Injection Enthalpimetry

When a chemical reaction occurs at constant pressure, heat is liberated or absorbed. The heat flow into or out of a system at constant pressure is quantified using a quantity called enthalpy, H. We usually measure the change in enthalpy, AH, called the enthalpy of reaction. It is a reproducible physical property for a given reaction A -I- B C -I- AH. Therefore the magnitude of AH depends on the quantity of reactants involved in the reactions (e.g., multiplying aU of the reaction coefficients by 2 means we have to multiply AH by 2). Excess amounts of any of the reactants do not take part in the generation or absorption of heat. 

Direct injection enthalpimetry (DIE) is similar in many respects to thermometric titrimetry. One essential difference is that an excess of titrant is added very rapidly to the sample and the reactants mixed vigorously. The temperature is then measured against time following the injection of the titrant, as shown in Fig. 16.32. We may suppose that the following exothermic reaction takes place  

The quantity of heat generated is a function of the number of moles that take part in the reaction. This, in turn, is controlled by the amounts of A and B. Any excess of either A or B does not react. In this instance, we have added an excess of B therefore the amount of heat generated must be a function of the amount of A present in the mixture. If the amount of A present were halved, then the amount of heat generated would be halved. Since there is an excess of B present in both cases, only the amount of A affects the amount of heat generated. 

The method is quite useful, particularly if the rate of chemical reaction between A and B is slow. The final quantity of heat evolved is not a function of time, but a function of the concentration of sample. This is a distinct advantage over conventional volumetric analysis and, in some instances, thermometric titrations. As stated earlier, slow reactions give rise to errors in the endpoint determination in thermometric titrations. 

The equipment used for DIE is identical to that for thermometric titrimetry. The titrant must be at the same temperature as the sample at the start of the experiment, and the syringe is emptied rapidly into the cell to deliver the titrant instantaneously . 

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Almost all kinetic investigations on azo coupling reactions have been made using spectrophotometric methods in very dilute solutions. Uelich et al. (1990) introduced the method of direct injective enthalpimetry for such kinetic measurements. This method is based on the analysis of the zero-current potential-time curves obtained by the use of a gold indicator electrode with a surface which is periodically restored (Dlask, 1984). The method can be used for reactions in high (industrial) concentrations. 

Rogers, D. W. (1973). Direct-injection enthalpimetry of micromolar quantities of unsaturated fatty acids, Analytical Biochem., 56, pp. 460-464. 

In thermal methods of analysis, either temperature change is measured or the temperature is manipulated to produce the measured parameter. Thermogravimetry (TG), differential thermal analysis (DTA), and differential scanning calorimetry (DSC) are the three major methods that use temperature change as the independent variable. Thermometric titration (TT) and direct-injection enthalpimetry (DIE) use temperature as the dependent variable. These five methods will be discussed primarily from an analytical point of view. Each method has its unique characteristics and capabilities for that reason, the major aspects of each method are considered individually. 

Direct-injection enthalpimetry data are similar to those from TT. However, titration is replaced by a virtually instantaneous injection of reagent, and temperature is monitored as a function of time. As a result, more rapid analysis is possible. Heats of reaction can be readily deduced, and kinetics may be studied in favorable situations. 

Thermometric titrations (TT) and direct-injection enthalpimetry (DIE) are both calorimetric techniques the heat evolved or absorbed serves as an indicator of the progress of the reaction. Nowadays, TT and DIE are used for routine analysis and in fundamental research involving the chemical equilibrium, reaction kinetics, and thermochemistry of processes not readily studied by other methods. 

Thermometric titration plots are characteristically graphs of temperature change versus titrant added. Direct-injection enthalpimetry yields plots of temperature... 

Figure 17.11. Characteristic curves obtained from thermometric titration and direct-injection enthalpimetry.

In direct-injection enthalpimetry, the titrant at the same temperature as the sample is injected rapidly into the sample and the data obtained as a temperature-versus-time curve. No endpoint is obtained, but the magnitude of the temperature change is proportional to the concentration. Also, one can generate kinetic curves to evaluate slow reactions. The speed of analysis is enhanced, and processes with equilibria unfavorable for titration are readily studied by using a large excess of one reactant. 

The analytical techniques used to study changes in physical properties with temperature are called thermal analysis techniques. They include thermogravimetric analysis (TGA), differential thermal analysis (DTA), differential scanning calorimetry (DSC), thermometric titration (TT), and direct injection enthalpimetry, dynamic mechanical analysis (DMA), and thermomechanical analysis (TMA). Thermal analysis techniques are used in... 

A word of caution should be mentioned about possible interferences, particularly if two simultaneous reactions take place. Since both reactions may generate heat, a direct error will be involved with injection enthalpimetry methods. However, with thermometric titrations, frequently consecutive reactions take place and can be determined consecutively without prior separation. This has been demonstrated in the titration of calcium and magnesium with oxalate. 

Direct injection enthalpimetry involves the reaction of a small portion of sample with a large excess of acetic anhydride under conditions where the reaction is rapid (<1 s), and the change in temperature associated with the reaction (dT) is recorded using a thermistor bridge. Under conditions of constant heat capacity, dT should be directly proportional to the number of reactive groups per unit mass of the sample. The main advantages of the technique are that (i) it is relatively simple to operate and (ii) only a few minutes are required to carry out an analysis. 

Kaduji and Rees [10] and others [11] employed direct injection enthalpimetry to determine the hydroxy value of glycerol-alkylene oxide polyethers and butane-1,4-diol-adipic acid polyesters. 

Direct injection enthalpimetry has great potential for determining the hydroxyl values of polyethers and polyesters. 

Method 3.2 Determination of Hydroxyl Number of Glycerol-Alkylene Oxide Polyethers and Butane, 1,4-Diol Adipic Acid Polyesters. Direct Injection Enthalpimetry [9]... 

Direct injection enthalpimetry has great potential as a method for determining the hydroxyl values of polyethers and polyesters. The method is rapid, the temperature rises for two samples and a standard being recorded in duplicate in about 10 minutes. 

METHOD 58 - DETERMINATION OF HYDROXYL NUMBER OF GLYCEROL-ALKYLENE OXIDE POLYETHERS AND BUTANE, 1,4-DIOL ADIPIC ACID POLYESTERS. DIRECT INJECTION ENTHALPIMETRY. ... 

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