Joseph Jordan, J. Keith Grime, and Dan H. Waugh Department of Chemistry Pennsylvania State University University Park, Pa. 16802
C. David Miller, Herbert M. Cullis, and D. Lohr American Instrument Co. 8030 Georgia Avenue Silver Spring, Md. 20910
Enthalpimetric Analysis I
I
Titrimetric Moae
Calorimetric
Thermometric Enthalpy Titrations
Direct injection Enthalpimetry
Mode
on fast processes, both thermometric and direct enthalpimetric procedures can be adapted t o study rate-controlled processes. When the latter type of conditions prevails, the designations kinetic titrimetry and kinetic DIE are appropriately used. The kinetic approach to enthalpimetric analysis, in general, can be referred to as thermokinetic analysis (TKA). In all the above instances the amount of heat evolved (or absorbed), Q, in the reaction may be calculated from Joule heating calibration experiments. The calibration circuitry is described later. Fundamental Principles T h e principles involved in enthalpimetric analysis are based on fundamental correlations inherent in adiabatic calorimetry:
Q = -n,-AH
Kinetic Titrimetry
(1)
where Q denotes the total amount of heat evolved (or absorbed), n,, the number of moles of product formed, and AH the molar enthalpy of reaction. Furthermore, the temperature change, A T , observed in an adiabatic cell can be explicated as in Equation 2:
Kinetic DIE
Figure 1. Relevant n o m e n c l a t u r e
A T = Q/K In view of the multiplicity of terms used to describe this field of analytical chemistry, a brief discussion of the relevant nomenclature is warranted. T h e terms used in this report are explicated in Figure 1. T h e two main types of enthalpimetric analysis, thermometric enthalpy titrations ( T E T ) and direct injection enthalpimetry (DIE), can be delineated as titrimetric and calorimetric methods, respectively. In practical terms they can be differentiated by the mode of introduction of the reactants into the adiabatic cell. T E T experiments are characterized by the continuous addition of the titrant to the titrate under effectively adiabatic conditions. The total amount of heat evolved (if the reac-
tion is exothermic) or absorbed (if the reaction is endothermic) is monitored autgmatically using the unbalance potential of a Wheatstone bridge circuit, incorporating a temperature sensitive semiconductor (thermistor) as one arm of the bridge (vide infra). The virtual cessation of heat evolved (or absorbed) is used to locate the equivalence point of the titration. DIE, on the other hand, involves the injection of a single “shot” of a reactant into the solution under investigation, the reagent being in stoichiometric excess. T h e corresponding caloric “pulse”, which is directly proportional to the number of moles of analyte reacted, is recorded using identical circuitry to that employed with T E T . Although initially the emphasis was
(2)
where K denotes the effective heat capacity of the system. Combination of Equations 1 and 2 yields
-AH-n,
AT=-
(3)
K
The heat capacity of the system, K , will remain essentially constant if the change in volume of the solution is minimized. Moreover, if it is assumed that A T is caused entirely by the reaction and that AH = AHo = constant, the observed temperature change, A T , is obviously proportional to the number of moles of product formed, np (and hence of reactant consumed):
AT = constanten,
(4)
A typical T E T enthalpogram (a thermometric titration curve) is
ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
427A
shown in Figure 2a. Curvature in the region of the equivalence point may be caused by less than quantitative conversion of reactants to products (equilibrium curvature) or by the slow attainment of titration equilibria (kinetic curvature). In general, prohibitive limitations may arise if the relevant equilibrium constant is unfavorable. Furthermore, conventional thermometric enthalpy titrations are normally restricted to reactions whose effective rate is fast compared to the rate of addition of titrant. An end-point correction procedure for slower rates has been described. Appropriate computations can be used for the thermometric determination of first order rate constants up to a limit of 600,000 M-I s-l ( 1 ) . In a recent communication (2),it has been suggested that the hitherto universally accepted linear relationship between A T and the volume of added titrant, hu, is valid only over a narrow range of volume if one takes into account that some heat transfer actually occurs between the ambient medium and the reaction cell which is necessarily not quite 100% adiabatic. It is recommended that a more practical representation of a thermometric enthalpy titration would be achieved by a plot of llhT vs. l f u . A typical direct injection enthalpogram is shown in Figure 2b. T h e technique is based on the measurement of the total heat evolved (or absorbed), Q, rather than of an end point. The application of T E T and DIE enthalpograms to the determination of an analyte is contingent upon the knowledge of certain fundamental quantities-the stoichiometry of the titration reaction with the titrimetric technique and the heat of reaction with the calorimetric approach. Fundamental thermodynamic information (the heat, AH, the equilibrium constant, K , and hence the free energy, AG, and entropy, AS,of a reaction) can be deduced from thermometric titration curves ( 3 ) . Practical Aspects The methodology in its present form has been the subject of several reviews (3-10). The salient features of the experimental apparatus are a reagent addition system, an adiabatic cell incorporating a temperature sensor, a calibration heater and a stirring device, and a temperature-measuring circuit and a recorder (Figure 3). Reagent Addition. Titrimetric Procedures. Reproducible, constant titrant addition is generally achieved using either a motor-driven syringe or in some instances a pulse-free peristaltic pump. Uniformity of titrant and titrate temperatures is routinely obtained by passing the tubing conveying the titrant to the adiabatic 428A
* ANALYTICAL CHEMISTRY, VOL.
dT dV
d2T dV2
0
Volu
rant added
d 4 dV3
" Votume of titrant added
Volume of titrant added
Figure 2. Typical enthalpograms. TET, thermometric enthalpy titration; DIE, direct injection enthalpimetry
cell through a constant temperature bath. The disadvantages of liquid titrants can be circumvented by the injection of gaseous reagents into liquid titrates (6, 14). The heat capacity of a gas is negligible when compared to that of a comparable volume of liquid; therefore, the temperature of gaseous titrants need not be regulated. Furthermore, changes in the heat capacity of the adiabatic cell and its contents will be small, thus minimizing a nonlinearizing effect on the resultant enthalpogram. The idea may be further extended to the thermometric determination of gaseous samples with gaseous titrants (11).The titrants, which can be diluted if necessary with an inert gas, are standardized by injection into a liquid absorbate and subsequent titration with an appropriate reagent. The fundamental requirements for a gas-phase titration are identical to those for liquid-phase experiments-a favorable equilibrium constant and relatively fast kinetics. Calorimetric Methods. For conventional liquid-phase direct injection enthalpimetry, the emphasis is on quasiinstantaneous injection of the reagent. As the injected reagent is usually in 48, NO. 4, APRIL 1976
excess and the heat capacity of the system is conveniently determined after each injection, precise volume measurements are not a prerequisite of this approach. Accordingly, a manually operated syringe is adequate. For sequential analysis, in which a series of samples is injected into an excess of reagent, gravimetrically calibrated syringes with Chaney adaptors are recommended. The same thermostating considerations as in T E T experiments obviously apply. Ingenious immersion pipets (where the titrate solution itself serves as thermostat for the titrant) have been used to obviate any temperature mismatch between reactants. Such a system is indeed the basis of a versatile direct injection enthalpimeter for multicomponent analysis ( 4 ) . Adiabatic Cell. Successful enthalpimetric determinations are contingent upon the measurement of small temperature changes. Consequently, it is important that extraneous heat effects in the reaction cell be minimized. A conventional silvered Dewar flask has a relatively large heat transfer coefficient. However, special thinwalled Dewar flasks have been de-
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pressed in calories per chart ordinate unit, Co t h e calibration factor effective before t h e start of t h e titration (when the volume is U O ) , and Cfinalthe calibration factor measured after the titration (volume = ufinaJ. T h e fraction of total titrant added required to reach the equivalence point is denoted by a: a
E
uep - L'O ('final
Figure 3. Diagram of apparatus and circuitry used in enthalpimetric analysis
signed (22,22). incorporating a n appropriate reaction cell which has a heat transfer modulus of 11 X min-' and an equilibration time of only 3 s. This cell now forms the basis of a sophisticated isothermal titrationmicrocalorimeter ( 1 2 ) (v.i.). Analytical data can be obtained by using simple styrofoam insulated reaction cells which maintain pseudo-adiabatic conditions for the short period of a titration. Thi:, method is not recommended for fundamental thermochemical investigations. At the heart of enthalpimetric analysis lies the temperature measurement system. Thermometric titrations were performed a s early as 1913 utilizing a conventional mercury thermometer as temperaturf. sensor (8). Modern interest in enthalpimetric analysis was engendered bq thta simple. inexpensive, rapidly respondling, automated instrumentation made possible by t h e use of thermistors as temperature transducers. Thermistors are semiconductor devices which have a large temperature coefficient of resistivity, on the order of -4% p m degree. Even though the thermisl or resistance, RT, varies mponential'lj with temperature:
RT == A exp R / ( C + T )
(5)
(where A, R , and C are material constants characteristic of a given thermistor). RT approximates a linear function of temperature over a small range ( T 5 0 . 1 " C ) . The changing resistance is usually monii ored with the aid of a Wheatstone bridge circuit, whose unbalance potential is proportional to AT (and to Q if K is constant) and plotted automatically on the ordinate scale of a recording dc millivoltmeter. T h e corresponding chart abscissa is a measure of volume and/or time. T o 430 A
avoid localized temperature and concentration gradients in the adiabatic reaction cell, efficient stirring is imperative. Calibration Heater. Generally, the caloric axis (ordinate) of enthalpograms is calibrated chemically or electrically, though this is not strictly necessary for a purely end-point determinative titrimetric use. Chemical calibration involves reliance on a n accurately known heat of reaction and requires stringent reproducibility of reaction volumes in successive experiments. This limitation is circumvented by electrical calibration with occasional verification against suitable thermochemical standards. T h e circuitry for electrical calibration by Joule heating appears as part of Figure 3. The calorimeter is calibrated by passing a constant current, i, through a calibration heater, RH (which is immersed in t h e solution), and a standard resistor, R q . T h e heat dissipated within the calorimeter may then be calculated from Equation 6:
i2RHt VSVHt ( 6 ) 4.184 4.184 Rr where V s and V Hare potential drops measured across the standard resistor and the calibration heater. Changes in the heat capacity of the system occur as titrant is added. Accordingly, calibrations are generally performed both before and after the experiment. T h e desired caloric quantity is the total reaction heat evolved (or absorbed) u p to the equivalence point. A weighted average of t h e preand post-titration calibrations is used:
Q (calories) = --
Ce, =
CO+ a ( C f i n a 1 - C O )
where C,, is the desired calibration factor a t the equivalence point, ex-
ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
(7)
- UO
In the calorimetric mode of enthalpimetric analysis, u,, = ufinal and only one calibration is required. Temperature Measuring Circuits-dc. Conceptually, the simplest method for continuous measurement of t h e smaH resistance change of t h e temperature sensing thermistor is its incorporation as one arm of a dc Wheatstone bridge (Figure 3). In this configuration the bridge unbalance potential, Vunba121nce,may be related by Equation 8 to the resistances of the three unchanging arms, R1, R?, R:i, to the variable resistance of the thermistor-containing arm, RT, and to the applied bridge potential, V a p p l i e d : Vunhalance = VapIilied
x
The variation of the unbalance potential is, for all purposes herein described, linear over a small temperature range (AT 5 0.1 "C). An unavoidable feature of such bridge circuits is t h a t a finite current must pass through the thermistor which is, as a result, self-heated. T h e sensed temperature is therefore not strictly t h a t of t h e solution in which the device is immersed but rather is the temperature of the thermistor itself, the difference between the two being dependent upon t h e rates of self-heating and of heat dissipation a t the stirred solution interface. (Stirring itself imparts heat to the solution.) T o minimize this temperature differential, the applied bridge potential is either pulsed (v.i.) or minimized. Methods for bridge potential optimization have been described ( 1 3 ) .In ultrahigh sensitivity experiments, alinear ramp generator circuit may also be necessary. In these situations the temperature increase due to stirring and thermistor heating may be prohibitively large, resulting in a steeply sloping baseline. This problem can be obviated by the introduction of an equal but opposite electrical signal ( 3 ) .Low bridge voltages require that the entire bridge circuit including the thermistor leads he carefully shielded to avoid noise pickup. T h e small unbalance potential is amplified by a low noise dc amplifier, the output of which is usually fed into a recording dc millivoltmeter.
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Figure 4. Schematic diagram of pulsed dc (partial duty) Wheatstone bridge circuit
Differential Titrations. There have been numerous modifications of t h e standard dc amplified bridge circuit. Differential titrations involve a simultaneous blank titration in an identical adiabatic cell, where titrant is added t o t h e pure solvent. The temperature difference between the actual titration a n d the blank is recorded on t h e ordinate of corresponding differential enthalpograms. In this way, extraneous heat effects du'e to undesired simultaneous or seque:ntial reactions, heats of dilution, and temperature mismatch between titrant and titrate may be eliminated (10 ). Differential titrations are implemented by the replacement of bridge resistor R1 with a thermistor possessing characteristics equivalent t o those of RT, and its placement in t h e blank titration cell. By an examination of Equation 8, equivalent temperature changes of t h e two thermistors will cause :no change in t h e bridge unbalance potential. Derivative Titrations. (Figure 2, curves c, e, g.) In a n attempt t o improve thermometric end-points, first derivative, second derivative, and even third derivative plots of t h e bridge output signals have been obtained through t h e use of differentiating circuits. By the use of such derivative technicluesi, end-points may yield well-defined nonmonotonic pulses amenable to trigger digital threshold readouts. Resolution Enhancement. The ultimate resolution of the temperature measurement circuitry is limited by omnipresent noise sources (Johnson and pickup) and dependent upon t h e temperature coefficient of the thermistor and its heat dissipation. Theoretical treatments of the ultimate resolution of thermistors and thermistor bridges have very recently become available (13).'The practical resolution is 10 microdegrees. Investigations have shown t h a t a barium titanate ceramic device, commonly called a positive temperature 432 A
coefficient ( P T C ) device, has a temperature coefficient on the order of +16% per degree (15).This inherent fourfold sensitivity enhancement over thermistor technology has proved useful in high-sensitivity thermochemical studies. The problem of thermistor selfheating has been attacked via the development (27) of a pulsed (partial duty) dc Wheatstone bridge (Figure 4).The applied bridge potential is greater than t h a t used in conventional dc bridge circuitry ( a gain in sensitivity), but t h e concomitant temperature differential between the thermistor and the measured solution is minimized by pulsation of the bridge power. In practice, the bridge is powered for 10%of the total duty cycle of 50 ms. T h e basic duty cycle of t h e bridge is set by a master timing gener-
- 1 -
ator, t h e output of which is a rectangular wave train. This train is used t o periodically energize a dc power supply which in turn powers the thermistor bridge. T h e power dissipated in the thermistor bridge is ccintrolled by the output of the timing generator. After amplification of t h e bridge unbalance potential, the still pulsatile train is fed t o a sample-and-hold circuit which is triggered by the master timing pulse. The triggering is adjusted so t h a t the sampling time occurs in t h e middle of t h e duty cycle of t h e power supply, thus eliminating transients which occur a t either end of t h e master pulse. Thermistor Bridges-ac. Several ac-powered bridge circuits have been described in t h e literature during t h e past several years (3,16, 17).This may be related to t h e ready availability of highly stable ac amplifiers and to t h e improvement of signal-to-noise ratios using phase sensitive detectors. A schematic diagram of a typical ac thermistor bridge is illustrated in Figure 5 . However, the effectiveness of the ac system is somewhat limited by the introduction of stray capacitance. In our opinion, the advent of stable d c amplifiers and the application of good shielding techniques to dc bridge circuitry minimize the advantages of ac circuits. Other Thermistor Circuits. Some novel bridge designs have been proposed but remain generally unaccepted. These include linearized Wheatstone bridges and circuits utilizing operational amplifiers ( 3 ) ,which exhibit improved linearity and increased sen-
+ .
~~
Phase Sensitive Detector
Filter
Figure 5. Schematic diagram of ac thermistor bridge
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sitivity and ease of grounding. These advantages notwithstanding, the simplicity and general lack of noise of the standard dc bridge circuit have insured its continued use for most enthalpimetric instrumentation. Quartz Resonator Thermometer. Because of the scarcity of temperature transducers possessing a high sensitivity to very small temperature changes, thermistors have remained the mainstay of enthalpimetric temperaturemeasuring devices. Recently, the quartz resonator thermometer (QRT) has been proposed as a new instrument for enthalpimetric analysis ( 3 ) . This device consists of a small quartz crystal incorporated as part of an oscillator circuit. T h e resonant frequency of this circuit (about 28.2 MHz f 1 kHz per degree) as monitored by a frequency counter, is a linear function of the temperature of the quartz crystal. The degree of resolution is dependent upon the length of the counting period. With highly sophisticated frequency counters, a resolution of a few microdegrees may be obtained with a counting time of 10 s. The merits of this system are twofold: T h e measurement is highly insensitive to external noise sources, and the output may be directly calibrated in terms of a standard temperature scale. In contradistinction, thermistor bridge circuitry previously discussed is capable only of recording outputs proportional to temperature change. The use of the Q R T for enthalpimetric analysis is en cumbered by several inherent disadvantages. These include extended counting periods necessary for the attainment of adequate resolution, the large physical size of the probe coupled with its considerable thermal mass, and inadequate chemical resistance of the probe enclosure.
OUT'PUT
I
I
1
I
Figure 6. Schematic diagram of flow enthalpimetry
as a change in T p of the product stream. A process stream can be assayed by monitoring the heat pulse obtained when a sample of reagent is injected into the stream a t periodic intervals. The shape of the resultant enthalpogram is dictated by sample size. A typical continuous flow enthalpogram is shown in Figure 2d. One of the latest and potentially significant advances in the development of enthalpimetric flow systems is the immobilization of a large stoichiometric excess of reagent (in the form of a n enzyme) on the walls of the reaction chamber ( 1 8 ) .As the analyte flows through, it is catalytically converted to products with zero order ki-
netics prevailing. The corresponding heat change is monitored in the usual manner. The enzyme (reagent), acting as a true catalyst, remains unchanged, thus permitting sequential analyses. In a similar vein, an enzyme thermistor based on the measurement of heat generated in an enzyme-coated PVC coil in intimate contact with a thermistor has recently been described (19). Peak E n t h a l p i m e t r y . A new method called peak enthalpimetry has been reported (20) in the context of contemporary requirements for sequential analysis with particular reference to clinical analysis. A typical peak enthalpimetric apparatus is schematized in Figure 7. A carrier liquid (an inert
Modifications of Enthalpimetric Analysis
Flow Systems. Continuous Flow E n t h a l p i m e t r y . Continuous flow enthalpimetry has proved to be a valuable modification of enthalpimetric analysis, particularly when utilized for the on-line analysis of industrial process streams ( 1 0 ) .T h e technique, based on the fundamental principles inherent in flow calorimetry, consists of passing two reactant solutions a t a constant rate through a mixing chamber (the reagent being in stoichiometric excess) and continuously monitoring the heat output of the product stream (Figure 6). The temperature of the product stream, T p ,is directly related to the temperatures of the sample and reagent streams, to the molarity of the sample, and to the ratio of the sample and reagent flow rates. Accordingly, any fluctuation of the sample concentration will manifest itself 434A
Figure 7. Schematic diagram of peak enthalpimetry apparatus
ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
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solvent) and a reagent solution are made to flow into a mixing cell from constant pressure reservoirs. The two separate streams merge and are homogenized by the vibration (60 Hz) of glass beads contained within the mixing cell. A discrete volume of sample is injected into the carrier stream, producing a cylindrical plug of sample so-
lution bounded by the solvent. As the sample is swept toward the mixing chamber, diffusional spreading occurs, resulting in an essentially Gaussian distribution profile as it reaches the reaction cell. The pseudo-Gaussian temperature profile of the product stream, measured in a differential mode, reflects the concentration pro-
file of the analyte entering the reaction cell. Moreover, the output from the thermistor bridge is a measure of the instantaneous concentration of the sample. Thus, an integration of the area bounded by the peak will be a measure of the amount of reactable material in the sample (Figure 2f). With the incorporation of automatic
Tabte I. Selected Applications of Enthalpimetric Analysis Analyte(4
Type of determination
Glucose, ATP Sodium pyruvate, NADH Hexokinase, ATP
Enzyme assay Substrate determination using enzyme catalysis
PHARMACEUTICAL
Alkaloids Sulfonamides
INDUSTRIAL
Petrochemical
Polyesters, polyethers
Fer t i l izer s Ceramics Bayer Process control Nuclear technotogy Minerals Bauxites Coal
INORGANIC
Complexometric
Anions
0 R G A N IC FUNCTIONAL GROUP
436A
[El (25)
Urease Phosphotungstic acid Standard antigens and haptens
Kinetic DIE TET
(281
Codeine phosphate, morphine sulfate, etc. Su If at hiazo le, sulfisoxazole
Silicotungstic acid
TET
KOH in propan-2-01
TET with catalyzed end-point
Water, oxygen, and low molecular weight alcohols in hydrocarbons GI ycerole-al kylene oxide polyethers, butane-1 ,&dioladipic acid polyesters PO:-, K+, NH;, NO; car bam ide
Triethylalum inum
Continuous Flow Enthalpimetry
Acetic anhydride
DIE
(33)
Several sequentially added reagents H*O, H+
Dl E
(341
Free acid in nuclear fuels
NaOH
T ET
Si.O,, AI,O,, Fe,O,, TiO,, and others Phenol
Various
DIE
KOH in propan-2-01
TET with catalyzed end-point
(6)
M n ( l l ) in presence of Ca(ll) I-, NO;, SO:-,and others
EDTA
Differential TET
(381
Various
Peak enthalpimetry
(39)
NH,, aliphatic amines
HCI (9) KOH
TET DIE
Antibodies {IgG, IgM)
NO:- in glass Na OH /A I ,OJN a, CO 3 ,
TET
(16)
(29,301
DIE TET
(37)
Mntlll in oresence of
zn(tlj
'
GASEOUS
Ga s/ga s Gasp iq u id
DIE
DIE kinetic
Ref
Kinetic DIE
Proteins Immunological
Method
Reagentk)
BIOCHEMICAL
cow so,
For an exhaustive review, see Ref. 10
ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
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ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
4 3 7 ~
injection and digital printout, t h e seriatim performance of many analyses is feasible. Catalimetric Enthalpy Titrations. Standard enthalpimetric analysis has sensitivity limits dictated by t h e heat liberated (or absorbed) during a reaction. For titrates present a t low concentration and those with small heats of reaction, a catalimetric technique has been developed (4,101. This methodology takes advantage of a secondary indicator reaction which occurs only after virtual completion of t h e analytically interesting primary reaction: T h e sequence is controlled by selecting an indicator reaction which has an appropriate equilibrium constant. A judiciously chosen catalimetric reagent which reacts with excess titrants is added. If the heat of reaction between the catalimetric reagent and titrant is large, t h e heat of the analysis reaction becomes unimportant per se, but t h e correct equivalence point is indicated by t h e commencement of heat evolution (or absorption) engendered by the secondary reaction. In a recent application an alkali-catalyzed polymerization has been used as t h e indicator reaction in the determination of acidic functions of sulfanilamide derivatives (and of sulfonamide formulations) by use of catalimetric enthalpy titrations (21). Isothermal Titration Calorimetry. Evident fundamental assumptions underlying the concept of enthalpimetric analysis are t h a t the relevant system is effectively adiabatic and t h a t AH is independent of temperature. If an efficient adiabatic cell is used, these approximations hold for most analytical measurements using virtually instantaneous reactions. For precise fundamental measurements and for reactions with slower kinetics, isothermal calorimetry has been utilized. T h a t method is beyond the scope of t h e present review. However, a technique combining titration calorimetry and isothermal calorimetry has been reported (4, 12,221, a description of which is warranted. T h e method closely parallels t h a t of thermometric enthalpy titrations, with the exception t h a t the temperature of the adiabatic calorimeter and its contents is held constant throughout t h e titration. This is achieved via the incorporation of a thermoelectric cooling device and a variable heater (Figure 8). T h e heat of reaction can be correlated to the powcr required to maintain isothermal conditions. Both endothermic and exothermic reactions have been studied in this manner a t temperatures held constant to f0.0002 O C and heat effects measured with a precision of &O 02 calories. T h e isothermal method, albeit very precise, is time consuming. a restriction that may 438A
Figure 8. Isothermal titration calorimeter (from ref. 22) Courtesy of the American Institute of Physics
limit its applicability from an analytical viewpoint. Applications
A brief summary of some selected applications is shown in Table I. T h e survey outlines a few innovative and topical applications of enthalpimetric investigations taken from recent reports. This list should not be regarded as an exhaustive review. More complete surveys of enthalpimetric determinations can be found elsewhere (6, I O , 23, 2 4 ) . As the varied nature of the survey suggests. enthalpimetric analysis has found acceptance in many fields. T h e high tolerance level of the technique to nonreactable impurities has resulted in numerous industrial and commercial applications where matrix problems might otherwise be prohibitive. Indeed, enthalpimetric flow methods have been particularly successful in continuous quality control of industrial process streams. An enthalpimetric approach to clinical analysis has been recently developed ( 2 6 ) .Its salient feature is reliance on a fundamental thermodynamic property, A H , in lieu of empirical standards. Specificity has been achieved by the utilization of t h e inherent selectivity found in enzymecatalyzed reactions. Moreover, t h e possibility exists for serum and, in
ANALYTICAL CHEMISTRY, VOL. 48, NO. 4 , APRIL 1976
some cases, whole blood analysis without prior deproteinization (25). Clearly, the volume of reports in the literature suggests that there is growing interest in the utilization of enthalpimetric methods for both analytical and fundamental studies. T h e versatility and simplicity of the technique, combined with a classic foundation in fundamental thermochemistry, make enthalpimetric analysis an inherently attractive methodological approach. References ( 1 ) P. M’. Carr and J . 45, 634 (1973).
.Jordan. Ancil. Chem..
(2) G . 0. Piloyan and Yu Y.Dolinina. Tn/ a n t a . 21. 955 (1974) ( 3 ) P. If-. Carr, C‘rit. Rei. Anal. (’hcm., 3,
491 (1972). J.Barthel. “Thermometric Titrations”. LVilev, Kew York. Y.Y., 1975. (5) I,. S.Bark and 5.lf.Bark, ”Thermometric Titrimetry”. Pergamon Press. Oxford, England, 1969. (6)L. S.Bark, P. Bate, and J. I(.Gfime. “Thermometric and Enthalpimetric Titrimetry”, in “Selected Annual Reviews of the Analytical Sciences”,Vol 11. L. S. Bark, Ed.. Society for Analytical Chemistry, London, England, 1972. (7) J . J . Christensen and K. hi. Izatt. ”Thermochemistry in Inorganic Solution Chemistry”, in “Physical hlethods in Advanced Inorganic Chemistry“. p 548, H.A.O. Hill and P. Day. Eds.. Interscience, h-ew York, K.Y., 1968. (8) ,J. .Jordan. “Thermometric Enthalpy Titrations”, in ”Treatise in Analytical (4)
Chemistry”, I. M. Kolthoff and P. J. Elvings, Eds., Part I, Vol 8, p 5175, Interscience, New York, N.Y., 1968. (9) H.J.V. Tyrrell and A. E. Beezer, “Thermometric Titrimetry”, Chapman and Hall, London, England, 1968. (10) G. A. Vaughan, “Thermometric and Enthalpimetric Titrimetry”, Van Nostrand Reinhold, London, England, 1973. (11) B. J. Duffield and D. N. Hume, Anal. Lett., 7,681 (1974). (12) J. J. Christensen, J. W. Gardner, D. J. Eatough, R. M. Izatt, P. J. Watts, and R. M. Hart, Reu. Sci. Instrum., 44, 481 (1973j . (13) L. D. Bowers and P. W. Carr, Thermochim. Acta, 10,129 (1974); 11,225 (1975). (14) P. G. Zambouin and J. Jordan, Anal. Chem., 41,437 (1969). (15) J. A. Lynch and J. Jordan, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Paper No. 54, Cleveland, Ohio, March 1974. (16) E. B. Smith and P. W. Carr, Anal Chem., 45, 1688 (1973). (17) R. L. Berger, W. S. Friauf, and H. E. Cascio, Clin. Chem., 20, 1009 (1974). (18) L. M. Canning, Jr., and P. W. Carr, Anal. Lett., 8,359 (1975). (19) K. Mosbach and B. Danielsson, Biochim. Biophys. Acta, 364,140 (1974). (20) A. C. Censullo, J. A. Lynch, D. H. Waugh, and J. Jordan, “Biochemical and Clinical Applications of Titration Calorimetry and Enthalpimetric Analysis”, in “Analytical Calorimetry”, R. S. Port and J. F. Johnson, Eds., Plenum Press, New York, N.Y., 1974. (21) E. J. Greenhow and L. E. Spencer, Anhl. Chem., 47, 1384 (1975). (22) J. J. Christensen, H. D. Johnston, and R. M. Izatt, Reu. Sei. Instrum., 39,1356 (1968). (23) L. D. Hansen, R. M. Izatt, and J. J. Christensen, “Applications of Thermometric Titrimetry to Analytical Chemistry” Treatise on Titrimetry”, Vol 11, J. Jordan, Ed., Marcel Dekker, New York, N.Y., 1974. (24) R. N. Heistand, Reu. Anal. Chem., 2, 193 (1975). (25) C. D. McGlothlin‘and J. Jordan, Anal. Chem., 47,786, 1479 (1975);Clin. Chem., 21, 741 (1975). (26) L. Donnovan, K. Barclay, K. Otto, and N. Jespersen, Therrnochim. Acta, 11,151 (1975). (27) D. Lohr, unpublished. (28) N. D. Jespersen, J . Am. Chem. S o c . , 97, 1662 (1975). (291 J. Jordan and N. D. Jespersen, Coll. Int. Cent. Nat. Rech. Sei., 201, 59 (1972). (30) J. A. Lynch, thesis, Pennsylvania State University, University Park, Pa., 1976. (31) L. S.Bark and J. K. Grime, Anal C h m Acta, 64,276 (1973). (32) E. J. Greenhow and L. E. Spencer, Anal. Chem., 47,1384 (1975). ( 3 3 ) I. I. Kaduji and J. H. Rees, Analyst (London),99,435 (1974). (34) I. Saj6 and B. Sipos, Talanta, 19,669 (1973). (35) K. Doering, Therrnochim. Acta, 8,485 (1974). (36) F. G. Bodewig,Anal. Chem., 46,454 (1974). ( 3 7 ) M. Mike, Hung. Sci. Instrum., 28,47 (1974). ( 3 8 ) Doi Kunio, Anal. Chim. Acta, 74,357 (1975). (39) R. Geyer, J. Birkahn, and G. Eppert, Chem. Technol., 12,775 (1974).
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