SEPTEMBER 15, 1935
ANALYTICAL EDITION
consists largely of the resin and identification is more definite. As the number of resins used commercially is very limited, identification is usually simple. Determine ammonia on an original sample by distilling with sodium hydroxide after addition of calcium chloride to reduce foaming. The difference between the nitrogen due to the ammonia and the total nitrogen obtained by a Kjeldahl deterrnination gives the nitrogen due to triethanolamine. Obtain the borax on an original sample, preferably by the Ross-Deemer method. A quicker, though possibly less accurate method, is to titrate the carbonate and borax together on an ash of the original sample. Add glycerol and titrate the free boric acid with caustic soda. It is essential to boil off the liberated carbon dioxide after the total alkali titration. If alkali soaps are absent, the ash consists almost entirely of any borax present. Titrate directly with acid. There is always some extraneous ash from impurities in the commercial ingredients used, so that the weight of ash cannot be assumed to represent the borax content.
319
The methods given above have developed over a period of years and include suggestions from various staff members of Foster D. Snell, Inc., in particular from Cyril S. Kimball and Harry J. Hosking.
Literature Cited (1) (2) (3) (4)
Biffen, F. M., Chemist-Analyst, 22, 17 (1933). Ibid., 20, 8 (1931). Grimshaw, A. H., Textile World, 79, 1212-14, 7245 (1931). Jacobs, M. B., and Jaffe, Leon, IXD. ENG.CHEM.,Anal. Ed., 3, 210-12 (1931).
( 5 ) Katrakis, C. G., and Megaloikonornos, J. G., Praktika Akad. Athenon, 5, 267-9, 311-14 (1930). (6) Leaper, J. M., Textile Colorist, 5 5 , 601-2 (1933). REWIVEDMarch 12, 1935. Presented before the Division of Colloid Chemistry at the 89th Meeting of the American Chemical Society, New York, N. Y., April 22 to 2 6 , 1935.
An Electronic Bridge Balance Indicator for Conductance Measurements R.
E
,. GAR3
4N AND G. F. KINNEY, Washington Square College, New Yorlc University, New York, N. Y.
LECTRON tube devices employed as alternating current bridge balance indicators have been described by Tulauskas (8), McNamara (8), and others (1). Tulauskas’ device contains seven tubes and though McNamara’s uses but two tubes, it has the disadvantage that the balance point is not indicated directly. A single tube may be used to indicate the balance point of a n alternating current bridge, as shown in Figure 1. The Type 75 tube functions first as an amplifier, then rectifies the alternating component of the plate current, and finally indicates on the meter in the plate circuit lead the magnitude of the impressed alternating voltage.
small steady or “null-point” input potential may be balanced out by reducing the negative bias on the grid, the latter being accomplished by adjustment of R4, which has the effect of reducing the potential on the grid and diode simultaneously and in the same ratio.
This arrangement has an advantage in addition to its comparative simplicity, in that the meter reads maximum at zero impressed potential and the readings decrease as the potential is increased. Reasonable overloads normally encountered in bridge methods are therefore not harmful. The sensitivity of this device is in the order of microvolts with a primary impedance of 100 ohms, which is probably comparable t o that of untuned earphones. Since earphone This arrangement is particularly efficient because the voltage ratings are not generally given because the sensitivity varies from the bridge is first stepped up with transformer T I ,then apwith the observer and the noise level in the laboratory, close plied t o the grid of the tube, amplified, and again stepped up with comparison is not possible. Under ordinary laboratory contransformer Tz. This amplified in ut voltage is now applied between diode and cathode through t\e high resistance, Rs. The ditions-that is, without sound-proof rooms-this singlerectified pulsating current flowing across Rz produces a negative tube device is more sensitive than earphones without a voltage which is converted t o a direct current voltage by conpreamplifier and convenience of use is considerably greater. denser C1 and resistances RI and RP. This direct current voltFigure 2 is a p l i t of bridge age is then applied to the grid and its magnitude indis e t t i n g against electronic cated on the direct current i n d i c a t o r readings. The I , r 75 0-1 milliammeter in the plate balance point can easily be circuit. Thus the tube funcl o c a t e d with an accuracy tions simultaneously both as an alternating and d i r e c t of 2 parts in 100,000, which current amplifier. corresponds to a resistance This arrangement is diffimeasurement with a precult t o design, as degeneracision of *0.01 per cent and t i v e o r regenerative effects h a v e t o b e avoided. In m a y b e determined withp r a c t i c e it is best to use o u t g r a p h i n g the results. transformer Tz so that the W i t h ordinary earphones voltage on the diode will be 7 under u s u a l l a b o r a t o r y in phase with the alternating current voltage on the c o n d i t i o n s t h e apparent grid. The network RQ-R4-R6 250 dead silent region extends is used to bias the diode as over practically the entire well as the triode and the FIGURE1. DIAGRAM region shown in this plot. values are chosen to bias the RI. 1 megohm volume control TP. Interstage transformer diode suitably, in order that Since t h e r e s p o n s e of Rn Rs 1 megohm 1 watt Cn 0 01 mf‘d aper condenser Rz: Ra: 10,000 oh& 1 watt Si.’ 6: P. D. T.aaxIey jackswitch it may be operated a t the t h e h u m a n e a r is logaR4. 75,000-ohm 4olume control Sa. S. P. S. T. switch on R4 point of maximum curvature Re. 6000-ohm volume control K . Kohlrausch slide wire rithmic, precision measureof the diode current-diode R7. 100-ohm potentiometer M. 0-1 milliammeter m e n t w i t h the earphones Ti. Input transformer volts curve, thus insuring e f f i c i e n t rectification. A r e q u i r e s a zero signal at
p -
-+
INDUSTRIAL AND ENGINEERING CHEMISTRY
320
the balance point. This limitation does not apply to the electronic indicator, as residuals of a low order of magnitude may be compensated for by adjusting Rd, without any sacrifice in sensitivity. It must be remembered, however, that a faint note in the earphones corresponds to almost full scale on the indicator. In any alternating current bridge earthing is important and too often neglected with resulting “head effect” and a minimum sound rather than silence a t the balance point. Resistors Rs and R7 are the earthing network, a Wagner ground RESISTANCE UNDER MEASUREHENT 15,000
OHUS
FIGURE 2. PLOTOF BRIDGESETTINGAGAINST ELECTRONIC INDICATOR RE.4DINGS L. and N. Type K Kohlrausch slide wire with end coils.
(9) as used by Jones (6) and others (7). The single-pole double-throw switch offers a convenient way of adjusting the Wagner ground for each particular measurement. If the output transformer of the oscillator is not electrostatically symmetrical, precision measurements require the use of a variable condenser or inductor from ground to one of the output posts of the oscillator. Condenser C1 is necessary to compensate for the capacity effect of the conductance cell (6). Its importance is generally overestimated, for with a well-arranged bridge, proper treatment of the conductance cell, and the Wagner ground it can sometimes be omitted when only ordinary precision is required. When this is true, the effect of small phase angle shift can be compensated for by adjustment of resistor Rd. The electronic indicator is connected to the bridge network through an impedance-matching transformer (2, 3, 4). While this transformer is necessary, its exact function is not widely understood. In this case its functions are, first, to isolate bridge and electronic indicator so that each may be properly grounded; second, to eliminate any possible electrostatic pick-up with the electrostatic shield of the transformer; and third, to provide a maximum transfer of energy from the bridge to the detector in order that the full sensitivity of the detector may be realized. For maximum sensitivity the primary of this transformer should match the impedance of the bridge and the secondary should match the input impedance of the vacuum tube. The impedance of the bridge is, of course, variable and changes with the resistance under measurement. In practice, two arms of the bridge are usually a Kohlrausch slide wire, the resistance of which is comparatively low. As long as this is true, the slide wire becomes the deciding factor, so that to a first approximation the impedance of the primary should approximately equal the resistance of the slide wire and should be connected as shown in Figure 1. When the transformer is to be used with various slide wires, or with one slide wire with and without end coils, it should have a different primary winding for each, all electrostatically shielded from each other and from the secondary. The indicator n a y also be connected to the opposite side of the bridge by
VOL. 7, NO. 5
interchanging the oscillator and indicator. The Wagner ground must also be changed and the value of Re reduced to match the new bridge-arm ratio. Impedance-matching transformer TI should then be properly designed to match the bridge with this connection. In general, the bridge balance indicator may be used in any suitably arranged bridge as a substitute for the conventional amplifier and head phones. Several of these indicators have been in use in the authors’ laboratory for the past year and the balance point has been compared repeatedly with that obtained with a preamplifier and earphones. With the precautions outlined the authors have never observed a discrepancy between the two balance points exceeding the experimental error. With this electronic indicator measurements are not restricted to any one frequency. The response a t any particular frequency depends largely upon the characteristics of the transformers used. Satisfactory measurements have been made as low as 10 cycles per second. The upper limit is about 10,000 cycles, but, of course, for precise work in this region the bridge must be designed for use a t these frequencies. The sensitivity of tlie device may be increased further by the addition of a high-gain tube as a preamplifier. Preamplifiers used in conjunction with earphones are usually tuned in order to suppress harmonics produced by the oscillator and to minimize the effects of tube noise. The noise level of tubes becomes the limiting factor for amplifiers and may easily be reached by two stages of high-gain amplification feeding a pair of phones. Since the noise level of tubes in a properly constructed amplifier is fairly constant and since it is possible to compensate for low alternating current inputs by adjusting Rd, preliminary experiments indicate that the lowest potential which may be detected with this arrangement is somewhat smaller than with the aural method. Experiments on further details of this arrangement and others are in progress in the authors’ laboratory.
Summary A single tube may be used as a bridge balance indicator which in a suitable arrangement has a sensitivity equal to that of the best headphones. I n using this electronic indicator to its best advantage, important factors are shielded impedance-matching transformers and proper earthing arrangement such as a Wagner ground. Literature Cited (I) Britton, “Conductometric Analysis,” London, Chapman and Hall, 1934. (2) Chaperon, Compt.rend., 108,799(1889). (3) Hague, “A. C. Bridge Methods,” London, Sir Isaac P i t m a n & Sons, 1932. (4) Jones and Bollinger, J . Am. Chem. Soc., 51,2407 (1929). (5) Jones a n d Josephs,Ibid., 50,1049(1928). (6) McNamara, Rev. Sci. Instruments, 2,343(1931). (7) Shedlovsky, J . Am. Chem. Soc., 52,1793(1930). (8) Tulauskas, Electronics, 2,478 (1931). (9) Wagner, EZektrotech.Z., 32,1001 (1911),33,635 (1912).
RBCEIVED June 8, 1936. Presented before the Division of Physical and Inorganic Chemistry, Symposium on Recent Advances in Microchemical Analyeis, under the title “Use of Multipurpose Radio Tubes in Analytical Chemistry,” at the 89th Meeting of the American Chemical Society, New York, N. Y., April 22 t o 26,1935.
CORRECTION. In the article on “Colorimetric Methods for the Determination of Phosphorus” [IND. ENG.CHEW.,Anal. Ed., 7, 228 (1935)]in paragraph 4 under the heading “Description of Methods,” the second sentence should read “Exact quantity of MOOSin it should be known.” CH. ZINZADZE
