Results corresponding with vacuum fusion analysis are obtained because the reaction is the same in both methods. In the inert gas fusion method the presence of additional graphite in the crucible has been found to be the key to satisfactory results. This may also be suggested as an aid to certain vacuum fusion difficulties. Although a 5-minute fusion time was used on the analyses reported in this work, even shorter times can be used on some steels with low contents of stable oxides. For regular analysis of low-
oxygen material, greater sensitivity of measurement can be obtained by making the manometer of smaller sized capillary. The advantages of inert gas fusion analysis are its rapidity and simplicity over any other available method. Agreement with vacuum fusion analyses should not be mistaken for proof of accuracy in measurement of the true oxygen content of samples. No satisfactory means of establishing the oxygen content of steel samples on an absolute basis has been found, and vacuum fusion
analysis has bccn accepted with rescrvations on this point. LITERATURE CITED
(1) Natl. Bur. Standards, Circ. 398 (1954). (2) Singer, L., IND.ENG.CHEM.,API'AL. ED. 12,127-30 (1940). (3) Smiley, W. G., ANAL. CHEM. 27,
1098-1102 11955).
( 4 ) Smiley, IT. G., Y'uclear Sci. Abstr. 3. 391-2 - - - - ilR491. ( 5 ) Thompson, J. G., Vacher, H. C., Bright, H. A., J . Research Satl. Bur. Standards 18, 259-93 (1937). RECEII-SDfor review July 30, 1957. .Iccepted January 10, 1958. - 2
\ -
Evaluating Concentrations of Spectrally Absorbing Vapors in Dynamic Systems Spectrophotometric Techniques and Equipment F. A. GUNTHER and R.
C. BLINN
Department of Entomology, University of California Citrus Experiment Station, Riverside, Calif.
M. J. KOLBEZEN Department of Plant Pathology, University of California Citrus Experiment Station, Riverside, Calif.
C. W. WILSON Research Department, Sunkisf Growers, Inc., Ontario, Calif,
R. A. CONKIN Monsanto Chemical Co., St. louis, Mo. ,Vapor-phase fumigants supplied by in-package sources are used to protect citrus fruit against decay during shipment and storage. These fumigants may be sorbed by both the fruit and the package, and rates of generation from the source, rates of disappearance within the package, and magnitudes of sorbed residues must be known to permit design of a satisfactory release system as the in-package source. Dynamic techniques were used to evaluate one of these fumigants, ammonia in air. They were directly applied to the determination of both sorption and desorption of ammonia by various fruits, vegetables, and packaging materials as well as to the evaluation of release patterns from candidate ammonia generators designed for in-package use. The techniques are based upon both a manually operated spectrophotometric apparatus and an automatic multisampling apparatus incorporating a ratio-recording spectrophotometer.
K
fumigants for preventing or suppressing the growth of the blue-green molds Penicillium digitatum NOWN
Saccardo and P . italicum Wehmer during shipment and storage of citrus fruits are most effective when continuously present in sufficient quantity in the vapor phase. Development and exploitation of these fumigants require information on concentrations of these vapors in the atmospheres within commercial packages and on depletion resulting from sorption by both fruit and packaging materials. Direct ultraviolet spectrophotometric measurement a t 204.3 mp (1) has been applied to ammonia in small concentrations in air. This procedure promises to be applicable to any fumigant or vapor selectively absorbing spectral energy in any portion of the available ultraviolet regions. AMMONIA AS A FUNGICIDE
I n its present application as a fumigant, ammonia is a fungicide (3, 6). Both as vapor and as aqueous solution, it will kill many microorganisms, including a number of fungi, if the concentration is sufficiently high ( 5 ) . However, ammonia vapor can damage many fresh fruits and vegetables (4). For optimum preservation of citrus fruit, careful control of duration of ex-
posure and concentration of ammonia is required, for they must be great enough to be effective but low enough to minimize rind damage ( 7 ) ; how these requirements are met by the present inpackage generators of ammonia is reviewed elsewhere ( 2 ) . To be practicable, the source or generator must establish and then maintain an optimum concentration of ammonia. This requires supplying the amounts sorbed by both fruit and fiberboard package or carton, in addition to that required for satisfactory fungicidal action. Study of the rate of depletion of continually renen-ed atmospheres of constant ammonia contents was selected to give results most easily translated into commercial practice. I t seemed best to treat the fruit and the packaging materials separately and then to apportion the results to individual package units in any combinations required. BASIC TECHNIQUE
To determine amounts of ammonia in an ammonia-air stream before and after it had been depleted by exposure to either fruit or fiberboard an ultraviolet spectrophotometric procedure was used VOL 30, N O . 6, JUNE 1958
1089
Table I.
Absorption
by
Carbon Dioxide, Water Vapor, Air, and Ammonia from 202.3 to 206.3 M p
Absorption* a t Several Wave Lengths 202.3 mp 203.3 mp 204.3 r n w 205.3 mp 206.3 nip Carbon dioxidec 0.056 0,055 0.053 0.051 0.051 Water vapord 0.050 0.049 0,048 0.047 0,047 Air8 0.058 0.057 0 054 0.053 0,052 Ammonia*)f 0.036 0.050 0 568 0.087 0.000 a In 10-cm. sealed silica cells, in Beckman Special Model DUS spectrophotometer purged with dry nitrogen (1). In absorbance units, corrected for instrument background of 0.043 unit. c Anhydrous and not diluted (25" (3.). d In unfiltered air of 97% relative humidity (25' C.). e Anhydrous and filtered (25" C.). f One volume of anhydrous ammonia in 19 volumes of dry nitrogen (25" C.) and in 1-cm. sealed silica cell. Gas.
(1). This method is based upon strong absorption of ultraviolet energy a t 204.3 mp by ammonia vapor in air. Respiration products from living citrus fruits do not interfere, as shown by the comparative absorbance data in Table I ; volatile terpenoid products from intact or wounded citrus fruits do not interfere (2). In principle, an air-ammonia stream was spectrophotometrically analyzed for ammonia content, then passed over the sample and re-analyzed for ammonia depletion. In principle and under full equilibrium conditions, then, the difference between the incoming and outgoing ammonia concentration represents the amount of ammonia sorbed by the Sample. Appropriate calibration of e q u i p ment can afford this value in milligram. of ammonia ( I ) per unit time. AIR-AMMONIA MIXTURE
Air containing the required concentration of ammonia mas introduced into the sample container a t rates such that small but readily measured depletion of ammonia took place upon contact with the sample. The parent air stream was metered by passing the laboratory air
supply through a standard pressure regulator followed by a large surge tank. a filter, a standard needle valve, and n flowmeter to obtain a constant flow rate of 2.0 liters per minute for each sample container in the system. Ammonia n a s not easily regulated. Less than 0.5 gram per day was required to produce atmospheres of the mayimuni usable concentration. A regulator that would deliver uniformly a t adjustable rates of this order of magnitude and a t a constant low pressure was not commercially obtainable, even if ammonia diluted n-ith 99 volumes of dry air was used. The solution lay in the constant preqsure in a therniostated lecture bottle of liquid ammonia. Barely cracking the cylinder valve afforded a very nearly constant flow, which was then put under a slight and constant pressure by a 50cm. column of light mineral oil. From this very lov pressure source, the required amount of ammonia was bled through a special valve made from a
long-taper sewing needle, spring-loaded to oppose the adjusting screw (Figure 1). SORPTION APPARATUS
Preliminary trials indicated that sorption by fiberboard was very rapid initially but reached a slow steady state in less than 200 minutes. Fruits required more time for equilibration and sorbed less ammonia per unit weight of sample. Because of the greatly different periods of time required for approaching sorption equilibrium, the apparatus for each tJ-pe of evaluation was specifically deqigned for the purpose. Fiberboard Apparatus. Amounts of amnionia sorbed by fiberboard are logically related t o moisture content (H-bonding and "solution") of the fiberhoard as well as to the presence of acid-Qizing materials (salt formation). The moisture content of a sample of fiberboard n.ill, in turn, depend upon the relative humidity of the atmosphere surrounding the sample. Relative humiditj- n ithin cartons of citrus fruits under simulated commercial conditions 1.arie.; from about 50 to 100% in the temperature range 47" to 75" F. ( 2 ) . Because a saturated solution of ammonium sulfate maintains a nearly constant relative humidity of S070 over a n ide range of temperatures, this mixture n a s used to obtain and maintain moisture relationships in these studies. The 30-gram fiberboard samples n ere therefore conditioned for 2 days a t room teinperature over saturated ammonium sulfate solution, and the air stream n-ithin the apparatus described below was similarly humidified before addition of the ammonia.
@-- - - - - - S c r e w Gum Rubber Tubing
--Sewing
--3/16"
L L - - i , 8 ' '
Needle
Copper T u b i n g
C o p p e r Tubing
Solurnled
(NtlqI2S04 SOIYtlO"
Figure 1. Special needle valve to regulate microflow of ammonia gas from thermostated lecture bottle Ammonia entering at bottom of valve
1090
ANALYTICAL CHEMISTRY
Figure 2.
All-glass fiberboard apparatus
Filter, humidifier, and 2-liter sorption chamber immersed to stopcock A in constant-temperature water bath. Desired bath temperatures obtained: 2 5 ' C., by thermostat and heater; 1 2 ' C., b y circulation of refrigerated water through large copper coil in water bath opposed b y heater; 0 ' to 4' C., b y continual addition of shaved ice
Figure 3. Four-unit fruit sorption a p paratus with recording spectrophotometer
EQUIPMENT.A schematic diagram of the all-glass fiberboard apparatus is shown in Figure 2. The analytical sample of incoming air-ammonia mixture was withdrawn through stopcock R from the tube attached at the entrance of the stream into the sample chamber, circulated through the spectrophotometer cell by means of a diaphragm pump, then returned t o the system through stopcock C . The outgoing analytical sample, after partial depletion of ammonia content by the fiberboard specimen, was withdrawn through stopcock B from the exhaust vent. After passage through the spectrophotometer, this outgoing sample !\as vented through stopcock C. To avoid drawing laboratory air back into the system, care was taken t o ensure that the volume of sample drawn
Ai.
*IOC"I3
1 r L CirC
Figure ratus:
4. Single unit of automatic fruit sorption a p p o -
A-A'.
Glass rods l o support fruits in Con.t.nt-temper.tule, wateriacketed chamber, D B. Cyclone reporator to remove oil droplets entrained in ~irculating pump exhaust C. Gas-rtreom b y p o s into pump D. Water-iocketed fruit chomber E, F, G. Pressure-equalizing vent, (system operaterunder slight p o t i t i ~ e pre.surel
H.
High-Row, rater-jacketed condenser 1. A i l d a s s solenoid v o l ~ e . 1. Air inlet to permit reproducible settings of Rowmeters since yacuum pump is constant speed K. Capillary condensed-moisture return to gor rtreom
through the spectrophotometer in unit time was always less than the volume per unit t i e of the incoming stream. OPERATION. T o operate this apparatus, clean air was introduced into the system at the required rate-.g., 2.0 liters per minute-until the instrument blank a t 204.3 mrr became constant. Ammonia was then turned into the system by judicious regulation of the fine needle valve (Fignre 1) and was adjusted to the precisely needed amount as drtermined by spectrophotometer reading relative to a calibration curve ( I ) . During this period the effluent air from the spectrophotometer was exhaustcd t o the laboratory ventilating system. The sample of fibcrboard had 2 days previously been cut into a/4 x 8 inch strips for passage through the lefthand 24/40 standard-taper opening in the specimen chamber and then conditioned at the desired humidity. When thc atmosphere within the system had reached equilibrium, as shown by spectrophotometric comparison of incoming and outgoing air streams at the desired level of ammonia, the conditioned sample was quickly weighed and charged into the apparatus; periodic readings of the effluent air were then begun. Essential steps in the operation of the apparatus are: The beginning time is noted. Stopcocks B and Care turned so that the ammonia stream exhausting from the sorption chambcr is directed to the spectrophotometer cell and then vented (position I). Readings of this outgoing ammonia are made on the absorbance scale every 5 minutes (y axis) and are plotted against time (z axis). Every 30 minutes stopcocks B and C are adjusted t o obtain a check reading of the incoming ammonia stream (position 11). Thc stopeocks are then readjusted t o position I to obtain outgoing readings at 5-minute intervals. 4 t 30-minntc intervals, 1 ml. of water is poured into the tube through which the air stream enters the humidifying c h a m k r (entry port not shown); this is necessary to keep this air-entrance tube free of ammonium sulfate crystals which can plug the opening and impcde air flow. Aftcr 200 minutes, stopcock A is turned so that ammonia may not enter the system. Readings are made on the absorbance scale as rapidly as possible during the next 10 minutes, with stopcocks B and C in position I, as initial desorption of ammonia by fiberboard is rapid. After a 200-minute desorption period the sample is removed, immediately wrapped in a tared piece of aluminum foil, and weighed. The sample is then nnwrapped, dricd a t 100' C. for 5 hours, remrapped in the foil, and reweighed to establish moisture content. Results obtained by this scnsitive technique have been reported (2). Fruit Apparatus. Additional mechanical problems associated with sorption of ammonia by intact citrus fruits included t h e fact t h a t longcr exposures and much larger samples were required, because sorption was VOL. 30, NO. 6, JUNE 1958
1091
both less and slower than with fiberboard; use of intact fruits required a large opening in the sample chamber, which precluded establishing the airammonia equilibrium within the chamber prior to introduction of the sample. The moisture given off by most fruits and vegetables from normal respiration sufficed to maintain the internal relative humidity in the desired range, obviating necessity for addition of moisture to the incoming air streaIu.
~
-F"--
--
- - - - - - - Solid GasketTo Seal Vacuum Control
_-__
4 - - - - --,+
-=
Cross-section Through A - B
~
I
\
- _ - - - - - - - -O i l i
ANALYTICAL CHEMISTRY
Wick
Splash Guard
---_
EQUIPMENT.To acconunodate the
1092
Vent From Pump C h a m b e r
- - - - - - - - 1/32''O i l R e t u r n F r o m T h i s O i l S e p a r a t o r T o B o d y O f V a n e Pump
\
long intervals required to evaluate sorption equilibria as well as the anticipated large number of replicates, a four-unit ammonia apparatus was used for fruits and vegetables. This apparatus incorporated an automatic Beckman DK-2 ratio-recording spectrophotometer ( I ) , modified in these laboratories to contain B 17-hour time drive in place of the usual wave length drive and t o be continuously purged with dry nitrogen. The complete apparatus is shown in Figure 3. A schematic diagram of a single unit of this essentially all-glass apparatus is reproduced in Figure 4. The sample chamber measures 4 X 42 inches, with a volume of nearly 8 liters. With this large volume the usual flow rate of only 2.0 liters of air-ammonia mixture per minute would result in sorption strata among the units of the sample; rapid internal circulation of the air mixture within the chamber was therefore established at 350 liters per minute, by means of special circulating pumps, with 2.0 liters of fresh mixture per minute being introduced into this circulating flow. These circulating pumps shown schematically in Figure 5 were truck windshield wiper booster pumps modified for continuous duty by drilling several l,'Tinch ventilating holes in both heads of the motor case and a 'lsrinch hole from the oil separator in the air-discharge stream to the body of the vane pump close to the shaft to provide for continuous lubrication of the pump by rapid return of the oil entrained by the air stream. If left in the system after shutdown, this circulating oil would drain into the body of the pump and prevent restarting. Oil \r as therefore removed a t the end of cach run by a small drain through the periphery of the pump case, as shomn in Figure 5 ; this drain WVBS stoppered during operation. For operation, 1 nil. of light oil was injected with a hypodermic syringe and needle through a cork plug inserted into the oil-separator chamber of the pump. This quantity of oil was small, to reduce errors due to the slight solubility of ammonia in the oil used, and was constant to provide compensation when comparing fruit runs and blank runs by the procedure described below.. The motors on these pumps were designed to use 6volt direct current power, but when force-ventilated with compressed air jetting into one of the bottom ports (Figure 5 ) they ran continuously without overheating on 115/6 volt alternating current radio filament
Off
Figure 5. E l e c t r o - V o c Model
Upper Bearing Housing
_ _ - _ - Oil Drain
To E m p t y P u m p After E a c h U s e
Rebuilt high-capacity circulating pump EV-105 for 6 volts; T r i c o P r o d u c t s Corp., B u f f a l o , N. Y.
* - - -Microswitch
Metal Washer
@
! ?
Q
8
(f+----Solenoid
Valve
I
Figure 6. Clock-driven actuator for sequentially diverting air (solenoid 5) or air-ammonia mixtures (solenoids 1 to 4) through spectrophotometer cell
transformers with output ratings of 10 amperes. The sample chamber and the circulation return tube (D and H, Figure 4) were jacketed for temperature control. Overcooling of the air stream entering the pump was necessary to compensate mechanical heating by the pump. When operation was a t low temperatures, some of the moisture in the circulating air was condensed in the descending heat exchanger; if permitted to pass through the pump, this water became emulsified in the oil and stripped out some of the ammonia. This condensed water was collected in separator C, returned to the pump discharge through the watersealed capillary tube, K , and thereby evaporated again in the warmer pump discharge. OPERATION. A constant 1.5-litersper-minute of the incoming air stream was sent through that spectrophotometer cell ordinarily used for the sample. Through the reference cell were sequentially sent the gases exiting from the sample chambers a t a constant flow
of not more than 1.5 liters per minute, interspersed with air purges. This sequential action was obtained by means of all-glass solenoid valves ( I , Figure 4) in the air-ammonia streams activated by a clock-driven mechanism shown schematically in Figure 6. The above arrangement of spectrophotometer cells, in which an air-ammonia mixture of constant composition introduced at constant rate was periodically and automatically compared to a series of varying references, had several advantages. Comparisons of the incoming airammonia mixtures with the air-beforeammonia as reference afforded ammonia concentrations as positive plots. Comparisons of the incoming airanimonia'mixtures with outgoing mixtures as reference afforded sorption values as positive numbers. During desorption the air supply without ammonia passed through the sample
Figure 7. Typical chart record of sorption-desorption by lemons, at two concentrations of ammonia, with ancillary data
cell and was compared with stripping air that had passed through the sample chambers; desorption-plus-dilution therefore plotted as negative values, as Model DK-2 is a ratio-recording instrument and is readily set to plot both above and below a zero axis. The DK-2 spectrophotometer was calibrated for trace quantities of ammonia under operating conditions of the entire apparatus by diverting the amiiionia-air sample stream emergent from an empty sample chamber through a Reckman DGS spectrophotometer used for fiberboard studies (I). The operating calibration curve for the DK-2 instrument in parts per million of amriionia per unit vertical displacement of the recorder pen was represented by a slope value of 7.4, with origin offset from zero deflection by about 0.05 unit. This method is sensitive to 7 p.p.m. of ammonia, or 5 y per liter of air ( I ) . Essential steps in the operation of the apparatus are as follows: Allow spectrophotometer to warm up for 1 hour. Insert stainless-steel wire baskets with samples and close each sample chamber securely. Turn on water-circulating pump for all condensers. Turn on reference sample pump and air-purge solenoid (No. 5, jump circuit not shown in Figure 6), 11hich is the sample chamber bypass; adjust air flow through spectrophotometer cells to not more than 1.5 liters per minute, using flowmeters and bleeder stopcock on reference sample pump. Adjust incoming air pressure
to distributing manifold to 2.75 pounds per square inch; adjust each flowmeter for sample tubes to 2.0 liters per minute and for spectrophotometer to not less than 1.75 liters per minute. Turn on circulating pumps for all sample cham' T , 0.1 bers. With instrument on % time constant, range 0 to loo%, sensitivity 0.50, and sample shutter closed, adjust zero; open sample shutter and adjust 100%; switch instrument to absorbance and to -0.3 to f0.7 absorbance range. Open ammonia cylinder valve and adjust special needle valve (Figure 1) to desired rate of ammonia flow. lF7ith scale expansion in neutral, set recorder-pen carriage to desired position Adjust wave length to maximum response in the 204.3-mp region Turn on chart drive and timer (Figure 6) and turn off solenoid 5. Re-engage scale expansion to the 2>( position and loner recording pen to the paper. The instrument will now autoniatically and continuously record in sequence the ammonia concentration entering and leaving each fruit-sample chamber. Normally, samples are exposed to ammonia for a t least 4 hours, n hich is sufficient time to indicate absorption characteristics of the sample. The completed absorption chart n-ill consist of two essentially parallel rov s of "points," with the vertical span between consecutive points proportional to the ammonia concentration in a given chamber. Typical charts ( 2 ) of ammonia sorption-desorption records are reproduced
in Figures i and 8. .4ir purges introduced after each sample draught are represented by the uppermost rows of points. Air purges were of t x o durations: Each narrow point indicated change of sampling from one chamber t o another and represented a 1-minute purge of the referenw cell; each broad point indicated the start of a new cycle and represented 3 5-minute purge of the rcferencc cell t o bring the spectrophotometer into true equilibrium with air as reference and thereby to indicate the ammonia content of the incoming air stream. Sample chambers recorded squentially across the bottom of the chart, but because chamber 1 was always the reference chamber, the series of points representing it served as the base line. Results obtained by this technique h a m been reported ( 2 ) . DISCUSSION
The objective in the design of both types of apparatus was maintenance of continuous and constant flow rates of air-ammonia mixtures across the saniples, nith periodic deflection of portions of these streams through the measuring spectrophotometer. Dilution of gas mixtures in sample chambers a t the beginning of both sorption and desorption portions of a run required elimination or compensation to establish true rates of ammonia eschange with the substrate. VOL. 30, NO. 6, JUNE 1958
1093
c
I
12
Figure 8.
Typical chart record of protracted sorption run with lemons at a single concentration of ammonia
.
7
.C.
.- .
.
. .
.
.
-
Y Figure Chamber follows: Zone A. Zone E. Zone C. Zone D. Zone E. Zone F.
1094
9.
Dilution-compensation run
1, empty; chamber 4, fllled with paraffined light bulbs to approximate size and volume of sample; chambers 2 and 3 as Instrument blank run with chambers 2 and 3 containing lemons 250 p.p.rn. of ammonia in sorption cycle with chambers 2 and 3 containing lemons Desorption cycle from exposure to 250 p.p.m. o f ammonia with chambers 2 and 3 containing lemons Same as zone A Same as zone 6 but with chambers 2 and 3 empty Same as zone C but with chambers 2 and 3 empty
ANALYTICAL CHEMISTRY
1
I
I
1
n ere comparatively small in practice. Deviations from a standard dilution curve prepared M ith the sample chamber empty were usually not significant. These sets of curves were always used to determine net fiberboard desorption
1
6% *
I
1
C2
C4
I 06
l l 38 Z
I 23
42
HOLr5
Figure 10. Effect of diluting airammonia mixture with air or of diluting air with ammonia in a light-bulb filled chamber vs. an empty chamber Dilution of air-ammonia with air Chamber with light bulbs Dilution of air with ammonia 0 - - -Chamber empty X-.- Chamber with light bulbs
0-
K i t h fiberboard, significant dilution of the air-ammonia stream a t the start of a sorption run was avoided by charging the sample into the chamber with minimum disturbance of the previously established internal atmosphere. Contamination of the stripping air stream with residual air-ammonia mixture during a desorption cycle could not be avoided, but the volume of the sample and the total volume of the system relative to the rate of introduction of gas mixture
The second much larger system presented different problems. For example, the fruit or vegetable sample could not be introduced into a prestabilized atmosphere, and the volume of the system compared to the over-all throughput of air-ammonia mixture usually represented a ratio of about 6 to 1. Dilution of the ammonia stream with air a t the start of each cycle was therefore a large factor; deviations of this factor from one run to another n-ere”often significant and required compensation ( 2 ) , as shown in Figure 9. Similar considerations applied to desorption cycles. In general, first-hour sorption and desorption data required correction for this dilution factor, but after the first hour corrections for both dilution and variation among chambers were negligible. Detailed scrutiny of the desorption cycle in chambers filled with paraffined light bulbs indicated that dilution of the air-ammonia mixtures with air, or of air with ammonia, was at substantially the same rate as for completely empty chambers, as shown in zones C and F in Figure 9 and analyzed graphically in Figure 10.
ACKNOWLEDGMENT
The authors gratefully acknowledge technical and other assistance from Beckman Instruments, Inc., the (California) Citrus Industry Committee on Fungistats, and the (California) Citrus Industry Research iidvisory Committee. LITERATURE CITED
(I) Gunther, F. -I.,Barkley, J. H., Kol-
bezen, RI. J., Blinn, R. C., Staggs, E. A , , BSAL. CHEII. 28, 1985-9 (1986). Gunther, F. A, Blinn, R. C., Kolbezen, 11.J., Barkley, J. H., Staggs, E. A., Conkin, R. A , , Kjlson, C. IT.,J . d g r . Food Cheni., in press. Klotz, L. J., personal communication. Klotz, L. J., unpublished data for Project 808, University of Callfornia, Riverside, Calif. Rideal, S., “Disinfection and Preservation of Food,” p. 216, Wiley, Kew York, 1903. Roistacher, C. S., Eaks, I. L., lilotz, L. J., U.S. Dept. ,4gr. Plant Disease Reptr. 39, 202 (1955). Roistacher, C. N., Klotz, L. J., Eaks, I. L., Hzlgardia, in press.
RECEIVEDfor reviev- June 20, 1957. hccepted January 13, 1958. Division of Analvtical 131st hfeetinr. . ~ ~Chemistrv. . . ~ ACS; Miami, Fla., April 1957. Pap;;. KO.993, Cniversity of California Citrus Experiment Station, Riverside, Calif. Contribution to the University of California College of Agriculture Project S o . 808, under the guidance of L. J. Klotz. ~~
~
“
I
Potentiometric Titration of Free Amine and Amine Ca rbo nute in Carbonated Monoetha nola mine Solutions YI-CHUNG CHANG low-Temperature Tar laborafory, Bureau o f Mines, U. S. Department of the Interior, Morganfown, W. Va.
b Determination of free amine and amine carbonate in carbonated monoethanolamine solutions by means of a double end point titration seems to have great simplicity. Results of potentiometric titrations for mixtures of monoethanolamine and its carbonate are presented. End points are found to occur a t pH 7 and 4. Using a sample of 0.5 to 1 gram of amine and 1 to 2 N hydrochloric acid, a titration can b e performed within 15 minutes. Titrated values agree within 2y0of the calculated percentage compositions for synthetic mixtures, indicating an accuracy adequate for routine control purposes.
D
a study on the regeneration of carbonated monoethanolamine solutions used in a phenol extraction URIKG
process, numerous determinations of the amine and carbon dioxide contents were necessary and a rapid, accurate method was sought to facilitate the work. A number of papers have been published on methods for determining free amine and amine carbonate. A procedure widely used for this analysis consists of determining the total amount of amine in solution by titrating with strong acid, using either methyl orange or methyl red as a n indicator, and determining the carbon dioxide content by liberating the carbon dioxide with acid, followed by either measuring the volume of the gas or absorbing it in Ascarite or baryta (1, 2 , 6-6). Cryder and llaloney determined the carbon dioxide content of a n amine solution from a chart prepared by plotting p H 2’s.
percentage conversion to amine carbonate ( I ) . Wagner and Lew made amine carbonate react lvith excess sodium hydroxide and back-titrated the excess base in 75% ethyl alcohol solution ( 7 ) . Mason and Dodge reported a double indicator titration procedure but did not get reliable results (6). However, the double end point approach based on the follorving reactions seemed to be the simplest, the question of reliability depending on the conditions of titration and the method of locating the end points: For the first end point:
+
HOCH2-CH2SH2 HCl + HOCH2-CH2PiH3C1 (HOCHZ-CHzNHa),CO3 HC1+ HOCHZ-CH~NH~HCO~ HOCH2-CH2NH3C1
+
VOL. 30, NO. 6, JUNE 1958
(1)
+
(aj
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