ANALYTICAL CHEMISTRY
1008 _ _.
Table 11. Sample So. 1 2 3
p-Xylene Known Exptl. 2 5 . 0 23.7 4 0 . 0 39.2 55.0 53.4
Error -1.3 -0.8 -1.6
Analytical Results of Synthetic Mixtures m-Xylene Known Exptl. 25.0 25.4 20.0 19.2 15.0 13.4
4 5 6 7 8 9 10 11 12 13
20.0 45.0 47.5 60.0 35.0 25.0 23.8 25.0 25.0 13.5
19.2 44.2 46.1 58.8 33.6 24.8 22.4 25.4 26.8 12.9
-0.8 -0.8 -1.4 -1.2 -1.4 -0.2 -1.4 +0.4 +1.8 -0.6
30.0 25.0 23.8 25.0 25.0 60.0 33.3 40.0 5.0 0.0
28.0 23.6 24.2 25.2 23.0 59.2 33.9 40.8 5.6 0.6
14 15 16
151.. 01
151 .. 21 0.2
0.0 4-0.2 f0.2
150 .. 50 15.0
140.. 90 14.4
0.0
. -
Error +0.4 -0.8 -1.6 -2.0 -1.4 +0.4 fO.2 -2.0 -0.8 f0.6 f0.8 f0.6 f0.6 - 00. 6 .0
-0.6
o-Xylene Known Exptl. 25.0 24.6 25.0 24.4 25.0 24.8 25.0 27.6 15.0 16.4 19.0 18.8 10.0 10.0 5.0 4.0 1.4 0.0 28.6 29.6 3.5.0 3 6 . 6 40.0 41.6 51.6 05 a5..o0 5 55 6 .. 3 8
;:.O
60.0
60.8
Ethylbenzene Error Known Exptl. Error-0.4 25.0 25.2 +0.2 -0.6 15.0 14.4 -- 00 .. 64 4.6 -0.2 5.0 +2.6 25.0 24.8 -0.2 +1.4 15.0 14.8 -0.2 -0.2 9.5 9.5 0.0 0.0 5.0 4.4 -0.6 -0.8 -1.0 3.5,O 3 4 . 2 +1.4 15.0 l5,4 +0.4 16.1 +1.8 f l . 0 . 14.3 +1.6 0.0 0.0 0.0 +1.2 30.0 3 1 . 2 f1.6 30.2 11.0 -1.4 29.2 0 .. 28 2 2 63 .. 94 f- 1 27 5 .. 7 0 2 -- 10 .. 68 +0.8
25.0
24.2
-0.8
_concentrations of the compounds in an unknown mixture is made with the following absorption coefficients: Wave Length, Microns 12.54 12.98 13.44 14.30
@-Xylene 21.0 0.4 0.2 0.2
m-Xylene
+Xylene
Ethylbenzene
0.3 18.0
0.2 0.5 31.0 0.2
1.0 2.0 5.0 12.0
0.3
1.2
Not more than two approximations are usually necessary to obtain the concentrations of the xylene isomers and ethylbenzene within an accuracy of 1% of the total concentration of the constituents. The use of a spectrocomputer facilitates rapid calculations.
EXPERIMENTAL RESULTS
The analysis of sixteen synthetic mixtures which were prepared from the National Bureau of Standards’ reference samples gave experimental results with a standard deviation of 1.05% for the 64 determinations reported in Table 11. The total concent’rationof 1 ml. of the mixture to 20 ml. of carbon disulfide solvent was held constant as the concentrations of the constituents were varied in this series.
LITERATURE CITED
(1) Barnes, James, and Fulweiler, W. H., J.;lm. Chem. ,SOC.,49, 2034 (1927). 12) Dadieu and Kohlrausch, Monatsh., 52, 220 (1929). (3) Ellis, J. W., Phgs. Rev., 27, 298 (1926). (4) Fenske, M . R., Braun, R. V., Quiggle, D., McCormick, R. H., and Rank, D. H., ANAL.CHEM.,19, 700 (1947). (5) National Technical Laboratories, South Pasadena, Calif., Beckman Bull. 153. (6) Oronite Chemical Co., San Francisco, Calif., “Xylene Technical Review,” 1947.
.-,
RECEIVED dpril 14, 1948.
Instrument for Automatic Continuous Titration PHTLIP A. SHAFFER, JR.’, ANTHONY BRIGLIO, JR., AND JOHN A. BROCKMAN, JR. California Institute of Technology, Pasadena, Calif. There is described an automatic continuous titrating instrument, originally developed for the determination of mustard gas in air. The unknown gas sample is continuously aspirated through a titration cell in which it is absorbed in solution. Titration is effected by electrolytic generation of the titrating agent in the cell; the electrolysis is so con-
D
URIXG the war the need arose for instruments capable of
measuring and recording, automatically and continuously if possible, the concentrations of toxic gases obtaining in chemical warfare. Because of the importance of such instruments in field trials, the development of the instrument herein described was undertaken. 4 complete description of the instrument is given in a wartime report ( I ) ; therefore sufficient information is not given here t o enable the construction of an instrument. The aim of this paper is to describe the novel aspects of the instrument and to discuss the principles of its operation, to the end that these principles may be applied further to chemical problems. In earlier types of titrating instruments ( I S ) the titration takes place a t a constant rate for a measured period following the absorption of the sample from the air. In contrast to this sequential operation, the device described here achieves continuous titration by the maintenance of an end-point condition by introducing the titrating agent continuously a t a rate which closely approximates, stoichiometrically, the rate of absorption of the unknown gas sample. The general relations between the several elements in the con1 Present address, E. S. Naval Ordnance Test Station, Pasadena, Calif.
trolled that a ver3 small excess of the titrating agent is maintained in the cell. The control is achieved by means of negative feedback, characteristic of servomechanism applications; the feedback loop involves chemical reaction. The dynamic behavior , is analyzed with the aid of equivalent electrical circuits.
tinuous titrating system are shown schematically in Figure 1. The diagram makes evident the feedback loop (6), by means of which the rate of addition of titrating agent is made to follow very closely the rate of absorption of the sample. The direct current amplifier of Figure 1 may be replaced by other equivalent devices; the authors’ first design made use of a relay-controlled reagent pump. Upon surveying the known quantitative methods for the determination of mustard gas, the authors decided that the bromine titration method utilizing a potentiometric end point described by Northrop ( 7 ) was perhaps the best suited for use in an automatic instrument. While work on a mechanical instrument making use of a titrating reagent pump ( 2 ) was under way, it occurred to them that the titrating agent might be generated electrolytically (12). Automatic equilibration of the rates of introduction of the two reactants into the reaction cell could be achieved then by using an electronic power amplifier driven by the end-point potential and delivering current to the generating electrodes. Because of the many important advantages which this new basis of operation holds over the mechanical one, attention xas turned from the titrating reagent pump to the electrolytic type of instrument. Detailed results of the developmental work have been reported ( 8 ) .
V O L U M E 20, NO. 11, N O V E M B E R 1 9 4 8 SAMPLE ADDED CONTINUOUSLY OR I N T E R M I T T E N T LY
TITRATING AGENT ADDED A T A RATE SUFFICIENT TO MAINTAIN ENDPOINT
m TITRATION
REACT10 N PRODUCTS DISCHARGED
RECORDER, INTEGRATOR O R CONTROL ACTUATOR
,
4 D C AMPLIFIER
AMPLIFIER DELIVERS TITRATING AGENT A T
1009 vessel. In the inner cell are located a platinum observer electrode, which responds to free bromine in the reaction mixture, and the generator anode electrode, a t which bromide ion is oxidized as bromine is needed. In the outer cell is placed a reference observer electrode, which may conveniently be a saturated calomel half-cell, and a platinum generator cathode, at which hydrogen ion is reduced in completion of the generator cell reaction. The circulation of the liquid through the absorbing cell is important in the operation of the titrimeter; it corresponds t o an exponential decay factor applying to each instant in the past, the argument of the factor being proportional to the fractional circulation and to the interval between the instant the sample was taken and the present. The circulation is effective in reducing fluctuations which would other-ise result in instrumental instability.' Xorthrop has reported the high potential sensitivity of a platinum electrode to free bromine in the mustard-bromine reaction (8). In the absence of free bromine the half-cell potential of a platinum electrode in the absorbing solution is relatively insensitive to the concentration of reducing agents in the solution, but the potential of the electrode very rapidly becomes more positive with respect to the solution a i t h increasing free bromine concentration. The sensitivity of the electrode in 1 formal sulfuric acid and 0.05 formal potassium bromide is approximately l o 5 volts liter per mole a t a point on the sigmoid potential curve roughly 50 millivolts above its foot, which corresponds to no free bromine ( 2 ) .
A RATE DETERMINED BY THE CONCENTRATION OF EXCESS REACTANT
Figure 1. Operating Principles of Automatic 'Titrating Instrument
D C AMPLIFIER
RECORDING METER
OPERATING PRINCIPLES
The operating principles of the automatic recording titrimeter as designed foi the determination of gaseous constituents are illustrated iir Figure 2. The titrimeter consists of a pump for drawing an air sample through the instrument continuously, an absorption-titration cell, a vacuum-tube amplifier in which may be included meters, integrators, and recorders for presentation of the analytical results in the desired form, and power supplies for both the sampling pump and the amplifier. Only the cell and the amplifier need be considered in detail in studying the way in which the instrument works. The cell consists of two parts: (1) an inner cell in which the reactive portion of the sample is absorbed in solution and in n-hich the titration takes place, and (2) a larger volume surrounding the first and constituting a reservoir in which absorbing solution is stored and cleansed of the products of past reactions. In this reservoir volume are located those electrodes which are not required to be within the titration cell proper. The two sections of the cell structure are connected by perforations in the bottom of the inner vessel and by a spigotlike opening near the top of the inner cell. As air is drawti into the inner cell through the dkperser near the bottom, the air bubbles rush toward the top of the cell, producing vigorous stirring as they go, and pass out into the top region of the outer cell carrying some of the liquid from the inner cell with them. The bubbles streaming through the inner cell act as an air lift which continuously displaces liquid from the titrating cell. The liquid ejected from the titrating cell by the air stream is replaced by freshly filtered absorbing solution which f l o s~ through the perforations a t the bottom of the inner cell, and a continuous circulation of the liquid results. The filtering is achieved by passage of the solution through a bed of granular charcoal which covers the entrance to the cell and removes excess reactants that may be present in the absorbing solution. The concentration of the absorbing solution may be varied within wide limits without detrimental effect in operation; a solution 3 formal in sulfuric acid and 0.05 formal in potassium bromide is satisfactory for the titration of absorbed mustard gas by bromine. The four electrodes used in the complete cell structure are an observing electrode pair for quantitative potentiometric measurement of concentration in the vicinity of stoichiometric equilibrium, and a generator electrode pair by means of which the titrating agent is produced within the titrating cell. One member of each pair is situated in the inner cell, the others are in the outer
OBSERVING
ELECTRODE GENERATOR
CIRCULATION
Figure 2.
Schematic Diagram of Titration Cell
The observer electrodes are connected to the input terminals of the power amplifier, and the generator electrodes are connected to its output terminals. The relative phases of the input and output signals are such that more current is forced to flow through the output circuit when less free bromine is detected by the observer electrodes. In order to perform satisfactorily in this role, the amplifier must have adequate transconductance, must be immune to changes in the relative average potentials of the output terminals with respect to the input terminals, and must provide isolation between the two pairs of electrodes to prevent polarization of the observing electrode pair by the generating current. Current recording or metering devices are placed in series with the generator electrodes and thus record or indicate the rate a t which bromide is being electrolyzed in the cell
1010
ANALYTICAL CHEMISTRY
The instrument is adjusted so that when no reactant is present in the sampled air stream there is maintained in the eel1 a very small concentration of the titrating agent by equilibration of the loss by circulittion and the generation of bromine a t a very slow rate by the passage of B small current, called the zero current.
Figure 4.
Automatic Titrim,3ter
DESCRIPTION O F INSTRUMENT
The titrating cell, illustrstad in Figure 3, oonsists of a 16-aunce screw-cap glass bottle fitted with B special cap msobined of methyl methacrylate plastic. The arrangement of the electrodes and the geometry of the cell design are approximately those shown in Figure 2. The portable titrimeter unit is shown in Figures 4 and 5. I n Figure 4 the titrimeter is shown without attsehed auxiliary apparatus and with the transparent amplifier panel open. In this form the titrimeter can he used to indicate or integrate the gas concentration while the instrument is hoing carried ahaut. In Figure 5 the titrimeter is connected to the recording milliammeter and to an auxiliary battery unit ( I ) , which extends its running time from approximately 10 to roughly 30 hours. Figure 3.
Titrating Cell
".
."
.."... _.
The unit measures 28 X 24 X 31 em. and weighs 18 kg. The case is constructed of phenolic bonded canvas sheet joined by riveted aluminum angles and hinges. The unit contains the cell, motor-pump unit, power amplifier, and power supplies for both pump and amplifier. The gas sample is drawn into the
When a small amounu ",..earn in the reaction cell. it, r e d s verv ranidlv wit,h the minut.e ouantity of bromine present. The obsemer electrodes resporId to the consumption of bromine with a potential change which, nmrlnro. .1 . vel. applied t o the amnlifior _._r...._., r .lllll. I. atively great increase of current in the generating electrode circuit. As a result of the oompensative generation, the original slight excess of bromine is almost maintained: the only decrease is that required to sustain the higher generating current necessary for the oxidation of the sample being absorbed from the air stream. Because the observing electrode potential change produced by the absorption of a small quantity of reacta n t can he amplified to deliver a generating current many times that necessary t o titrate the sample, the titration error m y be reduced to any degree required. Figure 5. Titrimeter with Recorder and Auxiliary Battery Unit "
1
I
"
1011
V O L U M E 20, N O . 1 1 , N O V E M B E R 1 9 4 8
Figure 6.
Circuit Diagram of Power Amplifier
411 filament circuits are controlled by means of a n on-off gang switch which also controls biae circuit at grid of firet tube and zero current control i n output mtage
w l l thruugh a glass tube which protrudes through the top of the case, and is protected by a perforat,ed metal screw cap. Two otht,r fistures on the top of the unit are receptacles for use in cvupling the recorder and an auxiliary pump power supply to the unit. The amplifier supply batteries are housed beneath tht, instrummt panel. On t hti amplifier ,panel are the instrument controls, the panel InettLr for direct observation of the determined concentration, and a gas typt' integrator from which the total sampled quantity can i)c read. The controls permit the selection of scale and of speed
( w e 3 D C AMPLIFIER
of rrspollst'. Thr. p o w n amplifier 'used in the titrimeter is a 'direct-coupled,
A
fivr-t ulie amplifier powered by small 1.5-volt and 67.5-volt dry c*ells. Tht, circuit diagram of the power amplifier is shown in Figure 6. The amplifier consists of three stages: the first stage obtains voltage amplification (approximat,ely 1250) by the use of two pritodes in series balance; the second stage provides isolation of' thr input and output stages by means of a push-pull vathotie follonw circuit : the third stage, the power stage, furnishrs this output current to the generator electrodes. The RC iietwork associated with the grid of the second pentode in the amplifier stage makes this tube highly capacitative and thus tLffects high-frequency cutoff at 0.1 t o 0.01 cycle per second. The isolation stage is necessary to prevent the completion of a circuit bt.tween the oliserving and the generator electrodes external to tl1t. Wll.
T l w circuit, and its performance have been treated in detail else\vilere ( 2 ) . Recently several highly stable direct current amplifiers have been described (5, 9) which might be readily adapted for use in a titrimeter; for this reason the original circuits used are not, described further here. Because of the formation of bubbles due to the passage of air through the cell, and, perhaps, because of slight random departures from homogeneity throughout the titration cell, there are superposed upon the average potential of the observing electrode system relatively large potential fluctuations which would vitiate the rrsults if they reached the recording end of the titrimeter system. .Uthough it is preferable not to remove these unwanted signals from the generator current but instead to remove t,hem from a subsidiary metering circuit which is not part of the feedliach I o o ~ , use was made of a lory-pass amplifier of special con5trucstion for reasons of convenience and portability. The lowpass character of the amplifier and the effective capacity of the titrating cell give rise to damped oscillations following a very nbrupt change in the concentration of the gas being titrated. THEORY OF OPERATIOY
equivalent circuit concept has been found very helpful in tl(4qn of an instrument with ioughlv optimum properties for
\II
1tics
6
C Figure 7.
Equivalent Circuits of Titrimeter
given components and size. The equivalent circuits shown in Figure 7 simplify the deduction of algebraic relations describing the operation of the titrimeter and show clearly the source of damped oscillations in the present simplified instrument.
As there is a definite amount of electrical charge associated with each reactant molecule being sampled, either because of its impending role in an oxidation-reduction reaction or because of its subsequent reaction with or neutralization of ions produced by electrolvsis at the generator electrodes, we equate the rate of input
ANALYTICAL CHEMISTRY
1012 of reactant molecules in the sampled stream to an equivalent input current in amperes, I,, defined by the relation
IC = nFLc
(1)
where n is the number .of electrical equivalents involved per mole, F is the Faraday in coulombs, L is the sampling flow in liters per second, and c is the molar concentration of the reactant in the sampling stream. It can be shown easily that two quantities which represent the idealized properties of the titration cell are:
__-
-. -
.
Figure 8.
the equivalent resistance in ohms,
Re
._ .. .-
Section of Typical Record Obtained during Saturation Line Run at Low Damping
9
=
and the equivalent capacitance in farads,
nFV c. = 7 Here 6 is the end-point sensitivity in volts liter per mole, V is the volume of the titration cell in liters, v is the liquid circulation in liters per second, and cy is the fractional aeration per second. The power amplifier has a transconductance of G ohms. The recorded current is represented in all cases by the symbol I, adjacent to the meter, M . The relation of the amplifier to the titration cell is shown in Figure 7, A , where F. and C, represent the titration cell in the equivalent circuit diagrams. The following relations are easily obtained: Titration efficiency = Titration error =
IIc.. = 1 +GR,GRe ~
IC - I, ~
Time constant
I, =
=
1
~
1
+ GR,
(4)
1
20
40 60 80 NOMINAL DOSAGE MICROGRAMS OF MUSTARD PER MINUTE
la,
Figure 9. Comparison of Concentrations Read from Record with Concentrations Presumably Delivered by Saturation Line
(5)
C& ~
c
OO
+ GR.
The time constant in the absence of the amplifier is C,R.. The effect of the power amplifier is seen in these diagrams to be the forcing of the input current through the meter branch of the circuit rather than allowing it to flow through R, (corresponding to discharge from the cell by liquid circulation). I n fact, the amplifier may be replaced by a resistor of value 1/G ohms; this has been done in Figure 7 , B and C. The alternating potential components arising from bubbles and concentration inhomogeneities in the cell necessitated the use of a low-pass amplifier in the portable instrument to prevent serious interference in the record. Figure 7 , B , includes an effective amplifier inductance which arises because of the frequency discrimination of the amplifier; the shunt resistor, lV/G, accounts for the fact that signal attenuation does not continue in greater degree as the frequency is increased but that an asymptotic level is approached rapidly. Damped oscillations occur in the resonant circuit composed of C,, &, and 1/G; the meter is in this circuit and therefore records the oscillation. Decreased values of Re and N / G result in increased damping, and near-critical damping can be achieved by careful adjustment of the circulation and the amplifier gain characteristics. Increased amplifier transconductance tends to diminish the damping, but damping due to 1/G is
negligible under normal operating conditions and the favorable effect due to decrease of S / G more than compensates for the instability which might otherwise result. The circuit of Figure 7 , C, corresponds to a preferred arrangement in which the fluctuations are bypassed around the metering elements or stages, but no frequency discrimination occurs in the generator circuit which constitutes the feedback loop. Unfortunately, condensers of sufficient size to bypass the frequencies involved are much too large to be considered; however, the same effect is achieved easily by the use of a separate vacuum-tube stage to provide high effective dynamic resistance in the meter. Typical values of the parameters involved in the equivalent circuit representation are: RE
CI G
Lm
iv
100 t o 300 ohms About 0.1 farad About 0.3mho 100 to 1000 henrys 0 t o 15
These values lead to titration efficiencies of 97 to 99% and a time constant of a few tenths of a second in the absence of damping and inductive elements. It is of interest to evaluate the sensitivity of the recorded current value to changes in several instrumental parameters. Such changes may be due to thermal drifts, evaporation of the solution, electrode instability, or drifts in the operating characteristics of the amplifier. Errors in the titration efficiency can occur as
V O L U M E 2 0 , N O . 11, N O V E M B E R 1 9 4 8 well as shifts in the zero of the record. It can be shown that the dependence of zero current upon changes in cell potential, operating bias, and R, are all reduced by a factor of approximately l/GR,, n-hile the dependence of the zero current upon G is reduced by roughly (1/GR,)2 compared to the dependence when feedback or null-point operation is not used. Shifts in operating bias do plot affect the titration efficiency, but both R, and G are involved in this quantity. However, a 1% change in Re or G produces a l/GR, 70change in the titration efficiency. Since GR, is generally about 50, all sources of error are reduced by a factor of 50, and some are diminished to 0.047c of the values which they would have in a system not utilizing feedback (6). PERFORMAh-CE
T h e e types of experimental tests of titrimeter performance have been made ( 2 ) . The greatest number of tests consisted in the determination by the titrimeter of the rate of delivery of mustard gas in air from a gas saturation line ( 4 ) consisting of flowmeters and glass bead bubblers. The nominal dosage was computed from the air flow rates and the vapor pressure of mustard gas a t the controlled operating temperature; because of the uncertainty in the vapor pressure of mustard gas, however, the nominal values are not sufficiently accurate to permit the evaluation of the titration efficiency. Response of titrimeters to nominal dosages is useful in testing linearity and transient behavior. A section of record typical of those obtained in nominal dosage runs is shown in Figure 8. A plot of points taken in three different test runs of two instruments over a period of several weeks is shown in Figure 9. I n an attempt to resolve errors in the titrimeter from errors arising in the gas saturation line, several experiments were performed in which two titrimeters and two sets of bubblers sampled from the same volume containing a gas mixture fed continuously from the saturation line. After sampling had occurred for an
PIPET O R MICROBURET7
Table I. Run 1 2 3 4 5
Average Concentrat.ions, Parts per Million by Volume in Air Titrimeter 1 3.9 3.8 1.7 1.9 2.8
Titrimeter 2 4.3 4.1 1.9 1.9 2.9
Bubbler 1 4.2 4.4 2.5 2.0 3.3
Bubbler 2 4.2 4.2 2.0 2.0 3.3
Nominal 5.5 5.0 2.4 2.4 3.5
appropriate interval, the titrimeter records were integrated and the samples accumulated by the bubblers were determined by electrolytic bromine titration (IO). Because the saturation line mixture fluctuated throughout the sampling period, only the total integrated values are comparable; these values may involve errors in the integration of the records. The totals are tabulated in Table I as average concentrations over the sampling periods, M hich ranged from 15 to 30 minutes to afford the material necessary for the bubbler titrations. To the extent that the tabulated data reveal titrimeter rather than experimental errors, the following conclusions may be drawn: Despite the fact that the titrimeters provide instantaneous values of the concentration, the reproducibility between the titrimeter values is about as good as that between the results of the bubbler titrations. The principal error in the titrimeter values is an absolute one of about 0.3 p.p,m. This is apparent in Figure 9 from the curvature of the line near the origin, and it can be seen that the actual curve over most of its length runs parallel to a straight line through the origin. The titration efficiency is higher than indicated by nominal dosage values, The slope of the curve in the upper concentration range, when corrected in terms of the values given in Table I, is unity within experimental error.
It is probable that the titration efficiency is roughly 97 t o 99%. The constant error perhaps arises from the adsorption or reaction of a small amount of the sample on the glass walls of the intake system. TITRATION OF DISCRETE SAMPLES
OBSERVERS
1
U’65
MICROCOULOMMETER
1013
’
Figure 10. Schematic Arrangement of Components for Automatic Titration of Discrete Samples
By means of the system shown in Figure 10, consecutive titrations of small dissolved samples of thiodiglycol have been performed automatically very rapidly. The addition of a sample containing a few micrograms of titratable material causes the generator current to rise very suddenly at first and then to fall off rapidly as the end point is neared. The generating current pulse can be integrated, for example, by an electronic microcoulometer which registers the integral on a clock-type counter ( 2 ) . I n this 71-ay a series of titrations of samples, each containing from 25 to 100 micrograms of thiodiglycol, was performed in 15 seconds per sample, much of the time being required for addition of the sample and recording of the analytical results. The root mean square error of a set of 22 determinations was 3y0,corresponding closely to the known precision of the microburet ( 1 2 ) used to introduce the samples. At the beginning of this work some titrations were carried out using a battery-resistor combination in series with a sF-itch to provide a constant rate of generation of the titrating agent, a stop watch being used to time the interval during which titration takes place. This is an exceedingly simple and inexpensive way of performing such titrations, and it may suffice in many cases where more elaborate, automatic equipment is not justified. The battery-resistor current source was used in the conversion of existing field instruments of Northrop’s design ( 8 ) to electrolytic generation. The constant-current generation technique has been further applied (IO) in this laboratory in amperometric titration, and elsewhere in the titration of acids (3). APPLICATIONS
Although the methods presented here were developed to provide an answer to the specific problem of mustard gas determina-
ANALYTICAL CHEMISTRY
1014
tion under field conditions, the methods appear to be sufficiently general to provide a basis for the design of a variety of instruments for many chemical metering, analysis, and control uses Continuous titration by competitive rates of addition of the titrating agent can be achieved in several ways, but the electrolytic method of introduction has much to recommend its use. It iq not difficult to devise ways in which electrolysis can enter into many analytical titrations (12) in either static or continuous flov systems. The instrument described in this report can be used for the continuous determination of gases such as hydrogen sulfide, sulfur dioxide, and acrolein in the range above 0.1 p.p.m. By slight modification of the apparatus, the halogens and carbon monoxide may be determined. ACKNOWLEDGMENT
The authors R ish to acknou ledge the assistance and eiicouragenient of their colleagues C. IT. Gould, Jr., \V. H. Eberhardt, J. IT-.Sease, G. Holzman, W. Schlinger, and R. Mills. They are indebted to C. G. Siemann and E. H. Su-ift, official investigators, for their sponsorship and consultation throughout the period of development of the instrument. They were assisted by H. Y. H Aircraft Assemblies, Glendale, Calif., in the construction of thts titrimetcr illustrated. LITERATURE CITED
Briglio, A., Jr., Brockman, J. A., Jr., Schlinger, W., and Shaffet,
(1)
P. h.,Jr., U. S.Dept. Coninierce Offive of Publication Board. OSRD R e p t . 6047, PB 5925,1945. (2) Briglio, A, Jr., Brockman, J. -4., Jr., and Shaffer, P. A,. .TI , Ibid., OSRD Rept. 6183, PB 5940, 1945. (3) Epstein, J., Sober, H. A., and Silver, S. D., .4x.4~.CHEY..19, 675-7 (19471.
Gould, C., Re’denian, C., Shaffer, P. A,, Jr., Brocknia~i.J. -1.. Jr., Holzman, G., and Lee, T. S.,U. S.Dept. Commerce. Office of Publication Board, OSRD Rept. 4627, PB 5939, 1945. (5) Liston, M. D., Quinn, C. E., Sargeant, W. E., and Scott, 0 . G., Rev. Sci. I n s t r u m e n t s , 17, 194-8 (1946). (6) JIacColl, L. A, “Fundamental Theory of Servomechanisms,” New York, D. Van Nostrand Co., 1945. Discussion of feedback in automatic control systems. (7) Northrop, J. H., U. S.Dept. Commerce, OSRD R e p t . 401, 1942. (4)
(8) Ibid., 1444, 1943. (9) Palevsky, H., Swank, R. K., m e n t s , 18,298-314 (1947).
and Grenchik, R., Rev. S c t . Insti i i -
CHEM.,19, (10) Sease, J. W., Niemann, C., and Swift, E. H., A N ~ L 197-200 (1947). (11)
Shaffer,P. A,, Jr., Farrington, P. S.,and Niemann, C., I h i d , 19,
(12)
Szebelledy, L.; and Somogyi, Z., Z. anal. Chem., 112, 313. 323,
(13)
Thomas, M. D., Ivie, J. o., and Fitt, T.C., ANAL.CHEM..18, 383-7 (1946), and references given there.
492-4 (1947). 332,385,391,395,400 (1938).
RECEIVED,\:arch 1. 1948. T h e instruments and experimental results described in this paper are the result of work carried out a t the California Institute of Technology on behalf of the Office of Scientific Research and Development under Contract O E X s r 323. Contribution 1180 froin the Gates and Crellin Laboratories of Chemistry, California Institute of T w h nology.
Automatic Distillation Apparatus for A.S.T.M. Method D-86 F. B. KOLFSON, C. J. PEKTHEK, AND D. J. POMPEO Shell Development C o m p a n y , Emeryzille, Culif. A n apparatus is described which automatically plots a complete distillation curve in accordance w-ith the standard il.S.T.41. D-86 procedure. The operator need only fill the distillation flask with liquid, insert the chart paper, and set the initial heat; the apparatus performs the distillation, recording all required data, such as correction temperature, initial boiling point, end point, and distillation rate. When the distillation is completed the apparatus automaticall? resets itself for the next test.
B
ECAUSE labor expended in checking refinery products and
unit streams for control purposes represents a sizable part of total refinery labor costs, means are constantly being sought to simplify test procedures and release laboratory and operating personnel for other work. One such development is this automatic distillation apparatus. The standard method of test for the distillation of gasoline, naphtha, kerosene, and similar petroleum products as crtahlished by the American Society for Testing Xaterialr provides for manual operation of a simple laboratory still. The procedure is tedious and demands constant attention from the operator if all provisions of the method are strictly followed. Too fast distillations result in temperature readings being misset! and then estimated, errors may be made in reading and recording temperatures, techniques vary from one operator to another, and the number of samples that an operator can handle is limited. h c cordingly, the development of this automatic apparatus was undertaken so that the difficulties encountered in manual operation would be avoided. Suggestions from refinery laboratory staffs as to how an automatic device was to operate were carefully considered and most of them were successfully incorporated in the apparatus described.
This apparatus was tested both in the development laboratory and at a refinery control laboratory on a variety of materials including cracked and aviation gaqolines, kerosenes, insecticidr bases, special solvents, and benzene. In general, the distillation curves obtained by the automatic apparatus checked, well withiir the A.S.T.M. tolerancrb, those obtained manually. Initiallv some discrepancies were found betxveen initial boiling point and end point obtained by the recorder and manually, although check runs made with the recorder on the same material were practicallxexact duplicates of each other. Subsequent work on the thermocouple mounting eliminated even these discrepancies. The equipment should find wide use, not only in refinery control laboratories, but also in still control houses where the operatoi can save considerable time by running a distillation to check thc performance of a column. Ill addition, although the present apparatus was specifically designed to perform only B.S.T.11 I)-86 distillations, slight modifications such as variable distillation rate control and substitution of warmer fluid in the condenser bath in place of ice, can easily adapt it to a variety of specification distillations encountered in the refinery. It is expected that its ultimate capabilities can be determined only by extended usc in laboratories and refineries.
