NOVEMBER 15, 1940
AIVALYTICAL EDITION
COLOR DEVELOPMEKT. The relative quantities of molybdate and acid present must be maintained within close limits (6, 6, 8, 9). At low acidities, color develops in the blanks, largely owing to silica. Too great acidity markedly decreases the intensity of the color. The optimum ratio is about 5.0 cc. of 1 N hydrochloric acid to 2.0 cc. of 0.5 per cent ammonium molybdate a t a volume of 20 cc. Ammonium molybdate was found definitely superior to the sodium molybdate, since the former allowed a somewhat wider range of acidities without change of color intensities. It is also important to develop the color a t the correct volume in order to maintain the acidity in the proper range. I n harmony with other investigators, the authors feel that hydrazine sulfate is the most suitable reducing agent for developing the color (6, 7 ) . More hydrazine sulfate than that previously recommended is used in order to reduce the excess potassium iodate used in oxidation of the arsenic. Ten minutes’ heating in a boiling water bath is sufficient to develop the color, which is then stable for several hours. A blank should be run on distillate from sulfuric acid alone, and should correspond to less than 0.5 microgram of arsenic. Either a 660-millimicron or 725-millimicron filter may be used for reading the color, though the latter is preferable. The very slight yellow color due to excess hydrazine sulfate does not interfere a t these wave lengths. Between 1 and 100 micrograms the color intensity follows
693
Beer’s law within 2 per cent. A 2-em cell is used up to 50 micrograms and the 1-em. cell for quantities of 50 to 100 micrograms. IKTERFERISG SUBSTANCES.Antimony, germanium, and selenium may accompany arsenic Tvhen distilled with hydrochloric acid, but do not give color with molybdenum reagents under the conditions described. Phosphate, which reacts like arsenate m-ith the molybdenum reagent, is nonvolatile and is thus prevented from interfering. Kitrate and perchlorate interfere with color development and should be removed prior to distillation.
Literature Cited Assoc. Official Agr. Chem., “Official and Tentative Methods of Analysis”, p. 372, 1935. Chaney, A. L., ISD. EXG.CHEM.,Anal. Ed., 12, 179 (1940). Deemer, R. B., and Schricker, J. A,, J . Assoc. Oficial Agr. Chcm., 16, 226 (1933). DenigBs, G., Compt. rend.. 171, 802 (1920). Kuttner, T., and Cohen, H. R., J . B i d Chem., 75, 517 (1927). Maechling, E. H., and Flinn, F. B., J . Lab. Clin. Med., 15, 779 (19301. Morris, ’H. J., and Calvery, H. O., IND.ENQ.CHEM.,Anal. Ed.. 9, 447 (1937). Truog, E., and Meyer, A. H., Ihid., 1, 136 (1929). Zineadze, C., Ihid., 7, 230 (1935). THIS work was done b y H. J. Magnuson on a research fellowship of t h e American College of Physicians.
Removal of Static Charges from Glassware by Ultraviolet Light CLERIENT J. RODDEN, National Bureau of Standards, Washington, D. C.
THE
and distance of the ultraviolet light source are given in Table I. The vessel was 15-iped with a dry chamois and was considered to have reached constant weight when the variation was less than 0.05 mg. using a macrobalance. Weighings were made usually after exposure to the light a t 5-minute intervals. I n some instances this was reduced to 2-minute intervals For general practice it is recommended that the wiped article be placed about 60 cm. (2 feet) from either a Hanovia Alpine sun lamp or General Electric Lab-Arc, for 10 minutes.
static charge acquired by glassn-are when wiped may cause a serious error in weighing the vessel. In the winter months, when the moisture content in the atmosphere is low, this difficulty is more apt to be encountered. In the Pregl (1) method for the microchemical determination of carbon and hydrogen the absorption tubes are wiped with a chamois. This causes a static charge which does not leak off soon enough, when the humidity is low, to maintain the time schedule (usually 10 to 15 minutes) between completing a combustion and weighing the absorption tubes. Van Straten and Ehret ( 2 ) state that the charge can be removed by the high-frequency discharge from a device commonly used to detect pinholes in evacuated apparatus. The author has found that if ultraviolet light is used to ionize the air surrounding a glass vessel, the charge can readily be dissipated. Data on the effect of time of exposure
Literature Cited (1) Pregl, F., “Quantitative Organic hdicroanalysis”, 2nd ed., p. 46, Philadelphia, P. Blakiston’s Son & Co., 1930. (2) Van Straten, F. W., and Ehret, W. F., IND. ENQ.CHEM.,Anal. Ed., 9, 443 (1937).
TABLE I. EFFECT OF TIMEOF EXPOSURE AND DISTANCE OF ULTRAVIOLET LIGHTSOURCE Vessel
Relative Humidity
% Pyrex tube, 10 mm. X 11 cm. 150-ml. Erlenmeyer flask (Pyrex)
Temperature O
c.
40
75
29 28 2s 34 23
70 70 76 73 73 76 80
23
SO
29 38
Increase in Weight Caused by Wiping Mg.
20 5
Source of Ultraviolet Light
Hanovia Alpine sun lamp, 110 volts, alternating current
18
General Electric Lab-Arc, IlOvolts, direct current
10
G-5 in special envelope, General Electric
Distance from Source Feet 2
Time Required for Discharge Min. 10
1 3 2
10 10
5
8 10
3 1
10 10
Still changing in wgight after 20 min.
