480
ANALYTICAL EDITION
The scrubbing is done by the bead column wetted with the scrubbing solution which is umped over the beads in closed cycle. By the arrangement oAhe tubing in the apparatus the as sample is caused to do the pumping. The gas enters througi a 6-mm. tube bent in the form of a trap, in which there is a carefully made orifice (0.5 mm. in diameter) situated below the liquid level. Through this orifice the scrubbing solution seeps into the trap, is quickly driven to the top where it is sprayed upon the bead column, and slowly finds its way down to the reservoir whence it came. The bulb may be sealed at the top or closed by a stopper of glass, rubber, etc. For gravimetric work, glass wool may be used instead of beads. The bulb can most readily be washed with a solution from a gravity reservoir fitted with a rubber tube and pinchclamp. After the solution has been let in, and before disconnecting, the tube is pinched between the bulb and the clamp to collapse it. This will prevent an annoying drip from the tube upon disconnecting. The bulb could doubtless be used for extremely low rates of gas flow by reducing the internal diameter of the 6-mm. inlet tube. For any given assembly, the height of the scrubbing solution above the orifice modifies within narrow limits the rate of pumping the solution. In most cases the rate should be such as to spray the solution over the beads, since flowing results in flooding which in turn causes loss of effective surface and efficiency.
Vol. 6 , No. 6
Gas passed through this bulb at a rate of 1.0 to 1.5 cubic feet per hour (approximately 30 to 45 liters per hour) develops a back-pressure equivalent to less than 0.5 inch (13 mm.) of water. It can be designed to use less than 5 ml. of scrubbing solution. The scrubbing efficiency and capacity have proved t o be entirely satisfactory. In two tests made upon the absorption efficiency of these bulbs, the rate of gas flow was between 1and 2 cubic feet per hour. In one case a bulb charged with 20 per cent sulfuric acid was subjected to a gas flow carrying a fairly high concentration of ammonia (similar to raw coke-oven gas). On the outlet of the bulb a trap was placed containing 0.1 ml. of 0.1 N sulfuric acid, in a little water colored with methyl orange. After 0.5 hour when the test was stopped, the solution in the tra had not changed color. Upon titrating the solution in the fulb it was found that the acid had become so nearly spent that only 0.2 ml. of 1 N sodium hydroxide was required to neutralize it. The second test was a series of determinations of nitrogen tetroxide in gas. Nitric oxide was added in fractional parts per million to the gas which was then treated as in the Schuftan procedure. Two bulbs charged with m-phenylene diamine solution were placed in the outlet in series arrangement. More than 95 per cent of the nitric oxide was accounted for in the fist scrubber. RE~CE~IVBID August 18, 1934.
A Salt Bridge for Use in Electrometric Measurements GEORGEW. IRVING,JR., AND N. R. SMITH,Bureau of Chemistry and Soils, Washington, D. C.
I
N MAKING a large number of routine p H determinations
on soils and other materials by means of the quinhydrone electrode, the faults of the commonly used agar bridge became so troublesome that the authors searched for a more suitable bridge. The bridge finally adopted has proved very satisfactory for their work and is offered with the hope that it may prove useful to others who employ electrometric methods. The bridge shown in the figure is a modification of that described by LaMer and Raker (4, 6),and is based upon ideas presented by Crowther (I), Cu r (2), Kohn (S), a n i S c h o l l e n b e r g e r (6). It is made of heavy-walled Pyrex glass tubing of about 1cm. diameter. The lugs are of Pyrex &ass rod and are ground into the tap e r e d e n d s of t h e bridge. The bulb is about 4 cm. in diameter and has a capacity of about 25 cc. For use the bridge is tilled to the groundg l a s s stopper w i t h saturated potassium chloride solution; the plugs are loosened momentarily t o insure a film of potas9 cm.sium chloride in the 1 ground-glass j o i n t s, and then are seated firmly. The tips are washed thoroughly with distilled water and dried with filt,erpaper to avoid dilution of the reference half-cell. Between determinations the washing and drying of the tips are re eated but it is not necessary to renew the potassium chloride in t6e ground-glass joints.
fik
LaMer and Baker (4)have presented proof that the groundglass form of junction gives a strictly reproducible potential and effectively prevents interdiffusion of saturated potassium chloride and quinone substances. The authors have compared the ground-glass with the agar bridge and have found them t o give identical results in buffer solutions, soils, and culture media. The new type of bridge has been used successfully and conveniently in several thousand pH determinations. Several advantages are apparent in this form of bridge: Preparation of the bridge for the day’s work involves only filling with saturated potassium chloride solution; one filling of the bridge is sufficient for a n unlimited number of determinations; between samples the bridge is easily and thoroughly cleaned by a stream of distilled water from a wash bottle and easily dried with a small piece of filter paper; and no special arrangements are necessary for keeping the bridge when i t is not in use, the potassium chloride being simply removed and the bridge filled with distilled water. T h e only caution to be observed in the use of this bridge is to avoid allowing the tips to d r y out, else they may stick. If this happens, the joints may be loosened b y soaking in warm water and by tapping gently on a solid surface. After loosening, they should always be well seated before proceeding; once seated, they are not easily loosened even if struck accidently against any part of the apparatus. While a determination is being made, the weight of the bridge is supported at the shoulders so that the tips are held several millimeters from t h e bottom of t h e vessel. In this position, the contents of the electrode vessel may be stirred without dislodging the plugs.
LITERATURE CITED (1) (2) (3) (4) (5) (6)
Crowther, J . Znst. Brewing, 33, 459-63 (1927). Cupr, Pub. facult4 sci. univ. Masaryk, No. 133, 1-50 (1931). Kohn, 2.angew. chem., 39, 1073-4 (1926). LaMer and Baker, J . A m . Chem. Soc., 44, 1954-64 (1922). LaMer and Parsons, 1.B i d . Chem., 57, 613-31 (1923). Schollenberger, IND.ENG.CHEM.,17, 649 (1925).
RE~CE~IVH~D July 27, 1934.
