T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
Aug., 1921
713
Modified Laboratory Apparatus' By C. C. Kiplinger Mr. UNION COLLEGE,ALLIANCE, OHIO AN IMPROVED BUBET AND HOLDER
TELEPHONE RECEIVERS FOB ELECTBOMETRIC WORK
The common form of buret and buret holder possess three objectionable features, namely, masking of a part of the scale by the holder, difficulty of obtaining vertical adjustments, and last, the trouble and time required to manipulate the support. An ordinary buret with a thick-walled, funnel.shaped top, 4 cm. in diameter, hung in a slotted wood funnel support mounted on a tall stand, is free from the above disadvantages and need not cost more than the usual type. Each opening of the wood funnel support has a slotted rubber crucible holder of the Bailey type cemented on it, so a s to reduce to a minimum the danger of breaking the funnel tops. The slots must be wide enough to permit the insertion or removal of the burets without binding. With this device, the scale is always unobstructed over its entire length. Gravitation holds the tubes vertical a d they may be instantly removed from the supports without fussing with springs or thumbscrews. The funnel tops obviously permit filling from stock bottles without the aid of auxiliary apparatus. Owing to the method of support, the tip may be moved over a certain portion of the table without disturbing the apparatus. Because of the above advantages, this instrument insures greater ease of manipulation and increased speed in titrations.
Two thousand ohm telephone receivers of the head-set type, such as are used for radio work, have been found of value in determining the null point in electrometric titrations, the writer preferring them to a galvanometer or a capillary electrometer in that they are dead beat and require no scale readings. This advantage permits the operator to confine his attention to the potentiometer and buret readings, thus decreasing the chance of error in the titration.
A MODIFIED CELL FOB T E E ULTRAMICROSCOPE
An ultramicroscope cell of simple construction, variable capacity and diasectible in type may be easily made of materials found in every laboratory. Two glass squares, 4 cm. on an edge, are cut from a good quality of plate glass 4 mm. thick. To the edge of one plate is cemented a strip of thin glass or clear mica 4 cm. long and 8 mm. wide, so that the edge of the strip extends 4 mm. above the surface of the plate. A similar strip 12 mm. wide is cemented to the top of the second plate so that i t projects 4 mm. over the edge. Hence,.when the latter plate is placed on the former and the two strips made contiguous, a long cell, 4 mm. by 4 mm. by 4 cm., and open at both ends, is formed, as shown in A and B in the figure. A spring clamp holds the two d a t e s in Dosition. The Eel1 shouli be supported on a wire or cork stand which raises it above the stage so that the condensing objective of the ultramicroscope may be brought t o the proper distance from the edge of the cell. The solution under examination is held in the cell by capillary attraction. The fl width of the cell can be changed to accommodate different quantities of liquid (C in the figure). Owing to its CELLFOR ULTRAMICROSCOPE small size and the large ratio of length to width, convection currents are inappreciable. This cell is more easily constructed and cleaned than several others which the writer has used, and the fields obtained are excellent.
'B
TELEPHONES FOR ELECTROMBTRIC WORK
The arrangement is as shown in the accompanying figure. The potentiometer P is arranged according to W. S. Hendrixson,' and the telephones (Murdock, No. 55, costing about five dollars are amply sensitive) are introduced into the circuit in place of a galvanometer, in series with a device for opening and closing the circuit, a telegraph key sufficing. The circuit is always kept open except when testing for the null point, which is indicated by the absence of sound in the receivers on tapping the key, hence the discharging of the electrode is reduced to a minimum. The high resistance of the receivers is especially favorable for hydrogen electrode use. The following checks indicate the sensitivity of the method, Ten cc. of approximateIy 0.1 N Fe(NH4S04)2.6H,0were titrated against 0.1 N K2Cr207.
i
c:
1
Received April 11, 1921.
TEST 1 2 3 4
KzCnOi cc
.
10.00 9.90 9.90 9.84
A small fixed condenser, C , shunted around the key and receivers increases the sensibility of the device. The operator should assure himself that the key generates no local or contact currents on closing the circuit, which is done by removing the key from the circuit, then connecting it with the head set, which should yield no click on opening and closing the key. The high sensitivity and permanent adjustment of the receivers, combined with the ease of manipulation and low cost of installation, should commend them to the workers in this field. For very precise and delicate determinations the writer is installing vacuum tube amplifiers, which will greatly increase the cost of the apparatus, but which are unnecessary for ordinary work. 1
J . Am. Chcm. SOC.,43 (1920), 14
