Estimation of Boron bv a Modified Flame Test J
H. C. WEBER
AND
R. D. JACOBSON, 3lassachusetts Institute of Technology, Cambridge, Mass.
T
equivalent of 0.02 to 0.10 mg. of boron in a test tube, evaporating almost to dryness, adding a mixture of 6 cc. of methyl alcohol and 1 cc. of sulfuric acid, and placing the test tube in the apparatus, a green color was imparted to the flame. The duration of the color was proportional to the amount of borate present. The results showi in Table I n-ere obtained by these tests, the times to the end points being recorded on a stop watch, the dial of which had been covered.
HE flame test, in which concentrated sulfuric acid and methyl alcohol produce a volatile boron compound, is commonly used for the detection of boron in some laboratories. The effects of the ratio of acid to alcohol, the temperature of the solution, and the amount of water present are presented by Stahl ( I ) , who has devised a quantitative measurement of boron based on the intensity of the green coloring in the flame u.ing a comparative standard. He claims i n accuracy of 15 per cent. Using the apparat'us herein described, although the intensity of color in the flame diminishes a t a marked rate when the amount of boron in the sample reaches 0.03 mg., the total time of duration of color is almost a straight-line function of the boron content up to about 0.1 mg. By successive dilution, a more concentrated sample of boron may be quickly analyzed, using this method.
T ~ B LI.E ESTIMATION OF B O R O ~ Boron Present
-KQ 0.02 0.04 0.06 0.08 0.10
Observed Duration of Sec. 80, 82, 80, i 3 170, 140, 135 265, 2 5 5 355, 275, 310, 330 390, 315, 340, 365, 360, 370, 410, 420, 366, 353, 360, 350, 325, 375, 330
.$Yerage Duration of Flame
Scc. 79 148 260 318 362
SCREW CLAMP
FLOW REGULATOR
AIR
PRESSURE REGULATOR
SAMPLE IN TEST TUBE
FLOW METER
The figure shows the apparatus used. Air passes at a rate of 150 cc. per minute through a calcium chloride drying tube and a into the bottom of a test tube, it bubbles through the 7 cc. of liquid containing the boron sample. Lkmixture of air, alcohol vapor, and methyl borate passes out the top of the test tube, through the nozzle, and through the thin part of a fan-shaped Bunsen flame, igniting and forming a small auxiliary flame at right angles to the other. It is this small flame that is colored distinctly green as long as an appreciable amount of methyl borate is present. The alcohol flame itself is bluegreen, but the green imparted to the flame by the boron is enough different so that one has but little difficulty recognizing an end point. Observation of the flame at intervals of a few seconds is recommended rather than continuou3 observation. That point when the constant blue-green of the alcohol flame is
The spread bet,ween the various tests is due in part to difficulty in holding the Bunsen flime in a single position. K i t h a little practice an accuracy better than one significant figure may be obtained even for traces of boron, especially if the tests be carried out in a some\That darkened room. Difficultly soluble materials such as glasses are analyzed by fusing the sample in a nickel crucible with potassium carbonate, dissolring the fusion in distilled water, taking a suitable aliquot, and elTaporat,ing almost to dryness. The residue is acidified \Titil sulfuric acid and then analyzed as out'lined above. Even ori glasses containing less than 1 per cent of boric oxide, checks of 10 per cent or better may be expect,ed,
noted at two successive observations is taken as the end point. After a few trials this point. is readily determined.
lcknowledgnient The authors wish to thank W. T. Hall for his helpful
T o check the accuracy of the method and its ease of application, an apparat'us was assembled and operated by an inexperienced observer. Two standard solutions x-ere macle u p - o n e Rith 0.570 gram of boric acid, 2 g r a m of potassium hydroxide, and distilled water to make 1 liter, and the other by diluting 50 cc. of the first to 250 cc. One cubic centimeter of the first had an equivalent of 0.10 mg. of boron, and 1 cc. of the second, 0.02 mg. By placing a sample having the
suggestions.
Literature Cited (1) Stahl, W., 2. anal. Chem., 83, 268-88 (1930); 101, 342-7, 348-56
(1935). RECEIYED October 2 0 , 1937
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