ANALYTICAL CHEMISTRY
1264 formic acid contain excess acetic acid. The method has been adopted in the determination of formic acid in sulfite u,asteliquor ( 1 ) . ACKNOWLEDGMENT
Support of this investigation by the Foundation for Forest Research is gratefully acknowledged. LITERATURE CITED
( 1 ) AhlBn, Lars, and Samuelson, Olof, Suensk Pnpperstidn., in press.
(2) Auerbach, F., and Zeglin, H., 2. physik. Chem., 103, 161 (1923). (3) Bed, Ernst, ed., “Berl-Lunge, Chemisch-technische Untersuchungsmethoden,” 8th ed., ‘Vol. 111, p. 764, Berlin, Julius Springer, 1932. (4) Euler, H. von, and Lovgren, T., 2.anorg. u. aZZgem. C h e m , 147, 123 (1925). ( 5 ) Gabrielson, G., and Samuelson, O., Acta Chem. Seand., 6 , 729 (1952). (6) Samuelson, O., “Ion Exchangers in Analytical Chemistry,”
New York, John Wiley and Sons, and Stockholm, Alniqrist and Wiksell, 1952. RECBIVED for review Fehruary 6, 1953. Accepted April 10, 1953.
Colorimetric Microdetermination of Boron by the Curcumin-Acetone Solution Method LOUIS SILVERMAN AND KATHERINE TREGO Atomic Energy Research Department, North American Aviation, Inc., Downey, Calif. I1 0 0
of variations of the amounts of reagents used in Tthe effects boron-curcumin reaction, as applied to the colorimetric HE
determination of boric acid or borate in solutions of low solids content, were studied. The procedure is effective in water solutions for determination of detectable amounts of boron (0.027, contained in a 10-ml. standard volume of solution plus reagents). Precautions are described which must be observed with reagents and apparatus in order to avoid contamination a t low boron contents (0.17 or less). Reaction of Boric Acid with Specific Organic Reagents. Korenman ( 4 ) reviewed the action of boric acid with several hydroxy derivatives of anthraquinone. It was shown that the colors of the sulfuric acid solutions of the selected quinones were changed when boric acid was added, and that these solutions become fluorescent. These quinones were characterized as having hydroxyl groups in peri-position with the carbonyl groups of the quinones. I n the case of anthraquinone, the hydroxyl group may be in the 1, 4, 5 , or 8 position.
LLU II
I
II
H 0 I
HO-
Curcumin is also found to have this basic structure, sinw it is believed that curcumin exishs in tautomeric forms:
0 I
0
h
H
eH
ci
I3
I
‘I
HC
H CH
HC
CH
II
HC
H 0
0
0
H
II
0
H
0 The boric acid reaction has been visualized as follows:
0.‘ Curcumin, which is pale1 yellow in acetone solution, is colored orange in the presence of boric acid, and the color is intensified in the presence of oxalic acid. Present Usage of Boron-
wherein the six-membered ring
or
forms, the boron being joined to the carbonyl oxygen by an auxiliary valence, or by chelate formation. Morin, the reagent which has been most used for the determination of boron, has a similar basic structure, with the hydroxyl group in pa’-position.
\
0
nn HaCO-(,,
O-OCH’ Curcumin-Oxalic tem. Allen and Acid Zies Sys(1) I 1 dipped strips of turmeric 0 0 paper in test solutions and H H dried the strips. The length and spread of color gave quantitative results for boron. Cassal and Gerrans ( 9 ) introduced oxalic acid but used solutions in place of paper. I n an improved method, Naftel(6) determined insoluble and soluble boron in soils and plants by fusing with calcium hydroxide, acidifying with hydrochloric and oxalic acids, adding cureurnin, and extracting with alcohol. The American Society for Testing Materials ( 2 ) uses a similar eystem for the determination of boron in steels, in conjunction with the methyl borate distillation. In this method, close control
V O L U M E 25, NO. 8, A U G U S T 1 9 5 3
1265 boron solution to 1 liter with water. One milliliter equals 0.01 mg. or 107 of boron. Standard Boron Solution B. Dilute 10 ml. of the stock boron solution t o 1 liter with water. One milliliter equals 0.001 mg. - or ly of boron. Curcumin Solution. Dissolve 0.0625 gram of curcumin in 5 ml. of Carbitol (monoethyl ether of diethylene glycol made by Carbide and Carbon Chemicals Co., New York), then dilute to 500 ml. with acetone. The solution is stable for 2 to 3 months. Hydrochloric Acid Solution, 1 to 4. Add 50 ml. of hydrochloric acid (specific gravity 1.2), to 200 ml. of water. Ordinary C.P. bottle acid is satisfactory. Oxalic Acid Solution, 5%. Dissolve 5 grams of oxalic acid in 100 ml. of reagrnt grads acetone. Discard after 2 days. Phenolphthalein Indicator. Dissolve 0.33 grams of the white powder in 100 ml. of alcohol. Sodium Carbonate. Anhydrous, boron-free reagent. Sodium Carbonate Solution. Dissolve 10 grams of sodium carbonate in 100 ml. of vater. Store in a plastic bottle. The solution may be delivered from glass pipets or burets, but the solution should not be stored in glassware in which it may dry and attack the container. The authors prefer to use the dry powder. APPARATUS
Beckman Spectrophotometer, Model DU. Other photometers are satisfactory. Burets. Glass burets are satisfactory. Filter Paper. Whatman No. 40, size 9 cm. Funnels. Glass funnels for filtering acetone solutions are satisfactory. Pipets. Glass pipets are satisfactory. Porcelain Crucibles. One-hundred-milliliter size for evaporating alkaline solutions. Volumetric Flasks. Glass-stowered flasks to retain acetone _solutions are satisfactory. Water Bath or Oven. Set a t 55" C. Oven set a t 110" C. Glassware which is to be used in the handling of boric acid solutions must be new and should be cleaned wTth C.P. hydrochloric acid, then with acetone, and rinsed with distilled water. Acetone may be used to dry pipets. Chromic acid cleaning solutions, soap, or detergents should never be used. Pipets cleaned with cleaning solution were found to retain about ly of boron. Boron-free glassware, when boiled with 100 ml. of
BORON MICROGRAMS Figiire 1. Effect of Varying A m o u n t s of Oxalic Acid Solution f r o m 0 to 20 Ml. Boron,
0
Y 0 5
0.000
10 15
20 25
30 35
40 45 50
0.051 0.077 0.094 0.079 0.137 0.189 0.151 0.105 0.089 0.039
Oxalic Acid Solution Used, Ml. 2 5 10 Absorbance 0.000 0.000 0.000 0.295 0,222 0.162 0.268 0.448 0.'787 0.598 0.358 1.137 0.828 0.461 1.297 0.848 0.480 0.860 0.578 1:447 0.878 0,590 1.587 0 963 0,679 1.497 1.045 0.761 1,537 1 063 0.732
20 -
0.000 0.036 0.080 0.153 0.177 0.209 0.230 0.319 0.289 0.299 0.368
of reagents aids in the over-all precision of the system, and 0.001 mg. of boron per 100 ml. of (water-acetone) solution is determined in a 2-cm. depth cell. Russell (6) distilled methyl _____ borate, then made an interestTable I. Effect of Variations i n ing study of the various mcthods for the determination 0.0 of boron. He favored the HCl, 0. 0 nil. Blank absorbancea 0.005 c u r c u m i n - a1c o hol solution Yellow Color of blank Yellow Color of tests method for boron. as did ... Follows Beer's law t o Differential readingsb None Winsor (7'). The present study offers as HC1, 0 . 5 ml. ... Blank absorbance5 new information for the coloriColor of blank Colorless Color of tests Colorless metric determination of boron ... Follows Beer's law t o Sone Differential readingsb (1) complete solution of the curcumin dye in CarbitolIICI. 1 . 0 ml. Blank absorbanceD 0.057 acetone, which solution is Yellow Color of blank Color of tests Orange stable for months, (2) use of FO~~O Beer's N S law t o 5y p h e n o l p h t h a l e i n to control Differential rradingsb Small (0 0 0 2 ) acidity, (3) optimum amounts HC1. 1 . 5 inl. Blank absorbancea 0.041 of oxalic acid, and (4) stable Yellow Color of blank Orange Color of tests c u r cumin-boron-acetone Follows Beer's law t o 1O y solutions. Differential readingsb Small (0,011) HCI. 2 . 2 5 ml. Blank absorbancea Color of blank Color of tests Follows Beer's law t o Differential readingsb
REAGENTS
Acetone. hnalytical reagent nrade: on evaporaa--- -- ~, residue tion about 0.001%. Stock Boron Solution. Dissolve 0.5716 grams of boric acid in 1 liter of water. One m i l l i l i t e r equals 1 mg. or 1007 of boron. Standard Boron Solution A. Dilute 100 ml. of the stock
HC1 3 . 5 ml. Biank absorbanceG Color of blank Color of tests Followw Beer's law t o Differential readingsb 5
~~~
...
... ... ... ... 0.060 Yellow Orange
Small
id. 016)
Amounts of Oxalic a n d Hydrochloric Acid Solution 0.125
Oxalic Acid Solution. M1. 0.25 0.50
...
...
...
... ..
... ... , . .
...
0.025 Very yellow Orange 10Y Small (0.015)
0.013 Pale green Orange
Calories's
ColoricSS Colorless
Colorless Sone
...
10Y
Small (0.012)
None
0.253 Orange Red 3 0 ~ Large ( 0 . 0 5 2 )
25r Large (0.068)
0.199 Orange Red
0.114 Orange Red 20r Large (0.024)
0.175 Orange Red 10Y Large (0.053)
2oy
... ... . .
...
...
...
... ... ...
...
0.148
Z Y A-"
%ge
~,
(0.043)
Colorless None
... ... ... ...
... ...
...
...
..,
_____________. ..-. .
...
0.126 Yellow-orange Red 15-2Oy Large (0.082)
0.096 Yelloworange Red
Color of curcumin in reagent solution. Geometrical tangent, absorbance units per microgram. . . .
... Colorless
...
0 235 Orange Red 25 y Large (0.047)
Large (0.054)
1.00 0.013 Pale green Orange 10Y Small (0.013)
'
0.064 Yellow-orange Red 10Y Large (0,004)
...
... ... ... 0.082 Yellow-orange Orange 107 Small (0.006)
1266
ANALYTICAL CHEMISTRY
water and 0.1 gram of boron-free sodium carbonate, was found to contain small amounts of boron. EXPERIMENTAL RESULTS
I n this study, the colorimetric readings were obtained from a 25-ml. solution of the boron and reagents in acetone. The general procedure involved addition of sodium carbonate to the test solution and evaporation to dryness a t 110' C.; addition of curcumin, hydrochloric and oxalic acids, and evaporation to dryness a t 55' C.; solution of the residue in acetone, filtration, and determination of the boron photometrically a t 535 mp. Absorbances were obtained with the Beckman spectrophotometer using a 1.0-cm. cell. Samples with no boron were used as blanks. Oxalic Acid. To study the effects of varying amounts of oxalic acid, measured amounts of oxalic acid solution were combined with 1 ml. of 1 to 4 hydrochloric acid, 1 drop of phenolphthalein, and 3 ml. of curcumin-acetone solution. Figure 1 clearly depicts the influence of oxalic acid in determining 0 to 507 of boron in the procedure, all other factors being held unchanged. The working concentration of oxalic acid is in the neighborhood of 2 ml. of the 5% solution, but results are obtainable with just 0.25 ml. (Figure 2). The largest differential readings are obtained with 0.5 ml.
content was less than lOy, but poor results were obtained for amounts greater than IO? of boron. Combined Oxalic-Hydrochloric Acid Mixture. Table I summarizes the results obtained when varying the acid constituents. The test solution contained 0.1 gram of sodium carbonate, for which about 0.6 ml. of hydrochloric acid is required for neutralization.
0.0V'
'
10
,
,
20
.
30
,
,
40
,
.
50
W
70
BORON, MICROGRAMS
Figure 3. Boron,
Effect of Amount of Excess Hydrochloric Acid 0.5
Excess Hydrochloric Acid, hll. 1.0 Absorbance
2.0
0.000 0.789 1,369 1.614 1 829 1,969
0.000 0.547 0.612 0.931 0.854
0.000 0.705 1.075 1,082 1.071
Y
0 10 20 30 40 50
ao'
Figure 2.
Boron.
'
/a
'
' so ' 40 ' i4 BORON, MICROGRAMS
20
do
'
io
Effect of Varying Amounts of Oxalic -4cid Solution from 0.125 to 1.0 M I . 0.125
Y
0 5 10 15 20 25 30 35 40 45 50
'
0.000 0.245 0.520 0:991 1.214 1.234 1.046 1.139
... ...
Oxalic Acid Solution Used, M1. 0.25 0.50 Absorbance 0 000 0.000 0 350 0: 672 0.669 1.000 1.292 i:i& 1.679 1.575 1:473 1:4+6
1:soo
1.00
0.000 0.479 0.791 1.293
i:ji4 1.862 1.922 2.032 I . 972 2.062
Hydrochloric Acid. To determine the effects of excess hydrochloric acid, 1 ml. of 10% sodium carbonate solution was combined with 0.6 ml. plus the noted excess of 1 to 4 hydrochloric acid solution, 1 drop of phenolphthalein, 0.25 ml. of 5% oxalic acid solution, and 3 ml. of curcumin-acetone solution. Figure 3 shows that the amount of excess (past neutralization) hydrochloric acid solution is important. In the test runs only 0.5 ml. of hydrochloric acid solution (past neutralization) was required. An additional 0.5 ml, of the acid could be tolerated when the boron
...
...
Solutions containing only 0 to 0.5 ml. of hydrochloric acid solution and 0 to 1 m]. of oxalic acid solution are insufficiently acidic, and give poor results. Solutions containing 1ml. of hydrochloric and 0.25 ml. of oxalic acid solution give sharper results-Le., closely follow Beer's law over a greater range-and variations from either of these figures are indicated by less favorable colorimetric readings. In Figure 3 the graphs show that an excess of 0.5 ml. of hydrochloric acid solution (past the phenolphthalein end point) is the optimum acidity. Curcumin. The effect of varying curcumin was tested with varying amounts of 0.01257, curcumin solution combined with 1 ml. of hydrochloric acid, 1 drop of phenolphthalein, and 10 ml. of 5 7 , oxalic acid solution. The range of concentration of curcumin may arbitrarily be selected, as shown in Figure 4. At high concentrations of oxalic acid a higher concentration of curcumin is permissible. A minimum of 3 ml. of curcumin is desired for a 0 to 1Oy range of boron. Sodium Carbonate. For aqueous solutions of low solids content, 0.1 gram of sodium carbonate is usually sufficient to neutralize the acidity and give a solution of residual alkalinity. If the solution is not alkaline, then significant amounts of boric acid (probably metaboric acid) may be lost by volatilization. Variations of 0.05 to 0.15 gram of sodium carbonate have no effect on the boron-curcumin system, provided the solution has become alkaline, and the subsequently dried residue is carefully neutralized with hydrochloric acid solution (phenolphthalein a8 indicator), and the specified excess of acid added. The importance of the specified excess of acid was noted above. Accordingly, this paper is limited to solutions which do not have high mineral contents of iron, aluminum, etc., but which
1267
V O L U M E 25, NO. 8, A U G U S T 1 9 5 3 nevertheless require precision analytical methods for boron in the range of 0.2 to 257. If amounts of sodium carbonate, such as 1 to 3 grams, are required, as in the determination of boron in steel, a modified technique must be used. Color Stability. The color density is constant for 8 to 24 hours. With one group of standards, the initial curve was plotted from solutions containing 10, 15, 16, 20 and 307 of boron, respectively. Seventy-two hours later the solutions were again read on the spectrophotometer, and the respective values had dropped to 9.6, 14, 15.9, 19.2 and 27.47. Precision and Reproducibility of Results. The precision for boron determination is dependent on the amount of boron prrsent. I n the 0- to 0.2-7 range, the final volume of the sample is limited to 10 ml. Kith a precision of 0.027; in the 0.2- to 2-y region the final volume is 25 ml. with a precision of 0.057; in the 2- to 25--( range the final volume is again 25 ml. with a precision of 95%; and above 25y, the operating limits are 50-ml. final volume for 5 0 of~ boron, 100-ml. final volume for 1007 of boron, etc., n i t h a precision of 95%. Tests show that this colorimetric method is superior to the potentiometric method for boric acid in the lower concentration range of O-to-25y, equally precise in the 25-to-100y range, but inferior above this concentration. In an extended series of tests, 900 liters of an aqueous boric acid solution (50 p.p.m. of boron) were poured through an anion exchange column, and the effluent was tested for boron content by the procedure as outlined. Thirty consecutive test samples (6 per day) indicated boron content in the range of 0.1 to 0.27, using the 25-ml. final volume technique. Duplicates did not vary by more than 0.05r. This shows that a series of tests will give consistent results over a period of time.
curcumin lies in extended boron ranges. On the other hand, the higher the curcumin concentration the higher the curcumin blank, and the more difficult it is to detect and determine fractional micrograms of boron. Therein lies the advantage of lower concentrations of the dye.
LO
I
DISCUSSION
The four figures in this paper graphically explain why there are strict limitations in the use of the reagents. Figure 1 shows that 20 and 10 ml. of oxalic acid must not be used, but that 5 ml. approaches the desired range. On the other hand, the absence of oxalic acid is noted in the “0 ml.” curve. Figure 2 is the companion to Figure 1. Figure 1 gives *the over-all view regarding oxalic acid while in Figure 2 the detailed picture of the importance of accuracy in the use of oxalic acid is clearly presented. The curve for 0.5 ml. of oxalic acid is easily reproduced if the oxalic acid is carefully measured. I n Figure 3, the importance of excess hydrochloric acid is stressed. A curve for “0 ml.” is not drawn, but Table I shows that good color development is not possible in neutral solution. In Figure 4, the variations in results due to curcumin content are shown. The advantage in the use of the larger amounts of
c
%sm/
CUQCUMIU
I
‘
Figure 4.
/o
’
i o ’ 50 ’ 40 ’ 30 BORON, MICROGRAMS
’
60
lo
Effect of Yarying Amount of Curcumin Solution
Boron.
1.5
Y
0 3 5 6 9 10 12 15 18 20
PROCEDURE FOR DETERMINATION OF BORON IN AQUEOUS SOLUTIONS
By pipet, transfer the aqueous solution (0 to 207 of boron) to a porcelain crucible and add 0.1 gram of sodium carbonate poivder. Prepare a set of standards to accompany the unknoTvns. Evaporate the solutions to dryness, a t 110’ to 130’ C., preferably in an oven. Cool. Add 1 drop of phenolphthalein indicator. Titrate 17-ith 1 to 4 hydrochloric acid solution to the disappearance of the red color and add an excess of 0.5 ml. of the acid. The total amount of 1 to 4 hydrochloric acid solution used is usually 1 ml. Add 0.5 ml. of 5% oxalic acidacetone solution and finally 3 nil. of curcumin reagent. Evaporate this acetone solution to dryness at 55” & 3 ” C. in an oven or water bath (6). (This is the critical stage of the procedure. The residue is dry when square crystals appear in the crucible.) Cool the residue. Take up the reddish residue in acetone, filter through a small paper into a 25-ml. volumetric flask, vash the crucible and paper LTith acetone, and make up to the mark, in the flask, o i t h acetone. Mix well. Thermostat a t 22” or 25” C., if desired. Read the colored samples, preferably n i t h a photometer. On a Beckman spectrophotometer, use a 0.3 micron .lit at 535 mp wave length.
,
25 30 35 40 45
50
Curcumin Solution Used, hll. 3.0 6.0 Absorbance
7.5
0.000 0.107
0.000
0.000
0.000
0:090
... o:ik
o:i56
o:iQ3 0.298
0 : iii
0: 300
0:225
0:252
0 : 536
0:383
0.305
0:7i4
0: 375 0.394 0.416
0:767 0.787 0.841 0.887
0:3i5 0.330 0.338 0.369
.. .. .. ... ... .. ..
0.515 ,
..
...
1.932
...
0:5i3
0.664 0.707 0.707 0.748 0.817 1,129
,4procedure is easily written which allows for duplication and accurate results for certain work. I n water, 0.27 of boron may easily be detected and determined in a 25-ml. volume, and the upper limit of determination is 25, per test sample, on a straightline graph; smaller amounts of boron are best detected in small volumes. The procedure may also be applied to small test samples in which fusion involving not more than 0.1 gram of sodium carbonate is required. In this way, iron and other insoluble carbonates are separated from the boron. Examples of this are boron in graphite ash and boron in inorganic and organic ashes. These viill be shown in a subsequent paper. ACKNOWLEDGMENT
This report is based on studies performed for the Atomic Energy Commission under Contract ilT-1 I-1-Gen-8. LITERATURE CITED
(6)
Allen, E. T., and Zies, E. G., J . Am. Ceram. Soc., 1, 739 (1918). American Society for Testing Materials, Philadelphia, “Methods for Chemical Analysis of Metals,” pp. 134-7, 1950. Cassal, C. E., and Gerrans, H., Chem. News, 87, 27 (1903). Korenman, I. Ll.,Zhur. Anal. Khim., 2, 135-8 (1947). Kaftel, J. A , , ISD. ENG.CHEX, A s 4 ~ ED., . 11, 407-9 (1939). Russell, J. J., University of Toronto, Canada, MC-47 (March 7,
(7)
Winsor, H. W., 4 x . 4 ~ CHEM., . 20, 176-81 (1948).
(1) (2)
(3) (4) (5)
1944).
RECEIVED for review September 29, 1952. .4ccepted hfarch 20, 1953.
