Solution Calorimeters

Solution calorimetry covers the measurement of the energy changes that occur when a compound or a mixture (solid, liquid or gas) is mixed, dissolved or adsorbed in a solvent or a solution. In addition it includes the measurement of the heat capacity of the resultant solution. Solution calorimeters are usually subdivided by the method in which the components are mixed, namely, batch, titration and flow. 

Figure Bl.27.5. A typical solution calorimeter with thennometer, heater and an ampoule on the base of the stirrer which is broken by depressing it against the ampoule breaker. (Reproduced with pennission from Suimer S and Wadso I 1959 Acta. Chem. Scand. 13 97.)...

The solution experiments may be made in aqueous media at around ambient temperatures, or in metallic or inorganic melts at high temperatures. Two main types of ambient temperature solution calorimeter are used adiabatic and isoperibol. While the adiabatic ones tend to be more accurate, they are quite complex instruments. Thus most solution calorimeters are of the isoperibol type [33]. The choice of solvent is obviously crucial and aqueous hydrofluoric acid or mixtures of HF and HC1 are often-used solvents in materials applications. Very precise enthalpies of solution, with uncertainties approaching 0.1% are obtained. The effect of dilution and of changes in solvent composition must be considered. Whereas low temperature solution calorimetry is well suited for hydrous phases, its ability to handle refractory oxides like A1203 and MgO is limited. 

High-temperature solution calorimeters [34-36] are in general of the twin heat flux type. They are applicable from around 900 K to around 1500 K and a... 

Solution calorimetry has been the most widely applied type of calorimetry because it needs relatively simple apparatus, and a wide choice of liquid reagents is available. Much of silicate thermochemistry, for example, has been based on the hydrofluoric acid solution calorimeters using platinum or silver vessels and fluorinated gaskets (90, 214). 

Figure 8.1 Scheme of a Dewar vessel isoperibol reaction-solution calorimeter. A ampule containing the sample B ampule breaking system C calorimeter head D temperature sensor E stirrer F electrical resistance G Dewar vessel H plunger of the ampule breaking system I, J inlets K plug connecting the calibration resistance to the calibration circuit. 

Various substances and reactions have been used to test the accuracy of reaction-solution calorimeters [3 9,40]. The solution of tris(hydroxymethyl)amin-omethane (THAM orTRIS) in 0.1 mol dm-3 HCl(aq), which was first proposed by Wadso and Irving [133] in 1964 and recommended by the Standards Committee of the U.S. Calorimetry Conference in 1966 [134,135], is perhaps the most widely used method. Several problems encountered in the use of the THAM+HC1 (aq) reaction to assess the accuracy of reaction-solution calorimeters have been... 

Figure 8.5 The vessel of the LKB 8700 reaction-solution calorimeter. A thermistor B heater used for calibration C stirrer/ampule holder D glass ampule E sapphire pin. Adapted from [132].

The accuracy of a titration calorimeter is normally assessed using the reactions of NaOH(aq) with HCl(aq) or HCICUjaq) [209,210], The dissolution of crystalline tris(hydroxymethyl)aminomethane (THAM) in HCl(aq) has also been employed when the apparatus is equipped with a system for the introduction of solid samples (e.g., an ampule breaking device) [210]. As mentioned in chapter 8, the latter method is commonly recommended for testing conventional reaction-solution calorimeters [39,40]. 

M. V Kilday. Systematic Errors in an Isoperibol Solution Calorimeter Measured with Standard Reference Reactions. J. Res. Natl. Bur. Stand. 1980, 85, 449—465. 

The design and operation of solution calorimeters is an extensive topic. Reference (125) reviews modem calorimetry and identifies earlier discussions. The thermometric titration type of calorimeter has been perfected during the past fifteen or twenty years. It is especially useful for measuring heats of reaction that take place in several steps. The availability of advances in thermometry has had a major effect on calorimetry. 

An additional benefit of the TAM system is that it is possible to purchase a range of apparatus that fit within the calorimetric chambers. Examples of such equipment include RH vessels, titration vessels, and a solution calorimeter. It is also not outside the bounds of possibility to construct a piece of apparatus to do a specific job oneself, if no such item is commercially available. 

In this experiment 500 mL of solution A, with a precisely known concentration in the neighborhood of 0.25 M, is reacted with 50 mL of solution B at a concentration sufficient to provide a slight excess over the amount required to react with solution A. The reaction is carried out in the solution calorimeter shown in Fig. 1. The calorimeter is a vacuum Dewar... 

Electrical heating circuit for solution calorimeter. The standard resistor should be a wire-wound resistor with a low temperature coefficient and rated for 2 W. If a heating coil of higher resistance (say, 60 2 is to be used, the current can be reduced to 0.5 A and a 1-H standard resistor can be used. 

In the high-temperature region, the main method of measurement is the drop calorimetry, where the sample is heated to the chosen temperature outside the calorimeter in a furnace and the heat capacity is calculated from the temperature dependence of the enthalpy changes measured after dropping the sample into the calorimeter. The application of this technique affects, however, the behavior of the sample heated in the furnace (decomposition, reaction with the crucible, etc. should be avoided) as well as at the cooling from the furnace temperature to that of the calorimeter. Sometimes the sample does not complete its phase transition at cooling (e.g. at the temperature of fusion, a part of the sample crystallizes while the other part becomes glassy). In such a case, the drop calorimeter must be supplemented by a solution calorimeter in order to get the enthalpy differences of all the samples to a defined reference state. 

The solution of the sample in a 2 1 mixture of concentrated hydrofluoric and nitric acids at Tref = 298 K was chosen as the reference state. The relative enthalpy, 7/rei(7m), was measured by indirect method of double calorimetry. This procedure enables us to determine Hiei(Tm) as the sum of enthalpy increase measured during the cooling of the system in a drop calorimeter (Acooi and during its dissolution in a solution calorimeter (Asoi//). Equation (4.34) can thus be written in the form... 

After measurement in the drop calorimeter, the crucible with the sample was opened by carefully cutting off the lid. The entire sample was removed, ground to the particle size less than 0.04 mm and homogenized. A part of the sample was subjected to X-ray powder diffraction and IR spectroscopic analyses. The heat of solution, Asoi77, of the sample was then measured in the solution calorimeter, described in detail by Proks et al. (1967). Approximately 0.05 g of sample was dissolved in 100 ml of the 2 1 mixture of concentrated hydrofluoric and nitric acids. The measurement of the heat of dissolution was repeated 3 times on an average. 

Kilday MV. Systematic errors in an isoperibol solution calorimeter measure with standard reference reactions. J Res Natl Bur Stand 1980 85 449-465. 

Solution calorimetry involves the measurement of heat flow when a compotmd dissolves into a solvent. There are two types of solution calorimeters, that is, isoperibol and isothermal. In the isoperibol technique, the heat change caused by the dissolution of the solute gives rise to a change in the temperature of the solution. This results in a temperature-time plot from which the heat of the solution is calculated. In contrast, in isothermal solution calorimetry (where, by definition, the temperature is maintained constant), any heat change is compensated by an equal, but opposite, energy change, which is then the heat of solution. The latest microsolution calorimeter can be used with 3-5 mg of compound. Experimentally, the sample is introduced into the equilibrated solvent system, and the heat flow is measured using a heat conduction calorimeter. 

Kullberg measured the heat evolved when small amounts of red HgO(cr) were dissolved in solutions of 0.3 and 1 M KSeCN in a reaction solution calorimeter. The HgO(cr) was contained in a thin-walled glass ampoule, which could be broken inside the calorimeter... 

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