron Determination
. A s . 101
Sample
D r y Ashing
25.0 33.0
32.4 41.2
24.0 208 181 106
48.0 285 278 188
B
Carrot tops
h
n C
6
13.3 10.3 62.5
n
The absorption spectrum of 1,l'-dianthrimide with and without added boric acid is shown in Figure 1. Added boron gives a broad absorption band centered near 620 mp and the reagent alone does not absorb light excessively a t this wave length. This is in contrast to quinalizarin and most of the other reagents tested, where reagent and reagent plus boron curves overlap to such an extent that the Lambert-Beer lam is not followed ( 7 ) . Figure 2 shows a standard curve for both 1,l'-dianthrimide and quinalizarin. The fact that 1,l'-dianthrimide gives a colored complex ivhich follows the Lambert-Beer law is advantageous for work with a photoelectric colorimeter, in that fewer points are required to define the standard curve and a greater change in optical density for a given change in boron concentration is obtained. A further advantage lies in the fact that a greater contamination of concentrated sulfuric acid with boron can be tolerated. 1 few tests for interferences were made. Sulfate, chloride, tungstate, arsenate, and arsenite were found not to interfere. Fluoride in concentrations found in plant samples does not interfere but gives low results a t higher concentrations. Nitrate dnd nitrite intei fere, but are usually eliminated during ashing or can be removed from solution by adding a little Devarda's slloy and evaporating to dryness with a calcium hydroxide solurion. Oxidizing agents surh as chromate, perioliate, and perchlorate interfere. The following cations were tested and fourid not to interfere: sodium, potassium, calcium, zinc, cupric, manganese, aluminum, and beryllium. The boron content of several alfalfa samples i w s found to be essentially alike when determined by the present method and by two others (Table I). The validity of the method i ~ a stested further and found satisfactory by determining the recovery of added boron using the wet-digestion procedure (Table 11). Winsor ( 8 ) by the analysis of fumes given off during the dry ashing of plant materials has shown that losses of boron occur during this operation. Indirect evidence presented in Table I11 indicates that plant samples vary considerably in this respect. The four samples of alfalfa give the same values when either wet or dry ashed. Both turnip and carrot tops, however, give considerably higher values when wet digested. Presumably the
Y/O.
12.5" 10.0 63.0 76.0
C D Turnip tops A
RESULTS AND DISCUSSION
K e t Ashing
7/@.
Alfalfa A
73.3
D Each value is average of two or more determinations.
Table IV.
Comparison of Two Dry-Ashing Procedures with Wet-Digestion Procedure
m htt l-b
Av.
Sample
61.3 77.3 60.0 75.6 59.5 63.8 70.0 70.0 128 131 148 113
69.3 67.8 61.7 70.0 130 131
hlethod of Ashing Ba(0H)zO Av.
73.0 62.3 61.2 58.8
...
78.3 71.5 71.3 110 129 129 110
67.7 60.0
78.3 71.4 120 120
Wetd
-. ---
.lr.
77.2 80.6 85.3 85.3 160 163 164 183 303 290
328 305
78.(1 85.3
161 174 29; 317
a Carrot tops. Boron content of nutrient solution, 0.5 p.p.m. for 1 a n d 2, 2p.p.m.for3and4,and5p.p.1n.for5and6. b Ashed according t o procedure of Austin a n d AIcHargue ( 1 ) in platinum crucibles. C Ashed a3 in a b u t without fixative and in porcelain crucibles. d I n samples 1 a n d 2, wet digested, values are for single determination. I n all other cases values are average obtained on duplicate aliquots taken from single ashing.
of boric acid. The reaction occurs about equally readily in porcelain and in Corning alkali-resistant glassware, while only a partial color development occurs in Evelyn colorimeter tubes. In platinum ware, the reagent is unstable and darkens with no blue color formation. At a temperature of 90" C. about 3 hours are required for maximum color development, while a t 80" C. about 5 hours are required. At room temperature the reaction is very slow. If a solution of the reagent is diluted with a little less than fin equal volume of either absolute ethanol or methanol, the color changes from a yellowish green t o colorless. This solution
ANALYTICAL CHEMISTRY L
tI
2.
v,
boron per molecule of dianthrimide is about 8, whereas in B the ratio is 16, and in C is 32. Using data found in a discussion of the quinalizarin method published by Weinberg, Proctor, and Milner ( 7 ) the comparable ratio for this reagent a t the concentration studied was calculated to be approximately 125. Curve D shows the relationship between the maximum color developed and reagent concentration and it can be seen that the Lambert? Beer law holds in this case.
/
Finally, the instability of the colored boron complex should br noted. If an excess of boric acid is added to a solution of the reagent and heated, a very deep blue color results. If the solution is then poured into water, the reagent precipitates out ae fine red crystals, leaving a clear colorless dilute acid phase The reagent may then be filtered, washed a few times with water, dried, and redissolved in concentrated sulfuric acid and found to have the same absorption spectrum (300 to loo0 mw) ap the original reagent.
1.2
z W
0 -I
0.8
Q.
0 I-
n 0.4
0
ACKNOWLEDGMENT
0
I 60
1
0 0
I
I
20 40 80 100 BORON MICROGRAMS/S M L . 50 100 150 200 250 REAGENT MICROGRAMS/5 ML.
-
-
Figure 3. Relationship between Color Developed and Concentrations of Reagent and Boron wlll give a blue color with boric acid after several hours a t room temperature. To obtain the same intensity of color, however, 5 to 10 times as much boron is required as in the regular prncedure. In Figure 3 a family of three curves is shown.
Curve A was obtained by running a series of reactions using II reagent concentration of 200 micrograms per 5 ml. of acid and with boron (as boric acid) concentrations as indicated. After heating a t 80” C. for 5 hours the optical density was determined using a 620 mp filter and Evelyn colorimeter tubes. Curves B and C were obtained similarly, except that the reagent concentration waa 100 micrograms per 5 ml. in B and 50 micrograms per 5 ml. in C. In each case the maximum color was attained a t a boron concentration of approximately 40 micrograms per 5 ml. Calculation q h n w that at this point in the rase of A , the ratio of atoms of
2nd Annual Summer Sgmpoeinm
The authors gratefully acknowledge the help of E. I. du Pun1 de Nemours & Company which furnished six or eight dyestuffs and a special preparation of 1-amino-4-hydroxyanthraquinone; of the General Dyestuffs Corporation for several samples; of W. Bergmann, Yale, IT.F. Bruce, Cornell, L. F. Fieser, Harvard. and J. &I.Andreas, Pasadena, Calif., for many reagents otherwise difficult to obtain; and particularly the help of V. hl. King, Calco Chemical Division, American Cyanamid Company, who wpplied a large number of dyestuffs and dyestuff intermediates LITERATURE CITED
(1) Austin, C. M., and McHargue, J. S.,J. Assoc. Ofic.Agr. Chemists,31,254-5 (1948). (2) Berger, K.C., and Truog, E., IND.ENG.CREM.,ANAL.ED., 11. 540-5 (1939); Soil Sei., 57, 25-31 (1944). (3) Eckert, A., and Steiner, K., Monatsh., 35,1129-51 (1914).
(4) Naftel, J. A.,IND.EXG.CHEM., AN~L ED., . 11,407-9 (1939). (5) Radley, J.A., Analyst, 69, 45 (1944). (6) Rowe, F.M., ed., “Colour Index,” Bradford, Yorkshire, England. Society of Dyers and Colourists, 1924. (7) Weinberg, S., Proctor, K. L., and Milner, O., IND. ENG.CHEM.. ANAL.ED., 17,419-22 (1945). (8) Winsor, H. W., ANAL. CHEM..20. 176-81 (1948) RECETVEn A l l K U S t
5 . 1949.
- Organ&Reagenta
Color Reaction of Beryllium with Alkannin and Naphthazarin Spectrophotometric Studies 4.
L. UNDERWOOL,
AND
M
W . F. NEUMAN, School of M d i c i n e ond D e n t i s t r y , University of Rochester, Rochester, N . Y
OST of the methods that have been proposed for the detarmination of beryllium have proved unsuitable for dealing with microgram quantities. The quinalizarin method of Fischer ( 8 ) can readily detect 0.5 microgram of beryllium, but the reagent is too unstable in the alkaline medium employed by Fischer to permit convenient quantitative work; furthermore, the color change from violet to blue is not a desirable one. The quinizarin2-sulfonic acid method proposed by Fairhall et al. (8) has been found extremely sensitive to salt concentration, and in general yields reliable results only when conditions are rigorously conrrolled (3). ilurintricarhnxylic acid (aluminon) has been tried in
?
chis laboratory as a colorimetric reagent for beryllium, but thr color reaction lacks sensitivity and is undesirable for several other reasons (9). The various fluorometric methods ( 7 , 8, I O , 11, 1 4 ) have been found inaccurate in the authors’ laboratory In the quinizarin method, for example, the optimal pH is about 11.5, and very small variations from this value lead to erratic results; it is difficult to buffer in this region (IS). Spectrographic methods (1, 2 ) are reported to be very good from the standpoint of sensitivity and specificity, but are somewhat lacking in precision. In view of the increasing interest in beryllium, and certain deficienciesin existing methods, it appears desirable to pub-
