NOTES ON ANALYTICAL PROCEDURES A GILBERT A S H B U R N
Specific Spot Test for Vanadium AND
J. H. REEDY,
Noyer Laboratory, University of Illinois, Urbana, 111.
I
N THE first issue of the famous Berichte der deutschen chemischm Gescllschnft, Rammelsberg ( 1 ) reportled that the yellow color of slightly acid solutions of vanadates is intensified by the presence of tungstates. More recently Willard and Young (3) have revived this reaction in the colorimetric determination of vanadium in steel. Other workers have modified the method by including hydrogen peroxide as one of the reagents. This modification limits the general applicability of the reaction, since several elements-e.g., chromium, titanium, etc.-interfere by giving strong colors. The original reaction, in the absence of peroxides, is now made the basis of a specific and sensitive test for vanadium.
PROCEDURE. A drop of the solution to be tested is mixed on a spot plate with a drop of 85% phosphoric acid, and the mixture is allowed to stand for a few seconds to allow the acid to form complexes with any interfering cations. A drop of 10% sodium tungstate soliltion is then added. The appearance of a yellowto-orange color indicates the presence of vanadium. The vanadium must be in the form of a vanadate t o respond t o this test. Vanadous compounds may be oxidized to vanadates by the addition of bromine and warming. Reducing agents interfere, and must be removed by oxidation with bromine or other suitable reagent. Strong acids also interfere, and for this reason phosphoric or acetic acid should be used in preparing the solution. The principle of the test is not certain. At first it was thought that. the product might be a tungstovanadic acid. Since phos-
phoric acid changes the color from yellow or orange and makes it more permanent, it seems that it should be included as one of the reagents. However, with traces of vanadates-e.g., 4 micrograms per drop-comparable tests were obtained by using dilute sulfuric or hydrochloric acid, instead of phosphoric acid. With larger amounts of vanadium, the color is much darker if phosphoric acid is used and strong acids are absent. Probably heteropoly acids ( 2 ) of very labile composition are formed. Besides intensifying the color, phosphoric acid is important in converting 2ertain ions into colorless solible materials that might otherwise interfere by forming colors or precipitateae.g., Fe+++,etc. The test, in the hands of an experienced SENSITIVENESS. analyst, will detect as little as 4 micrograms of vanadium in a drop, or 0.08 mg. per ml. of solution. It will detect 8 micrograms of vanadium in 1000 times that amount of Ag+, Pb++, Hgl++, Hg++, Bi+++, Cd++, Asoh---, AsOa---, Sn++++, AI+++, Fe+++, Mn++, MOO,--, Tl+++,or UOt++. The anions C1-, Br-, SO4--, NOa-, CHICOO-, and ClOa- do not interfere. Colored ions reduce the sensitiveness of the test. By use of comparative tests, it will detect vanadium in 500 times its weight of Co++ or Cu++, and 10 times its weight of Cr+++ or
w0,--.
LITERATURE CITED
(1) Rammelsberg,Be?., 1 , 161 (1868). (2) Rosenheim, Ann., 251, 197 (1889). (3) Willard and Young, IND.ENG.CHEM.,20, 768 (1928).
Quantitative Isolation of Hemicelluloses from Coniferous Woods Preliminary Communication L O U I S E. WISE, Institute of Paper Chemistry, Appleton, Wir. hydroxide in an atmosphere of nitrogen. I t was found expedient in the present study to begin with 4 to 5% and to end with 24% potassium hydroxide. The first of these extractions was carried out with 500 ml. of 4.6% potassium hydroxide in a wide-mouthed Erlenmeyer flmk carrying an outlet tube provided with a stopcock. The rubber stopper sealing the flask carried a dropping funnel and an inlet tube (reaching nearly to the bottom of the flask), also provided with a stopcock. The crude holocellulose was transferred quantitatively to the flask and dry nitrogen was passed through for 20 to 30 minutes, following which the aqueous potassium hydroxide (preheated to 85" C.) was added gradually without interrupting the gentle nitrogen strp2m, which was maintained for about 15 minutes longer. All stopcocks were then closed, and the flask was shaken a t intervals during the 24-hour extraction period. The final extraction was carried out similarly but with about 170 ml. of the cold, 24% potassium hydroxide. Each hemicellulose fraction was recovered by acidification with acetic acid and precipitation with alcohol and, when iiecessary, was freed from residual lignin by a slight modification of Anderson's bromination method ( 1 ) . The bromine was added dropwise to the cold acetic acid solution (or suspension) of the hemicellulose until, on shaking, a slight excess of bromine persisted. The time of contact between the bromine and the hemicellulose mixture was not permitted to exceed 20 to 25 minutes, a t the end of which the bromine was destroyed by the addition of a n excess of 95% ethanol which also served to precipitate the hemicellulose fraciioi) ; this was separated by centrifuging, and thoroughly washed with alcohol and with ether (and finally filtered on a 1G3 Jena fritted-glam crucible).
THE
ifficulty of isolating hemicelluloses quantitatively from coniferous d' woods has long been recognized. Recently, however, Jayme ( 2 ) has developed a technique for holocellulose isolation which appears to be carried out more readily than is the procedure of Van Beckum and Ritter (3). I t depends upon the partial delignification of wood by a n aqueous acetic acid solution of sodium chlorite, the chlorine dioxide from which removes a large part of .the lignin without seriously affecting the cellulose or hemicelluloses. The fibrous residue after such treatment is essentially an impure holocellulose, from which the lignin and lignin degradation products may be removed almost quantitatively in the course of subsequent operations. This "holocellulose" serves as a suitable starting material for the quantitative isolation of hemicellulose fractions and for their further study. Ten-gram samples of air-dried unextracted wood (prepared by passing through a Wiley mill) were heated to 60" C. with a solution containing 500 ml. of water, 50 nil. of acetic acid, and 50 grams of sodium chlorite. After the initial heatinz, the delignification was allowed to proceed a t about 30" C. for 24 hours, the mixture being stirred a t intervals. At the end of this period the nearly white residue, which retained its woody structure, was filtered by suction and washed with ice water. The hemicelluloses were removed (in any desirable number of fractions) by successive extractions with aqueous potassium
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