Total Heat Effect Research Articles

Thermal behavior of Landolt reactions in comparatively dilute aqueous solutions followed by heat exchange calorimetry of batch type

In batch-type Landolt reactions composed of potassium iodate and sodium hydrogen sulfite, the color of the solution changes suddenly at a time interval after mixing the component substances. This is often presented as chemical magic with the nickname “clock reaction”. The thermal behavior of more dilute solutions than in the demonstration is studied by heat exchange calorimetry of batch type. Sample and reference vessels are fixed differentially in a precision water bath. Heat evolved in the vessel is exchanged freely with the surrounding water. Crystalline sodium hydrogen sulfite is added to an aqueous solution of potassium iodate to start the reaction. The temperature change in the vessels is detected by a thermistor, and treated by an analogue computation to obtain the rate of heat evolution and total heat effect. The heat of reaction Q reac is estimated after correction for the negative heat of solution of sodium hydrogen sulfite. Free iodine may be produced after the consumption of sodium hydrogen sulfite, if [NaHSO 3] init, is not larger than three times [KIO 3] init Q reac increases in direct proportion to [NaHSO 3 init in the region of excess [KIO 3] init However, as expected, constant values of Q reac were observed in the region of excess [NaHSO 3] init when it is more than 3.6 times larger than [KIO 3] init. The reaction enthalpy at infinite dilution was estimated to be −228.4 kJ per 1 mole of [NaHSO 3] init.

Caloric behaviour in the induction period of Belousov- Zhabotinskii reactions monitored by heat exchange calorimetry of batch type and the simultaneous estimation of the potentiometric aspect

The induction period of the Belousov-Zhabotinskii (B-Z) reactions which occur in an aqueous solution composed of potassium bromate, sulphuric acid, malonic acid and ammonium cerium(IV) sulphate was studied in the same reaction by heat exchange calorimetry of batch type and potentiometry. The sample and reference vessels were fixed differentially in a home-made heat exchange calorimeter in a temperature-controlled water bath. The heat evolved in the vessel was exchanged freely with the surrounding water. The rate of heat evolution q and the total heat effect Q were estimated by the analogue treatments of electrical signals observed as temperature changes via thermistors placed in each vessel. A standard potentiometric cell was also present in the water bath in order to obtain simultaneously the potential difference E which reflects the bromide concentration in the reaction. When fine crystals of the cerium(IV) salt are added to start the B-Z reaction, a small endothermic peak is observed in the q versus t curve, followed by large exothermic double peaks. In the induction period, only the smaller change is observed in the potentiometry. The Q values in the induction period depend significantly upon the initial concentrations of KBrO 3 and H 2SO 4 in the solution, in contrast to those of malonic acid and cerium(IV) salt. Several minor fluctuations in Q and the E versus t curves are observed in the induction period of dilute KBrO 3 or H 2SO 4 solution. The Δ H values in the induction period were also estimated in several different concentrations of the initial constitutents.

Thermal behaviour of Landolt reactions studied by heat exchange calorimetry of batch type

The Landolt reaction, known by the nickname of a “clock reaction”, is exemplified by redox reactions in aqueous solution, including that between potassium iodate and sodium hydrogen sulphite. In the presence of starch, the colourless solution becomes bluish violet at a certain time interval after mixing. The thermal behaviour of the clock reaction was studied for estimating the rate of heat evolution and the total heat effect by heat exchange calorimetry of batch type. To start the reaction, the weighed sample crystals of sodium hydrogen sulphite were added to the aqueous potassium iodate solution in the sample vessel in the calorimeter. The heat of solution of sodium hydrogen sulphite was observed separately in water. The experimental concentration ranges of potassium iodate and sodium hydrogen sulphite were selected according to the requirements of the calorimeter. The total reaction time was also estimated from curves of the rate of heat evolution. The total heat effect was obtained as the difference between the observed heat of reaction and the heat of solution, and depended only on the amount of sodium hydrogen sulphite. The heat of reaction was estimated to be −239.9±5.5 kJ per mol of sodium hydrogen sulphite at 25.0±0.1°C.

A Smith thermal analysis and DSC study of the In-Cd system

The Smith thermal analysis technique has been used to study phase boundaries and invariant reactions in the In-Cd system. Results obtained from measurements at eleven compositions are in full agreement with the phase diagram proposed by Heumann and Predel. DSC measurements at low scan rates have been carried out over the temperature range including the eutectic and eutectoid reactions. Measurements of the total heat effect together with the lever rule have been used to obtain, (i) enthalpies of reaction for both the eutectic and eutectoid reactions, and (ii) compositions of the phases involved in these invariant reactions. The method used should have wide applicability to eutectic and eutectoid reactions in a range of systems.

Thermal behaviour of Belousov-Zhabotinskii reactions followed by heat exchange calorimetry involving simultaneous estimation of potentiometric oscillations

The chemical oscillations of the Belousov-Zhabotinskii reactions which occur in a solution composed of KBrO 3, H 2SO 4, CH 2(COOH) 2 and Ce(NH 4) 4(SO 4) 4 · 2H 2O, have been followed simultaneously by heat exchange calorimetry of a batch type and by potentiometry in order to study the relation between the thermal behaviour and the reaction mechanism. In the assembled calorimeter, the sample and reference vessels were fixed differentially in a water bath. The heat evolved in the sample vessel was exchanged freely with the ambient water whose temperature was controlled precisely. Because this reaction does not change the effective heat capacity, it was a very suitable subject for the calorimetric study. A normal potentiometric cell was also assembled in the bath to estimate the potential difference of the same reaction. The rate of heat evolved q, the total heat effect Q and the potential difference of the bromide ions, E, were measured against time t. After the start of the reaction, large Q values were measured in the induction period, in contrast to only a small change in E which increased linearly with initial concentrations of H 2SO 4 and KBrO 3. In the oscillation period, the amount of Q evolved in each cycle decreased exponentially with t. The measured q value depended on the initial concentrations of the components, and was compared with the values of E estimated simultaneously. The threshold value of [H 2SO 4] init obtained from the relation between q and E was compared with that calculated from the rate constants obtained from the literature; fairly good agreement was obtained.

Some improvements in the heat exchange calorimeter for estimation of small heat effects

Abstract In heat exchange calonmetry, sample and reference vessels are fixed in a temperature-controlled water-bath. Heat evolved in the sample vessel is exchanged freely with ambient water Therefore, the measuring temperature can be easily selected by changing the temperature of the bath water. In the estimation of small heat effects, the measurement of temperature via a thermistor used as a sensor must be accurate and precise Some improvements in the water-bath of the heat exchange calorimeter are reported To find the influence of close agreement between B constants on the thermal fluctuation range of the bath water, a pair of thermistors with B constants of 3396 and 3342 K, used in the sample and reference vessels respectively, were compared with thermistors with B values of 3404.09 and 3404.07 K, respectively. A hand-made heater block was introduced into the temperature control system of the water-bath, and was found to shorten the delay time of heat evolution. The heat evolution from an area instead of a point source was effective in mixing the water throughout the water-bath. To demonstrate the proposed improvements, the total heat effects by electric heating were compared in the range 0.5–0.05 J On the whole, reasonable repeatability and simple handling of the vessels were obtained.

Features of the interaction of cellulose with liquid and vapour phase water

The interaction of dry cellulose with H 2O liquid and vapour phases has been investigated by calorimetry and infrared. It is shown that there are two stages in the interaction of cellulose with water — the endothermic stage, that is the more rapid and less energy-intensive one, and the exothermic stage which affects the total heat effect.

Addition of Local Anesthetics to Contrast Media

Effects of equiosmolar solutions (osmolality 1.35 mol/kg) of the ratio 1.5 contrast medium metrizoate (250 mg I/ml) containing either the local anesthetic mepivacaine or saline were evaluated on subjective discomfort, aortic blood pressure and heart rate in 17 patients referred for aortofemoral angiography due to intermittent claudication. Each patient was injected with 30 ml (15 ml/s) of each solution in random double-blind order. Each patient was then injected with 45 ml (12 ml/s) of the ratio 3 medium ioxaglate (320 mg I/ml, osmolality 0.58 ml/kg). Ioxaglate caused significantly less total discomfort, pain, heat, vocal reactions, involuntary movements and hemodynamic effects than metrizoate-mepivacaine and metrizoate-saline. The effects of the two metrizoate solutions did not differ significantly. Previous contradictory reports on the effects of anesthetics in contrast media were reviewed with regard to experimental design, different osmolality of the test and control solutions, premedication, and pH dependence of local anesthetics. Ratio 3 media are recommended because they produce a more reliable relief of patient discomfort and fewer hemodynamic changes than the addition of local anesthetics to ratio 1.5 media.

Effect of the composition of the fuel on the stability of the heat input into a glass-melting furnace

The change in the concentration of the components of the natural gas burned in the furnaces of the Saratov Technical Glass Plant in the course of a year are presented. The authors consider the results of uncorrected deviations in the composition of the gas fuel, bearing in mind the component of the heat flow to the furnace obtained directly by the combustion of the gas. The fuel-combustion regime is organized on the basis of a calculation for a certain composition called a calculated regime. The dependence of the heat loss as a result of inadequate combustion of the gas, total heat effect of a change in the composition of the gas, and the overconsumption of fuel on the requisite excess-heat coefficient are presented. The authors found that any changes in the composition of the natural fuel which might lead either to an increase or decrease in the excessair coefficient will involve an over consumption of natural gas where no correction is made at the same time on the specified values of fuel consumption and the gas:air ratio. Allowing for these changes at the glass plants where the composition of the fuel is not constant could be one source of economies.

The kinetics and mechanism of the reaction of oligobutadienyl lithium with a secondary lithium alkyl

The kinetics of the reaction of lithium oligobutadienyl (OBL) with 2-hexyl-lithium in heptane have been studied by calorimetry and by GLC at 31–63°. It has been found that the initial reaction rate depends linearly on the square of the OBL concentration and changes weakly with the concentration of 2-hexyllithium. For a stoichiometric reagent ratio, the heat release kinetic curves obey a second order law. The total heat effect of the reaction, the effective second order rate constants and the effective activation energy have been found. Infrared spectroscopy has shown that metallation of oligobutadienyl lithium takes place with formation of both 1,1 and 1,4-lithiated derivatives. Besides metallation, side reactions occur, in particular, elimination. The probable reaction mechanisms are discussed.

Comparison of the detection limits of microcalorimeters

Different calorimetric methods are compared with regard to the minimum detectable total heat effect that can be measured. This heat effect is shown to be dependent on: (a) the peak-to-peak uncertainty in the primary measurement which should be expressed in either degrees Kelvin or watts depending on whether the primary measurement is of temperature or power, (b) the time constant of the calorimeter system, and (c) the total energy equivalent of the reaction vessel and contents if the primary measurement is temperature.

Microcomputer controlled experimentation in carlorimetry. Application to the determination of the partial enthalpies of 3 d transition metals in liquid aluminium

This paper describes the development of a total automation and cheap data acquisition system for a high temperature Calvet calorimeter or an isoperibolic calorimeter with a Commodore 3032 microcomputer. The system has been developed successfully to drive a two-phase stepper motor, to provide programmed control of a 181 Keithley nanovoltmeter and to calculate the total heat effect. The calorimetric precision for dissolution of 3 d transition metals in liquid aluminium at 1000 K is about +0.1%.

Microcomputer controlled integrator for Calvet-type twin calorimeters

An improved integration method has been developed for use with Calvet-type differential calorimeters. The new integrator is based on a voltage-to-frequency converter combined with a digital system interfaced to a microcomputer. The microcomputer provides programmed control of the integrator and, with little or no operator intervention, calculates the total heat effect making corrections for background and baseline drift. With this method a calorimetric precision of the order of ±0.1% has been achieved in experiments carried out at 651°C. This is about one order of magnitude of improvement over graphical methods and about a factor of 3 better than the analog integration techniques which we have used previously.

Principles of identification of the physical processes caused by variation of thermodynamic parameters in solids

A method is proposed which permits identification of the reversible and irreversible thermodynamic processes occurring in solids as thermodynamic conditions vary, and definition of the contribution of each process to the whole. Theoretical values have been obtained of the ratio of the rates of change of enthalpy and volume as well as the ratio of the total heat and volume effects for a series of processes in solids. The method is applied to some actual special cases.

Electrochemical Calorimetry: IV . Measurement of the Reversible Heat Effect Attending the Passage of an Electric Current Across a Liquid Junction

Measurements of the temperature changes resulting from the passage of current across the interface of two electrolytes were made at various distances from the junction so as to yield a temperature profile of the effect. The total heat effect was calculated from the area under the curve of the temperature profile. The coefficient obtained by dividing the total heat effect by the charge is reported as a voltage-equivalent. The following values were obtained for three systems of electrolytes and are probably correct to within about 7 mv: 0.04M hydrochloric acid and 0.4M hydrochloric acid, 30 mv; 0.90M hydrochloric acid and 3.0M potassium chloride, 80 mv; 1.15M hydrochloric acid and 1.21M lithium chloride, 80 mv. A hypothesis is advanced to relate the voltage‐equivalent of the heat effects to the diffusion liquid junction potentials and to the Peltier effect.

Calorimetric Determination of Half‐Cell Entropy Changes

The reversible heat effects of half‐cell processes have been measured in a twin calorimeter consisting of a Dewar flask divided into two halves by means of a vertical partition. Both compartments contain the same electrolyte and each has an electrode. A hole in the partition, covered by filter paper, permits the passage of electrolytic current between the compartments. The total heat effect and the heat due to irreversible phenomena are measured separately for each compartment. The difference between these two gives the desired reversible heat.The half‐cells studied include copper in acid copper sulfate, silver in acid silver perchlorate, and silver‐silver chloride in various chloride solutions. The effect of “transport entropies” on these measurements is outlined. Presently accepted theories of the “irreversible” or “steady‐state” thermodynamics of electrolytic cells predict the equivalence of half‐cell entropy data obtained by the present calorimetric method and data obtained from thermocell studies. This predicted agreement was not obtained in the case of the half‐cell silver‐silver perchlorate.

Liquid Metal Solution Calorimeter

A liquid metal solution calorimeter and its operation are described. The method of calculation is outlined and the sources of error are analyzed. In this calorimeter, heat effects upon dissolution of samples in liquid metals are measured; heats of solution are determined directly and heats of formation, energies of ordering, and the stored energy of cold work are found by a difference method. The calorimeter operates in the temperature range 240 to 450°C; samples are added from 0 or −195°C. Highest accuracy, of the order of ±2 cal/g-atom, is attained with suitably chosen solute-solvent combinations giving very small total heat effects upon dissolution.

A common misunderstanding of Hess' law

The statement, sometimes attributed to Hess, that In any series of chemical or physical changes the total heat effect is independent of the path by which the system goes from its initial to its final state is incorrect.

EFFECT OF PREHEATING TIME ON THE INACTIVATION OF PHOSPHATASE IN MILK

The authors consider the incongruities encountered when the phosphatase test is applied to milk and offer a practical mathematical solution for some of them. Experiments are described wherein it was possible to evaluate the individual effects of the preheating, holding and cooling segments of the heating (pasteurizing) curve on the inactivation of phosphatase in milk. Thus experimental data were obtained that satisfied the requirements of the mathematical equations used currently in the food canning industry to calculate the total lethal heat effect of a process.The authors report that when one minute preheating time with straight line heating is used, it will contribute 0.40 percent and 29.91 percent of the total lethal heat respectively when the holding temperatures are 146.3° F and. 163.4° F.

Combination of Wool Protein with Acid and Base: The Effect of Tem perature on the Titration Curve

A description has previously been given of the relation bctwcen the amounts of hydrochloric acid and of potassium Hydroxide bound by wool and the concentration of these substances in solution. These measurements were made at 0° C in order to avoid com plications due to decomposition. However, when wool is exposed to acid or base in the course of carbonizing, acid-dyeing, milling, and scouring much more elevated temperatures prevail. It is therefore desirable to determine the temperature dependence of the relations previously described. It is also desirable to compare the relative magnitudes of the effect of temperature on the amounts of acid and on the amounts of base combined, in order to establish the correctness of assumptions as to the mechanism of combination previously made in explaining the results obtained at 0' C.With the purpose of providing this information, measurements of the amounts of hydrochloric acid and potassium hydroxide bound by wool as a function of acidity and salt concentration, at 0°, 25° and 50° C, are now reported. In order to eliminate effects due to decomposition of the wool in alkaline solutions, a study was also made of the effect of temperature on the rate of decomposition of the wool. The results obtained support the assumption previously made that the carboxyl groups and amino groups of wool in the uncombined state are completely ionized. This conclusion follows from the observation that changes in the amounts of acid bound brought about by changes in temperature are small, which indi cates that combination with acid is equivalent to back-titration of the carboxyl groups, but changes in the amounts of base bound at at given hydrogen ion concentration are large, which indicates that combination with base is equivalent to back-titration of amino groups. Similar results obtained on analogous compounds, render it probable that the direction of the temperature effect on the curve of acid combination reverses at a temperature near 40° C. Thus curves at temperatures used in dyeing are probably not far different from the curves at 0° C, which have been studied in most detail.The heats of dissociation calculated from the magnitudes of the changes with temperature in the curves of combination for the two kinds of groups, are in good agreement with values for these groups in comparable compounds, and in soluble proteins. The value obtained in the acid range is also in good agreement with the results of calorimetric measurements on the combination of acid by wool.It is shown that approximately equal parts of the total heat changes in the acid range are associated with the dissociation of hy drogen ions and of chloride ions from the fibre. An appreciable part of the total heat effect is ascribed to a heat of transfer of the ions between the two phases of the heterogeneous titration system. Neither titrimetric nor calorimetric estimations of the heats of dissociation provides evidence for or against the existence of salt linkages in wool.