was the analytical technique used. The values reported by these workers are higher than those found in this study. Figure 3 outlines the acid and chloride concentrations which mark the division between chlorine and no chlorine production by radiolysis. There is a demarcation at 1.40 HCI concentration which represents the lower chloride limit. A higher acidity at lower chloride was not attempted because the addition of another acid besides hydrochloric would add a new factor to be resolved. As the chloride content increases, the critical acid concentration decreases. There is a fair amount of hydrogen peroxide produced in low chloride solutions which decreases with increasing chloride. This amount is of the order of the chlorine produced but far below that required to color o-tolidine and reacts rapidly with any chlorine that is produced as long as the acidity is low. The reactivity of peroxide with chlorine diminishes with increasing acid concentration. There is more peroxide formed at low chloride than at high chloride. Thus, a higher acidity will be required at low chloride concentration to decrease the reactivity between peroxide and chlorine and bring the net chlorine production to a level close to that at high chloride concentration and low peroxide formation. This explanation is essentially what was proposed by the Soviet workers, (IO) and a similar suggestion has been made for the radiolysis of aqueous bromide system
As stated before, this investigation was aimed primarily at the extent of radiolytic damage that can occur to analytical systems. Thus, all chemicals used were reagent grade with no further purification, and ordinary distilled water was used. No quantitative explanations with the radical and molecular yields in aqueous systems are attempted. The radiolytic measurements were made on solutions prepared from different batches of reagents at various times. There was variation between measurements made at different occasions, but the overall trends remained the same. The values usually ranged within 30% of the average. Thus, it is believed the data of Figures 1 and 2 describe rather closely what radiolytic effects can be expected in ordinary acid chloride media. Very little difference was found between lithium and sodium as the cation in these acid-salt systems, and it was concluded that these cations do not play significant roles. Similar behavior is expected of other alkali and alkaline earth cations. Lithium chloride was the choice here because its solubility makes possible a wider total chloride range. RECEIVED for review October 11, 1967. Accepted November 17, 1967. Research sponsored by the U. S. Atomic Energy Commission under contract with Union Carbide Corporation. (11) A. Rafi and H. C. Sutton, Trans. Faraday Soc., 61,877 (1965).
(14.
Measurement of Total Organoaluminum-Reactable Impurities in Hydrocarbons by Continuous Flow Thermometric Analysis T. R. Crompton and Brian Cope Carrington Plastics Laboratory, 'Shell' Research, Ltd., Urmston, Manchester, England An instrument is described for the continuous or semicontinuous measurement in hydrocarbons of total triethylaluminum-reactable impurities such as water, dissolved oxygen, and low molecular weight alcohols. The sensitivity of the instrument is such that it produces a measurable response for concentrations as low as 1 ppm of these substances in the sample. Although a specific application of the instrument is described, the principle of continuous flow thermometric analysis could be used in other problems connected with the on-line analysis of process streams.
FREQUENTLY, in chemical processes or in end-use applications, critical limits exist regarding the concentration in hydrocarbons of dissolved impurities such as water, oxygen, and low molecular weight alcohols. A thermal analysis procedure involving reaction of the hydrocarbon with an organoaluminum compound was considered a feasible proposition for the determination of these impurities at the required level. Triethylaluminum was chosen as the reagent because of its solubility in the hydrocarbon and because of the sensitivity afforded by its high heat of reaction with water (approximately 100 kcal/mole). In some experiments, with a thermometric titration apparatus using a thermistor end-point detector of the type described by Everson (1) and Everson and Ramirez (2) a sample of a CS (1) W. L. Everson, ANAL. CHEM., 36, 854 (1964). (2) W. L. Everson and Evelyn M. Ramirez, Ibid.,37,806 (1965).
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ANALYTICAL CHEMISTRY
hydrocarbon was titrated with an 8% v/v solution of triethylaluminum in the hydrocarbon. The titration was carried out under dry, oxygen-free nitrogen to eliminate thermal interference due to reaction of the reagent with atmospheric oxygen and moisture. This procedure revealed that the hydrocarbon contained impurities which liberate heat on reaction with triethylaluminum. A continuous method of analysis based on this principle had obvious advantages, such as high sensitivity and the fact that all substances present in the hydrocarbon which destroy triethylaluminum, would evolve heat upon reaction and provide an estimate of the concentration of total impurities present in the hydrocarbon without any knowledge of their chemical nature. Priestley et al. (3, 4 ) have described an apparatus for the continuous flow enthalpimetric analysis of aqueous streams. In this apparatus, the sample stream from a small reservoir and a stoichiometric excess of reagent stream are pumped at controlled, constant-flow rates into a reaction cell where thorough mixing occurs. This method is not truly continuous but does handle a flowing 25-ml sample. Priestley mentions the possibility of applying this technique to continuous process control. Three thermistors are used in conjunction with a suitable bridge circuit to monitor the difference between the temperature of the contents of the reaction cell and the average (3) P. T. Priestley, W. S. Sebborn, and R. F. W. Selman, Analyst. 90, 589 (1965). (4) P. T . Priestley, J . Sci. Znstr., 42, 35 (1965).
temperatures of the incoming sample and the reagent streams. Priestley applied this apparatus to various acid-base titrations in aqueous media and was able to calculate the concentration of the sample constituent after calibrating the apparatus with standard solutions of acids and bases. Priestley also mentions the possibility of equalizing the temperatures of the sample and the reagent streams before they enter the reaction cell, so that only two thermistors are needed with a simpler bridge circuit. He states, however, that such an arrangement would be inconvenient for rapid work owing to heat transfer problems. Wasilewski ef al. (5) discuss a thermal analysis technique which they describe as direct enthalpimetry and in which a small volume (300 ~ 1 of ) concentrated aqueous reagent is rapidly injected into the more dilute aqueous sample and very rapidly mixed. The concentration of the sample is deduced from the corresponding temperature increment or decrement. Wasilewski mentions the possibility of applying this technique to process control but does not discuss this further. This technique was also mentioned briefly by Priestley (3). Independently of our work Crespin (6) developed an apparatus called the continuous thermometric titrometer, based on similar principles to our own, for monitoring the concentration of sulfuric acid in aqueous effluents. In this apparatus, the sample and reagent (1.ON sodium hydroxide) streams are passed into a reaction cell through heat exchangers all of which are immersed in a thermostated water bath. The temperature difference between the incoming sample stream and the reaction mixture is continuously monitored by two thermistors linked to a bridge circuit. At maximum sensitivity this gives a full scale recorder deflection for a 1O C temperature difference between the two thermistors. The first important requirements of an apparatus for determining triethylaluminum-reactable impurities in hydrocarbons are that it must be rugged and be run with a'minimum of attention, and it should provide a continuous or periodic on-line analysis of solvent. The sensitivity requirement is such that the instrument should be capable of producing a measurable response for a hydrocarbon containing 1 ppm of impurity. This is equivalent to a temperature increase of less than 0.01 O C,-i,e., recorded temperature differences of f0.005 O C must be significant. This degree of temperature control requires precision thermostating of the reaction cell. Facilities are also required for preheating the incoming hydrocarbon stream from storage tanks from temperatures as low as 0" C during the winter months to the operating temperature of the reaction cell. The selection of construction materials is important as they must resist attack by hydrocarbons and the reagent. Easy calibration and standardization checks are mandatory in process control instruments. Finally it is necessary to develop a system which avoids difficulties due to blockages caused by insoluble products formed in the reaction cell. In principle, the instrument discussed in this paper consists of a system in which accurately metered streams of the hydrocarbon and the reagent pass separately through a precision thermostat so that the temperatures of the two streams are adjusted to within ~ l = 0 . 0 0 2C~of each other prior to mixing. The temperature rise which occurs when the reagent and hydrocarbon streams are mixed is measured differentially by two thermistors immersed in the incoming hydrocarbon stream and the mixed stream, respectively. Triethylaluminum ( 5 ) J. C . Wasilewski, P.T-S. Pei, and J. Jordan, ANAL.CHEM., 36, 2131 (1964). ( 6 ) G . Crespin, "Lapis," No. 71, (1964) p. 24.
(8% v/v in solvent) was used as the reagent in the work described in this paper, but other reagents can be used if required. The construction of the process analyzer discussed below is based on these principles, THEORETICAL CONSIDERATIONS
The relationship between the observed temperature rise occurring on reaction and the amount of impurity in the hydrocarbon is as follows. Single Impurity of Known Identity in Hydrocarbon. Heat entering the reaction cell system due to reaction enthalpy and unreacted solutions equals heat leaving the reaction cell system by way of product solution plus heat lost from the reaction cell system to surroundings (Ho). (FiMH
+ KFlTl + KFzTi) = KF,T2
+ KFzTz + H o (cal/second)
(1)
where (Hois assumed to be zero); M is the molar concentration of impurity in hydrocarbon (mmole/ml); FI and Fgare the flow rates of hydrocarbon and reagent, respectively (ml/second); K stands for the thermal capacities of the hydrocarbon and reagent solution before mixing and of the reaction mixture. It is assumed that no change in thermal capacity occurs upon reaction at the working dilutions. In this instance thermal capacity is defined as the product of the specific heat and the density of the hydrocarbon at the solvent temperature in question, H i s the heat of reaction between triethylaluminum and 1 mole of hydrocarbon impurity (kcal/mole). As the reagent is present in large excess, it is assumed to react on a 1:l molar basis with the impurity. TI and T z , respectively, are the temperatures of hydrocarbon and reagent streams before mixing and the temperature of reaction product after mixing ( O C). If " F2is kept constant, (Tz - T I )is denoted Fl
by AT and Hois assumed to be zero. Then M =
C X AT H = C' AT (where C ~
=
K
Thus, in the case of a single impurity, the instrument response is directly proportional to the concentration of impurity in the hydrocarbon. In practice, the hydrocarbon contains more than one triethylaluminum-reactable impurity. Under these conditions the relationship between instrument response and impurity level is more complex (except in the unlikely event that all the impurities have the same heat of reaction with the reagent); a-n
( M ~ H u= ) C'AT
(4)
a-1
where a=n
(MuHu) a-1
+ MZHZ+
(MIHI
a
M i , 2 , , , ,= , concentrations of impurities, 1,2 carbon (mmole/ml)
9
MnHn)
(5)
. . . n, in hydro-
= heats of reaction of impurities, 1,2 ethylaluminum (kcal/mole)
. . , n, with tri-
VOL. 40, NO, 2, FEBRUARY 1968
b
275
ELECTRICAL
TIMER
RESERVOIR
I
I
Figure 1. Continuous flow thermometric analyzer Layout of apparatus Thus the temperature increase (AT) is directly proportional to the summation of the products of molar concentrations (M) of each reactive impurity and their heat of reaction ( H ) with triethylaluminum. DESCRIPTION OF INSTRUMENT
The general layout of the apparatus is shown in Figure 1. Flow control of the hydrocarbon into the apparatus on the inlet and outlet controllers is achieved by means of a needle valve and rotameter combined with a constant differential relay, which has a constant predetermined flow rate regardless of any upstream or downstream pressure variations in the hydrocarbon stream. Any particles in the hydrocarbon entering the apparatus are removed by means of a fine pore size ceramic filter. The hydrocarbon is passed through a preheater to bring its temperature to 29" i 0.1 O C-i.e., within 1O C of the temperature of the precision thermostat (30" C) which followsto reduce the load on the precision thermostat. Precision thermostating ensures that the hydrocarbon and reagent streams are adjusted to within =t0.002" C of each other before mixing in the reaction cell and that the ambient temperature at which the two streams are mixed is 30' C, thereby diminishing any effect of ambient temperature on the heat of reaction of hydrocarbon impurities with triethylaluminum. After entering the thermostat, the hydrocarbon passes via a heat exchanger to a reference cell containing the first thermistor, then through the electrothermal heater (instrument standardization is discussed later) and the reaction cell containing the second thermistor, then via a second filter to remove any insoluble reaction products, thence to the outlet flow controller. The reagent consists of 8 v/v triethylaluminum dissolved in the hydrocarbon and is stored in a 5- or 10-liter capacity stainless-steel container. It is delivered to the reaction cell via a pump and heat exchanger. If necessary, the reagent could also pass through the preheater discussed above. The difference in temperature between the reference and the reaction cell thermistors is measured by a Wheatstone bridge circuit and the out-of-balance bridge voltage displayed on a potentiometric strip chart recorder. Reagent can be delivered into the apparatus either continuously to provide 276
0
ANALYTICAL CHEMISTRY
a continuous record or on a periodic timed shot basis achieved by a suitable electrical-timer circuit. Thermistors age with continued use and this affects their response characteristics. Their response can also be influenced by any build-up on the surface of insoluble reaction products (which is especially likely to occur if the hydrocarbon contains more than 20 ppm water which reacts with triethylaluminum to form aluminum hydroxide). As a check on the stability of the thermistor response and factors such as the flow control, an electrothermal heater with a constant heat output is inserted between the reference and the reaction cell thermistors. At predetermined intervals the reagent supply is cut off and simultaneously the electrothermal heater switched on by means of an electrical timer circuit. The hydrocarbon sample flow is continued unchanged. Any variation in instrument response occurring in these checks should be investigated immediately. The construction of various parts of the instrument is discussed below. In many cases of course, this equipment could be obtained from alternative suppliers. FILTERS. The ceramic filter inserted before the inlet flow controller, and the filter inserted after the reaction cell are obtainable from Aerox Ltd., Stroud, England (Pyrolith G 35). FLOW CONTROLLERS. Both the inlet and outlet controllers have a range of 0-50 ml/minute and consist of a needle valve and rotameter and a differential relay operating at 3 lb/in*. They were obtained from Brooks Instrument Co. Inc., Hatfield, Pa. SOLVENT PREHEATER. This heater consists of a bored-out cylindrical brass block, 9 X 3 inches in diameter, wound with 10 feet of '/*-inch 0.d. copper tubing. The bored-out portion of the block contains a cartridge heater (75-watt heater is convenient for maintaining a block temperature of 29 i 0.1" C) and a thermostat. The preheater is mounted in a box insulated with expanded polystyrene. PRECISIONTHERMOSTAT. The L.K.B. 7600 thermostat available from L.K.B. Instruments Ltd., L.K.B. House, 137 Anerley Road, London S.E. 20, is suitable. This water thermostat has a capacity of 5 liters and maintains a constant temperature within i0.001" C for prolonged periods. HEAT EXCHANGERS. The exchangers consist of 20 feet of 1/8-inch0.d. copper tubing.
F'
"&"* G Sample oullst G.J. \'Wade couplings
Hk'tW.de
couplings
$1
(b)
REACTION CELL
'i F,
I
I
F, 10,W n thermistor AI1 ends I o Ish.
Wade c o v p I h g s
A Rcagcnt inlet Malwal: 1.ddla. perrpex rod
B Sample inlet
Figure 3. checks
C Stream wllet A.&C.%'Wade couplings
Electrothermal heater cell for instrument stability
2" Material
1"dm p w s p . ~rod
B
l
Figure 2. Reference and reaction cells
REFERENCE AND REACTION CELLS. These were constructed from 1-inch diameter solid Perspex rod and are illustrated in Figure 2. This cell design gives efficient and rapid mixing of the solvent and reagent streams and is not subject to any back flow of reagent into the solvent streams entering the cell. The thermistors are a matched pair, lo4 ohm directly heated bead type obtained from Grant Developments Ltd., Toft, Cambridge, England. ELECTROTHERMAL HEATER.The heater is constructed from I-inch diameter solid Perspex rod (Figure 3). The heating element is Nichrome wire and is supplied from a 1.5-volt dry cell. The length and resistance of the Nichrome wire should be chosen so that the heat output is equivalent to 70 to 8Oz full scale deflection on the recording potentiometer at maximum bridge sensitivity. REAGENT RESERVOIR, The reservoir consists of 5 - or 10liter vessels constructed of stainless steel and containing 8 % vjv triethylaluminum in solvent. A dual reservoir system with nitrogen purging facilities is recommended to ensure uninterrupted instrument operation-Le., one reservoir can be recharged while the other is in use. REAGENT PUMP. A calibrated pump fitted with a PTFE diaphragm head with an adjustable delivery rate in the range 0 to 180 ml per hour is used. The DCL micropump with PT'FE diaphragm, Series I, available from F. A. Hughes & Co. Ltd., Great Burgh, Epsom, Surrey, England, is suitable. ELECTRICAL TIMERS.The timers for reagent addition and activation of electrothermal heater are available from Crozet (England), Ltd., Brentford, Middlesex, England. BRIDGE CIRCUIT.A circuit is shown in Figure 4. The continuous balance recording potentiometer had the following characteristics : input span-1 mV; span step response time1.2 seconds; source impedance-2.5 X l o 4 ohm maximum.
1 0
TO RECORDER RECORDER
b3
Figure 4. Continuous thermometricanalyzer Wheatstone bridge circuit
R1 = 5K Zero pot 1 R2 = 5K Zero pot 2 RB = 10K Resistor R4 = 10K Resistor Rs = 50K Attenuator Rb = 10K Recorder input TI = Ref thermistor Tz = Measure thermistor MI = Bridge current meter (mA) SlS2= Single pole changeover switches Sa = Single pole on/off switch, bridge volts V I = 1.5-Volt cell EVALUATION
A standard reagent Aow rate of 3 ml/minute was chosen, If the reagent were injected continuously, then a 10-liter reservoir of reagent would be used in about 2 days. However, practical experience has shown that a 2-minute injection VOL. 40, NO. 2, FEBRUARY 1968
277
STAR1
t
Figure 5. Plant run with continuous flow thermometric analyzer Measurement of total triethylaluminum-reactable impurities in hydrocarbon solvent
at 15-minute intervals provided an adequate record of solvent quality; under these conditions a 10-liter reservoir of reagent
SENSITIVITY AND CALIBRATION
would last about three weeks. A hydrocarbon sample flow rate of 30 ml/minute was adopted. Under these flow conditions the hydrocarbon flow rate past the second thermistor is approximately 10% greater than that past the reference thermistor. The thermistors were of the directly heated bead type and it was considered that this increase in flow rate might cause a differential thermistor output even in the absence of solvent impurities. Trial injections of the hydrocarbon (instead of reagent) into Aowing hydrocarbon, however, showed that no interfering signal was produced under these conditions even when the instrument was operated at maximum sensitivity. When the 8% v/v triethylaluminum reagent is injected into the sample stream, it is diluted to less than 1%. Any heat of dilution of the reagent therefore would cause a differential thermistor output which would interfere in the analysis. To check whether a heat of dilution effect existed the 8% triethylaluminum reagent was repeatedly injected into Aowing hydrocarbon which had been previously doped with 0.5% vjv triethylaluminum to kill any reactable impurities, The instrument response, at maximum sensitivity, was found to be zero, indicating no effect due to heat of dilution. The operational stability of the instrument was very good when it was run at maximum sensitivity under the prescribed conditions. The bridge signal did not vary by more than &2% during analysis or during the periodic checks with the electrothermal heater. At maximum sensitivity, full scale recorder deflection (100 divisions) is obtained when the temperature difference between the two thermistors is approximately 0.2" C. The variation of +2% referred to above therefore corresponds to a temperature variation of f0.004" C which is considered to be the maximum sensitivity of the instrument, although it could probably be improved. The circuit shown in Figure 4 has provision for carrying out temperature measurement over a range of sensitivities. The electrothermal heater can be used to obtain the sensitivity factors for an instrument. The heater has a constant output and it is possible to obtain sensitivity factors corresponding to various instrument settings by altering the attenuator and the recorder input potentiometer settings at a constant solvent flow rate with the reagent switched off.
The impurities most likely to occur in the hydrocarbon are water and dissolved oxygen. Water and oxygen present difficulties as primary calibration standards owing to the problem of preparing standard solutions in hydrocarbon solvents. Calibration was carried out therefore against a synthetic solution of absolute ethyl alcohol. An air-tight, 5-liter container of hydrocarbon is dried over molecular sieves and then dissolved oxygen is removed by purging with dry oxygen-free nitrogen. The solvent is then run through the apparatus for some time under standard conditions. Absolute ethyl alcohol (0.35 mmole per liter) is mixed with the hydrocarbon and the analysis is continued. The steady recorder responses obtained before and after the alcohol addition are recorded. The 0.35-mmole per liter (approximately 16 ppm w/v) ethyl alcohol solution produces a 50-scale division response. It can be seen that, at maximum sensitivity, the instrument is capable of detecting ethyl alcohol in amounts down to approximately 1 ppm w/v, while at lower sensitivities, analyses can be performed in the hundreds of ppm range. Moreover, the molar heats of reaction of ethyl alcohol, water, and oxygen with triethylaluminum are in the approximate ratio of 1 :1:2. Thus, water and oxygen are determined with molar sensitivity similar to that obtained for ethyl alcohol. This instrument can be calibrated against a range of solutions of ethyl alcohol of known molar concentrations in anhydrous, oxygen-free hydrocarbon. The calibration curve obtained is then used to obtain the purity of solvent samples as ppm ethyl alcohol equivalent. Of course, under these conditions, results obtained with hydrocarbon samples will be correct if the hydrocarbon contains only ethyl alcohol and no other triethylaluminum-reactable impurity. Dissolved oxygen for example, has approximately twice the heat of reaction with a mole of triethylaluminum as does ethyl alcohol. Thus, if the analyzed hydrocarbon contained any dissolved oxygen, the reported molar concentration based on the ethyl alcohol calibration curve would be higher than the true oxygen content of the solvent. For this reason, reported analyses can be up to two times higher than the correct value. However, this margin of error was acceptable for our purpose and enabled us to monitor solvent quality against the criterion of its performance in a chemical process.
278
ANALYTICAL CHEMISTRY
ELECTROTHERMAL HEATER
INSTRUMENT CHECK
1900 HOURS
1800 H O U R S
2400 HOURS
1700 HOURS
2300 HOURS
1600 HOURS
I500 HOURS
A
- STAR7
1900 HOURS
2200 HOURS
Figure 6. Triethylaluminum-reactable impurities in hydrocarbon Ten-hour continuous run with periodic analyses and instrument standardizations PERFORMANCE
In the earlier stages of development of the instrument, samples of hydrocarbon were taken at the plant in dry bottles under an atmosphere of dry oxygen-free nitrogen. The bottle was connected to the apparatus and a nitrogen pressure applied to feed the hydrocarbon into the flow controller. This method of sampling was impracticable because, despite the most careful precautions, some oxygen and water contaminations of the hydrocarbon occurred during sampling and subsequent transit to the apparatus. To overcome this problem, the apparatus was permanently connected on-line on the plant. An analysis program was set up using a system of electrical cam timers to provide a sample analysis period of 2 minutes at 13-minute intervals and an instrument standardization period of 20 minutes every 6 hours, Figure 5 shows the analysis record obtained with the on-line instrument operating at maximum sensitivity during a run following startup on a tank of hydrocarbon. Initially, an almost full scale recorder deflection occurs corresponding to approximately 0.7 mmole per liter ethyl alcohol equivalent which progressively decreases over 3 hours to a level which corresponds to less than 0.1 mmole per liter of ethyl alcohol equivalent in the hydrocarbon. Figure 6 shows a recorder trace for a continuous 10-hour run at maximum sensitivity. The trace clearly shows the hydrocarbon impurity level gradually increasing to off-scale then diminishing over a 6-hour period. This trace also shows the reproducibility of the electrothermal heater procedure for instrument standardization. As mentioned previously, blockages may occur in the instrument lines when the hydrocarbon contains higher concentrations of water which react with triethylaluminum to produce an insoluble precipitate of aluminum hydroxide. Blockages do not occur, however, if the hydrocarbon contains
less than 0 to 1 mmole per liter water. It is in this range that analyses were required. In practice, therefore, the instrument was used only at its maximum sensitivity setting and, to avoid blockages, a mechanism was installed for simultaneously cutting off the triethylaluminum reagent deliveries and activating an alarm system when the recorder signal exceeded 75% full scale deflection at maximum sensitivity. The hydrocarbon was allowed to flow uninterrupted during the cutoff to sweep any insolubles from the system. No blockages occurred in the instrument during several months continuous running under these conditions. FURTHER APPLICATION OF CONTINUOUS FLOW THERMOMETRIC ANALYSIS
Adaption of the continuous flow thermometric analyzer to the analysis of the individual components of multi-impurity streams can be envisaged, although this application has not been investigated in detail. Let us consider the case of a sample containing three reactive impurities. If, after the reference thermistor, the sample stream is split three ways and a different reagent added to each stream, a temperature rise will occur in each of the three reaction cells which can be measured by independent bridge circuits. Each of the three reagents must have an appreciably different heat of reaction with each of the sample impurities. If the three impurities are denoted by a, b and c, assuming that heat losses from the reaction vessel to the surroundings are zero, the process can be represented mathematically as follows: Reagent 1
+ HalMb + Hc1Mc Reagent 2 Ha?Ma HbzMb + HcpMc HalMa
=
CIATI
(6)
=
CzAT2
(7)
=
CATS
(8)
Reagent 3 HaaMa f HbaMb -I- Hc3Mc
VOL. 40, NO. 2, FEBRUARY 1968
279
Where H a 1 . 2 . 3 . Hb1.2.3, and H c 1 . 2 , 3 are the heats of reaction of compounds a, b and c with reagents 1, 2, 3 (kcal/mole), respectively, Ma, Mb and M , are the molar concentrations of compounds a , b and c in sample (mmole/ml). AT1, AT,, and AT, are the measured temperature increases obtained with reagents 1, 2 and 3 ("C). Cl, Cz,and Csare constants comprising the thermal capacity of the system (K equals the product of the density and the specific heat of the sample at the operating temperature) and the flow rates of the sample (Fs) and the three reagents (F,),
Fs
+ F,
=K I T - >
It is assumed that this ratio is kept constant for each of the three reaction cells.
These equations can be solved for Ma, M,, and M,, the concentrations in the sample of the three impurities. The arithmetical work involved in the solution of the equations involves considerable effort, especially if frequent analysis is required, and in these circumstances it would be advisable to arrange for the calculations to be handled by means of a data link to a computer. The principles of CFTA should have other applications in on-line analysis of organic or aqueous liquid streams and
possibly, with suitable modifications in cell design, of gas streams. Choice of suitable reagents would make it possible to determine different types of impurities or added substances in sample streams. The reagent might be specific for a particular component of the sample or, as in this paper, a nonspecific reagent might be used which determines the total of various impurities present in the sample. The principles of CFTA might well find applications in the monitoring of effluents from liquid column chromatographs using a stream splitting device to isolate a portion of the total effluent for reagent addition. With suitable modifications the technique might also be applicable as a gas chromatographic detector. ACKNOWLEDGMENTS
The authors thank A. G. Barclay for his assistance in the early stages of the mechanical and electrical design of the instrument and B. W. Heys, D. I. Walmsley, and Miss V. M. Overend for their assistance during the development of the instrument. RECEIVED for review July 21, 1967. Accepted October 23, 1967.
Polarography of Gallium in Acid Media Separation of the Effects Due to Anion Interaction and Ionic Strength E.D. Moorhead
and G . M. Frame I1 School of Chemistry, Rutgers-The State University, New Brunswick, N . J . 08903 Investigation of the SCN-.catalyzed ac and dc polarography of gallium in strong acid media has been extended to ascertain the separate effects on the Ga(lll) reduction process of (a) ionic strength; (b) thiocyanate concentration; (c) added halide; (d) pH; and (e) changing Ga(lll) concentration. Results(a) through (c) intimated that reversible reduction may occur via anionic gallium [probably Ga(SCN)*-1 rathe! than cationic, and results obtained in (d) were not inconsistent with this notion. A subsequent anion exchange study using the C104- form of Dowex 1-X8 indeed verified that anionic Ga(lll) could be present in the SCNcontaining 6.OM NaC104 test solutions. Variation of corrected dc diffusion current vs. Ga concentration was linear over the range 0.01 m M to 5.0 m M Ga(lll) which enabled direct polarographic measurement of Ga(lll) down to the 1.0-ppm level. The Ga(lll) diffusion coefficient, DGa,was calculated to be 3.06 X 10" cmzsec-l (30° C) referred to 6.OM NaCIOa.
formerly supposed (2). Simplifications in the experimental requirements afforded by this development enabled us in the present study to survey a number of the effects of solution composition on the gallium electrode reaction, information which we believe constitutes a useful step toward eventual description of the SCN--based mechanism but which was previously inaccessible because of the apparent need for extreme concentrations of the pseudohalide (2). Results are presented here on the manner in which the dc and ac polarography of gallium are influenced by ( a ) ionic strength, (b) SCN- concentration, (c) pH, (4 added halide, and (e) variation in Ga(II1) concentration, and a new value is reported for the Ga(II1) diffusion coefficient referenced to 6.OM NaC104.
INA RECENT COMMUNICATION (1) we demonstrated that operation of the mechanism underlying the thiocyanate-catalyzed reversible reduction of Ga(II1) at the dropping mercury electrode @ME) ( 2 ) is conjunctively dependent upon high solution ionic strength, From an analytical as well as a theoretical electrochemical standpoint a significant consequence of this is to shift the normally irreversible Ga(II1) wave [El,2 = -1.1 V US. SCE from noncomplexing media (1-2)] away from the masking effect of proton reduction toward less reducing potentials and to render the resulting G a (111) wave reversible at SCN- concentrations far lower than
Apparatus. All polarographic measurements were made using a 3-electrode cell and dual mode (ac, dc) custom-built operational amplifier circuitry of conventional design. A Moseley 2D-2 or 135 M X-Y recorder was used for the polarograms, and currents were evaluated from calibration markers derivable from the polarograph. Applied ac signals were supplied by a Hewlett-Packard 202A function generator, and a Hewlett-Packard 302A wave analyzer (6-Hz constant band width, frequency-locked) was used to extract the signal of interest. Potentials applied to the electrode were accurately measured using a Hewlett-Packard dual channel 5233-L counter in tandem with a Hewlett-Packard (Dymec) 221 1-BR voltage-to-frequency converter (100 kHz). To avoid possible formation of insoluble KClO, the saturated calomel reference electrode (SCE) was isolated from the
(1) E. Moorhead, J . Am. Chem. Soc., 87,2508 (1965). (2) W. MacNevin and E. Moorhead, Ibid., 81, 6382 (1959).
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EXPERIMENTAL
