Source of Error in Determination of Boron by Chromotrope=B
,
W. B. HEALY', Soil Bureau, Department of Scientijic and Industrial Research, Wellington, .Yew Zealand USTIN and BlcHargue ( 1 ) in their method for the deter-
A mination of boron in plants using Chromotrope-B (p-nitrobenzeneazo-1,8-dihydroxynaphthalene-3,6-disulfo1iic acid) describe a procedure for checking on interference of ions. This consisted of a visual evaluation on a spot plate of the effects of 5 to 10 mg. of a particular compound to be tested on a standard boron solution. I n this way 28 cations and 21 anions were tested; no interference by manganese was reported.
neqe content of the material used was very low. I t can be se'sri from Table I that even a t the 106-microgram level (212 p.p.m 1 an error of 30% will be introduced. The percentage error will, ot coulse, increase when the boron present is lower than that of the plant material used for theye tests. Manganese contents of 200 p.p.m. in plant material are common and more than 2000 p.p.m. have been recorded. When manganese (as manganese acetate in 70y0 acetic acid) was added after ignition little or no interference was observed even a t the 500-microgram level. The interference of manganese when added before igniting suggests that the effect is due t o higher valency forms. It v a s observed that the residue, especiallja t the 500-microgram level, appeared greenish, indicating the presence of manganate(I1) ion. Under the acid conditions of the determination manganate(I1) is converted to mangane.e dioxide and permanganate(1) according to the equation : 3Mn04--
100
200
400
300
MICROGRAMS OF YANGANESC PRESENT
Figure 1. Influence of Manganese on Boron Determination
Work a t this laboratory has shonn that manganese can he a serious source of error when boron is determined by the Chromotrope-B method. The interference \+astested as follows: To 0.5-gram samples of ground air-dry plant material low in manganese were added 0, 25, 50, 100, 200, and 500 micrograms of manganese as manganese acetate. The manganese was added in 5 ml. of solution in each case, and taken t o dryness under an infrared lamp, 5 ml. of distilled water being added to the sample receiving no manganese. I n this F a y the manganese(I1) ion was brought into intimate contact with the plant material. After addition of 3 ml. of saturated barium hydroxide the sample &'as taken t o dryness once more, and the residue was ignited a t 600" C. for 2 hours and taken up in 5 ml. of 70% acetic acid as in Austin and BlcHargue's method. The results are shown in Figui e 1.
Table I.
On the addition of 70% acetic acid the green residue became pink. It is suggested that the error is due to the oxidation of the Chromotrope-B reagent by the permanganate(1) ion. The w i t e r attempted to eliminate interference from permanganate(1) by a further ignition a t 300" C. after the ignited residue had been taken up in i O % acetic acid, and, by the action of hydrogen peroxide, excess peroxide being removed. I n each method interference was reduced but not completely removed. The first method gave marked improvement, but 1% as not entirely satisfactory. LITERATURE CITED
(1) dustin, C. ists.
&I., a n d McHargue, J. S.,J .
Assoc. O ~ c d. g r . C h e m -
31, 427-31 (1945).
RECEIVED March 8, 1981.
Microdetection of Carbon LEONARD P. PEPKOWITZ Knolls Atomic Power Laboratory, General Electric Co., Schenectady, N . Y .
Effect of Rlanganese on Boron Recoiery
l l n Added,
Mn Piesent,
Y
Y
Boron Determined Mean Value. P.P.hI a
6 31 56 106 206 506 506
37 34 32 23 10 0 35
Ob
25
50
100 200 500 500 (added
after ignition)
Boron Recovered,
%
~~
~
The extent of the interference is such that for this plant material 340 micrograms of manganese (680 p.p.m. on t h e plant material) are sufficient to depress to zero the boron estimated, Extrapolation of the curve to 340 micrograms seems justified since, as shown in Table I, 500 micrograms definitely reduce the boron determined to zero. For the purpose of calculation recovery of boron when no manganese was added has been taken a8 loo%, since the manga1
+ 4H+ = MnOz + 211110~-+ 2H20
Present address, Rutgers University, S e w Brunswick, S. J.
H E problem of detecting carbon contamination in solids and Tfilms is ever present in the analytical laboratory. None of the recent texts (1, 3-6) refers t o a microdetection method for combined and free carbon, although a very sensitive procedure was described by Emich ( 2 ) . This report presents a convenient and simplified modification of the Emich method that has found wide application in this laboratory because of the diversity of materials that can be handled, either organic or inorganic, such as graphitic carbon, iron carbides, silicones, etc. Inorganic carbonates present a special problem, because they are not decomposed at the temperatures that can be attained in borosilicate glass tubing. Quartz T-tubes have been used with the described procedure for detecting carbon in carbonates, but a modification of the method is described below for handling carbonates utilizing borosilicate glass tubing. The method simply depends on the combustion of the sample in an atmosphere of oxygen, absorption of the carbon dioxide formed in barium hydroxide, and identification of the carbon by the formation of the characteristic barium carbonate crystals. The sensitivity of the original method is retained. Emich ( 2 ) gives the limit of identification as "within &vera1 millionths milligram." I n actual use a few tenths of a microgram of carbon
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