The following ions did not interfere when 500 pg, were compared to 1 pg. of Be:
Compared to 1 pg. of Be, the following ions interfered a t 500 pg. but did not interfere with the test a t 50 pg.:
test for 1 pg. of Re but did not prevent the test: C T + ~ ,CrOc-', Rhf3, and Sb+3. DISCUSSION
LITERATURE CITED
The strength of the reagent solution did not influence the sensitivity of the test, although a concentration in the vicinity of O.lyois most convenient for ease of observation of the results. Although Eriochronie Cyanine R reacts with a number of ions, the combined masking effects of tartrate and EDTA make this test highly selective for beryllium. The use of a buffer in the reagent solution is required to control the p H to obtain the neutral color of the reaeent. I n acidic Or strongly basic media, the reagent color is sufficiently similar to
( 1 ) Hill, C . T., AXAL. CHEM.30, 321-4
-
the 50-pg. level the following four ions slightly distorted the color of the
the color of the reaction product \vith beryllium to make the reaction useless for identification purposes.
(19%). ( 2 ) West, P. IT., Mohilner, P. R., CHEM.34, 558-60 (1962).
.$SAL.
PATRICIA R. ~ \ I O H I I . S E R '
C'oates Chemical Laboratories Louisiana State University Baton Rouge 3, La. WORK wag supported by PHS research grant S o . 7481 to Philip W. West from the Xational Institutes of Health. Public Health Service. 1 Present address, Department of Cliemistry, C1i:itliam College, Pittsburgh 32, Pa.
A N e w Automatic End Point Detector for Constant Current Coulometric Titrations C. B. Roberts and H. A. Breicha, The Daw Chemical Co.,Midland, Mich.
A
end point device is designed to terminate coulometric titrations. I n our laboratory it is used n ith a Sargent coulometric current source which provides the desired remote control and a wide range of currents. It makes use of both amperometric and potentiometric principles and is especially useful for the determination of unsaturated compounds by electrolytically-generated bromine and iodine. It has the advantages of simplicity of operation, a minimum of interaction between generating and detecting electrodes, and high sensitivity. -1 50-pa. meter relay terminates the generating current at, or slightly before, the end point, while another meter regieters the relative solution potential a t all times during the titration. The two meters are synchronized, but the relay meter no longer functions as an indicating device after i t has been activated; thus, the second or monitoring meter indicates the end point of the titration. T w o adjustments are required before a titration is carried out: a zero control adjusts the meters to zero before titrant has been generated, and a span control adjusts the monitoring meter to full qcale after an e x e s s of titrant has been qenerated. ilfter these adjustments the titration is terminated a t midpoint (25 pa.) on the monitoring meter when the current control on the source is on remote. The relay needle may be set NEW AUTOMATIC
1104
ANALYTICAL CHEMISTRY
a t a reading slightly bclon- the midpoint to allow for overshoot. If, after generation has been a,utomatically terminated, the monitoring meter has not quite reached midpoint a few short increments of titrant may be added by the manual control on the current source until the midpoint reading is reached. If a fully automatic end point is desired, a few preliminary end points can usually det,ermine the correct setting of the relay meter needle to compensate for overshoot and result in a midscale reading. The instrument has been designed for slow reaction rates where solution potential may rise prematurely and shut off the titration. As titrant is consumed upon standing. the solution potential drops and current generation will start again to provide additional titrant. A delay circuit causes a 1ap.e of a few feconds between the time the relay meter becomes deactivated and the current is turned on; this makes possible a much smoother end point'. The schematic is shown in Figure 1. The signal from the detecting electrodes is impressed across the grids of the 12.2U'i tube. This results in a difference of potential across the plates of the tube, and causes a proportional amount of current to flow through the meters of A I l and AT2. The current relay in the constant current source is controlled by a Model RIR, 0- to 50-pa. Larson relay meter. When the contacts of the latter are open and the current control is on remote, titrant i. generated; when they
close, the titration stops. The delay circuit is controlled by the thyratron tube 2D21 and the RC circuit in its grid. .i plug-in alarm circuit may be energized to signal the end of the t'itration. ,4 50,000-ohm potentiometer (R1) is placed across the detecting electrodes and is used as the span adjust. -4s titrant is generated and the voltage rises across the detecting electrode-, current is d r a n x and a voltage develops across the resistor. This distorts the true pot,entiometric curve obtained under no-load conditions, but it re,.ults in the electrodes reaching equilibrium very quickly; this is most desirable for automatic titrations. Figure 2 s h o w voltage-time curves for various valiies of resistors when bromine is gcneratd at 193.0 ma. The ralue chosen ;till provides a sharp end point, but amids the sluggishness of respon.5e which higher resistances produce. JYhen cscess bromine is present (yellow electrolyte), a portion of the voltage across the potentiometer (R1)is used to provide a full-scale reading of the meters. Khen bromine or iodine is used as titrant, the amount of current flowing through bhe electrode system depends upon the t,hickness of the diffusion layer surrounding bhe detecting electrode; thus the ctirring rate should be kept constant for the blank and throughout the titration. -1smooth magnetic stirring bar without a center rib has been found to be verj- satisfactory.
I
,-
I
I
I I
i I I
J 115 V 4 C L I N E INPUT
I I S V. A.C. O U T P U T TO CONSTANT CURRENT RELAY
I
ELECTRODES
Figure 1 .
,
Circuit diagram of automatic end point detector
Zero adjust is provided by a 20K potentiometer (R5)in the plate circuit of the triodes. The detecting elecimdes for bromine and iodine titration are of standard deqign. small-diameter saturated calomel electrode (Sargent NS-30490) serve? a. reference. .it should be placed oppo&e a half-inch platinum wire detecting electrode and turned so that its contact point with the solution is away from the field set up by the generating current. The kick which results \Then the generating current is turned on and off should not exceed 2 pa. This slight deflection rrill not interfere with the operat'ion of the instrument. The generating anode is a 11/'*- by 3;'r-inch piece of phtinum foil (0.008 inch thirk). The generating cathode is designed for easy renioval and addit,ion of cntholyte ( 1 ) . PROCEDURE
-1 blanb is titrated by placing electrolyte in the cell and generating titrant manually for a few seconds until the monitoring meter read5 midscale. The timer iq set back to zero, the sample added, and the conitant current control set to remote. -4b the contacts on the meter relay open, generating current start- and the titration proceeds until it i q automatically terminated. RESULTS AND t)lSCUSSION
Aliquots of sodium arsenite were titrnted automaticall:,- with bromine at
01
0
TIME (SEC)
Figure 2.
Voltage-time curves for various load resistors
193.0 ma. with a meter relay setting of 10 pa. This is a fast reaction, and the meter reading remained constant after the generating current was stopped. I n 10 determinations the titration time ranged from 248.0 to 218.5 seconds
with a relative standard deviation of *0.07%. Accuracy was of a comparable degree. llliquots of 10-undecanoic acid were titrated with bromine automatically at 193.0 ma. with a meter relay setting VOL. 35, NO. 8, JULY 1963
1 105
of 15 pa. I n eight determinations the range was from 240.3 to 242.6 seconds with a relative standard deviation of =t0.3%. Three to five increments of titrant were automatically added before a constant potential was maintained. More precise results could have been obtained by manually adding the last few seconds of titrant.
The detector may be used with any constant current source provided with the necessary relays to control the current and the timer. It has a midscale sensitivity of approximately 15 nanoequivalents of double bond per microampere and thus is quite useful for microanalyses. By using higher currents (100 to 200 ma.), assay amounts can be
determined with good precision and accuracy. LITERATURE CITED
(1) Roberts,
C. B., in “Titrimetric Methods,” D. S.Jackson, ed., pp. 21-31, Plenum Press, S . Y., 1961.
Division of Analytlcai Chemistrv, 144th Meeting, ACS, L~~ ~ ~Calif , ~*4pril ~ 1963
A Simple, Inexpensive, Heated Infrared Gas Cell Eugene A. Burns,‘ Propulsion Sciences Division, Stanford Research Institute, Menlo Park, Calif.
SIMPLE,
inexpensive, heated in-
A frared gas cell has been designed
and fabricated for the purpose of elucidating the infrared-active gases evolved on thermal decomposition of high energy compounds. A survey of leading manufacturers of infrared equipment at the inception of these studies revealed that there was no heated gas cell commercially available. Because of the potentially explosive nature of some of the high energy fuel additives which were to be examined, it was desirable to fabricate a n inexpensive cell. Several possible cell designs were considered and evaluated, and the best features of these were incorporated in the design shown in Figure 1. The chemical requirements imposed b y the high energy compounds, in general, permit use of a glass container. The method of attaching the optical windows to the cell was best served by “O”-ring closures. -4 simple glass section was constructed with a stopcock connected to a male 9/18 ball joint attached in the center and two glass (size 15) “0”ring joints a t either end. The gas volume of the cell is approximately 15 ml. with an optical path of 8 em. Vacuum seal is made between the optical plate and the glass section with a choice of Silicone, Seoprene, Viton A, or Teflon “0” rings. The optical plates are held in place with lead gaskets maintained by the furnace itself. The furnace is fabricated in two sections and the closure is ensured by adjusting the two sections until they are as close together as practical. Each section of the furnace is prepared from an aluminum block, machined, and wrapped with 30 inches of No. 20 insulated Nichrome wire. This wire is held in place and uniform heating is ensured by a coating of KO.6 Sauereisen. The Sauereisen is applied in a paste form and heated to a hard finish. Thermocouple and thermistor wells are Present address, Space Technology Laboratoriee, Inc., One Space Park, Redondo Beach, Calif. 1106
ANALYTICAL CHEMISTRY
drilled in the block for measurement and regulation of the temperature. When 115 volts are applied across the heater terminals, the cell temperature is 225’ C. T o use readily obtained inexpensive “O”-ring groove glass and optical windows, the size of the cell was relatively small and hence some of the energy from the source is lost. It is necessary to align the cell in the light path to give maximum energy, and then attenuate the reference side of the spectrometer. T o obviate this difficulty, construction of a larger diameter cell would be required, which in turn would require a higher capacity furnace and a larger sample size. The disadvantages encountered with a larger furnace, larger windows (inefficient furnace and heat losses from the windows), and a larger sample size (when dealing with potentially explosive compounds the smaller the sample the better) offset the alternative attenuation requirement. The furnace may be powered from a 115-volt variable transformer (Variac or equivalent n-ith a maximum capacity of 7.5 amp.), with manual temperature control. For precise temperature control in the order of 0.02’ C., a thermistor temperature regulator similar to that of Gray and VanDilla (3) was constructed. A Victory Engineering
Co. Type 51A1 glass bead thermistor was used as the sensing device. Figure 2 shows the schematic of this device. By adjustment of the bridge balance with helipot R1 and ratio arms Ra, R,, and Rs to a prescribed setting, regulation of the temperature is effected by the phase sensitive control system. This system (3) operates in the following manner: The output of the bridge is fed directlv to a 6 SL7-GT dual triode connected as a two-stage resistance-capacitance coupled amplifier. The amplifier output is coupled through Ce and Rl1 to the grid of a 2050 Thyratron. The plate circuit of the thyratron is supplied with ax. power through switch Sz and the coil of the relay, so that contacts on the relay close when the thyratron conducts. If the bridge is unbalanced, a n alternating voltage having a phase dependent on the direction of the imbalance is supplied to the grid of the thyratron. When the temperature of the thermistor is below the preset value on the balance resistor, the phase of the thyratron grid voltage is then a few degrees ahead of the alternating supply to the plate of the thyratron. Hence, when the plate voltage goes into ita positive half cycle the grid voltage is already positive and the thyratron starts conduction a t the beginning of
WINDOW GASKET
I t -2
Figure 1.
I” 116
Cross-section diagram of heated infrared cell
l
