sensitivity to agent was achieved, on a dosage basis, as under ordinary conditions. However, the minimum detectable concentration was raised to 0.04 p.p.m. of GB. The other difficulty occurred a t high temperature and high relative humidity (100O F. and above 80% relative humidity). Under these conditions in a test chamber, moisture condenses on the unit and causes electrical leakage in the circuit, which creates a drift in the null point of the alarm and a serious decrease in sensitivity. If the unit is renulled after this, adequate operation is still obtained. Proper potting of the photocells and high impedance circuit will probably eliminate this difficulty. However, it has not been serious under actual operating conditions encountered in hlaryland. When the operating temperature drops below freezing, an antifreeze may be added to the aqueous solution to prevent a freeze-up. The antifreeze used is isopropyl alcohol. Table IV shows a number of different antifreezes that were tested and the results of these tests. If it is not desirable to use an antifreeze a t temperatures below freezing, an electrical heater is provided for alarm operation. ROUGH HANDLING TESTS
The instrument is extremely rugged and was subjected to rough handling tests similar to those given the AN/ PRC-7, commonly known as the Walkie Talkie radio. The alarm successfully passed these t)ests, shock. vibration, immersion, drop test, etc. At the request of Army Field Forces, a unit was air-dropped by parachute from a plane. The alarm was started up immediately after the drop and operated satisfac-
Table 111.
Compounds Used to Retain Moisture
Compound Glycerol Ethylene glycol Sorbitol Methyl Cellosolve Lithium chloride Lithium bromide Calcium chloride Colloidal silica gel
Cause for Rejection Colored soln. Colored soln. Ineffective Colored soln. Ineffective Ineffective Ineffective Ineffective
Table IV. Antifreeze Additives Remarks Additive Poor Ct, rapid drying, Methanol poor stability Fair stability, fairly Ethyl alcohol good sensitivity Poor stability, slush 1-Propanol point of 12" F. Satisfactory storage sta2-Propanol bility, blank time, and sensitivity Solubility in waterrtoo 1-Butanol low Solubility in water too 2-Butanol
+
10F
Very rapid color formation, high viscosity Poor stability, high viscosity Poor stability Low water solubility, rapid color formation Low water solubility, rapid color formation
Ethylene glycol Glycerol Acetone Methyl ethyl ketone Tetrahydrofuran
use in the field. It is lightweight and requires little electrical power for operation so that a portable power supply may be used with the unit. The instrument will operate unattended for 12hour periods and responds to extremely low dosages of G-agent. It is reasonably free from chemical interference in the field and rugged enough to withstand rough handling by combat troops. The alarm must be serviced every 12 hours with solution and paper tape, and the air knockout pot and platen must be cleaned. Other maintenance and lubrication duties are required every 1000 hours of operation. ACKNOWLEDGMENT
The authors wish to express their appreciation of the technical guidance given by Robert Picard, formerly of Radio Corp. of America, and Solomon Love, Army Chemical Corps. In addition, they are pleased to acknowledge the efforts of many colleagues in this work, particularly Saul Zelkind, Andrew Davis, Robert Gamson, for chemical assistance and Ronald Ruefenacht, Robert Jones, William Russell, Edward C. Luke, and William Keane for their engineering assistance. LITERATURE CITED
torily. The unit is completely waterproof when the caps and covers are closed. In operation, the alarm operated satisfactory when placed in a trailer attached to an Army truck and driven Over 250 miles of rough fields and roads. CONCLUSIONS
The alarm is satisfactory for portable
(1) Gehauf, B., Epstein, J., Wilson, G. B.,
Witten, B., Sass, S., Bauer, V. E., Rueggeberg, W. H. C., ANAL. CHEII.29, 278 (1957). (2) Gehauf, B., Goldenson, J., Zbid., 29,276 (1957). for revien- xfarch 23, 12157. Accepted January 8, 1958. Division of
Analytical Chemistry, 130th Meeting, ACS, Atlantic City, N. J., September 1956.
Alarms and Analyzers for Nerve Gas Vapors R. H. CHERRY, G. M. FOLEY, C. 0.BADGETT,' and R. D. EANES leeds & Northrup Co., Philadelphia, Pa.
H. R. SMITH Chemical Warfare laboratory, Army Chemical Center, Md. ,Nerve gases give no sensory warning when lethal concentrations are present. Automatic nerve gas alarms use the Schoenemann reaction with indole reacting with the nerve gas to form a fluorescent compound, indoxyl. An automatic chemical processing system is combined with a fluorescence photometer to operate as a continuous alarm or analyzer. Data are presented to
select the reagents giving maximum sensitivity in a given photometer. Studies on reagent solutions led to the selection of two separate solutions, and to the use of acid-stabilized hydrogen peroxide and carbonate-bicarbonate buffering. The instruments will give warning before any injury occurs, and may b e used as calibrated analyzers.
S
of nerve gases (Gagents) during manufacture, storage, and laboratory use requires sensitive detection equipment, capable of giving an alarm before personnel are exposed t o harmful dosages. The instruments and chemical systems deAFE HANDLING
l Present address, Tele-Tronics Co., Ambler, Pa.
VOL. 30, NO. 7, JULY 1958
1239
scribed here, which were developed for use as gas alarms, are sufficiently sensitive, stable, and convenient for calibration and use as continuous automatic gas analyzers in research and engineering work on the nerve gases. A review which discusses some of the properties of the nerve gases was given by Sartori (5). Among the agents with which the alarms and analyzers described below are useful are: GB, the isopropyl ester of methane phosphonyl fluoride, and GA, the ethyl ester of dimethylamidocyanophosphoricacid. CHEMICAL SYSTEM
The alarms and analyzers use the fluorescence resulting from the Schoenemann reaction (6) as the detection principle. “Schoenemann reaction” is the general designation for the oxidation of an indicator by peroxide, perborate, or similar oxidant in the presence of the G-agent. The reaction is conducted in an alkaline solution. Although the oxidation proceeds slowly when the G-agent is not present, it is greatly accelerated by very small quantities of the agent. Several indicators have been used, the most sensitive of which appears to be indole. Gehauf and Goldenson (1) describe a batch method which is very useful for the laboratory analysis of G-agents. An adaptation of this method was used in the instruments described. Indole is a relatively nonfluorescent compound. In alkaline peroxide and in the presence of G-agent, it is rapidly oxidized to indoxyl, which is highly fluorescent. In the range of concentration of interest, the amount of fluorescent material produced is proportional to the amount of G-agent present. The mixed reagents used in the batch method are not stable enough for use in a continuous analyzer. However, a buffered alkaline solution containing the indole and an acidified solution containing the peroxide have satisfactory stability. These two solutions are automatically mixed in the instruments before use.
a t the point marked Analyzer Sample Intake. The sample passes upwards through the absorption column, thence through a trap and flowmeter to the air sample pump. The exhaust of the air sample pump is led out of the analyzer to a safe hood system. The reagent flows in this system may be disturbed by sudden changes in sample pressure. The part of the sample intake system shown in dashed lines &as developed to improve operation under these conditions. -4primary air pump sucks in a total sample a t a rate considerably higher than that used by the main part of the analyzer. The excess sample is vented to a safe hood system. The vent tube is long enough to prevent back-diffusion from having a significant effect on the composition a t the analyzer sample intake. One of the reagent supply bottles in Figure 1 contains the indole solution; the other is filled with the solution containing the peroxide or perborate. These solutions are individually metered into the mixing chamber, from which they flow by gravity down the absorption column, countercurrent to the sample air. The solution strips any nerve gas from the sample air. It then passes through the fluorescence photometer and into a waste tank or drain, depending upon the requirements of the particular installation. The optical details differ somewhat among the various fluorescence photometers developed. In every case, the fluorescent light emitted at right angles
to the exciting light beam is detected by an electron-multiplier photocell. Optical glass filters passing only the near ultraviolet are used in the exciting light beams, and filters to reject the near ultraviolet are used in the path to the detecting photocell. Two of the instruments described use the direct current from the anode of an electron-multiplier photocell as a measure of the fluorescence of the solution. The electronic components of the direct current systems are fairly complicated. The instrument accuracy is directly dependent upon the stability of the electron-multiplier photocell, photocell power supply, and exciting lamp. The photocell cannot be used a t maximum sensitivity, because of the inaccuracy caused by fatigue effects. This requires the balancing amplifier for the recorder or indicator to have high gain. Although the instruments using the direct current measuring system give excellent service, a second kind of photometer has been developed which employs an optical null-balance method, basically like that applied by Hardy in a series of spectrophotometers (9, 3 ) . This system makes the calibration of the photometer essentially independent of variations in lamp brightness, photocell sensitivity, and amplifier gain. The electronic circuits are considerably simpler than those in the earlier instruments, a t the cost of some increase in mechanical complexity of the photometer.
Figure 1. Schematic diagram of nerve gas analyzer
PRINCIPLES OF CONTINUOUS FLUOROMETRIC ANALYZERS
The instruments described were specifically developed to detect nerve gases in air, but similar systems and techniques could probably be applied to other problems of gas, vapor, or liquid analysis, where change in the optical characteristics of a solution can be related t o the concentration of a particular component of the sample. Figure 1 shows a schematic diagram of the nerve gas alarms and analyzers. Two optional air sampling methods have been used. If the sample pressure is close to atmospheric and does not change rapidly, the sample is admitted 1240
ANALYTICAL CHEMISTRY
ELECTRICAL MEASURING AND POWER SUPPLY SYSTEM
1 W A S i i TANY
I
I
FLUORESCENCE OF REAGENTS
The fluorescence of the indoxyl produced in the Schoenemann reaction is excited best by near-ultraviolet light, in the wave length range of 350 to 400 mp. The resulting fluorescence peaks in the blue-green region of the spectrum, around 500 mp. Incandescent lamps are used in all the instruments. The stability of their output is good, and the control of scattered light in the photometer is better than can be obtained with gas discharge lamps. The near-ultraviolet output is selected by Corning glass 59TO primary filters; the main secondary filter is Corning glass 3387. An additional light blue secondary filter, Corning glass 4308, improves the rejection of scattered light. At least part of the effect of the 4308 filter is to reject some of the deep red light passed by the 5970 primary filter and scattered in the liquid cell. The fluorescence of the freshly mixed indole-peroxide solution, in the absence of the nerve gas, is low and very constant. As a result, a photometer designed to minimize the detection of Scattered exciting light will detect very small additions of nerve gas with high reliability.
sample cell and photocell. It was also necessary to interpose permanently a neutral attenuation of 99% between the source and sample to reduce the current through the phototube to a reasonable value of about 0.2 pa. (The fluorometer was always standardized against a standard quinine sulfate solution before use.) All reagents used in the tests were reagent grade, except the indole, which was Eastman No. 2773. There was no further purification. Two reagent solutions were used : Solution A was prepared by mixing 1000 ml. of water, 150 ml. of 2-propanol, 100 ml. of acetone, 2 grams of indole, and 4 grams of buffer salts. Solution B contained 1000 ml. of water, 250 mi. of 2-propanol, and the acid-stabilized hydrogen peroxide. A solution containing 5 y per ml. of GB in dry redistilled %isopropanol was prepared. One milliliter of the dilute GB solution was placed in a 25-ml. volumetric flask. Equal volumes, usually 25 ml., of solutions A and B were thoroughly mixed and added to the flask containing the agent, and the volume n-as made up to 25 ml. The
Table I.
The sensitivity of the alarm depends upon the kinetics of the fluorescence reaction, the solution flow rate through the cell, the photometer efficiency, the cell geometry, and the scrubber efficiency. Knowledge of the kinetics of the reaction, as affected by solution composition and pH, permits selection of a cell design and solution flow rate compatible with the sensitivity desired. The effects of pH, buffer composition, and peroxide concentration of the reagent solutions are reported beloF. Equipment and Reagents. The equipment used for studying fluorescence dynamics consists of a Farrand Model A fluorometer, a Leeds & Korthrup Catalog No. 9836 micromicroampere indicator, and a Leeds 8- Northrup Speedomax G recorder having a full scale range of 5 mv. The micromioroampere indicator has a sensitivity of 2 X ampere per scale division, the most frequently used sensitivity being 4 X 10-9 ampere per scale division. The noise of the system ~ 7 about s 1.5y0 of recorder full scale. The optical system and mercury lamp in the Farraad fluorometer gave a fairly intense source of illumination. Marked fatigue of the photomultiplier tube was observed due to the high current obtained JThen viewing a solution of 0.15 y of quinine sulfate per ml. of 0.1N sulfuric acid in the sample cell. For this measurement, a Corning No. 5860 filter was interposed between the source and the sample cell, and Corning No. 3389 and 4308 filters were placed between the
Substrate H20
50 50
500 ml. H20
I
-
SODIEM PERBORATE SOLUTIONS. Solutions of Merck C.P. sodium perborate were made up to have a concentration of approximately 0.035y0 hydrogen peroxide. Two substrates were used: deionized distilled water, and a substrate composed of 500 ml. of deionized dis-
yo of Original H2Oz Concn.
c.
2; 1
Stability of Reagent Solutions. A single reagent solution containing indole and sodium perborate in a substrate of water, acetone, and 2propanol was relatively unstable with time and temperature. Dividing the reagents into two separate solutions and including the alkaline buffer in the indole solution resulted in a marked improvement in the stability of both solutions. The following investigations were made on the stability of the various reagent solutions.
Rate of Decomposition of Sodium Perborate Solutions
Temp.,
INVESTIGATION OF CHEMICAL SYSTEM
contents were mixed and a sample was placed in the fluorometer. These manipulations were carried out as rapidly as possible, noting the time when the agent first came in contact with the reagents. The fluorescence-time curve was recorded.
2-propanol
Table II.
+ 125 ml.
10 hr. 80 60 28 78 75 50
24 hr. 60 0 0 48 38 0
48 hr.
60 hr.
0
15
..
... ...
.. 0
10 0
...
...
...
Rate of Decomposition of Hydrogen Peroxide Solutions
yoof Original Hz02 Concn. Temp., O
c.
Substrate
500 ml. H20
2-propanol Table 111.
Pretreatment of Soln. -4 T;mz., Duration, 50
40
days 0 1 3 3 10
0 1
2
3 30
6 0
3 5
10 20
+ 125 ml.
10 hr . 100 100 100 100 100 96
24
hr. 100 100 90
100 100
93
48
60 hr. 99
hr. 100 100 78 100 100
... 73
...
...
90
..,
Stability of Alkaline Indole Solutions
8
H of Mixed oln. at Room Temp. (Photometer Effluent) 10.7 10.7 10.7 10.6 10.6 10.4 10.6 10.6 10.6 10.6 10.6 10.5 10.6 10.6 10.6
Photometer Output, pa. Air Air X X POC1,Y Y0.4 0.8 1.1 1.1 2.0
3.1 3.5 3.6 3.4 3.6 2.7 2.5 2.5 2.5 2.8 2.9 2.8 2.7 2.4 2.1
0.5 0.4
0.4
0.5 0.7 0.5 0.5 0.4 0.5 0.5
Useful Sensitivity,
x
2.7 2.7 2.5 2.3 1.6 2.2 2.1 2.1 2.0 2.1 2.4 2.3 2.3 1.9 1.6
Y-x X 6.7 3.4 2.3 2.1 0.8 4.4 5.3 5.3 4.0 3.0 4.8 4.6 5.8 3.8 3.2
~
VOL. 30, NO. 7, JULY 1958
1241
IO0 TlUE (SECONDS1
Figure 2.
No significant difference was observed in the stability of the two samples as both samples showed essentially constant composition over the duration of the test period of three months. IYDOLE SOLUTIONS. Tables I and I1 shorn that an acidified hydrogen peroxide solution has a much greater useful life than an alkaline solution derived from sodium perborate. Because the optimum pH for the indole to indoxyl reaction is approximately 10.7, it was necessary t o add an alkaline buffer to the indole solution. The measure of the effective stability of the alkaline indole reagent solution was based upon the fluorescence developed when the solution was used t o scrub phosphorus oxychloride from air, in a prototype gas alarm system. The solutions used for this purpose were: A. 1000 ml. of distilled water 100 ml. of acetone 150 ml. of 2-propanol 2 grams of indole 1242
Figure 3.
Fluorescence intensity vs. time
tilled water and 125 ml. of >propanol. The solutions were kept in borosilicate glass bottles in a constant-temperature oil bath controlled a t 40", 45", 50", or 60" C. In each case the temperature control was good to i~0.05"C. Aliquot portions of the solutions were withdrawn periodically and titrated with ceric ammonium sulfate solution, using a potentiometrically determined end point. A summary of the data is shown in Table I. HYDROGENPEROXIDE SOLUTIONS. Hydrogen peroxide is known to have greater stability in acid than in alkaline solutions. Consequently, Merck & Co. C.P. Superoxol (nominally 30% HZOZ in 0.1N HzS04) was used to prepare solutions containing approximately 0.035% of hydrogen peroxide. These solutions were then tested by the same procedure used for the Sodium perborate solutions, and data are summarized in Table 11. A further study was made of the stability of two Merck samples: Super0x01, c.P., received and stored at room temperature in a brown bottle with vent cap; and Superoxol, reagent, received and stored a t room temperature in a polyethylene bottle with vent cap.
ANALYTICAL CHEMISTRY
200
I50
250
300
T I M E (SECONDS)
10 grams of borax (Na2B407. 10 HzO) 10 grams of'sodium carbonate B. 1000 ml. of distilled water 250 ml. of ZDroDanol 3.5 ml. df Superoxol (13.3%
HzOz) The contaminated air used in these tests contained approximately 175 y of phosphorus oxychloride per liter. The actual concentration was determined by titration for each test, and all values of the photometer output were corrected to be equivalent to 175 y per liter. Fresh B solution was made up for each test. The pH of the effluent solution from the photometer was measured by glass electrode. The results of the tests are given in Table 111. It is apparent from column X of Table I11 that the fluorescence of the alkaline indole solution does not change appreciably over several days when the solution is stored a t temperatures of 40" C. or less. Kor is the useful sensitivity, as judged by the test with phosphorus oxychloride, seriously affected a t these temperatures. At 50" C., hornever, the data indicate a marked increase in fluorescence of the solution and a consequent loss in useful sensitivity. As it is not well established that phosphorus oxychloride is a satisfactory simulant for G-agents in the Schoenemann reaction, the usefulness of the above conclusions may be subject to question. Adequate facilities for safe handling of G-agents were not available, nor were better simulants than the phosphorus oxychloride known to the investigators, at the time of these experiments. Effect of Reagent Solutions on Various Materials of Construction. Table IV shows the results of a study of the effect on the stability of the reagent solutions of various possible materials of construction for gas alarms. The materials were judged by two criteria: the degree of direct attack on the test specimen by the two reagent solutions; and the increase in fluorescence of the solution
Fluorescence intensity vs. time
due to contamination from the test specimen. FLUORESCENCE DYNAMICS
The fluorescence intensity curve is typical for consecutive reactions where an intermediate product is being measured. The reaction is represented by the equation:
(I) Indole
(11) Jndoxyl OH
OH
(111)
Indigo White (Diindoxyl)
.1
(IV) Indigo
Species I1 and I11 are reported to be very fluorescent ( I ) , whereas I and IV are nonfluorescent. It is the concentration of the fluorescent compounds that is being measured. A study of the effect of pH on the rate of ffuorescence development showed the sensitivity of the system to agent to be related to the total boron concentration a t constant pH.9 As a consequence of the behavior of borax-boric acid buffers, the final study was conducted with carbonate-bicarbonate buffers. Figures 2 through 6 are the fluorescence-time curves for carbonate-bicarbonate buffers a t various final concentrations of hydrogen peroxide. The ratio of carbonate to bicarbonate was
9.02
PH I
-
50
3
P
'
/'
\ I
20
I I .
1
I
h
&
'ti 10.5,
I
I 5G
1 I GO
F I N A L 1123;
,
=
O.OZ$
I50
I
I
I
200
250
300
10
IO0
50
IO.i(B.
0.10%
I50
200
250
300
TlWE (SECONDS)
Figure 5.
Fluorescence intensity vs. time
varied while the total salt concentration was held constant. These curves are generally applicable to any instrument in which the carbonate-bicarbonate buffer system is used in conjunction with hydrogen peroxide and indole, for the mixed solvent specified above. So attempt was made to determine the effect of solvent composition. In using the curves of Figures 2 through 6 to estimate available sensitivity, it was postulated that the sensitivity to agent is proportional to the integrated area under each curve between the time limits imposed by the viewing time of the sample cell-Le., from the lag time to the final response time-for the solution flow rate to be used. In order to measure the lag volume and the viewing volume of the cell under dynamic conditions, the sample cell was first filled with water; then, a solution of quinine sulfate was allowed to flow into the cell through the absorber column in place of the normal reagent solution. The times for initial recorder response and for maximum recorder reading were noted. The products of these times and the solution flow rate give the cell volumes in question. A typical set of data for a sample cell is given in Table V. The sample cell characterized in Table V holds approximately 7 ml. of liquid, of which 3.75 ml. are in the viewing field of the photometer and 3.25 ml. are in the entrance arm of the cell. These are static volumes and are not representative of operating conditions when solution is flo\Ting through the cell. From the above data it is apparent that the dynamic lag volume is about half the static value, while the dynamic viewing volume is more than twice the static volume. This behavior is consistent with the probability of mixing of a solution admitted to the cell with the solution already in the cell. It is apparent also that the dynamic lag volume and the viewing
I PH
FINAL H,O,'
TIME(SEC~NDS)
Figure 4.
I
1 -
Fluorescence intensity vs. time
volume are substantially independent of solution flow rate in the range covered. For a particular sample cell and photometric system, the desired or minimum tolerable initial lag time will determine the necessary reagent flow rate. The selected flow rate and the corresponding vien-ing time will then determine the reagent composition required to develop and maintain fluorescence for maximum photometer response to agent. As an example, the procedure given above was applied to a prototype instrument using the cell for which measurements are given in Table V for a solution flow rate of 4.0 ml. per
Table IV. Effect of Reagent Solutions on Various Materials of Construction
Material Tested Aluminum Bakelite Brass Cold rolled steel Compar Monel Neoprene Nylon Phosphor bronze Polyethylene Rubber Buna N Gum Hycar Natural carbon Stainless steel
Resultsa U U U
u
vA A A U
U
A
A A A A
(18-8)
a
Vinyl plastics U U, unacceptable; A, acceptable.
minute. Under these conditions the lag time is 30 seconds and the final response time, 180 seconds. The areas under the fluorescence time curves of Figures 2 through 6 between the time limits prescribed are plotted in Figure 7. Maximum sensitivity for the system is obtained a t pH 9.5 and 0.10% hydrogen peroxide or a t pH 9.8 and 0.02% hydrogen peroxide in the final solution. The application of these data were experimentally verified by actual use in a prototype nerve gas alarm. INTERFERENCE TESTS
Two different A solutions were tested for sensitivity to possible interfering substances which may be present in an atmosphere being tested for GB. One of these was the original A, boraxcarbonate, solution (2.00 grams of borax, 2.00 grams of sodium carbonate, and 2.00 grams of indole in 1000 ml. of water, 150 ml. of 2-propanol, and 100 ml. of acetone) and the other was A', a bicarbonate-carbonate solution (3.57 grams of sodium bicarbonate, 0.43 gram of sodium carbonate, and 2.00 grams of indole in 1000 ml. of water, 150 ml. of 2-propanol, and 100 ml. of acetone.) The B solution, in each case, contained 0.04% of hydrogen peroxide and 250 ml. of 2-propanol. The p H of the borax-carbonate-peroxide mixed solution was 9.7, whereas that of the bicarbonate-carbonate-peroxide was 10.6, the final peroxide concentration in both sets of mixed solutions being 0.0270.
Table V.
Flow
Rate, Ml./Min. A 9.36 4.28 1.89 0.74
Dynamic Characteristics of Fluorescence Sample Cell Lag Final Response Viewing T%e, Vol., Time, Vol., Time, Vol.,
min. B
0.18 0.44 0.90 2.30
ml.
AB 1.7 1.9 1.7 1.7
min. C
ml. AC
min. C-B
1.16 2.80 6.25 15.1
10.9 12.0 11.8 11.2
0.98 2.36 5.35 12.8
VOL. 30, NO. 7, JULY 1958
ml. AC-AB 9.2 10.1 10.1 9.5
1243
Several gases and vapors Tvere passed through the monitor and the readings obtained were interpreted in terms of GB concentration. Table VI summarizes the interference tests. G A S ALARMS A N D ANALYZERS
Three different designs of continuous analyzers have been developed to meet various operating requirements. The B-17 gas alarm (Figure 8) was developed under contract during 1950. The chemical reagents used initially were investigated and specified by the Detection Branch of the Protective Division, Chemical and Radiological Laboratories, Army Chemical Center, Md. The alarm was intended primarily for use in production plants for safety monitoring and leak detection. One of the two reagent bottles is marked 27 in Figure 8. The solutions flow under constant head through a pair of thermostated capillary tubes, 6, into the mixing chamber, 4. The mixed solution flows downward through the absorption column, 5, the design of which originated a t the Chemical Warfare Laboratories. The effluent from the absorption column then flows into the fluorescence photometer, 9. The sample air stream in which the nerve gas is to be detected is drawn into the instrument a t 10 and passes through the flowmeter, 20, to the bottom of the absorption column. The sample air passes up the absorption column, 5, countercurrent to the reagent stream, and is effectively stripped of any nerve gas present. The air then passes through a filter and mist trap, 25, into the reciprocating sample pump, 24, and is exhausted a t 23. The photometer cell is essentially a borosilicate glass U-tube, the inlet arm of which contains a section of precision bore square tubing. The light from an incandescent projection lamp is collimated by a lens and mirror system and passes through the primary filter. The light from the reagent solution, emerging a t right angles to the incident beam, passes through the secondary filters to the photocell. Careful baffling
and filtering reduce the photometer response to scattered exciting light to a negligible value. The fluorescent light is detected by a Type 1P21electron-multiplier photocell, supplied by an adjustable high-voltage power supply whose output is stabilized to better than *0.02%. This power supply is stabilized by using a highgain balanced direct-coupled amplifier to bias a rheostat tube, to maintain constant current in a series string of voltage reference tubes shunted across the high-voltage output circuit. The output voltage is adjustable over a range of 550 to 1100 volts. The anode current of the multiplier photocell is measured by a strip chart recorder with a range of 0 to 0.5 pa, The recorder and controls are mounted on the face of the alarm cubicle opposite that shown in Figure 8. Optional zero-suppression is available in the recorder to buck out the small signals which result from the fluorescence of the uncontaminated reagents. An alarm is given when the nerve gas concentration rises above a preset value through the operation of a mercury switch by an actuating disk coupled to the balancing slide-wire in the recorder. A trouble light is actuated if the recorder balancing amplifier, photometer lamp, photocell, or photocell power supply should fail. The range of the instrument can be adjusted by varying the high voltage supplied to the photocell. In recent models of the B-17, a reference light path is provided m-hich can be used periodically to check the condition of lamp, photocell, and measuring system. The use of nerve gas for checking the range is avoided, in ordinary service, by the use of acidified quinine sulfate solutions as fluorescence standards. These solutions fluoresce in roughly the same spectral region as does the nerve gas-indole-peroxide solution. The quinine sulfate solutions are stable for many months. The instrument has been used in GB production plants with a full-scale
3.0
range of less than 1 y of GB per liter of air, and has given reliable performance. The fluorescence of the reagent solution under these conditions, for full-scale indication of the instrument, is about the same as that of a solution of 0.25 y of quinine sulfate per ml. of 0.1iV sulfuric acid. The B-25 gas alarm uses the same chemical and measuring principles as the B-17 and has the same sensitivity and stability, but was designed to be rugged enough for field transportation. The instrument is built in two parts; it can be operated with the two parts fastened together or separated by a moderate distance. The chemical unit contains all pumps, reagent supplies, and the photometer, while the electrical unit contains power supplies, microampere indicator, alarm components, and all electrical controls. In the reagent and gas-handling systems, only the reagent bottles, the air sample flowmeter, and the photometer cell are glass. All other parts are stainless steel. The height of the instrument has been reduced by using a double pump to meter the reagents before mixing, in place of the gravity feed used in the former instrument. Figure 9 shows the optical principles of the photometer of the B25, The part of the photometer cell shown is a glass tube of circular cross section with a flat circular Findow a t one end. The solution inlet and outlet are both a t the end of the cell opposite the window. A collimated cylindrical beam of ultraviolet light, of diameter smaller than the inside diameter of the cell, enters through the window. The fluorescent light output is collected in a ivhite-walled cavity whose cross section is a half cylinder. The light passes through the secondary filters into a second white-walled cavity containing the electron multiplier photocell. In practice, axes of the two cavities are skewed with respect to one another, and the photocell cavity is considerably
I
I
I I
2.0
un
1.0
-
20
51.
I
I
I GO
I50
I 200
PH 9.79
I 250
I 300
9 0
1244
Fluorescence intensity vs. time
ANALYTICAL CHEMISTRY
10.0 F I N A L pH OF
ilME(SEC0NQS)
Figure 6.
9.5
Figure 7.
10.5
SOLUTIONS
Relative sensitivity to agent vs. pH
larger in radius than the photometric cavity. This photometer arrangement requires only about half the volume of fluorescent material to give a photocell current equal to that of theB-17 photometer, using the same lamp and photocell in each. The reagent flow through this photometer can be about half that required in the B-17, rithout sacrifice of speed of response. The two arrangements are about equal in rejecting scattered light. Because the B-25 gas alarm must be operable from rotary converters or field power supplies of poor frequency stability, a special lamp brightness regulator is required. The regulator is an electronic feedback system in which the lamp current is controlled by the amplified output of a vacuum photocell which monitors the light output of the photometer lamp. Stabilization of the lamp output is better than 10.5% for input voltage variations from 95 to 125 volts. The frequency may vary from 55 to 65 cycles without appreciable effect. The lamp consumes more than 30y0 of the input power to the regulator system, so this regulator has a relatively high over-all efficiency. The measuring and electron-multiplier photocell power supply circuits of the B-25 gas alarm are substantially the same as those of the B-17, but recording is not provided. The most recent instrument of the .group described here is the B-38 gas analyzer, designed primarily as an analytical instrument for laboratory use. The chemical principles of the B-38 are the same as those of the two alarms described above, but an improved airsampling system is provided and a selfbalancing photometric system is used. Most of the parts of the reagent handling system of the B-38 are identical with those in the B-25 alarm. The air-sampling system, however, was required to handle samples at pressures differing by at least + 6 inches of water from atmospheric. This was accomplished byemploying a primary sampling pump to deliver at least 2.5 liters per minute of sample to a tee, one arm of which led to the 1-liter-per-minute sample intake of the analyzer system, similar to the B-25 system. The excess, more than 1.5 liters per minute, was exhausted through the other arm of the tee. The materials in contact with the sample in this system were carefully tested to &&sure that there would be negligible holdup or loss of nerve gas sample. The instrument samples satis factorily from systems at pressures differing from atmospheric by as much as 30 inches of water. The photometer of the E38 gas analyzer employs the optical nullbalance system mentioned above. The sample cell, containing the flowing reagent solution coming from the gas absorber, is identical mith that used in the B-25 gas alarm. A reference cell, optically similar to the sample cell, contains a stagnant acidified quinine
Table VI.
Possible Interfering Substance COX HC1 Gasoline Coned. HNOI Coned. CHCOOH Concd. "&OH
1
I
Interference Tests Monitor Response in Equivalent y of UB per Liter BoraxBicarbonateConcentration or Sampling Time carbonate carbonate 100% 0.06 0.01 7000 ?/liter 0.14 0.03 0.01 0 3 minutes of vapor 0.16 0.11 saturation 0.02 *0.01 -0.02
*o .01
1 2 3 k I
b 1
a t
10
I1 110101 A 4
Figure 9. Schematic diagram of 8-25 photometer optical system 12
Figure 8.
8-17 gor olorm
sulfate solution as a fluorescence standard. The photometer arrangement is shown schematically in Figures 10 (illumination system) and 11 (light collection system). In Figure 10, the light from the projection lamp is collimated by a lens, and the near ultraviolet component, selected by the primary filter, passes through two separate circular apertures into the two cylindrical liquid cells. Figure 11 shows the light collection and flickerine svstem. Each liauid cell is contaiged "in a separate w&ewalled photometric cavity. The light from each passes through a secondary filter and through a common rotating Polaroid filter disk, turned by an 1800r.p.m. synchronous motor. The light from each cell then passes through a separate fixed Polaroid filter and enters the photocell cavity. The fixed Polaroid filter in the sample cell light path is oriented at right angles to the one in the reference cell light path. In addition to these parts, there is also a balancing shutter, positioned by a two-
phase servomotor, placed in the light path between the reference cell and the photocell 80 that it reduces the amount of light passing from reference cell to photocell. Twice during each revolution of the rotating Polaroid filter, the plane of polarization of the light is such that i t suffers minimum attenuation in passing through the fixed Polaroid in the sample cell light path; a t these times, however, the light passed by the rotating Polaroid is almost completely cut off by the fixed polarizing filter in the reference cell light path. Likewise, twiceduring each revolution of the rotating Polar*:.l nrh-n :+ VOL. 30, NO. 7, JULY 195
is a t 90" to the two positions just r e ferred to. the lieht from the reference i.rII siiflers iiiiiiinium attenuation in pwiiig tln. fixrd I ' h r o i d , and the light frointlir ?niiipkrrIl is highly nttrnuatrd. If the balancing shutter in the refer'ence cell light path is so adjusted that the maximum light falling on the photocell from the reference cell is exactly equal to the maximum light falling on the photocell from the sample cell, there will be no variation in the total light falling on the photocell during the revolution of the polarizing filter, and a steady current will flow in the photocell anode circuit. On the other hand, if the maximum emission reaching the photocell from one photometer cell exceeds that from the other, an alternating component of photocell anode current will he generated. This alternating current is applied to a balancing servo-amplifier. The amplifier drives the servomotor, referred to above, to position the balancing shutter. The amplifier output is so phased that the balancing shutters open or close as necessary to restore the system to balance. The B-38 photometer is shown in Figure 12. Some parts have been removed so that the liquid cells are visible. The quinine sulfate filled reference cell is shown a t 71, and the sample cell is a t 67. The reflecting part of the photometric cavities is shown, removed from the assembly, a t 58. As the quinine sulfate reference solution can be made any strength necessary, most of the space available is devoted to the cavity, 57, containing the sample cell, maximizing the light collection from this cell. The lamp housing is a t the right, 54. The rotating Polaroid is contained in the large circular housing, 69. The servc-halancing motor is behind the lamp housing. The position of one of the shafts connecting the servomotor to the balancing shutters is used as a measure of the fluorescent brightness of the reagent solution in the sample cell relative to that of the quinine sulfate solution in the reference cell. The shutters are positioned hy means of a cam, the shape of which is chosen to make the balancing shaft angle linear with respect to relative fluorescent brightness of the two solutions. A pair of potentiometers, driven by the balancing motor shaft, feeds signals to an indicating meter and an auxiliary recorder, respectively. The recorder may be located a t a point remote from the analyzer. The meter and recorder are calibrated in terms of nerve gas concentration. Two ranges are provided in the instrument. On the more sensitive range, the maximum possible amount of light from the sample cell of the photometer is allowed to reach the photocell. On the other, a range shutter between the sample cell of the photometer and the photocell is partially closed. The ratio of the two ranges is ordinarily set a t 1246
ANALYTICAL CHEMISTRY
10 to 1, but is easily adjusted to other values. if reanired. T h e elec&onic components of the B-38 are much simpler than those of the B 1 7 and the B-25. The photocell power supply is regulated only by a quiescent saturating-type voltage regulator, which also stabilizes the photometer lamD voltaee. The Dhotocell
Photocell f a t i e has no effect on the measurement in the optical null-balance photometer, so that the photocell output current can be made as high as is permitted by signal-to-noise ratio considerations. As a result, the balancing amplifier in the B-38 instrument need have less gain than the balancing amplifiers in the recorders and indicators in the previous instruments. The signal-to-noise ratio of the output current of the multiplier photocell used in this instrument is too low to give good servo-action using a conventional balancing amplifier. The effects of this poor signal-to-noise ratio, as well as those of some deficiencies in the behavior of the polarizing filter material, are obviated by the use of the quadrature-rejecting type of balancing amplifier invented by Williams (4). This amplifier p e d i t s the instrument to operate with a dead space equivalent to less than 0.01 y of GB per liter of sample air. The full-scale range of the instrument may be set below 1 y of GB per liter of air. USE OF INSTRUMENTS
The instruments described are the
types a t present used for personnel protection in the plants manufacturing GB for the U. S. Army Chemical Corps. Their sensitivity and stability are so good that they are used for detection of le& in large batches of loaded munitions, where the leakage from a single container must be detected in spite of very large dilution. These instruments are also used for air monitoring by the Armed Forces in other fixed installations. ACKNOWLEDGMENT
The alarms and analyzers described were developed by k e d s & Northrnp
Figure 11. Diagram of fluorescent light collection system, 8-38 nullbalance photometer
"9. FI
60
Figure 12.
8-38 null-balance photometer
Co. under research contracts with the U. S. Army Chemical Corps. Many persons contributed t o these developments, in particular: F. H. Krantz, Will McAdam, W. E. Proctor, J. F. Spear, J. Re Stewart, v. Underkofier, Joseph Klein, David Clements, and J. E. Stringer of Leeds & Northrup Co.; and Saul Hormats, Solomon Love, and v. E. Bowman of the Chemical Warfare
Laboratory (formerly Chemical and Radiological Laboratory), Army Chemical Center, Md. LITERATURE CITED
(1) Gehauf, B., Goldenson, J., ANAL. CHEM.29, 276 (1957). ( 2 ) Hardy, A. c.7 J . Opt. Am. 18, 96 (1929). (3) Ibid., 25, 30s11 (1935). (4) Payne, J. F., Jr., Williams, A. J., Jr.
Trans. Am. Inst. Elec. Eng. 72,
Part I, 611-15 (1953). (5) Sartori, M. F., Chem. Revs. 48, 225-57 (1951). (6) Wilson, G. B., Gehauf, B., Roeggeberg, H. C., “Analytical Method for Nerve Gas-,” Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., February 1956. for review October 26, 1956. RECEIVED Accepted December 18, 1957.
Titration of W e a k Bases in Strong Salt Solutions FRANK E. CRITCHFIELD and JAMES B. JOHNSON Developmenf Department, Union Carbide Chemicals Co.,Division o f Union Carbide Corp., South Charleston, W. Va. ,Weak bases, such as aniline, can b e titrated potentiometrically or by the use of indicators in strong aqueous solutions of neutral salts (6 to 8M). The indicator method has been applied to bases with ionization constants of 1 X 1 O-’I. The potentiometric method is applicable to bases with ionization constants as low as 1 X 10-l2. The method has been applied to the differentiating titration of the individual amino nitrogens of polyfunctional amines and to mixtures of strong and weak amines.
0
bases with ionization constants less than 1 X are too weak to be titrated acidimetrically in water. Compounds of this class are usually analyzed by titration in acidic or nondissociating-type solvents. Under these conditions weak bases give sharp potentiometric breaks and can also be titrated using indicators. A recent finding in these laboratories has shown that neutral salts enhance the potentiometric break for the titration of weak bases with aqueous mineral acids. This effect and some of its analytical applications are discussed here. RDINARILY,
APPARATUS AND REAGENTS
All potentiometric titrations were performed using a Leeds & Northrup line-operated pH meter equipped with glass and calomel electrodes. All salts were Baker’s Analyzed reagents, J. T. Baker Chemical Co. DISCUSSION
The ability of a neutral salt to enhance the potentiometric break in the acidimetric titration of a weak base is demonstrated in Figure 1. Curve 1 is the potentiometric titration of aniline in water with 0.5N hydrochloric acid, Obviously, the amine is too weak (R =
1.
In water
2.
In 7 M aqueous sodium iodide
3.8 X 10-lO) to be titrated satisfactorily under these conditions. The same titration in 7 M sodium iodide gives a potentiometric break that is satisfactory for precise analytical measurements. The potentiometric curves in Figure 2 were obtained for the titration of aniline in solutions containing various concentrations of sodium iodide. For sake of clarity, the curves have been displaced along the abscissa and only the end point portions are shown. These curves show that the enhancement of the potentiometric break by neutral salts is a definite function of the salt concentration and is noticeable a t a sodium iodide concentration of 1M. For this series of curves the sharpest potentiometric break is obtained a t a concentration of 8M sodium iodide. An examination of the curves in Figure 1 shows that the initial portions of the curves for the titration of aniline in water and in 7 M sodium iodide are superimposible. This suggests that the
pH of weak bases is independent of salt concentration. After aniline is neutralized, the pH of the titration solution is dependent upon the amount of excess hydrochloric acid present and is independent of aniline hydrochloride. The curve in Figure 3 shows that the p H of 0.0192M hydrochloric acid decreases linearly with sodium iodide concentration. At a concentration of 7 M the solution has a pH of zero. An aqueous solution containing the same concentration of hydrogen ion has a pH of 1.8. An inspection of curve 2, Figure 1, shows that, in the titration of aniline in 7 M sodium iodide, the addition of 4.0 nil. of excess hydrochloric acid, which makes the solution 0.0192M, lowers the pH of the titration solution to -0.10. This value corresponds with the value of zero predicted by the curve in Figure 3. Although the explanation of the enhancement of the potentiometric break by neutral salts is unknown, the mechanism must be associated with the decrease of pH of mineral acids by neutral salts, because the magnitude of this decrease shows up in the potentiometric break. Investigations are now in progress in these laboratories which should explain the nature of this effect. Any salt of a fairly strong base and a strong acid will enhance the potentiometric break for the titration of weak bases. Among the effective salts are sodium chloride, lithium chloride, sodium iodide, and calcium chloride. Salts of strong bases and weak acids inhibit the potentiometric break, as would be expected because they are appreciably basic. Sodium sulfate, normally considered a salt of a strong acid and a strong base, also inhibits the break in the titration of aniline. This suggests that the acid from which the salt is derived must have an ionization constant greater than 1 x lo-* in order to exhibit this effect. VOL. 30, NO. 7, JULY 1958
1247
