Patrick Picker

A high-precision digital readout flow densimeter for liquids

A precision densimeter for liquids, based on the oscillating-tube principle, has been designed. The apparatus allows relative density measurements to be carried out routinely with ppm precision, on 5–7 cm3 of solution, in a total time ranging between 5 to 10 min. The results of various tests reported here show the densimeter to be useful for investigation in aqueous as well as nonaqueous media; it is particularly well adapted to accurate measurements of small density increments, as required in excess-volume studies. Since the instrument operates in the flow regime, it is also adaptable to on-line monitoring of density in continuous processes.

Heat capacity of solutions by flow microcalorimetry

A differential flow microcalorimeter has been designed for the measurement of heat capacities of solutions. Under steady-state condition, it is possible to obtain directly the ratio of the heat capacity of a solution to that of its solvent without any knowledge of the heat capacity of the instrument, the flow rate, or the induced change in temperature. The steady state is reached within 1 min. The overall precision on heat capacity changes is 0.5 per cent and the limit of detectability is 7 × 10 −5 JK −1 g −1 . As a check for reproducibility, the apparent molar heat capacities of aqueous solutions of NaCl in the range 0.01 to 2 mol kg −1 and of urea in the range 0.1 to 10 mol kg −1 were measured at 297.3 K.

Reexamination of the heat capacities obtained by flow microcalorimetry. Recommendation for the use of a chemical standard

A series of measurements with aqueous electrolyte and nonelectrolyte solutions indicates that there is a small systematic difference between the heat capacities per unit volume determined with a Picker flow microcalorimeter and the original prototype. Through various tests and comparisons, it is, concluded that the commercial instrument gives results closer to the true values. Most of the previous data obtained in our laboratory have been corrected and expressed relative to aqueous NaCl at 25°C taken as a standard.

Heat capacities of some organic liquids determined with the Picker flow calorimeter

Abstract The Picker flow calorimeter was used to determine the molar heat capacities of ethanol, n -propanol, benzene, ethylbenzene, cyclohexane, and tetrachloromethane at 298.15 K relative to n -heptane (N.B.S. standard sample). Precautions and changes in operating technique, necessitated by the application of the calorimeter to organic liquids, are discussed.

Steady state and composition scanning differential flow microcalorimeters

Abstract Two different flow microcalorimeters have been constructed. One is of the adiabatic type and can be used to measure ΔTmixing for liquid phase reactions. The other can be operated under either adiabatic or isothermal conditions and serves for either gas or liquid phase investigations. These instruments have similar sensitivities, their thermal detection limit being ≈ 10−5 °C. Enthalpies of reaction involving temperature changes greater than 0.001 °C can be measured with an accuracy of 1 per cent or better; in the isothermal operational mode the heat flux sensitivity is ≈ 0.5 μW. The electrical calibration of the instruments has been checked against the enthalpy of neutralization of 0.005 M HCl with 0.0055 M NaOH in carbonate free water. To estimate the efficiency of mixing in the flow cell, the enthalpy of twofold dilution of aqueous urea solutions has been measured at 0.1 mol kg−1 and higher initial molalities. Both instruments reach steady steates within 1 min. This feature allows the flow rates of the mixing fluids to be varied continuously. Thus curves of enthalpy of mixing against volume fraction can be obtained directly in about 1 h. Some composition-scanning curves are reported for the aqueous systems: NaCl (2 mol kg−1) + KCl (2 mol kg−1) and HCl (0.02 M) + NaOH (0.02 M). Investigation of the latter system at various times of scanning shows that the scanning period can be reduced to 5 to 10 min without severe losses in the thermochemical information. With such characteristics, these instruments can be applied to a wide variety of analytical and thermodynamic problems.

Enthalpies of dilution of electrolyte solutions by flow microcalorimetry

The enthalpies of dilution of NaCl, Me4NBr, andn-Bu4NBr were measured in water at 25°C with a new flow microcalorimeter. The data were analyzed with a polynomial equation, and the derived relative apparent molal enthalpies φL are in good agreement with literature values. Provided care is taken that mixing is complete, flow calorimeters are as reliable and much less time-consuming than cell-type instruments for enthalpies of dilution measurements.

Enthalpies of mixing of organic liquids measured directly as a function of composition by means of scanning dynamic flow microcalorimetry

Abstract The dynamic flow microcalorimeter designed and built at the Universite de Sherbrooke has been used, with new pumps, to measure enthalpies of mixing of organic liquids. The curve for the enthalpy of mixing per unit volume is recorded directly against the volume fraction. Results are given for the standard mixture cyclohexane + n -hexane at 298.15 K.

Enthalpies of reaction and reaction rates by flow microcalorimetry: Ester hydrolysis in basic medium

The conditions under which the Picker flow microcalorimeter can be used to measure enthalpies and rates of reactions were investigated. For this purpose, systematic studies were made of the enthalpies of neutralization of HCl, HBr, HNO3, acetic, proprionic, and butyric acids with NaOH, enthalpies of hydrolysis of methyl and ethyl acetate with NaOH, and the reaction rates of the ester hydrolysis with NaOH. The general procedure and various sources of error are discussed and it is concluded that enthalpies of slow reactions can be measured to about 1% when the calorimeter is operated in the quasi-isothermal mode and the reaction rates to about 3% when operated in the quasi-adiabatic mode.

Direct continuous measurements of thermal expansion coefficients of liquids and solids using flow microcalorimetry

A differential heat capacity flow microcalorimeter is used to monitor in a continuous mode the thermal expansion of a sample during a programmed temperature scan. The sample may consist of liquids, suspensions, or bulk solids in a confining liquid and the typical temperature scanning rate is of the order of 1 K/min. The technique has a precision better than 1% and a detection limit of 10(-6) ml s(-1). In contrast to conventional dilatometers, this technique offers variable sensitivity and is not limited by the magnitude of the total volume change during the experiment. Various expansibility data obtained in the temperature range 10-55 degrees C are reported for several systems, namely water, benzene, carbon tetrachloride, and aqueous solutions of sodium chloride. The volume changes for the thermal transition of Teflon and the phase separation of 2-butoxyethanol/water mixtures further illustrate the possibilities of this new technique.

The concentration dependence of the enthalpies of mixing of some aqueous electrolytes at 25°C. A test of Young's rule

The enthalpies of mixing of a series of aqueous electrolytes having a common ion were investigated as a function of concentration to determine the limits of applicability of Youngs rule. Measurements were carried out with solutions of equal concentration and 0.5 mole fraction with a recently developed flow microcalorimeter. The systems chosen for this study were aqueous NaX+KX with X = F−, Cl−, Br−, I−, NO3−, AcO−, and CO32−. Except for the nitrates, the enthalpies of mixing in the concentration range investigated could be fitted to an equation of the type ΔHm=αc2+βc3, where c is the concentration of the cations. Plots of −ΔHmc2 against c indicates that Youngs rule is approximately obeyed only when the common ions compared are similar, e.g. Cl−, Br−; if they are very different, significant deviations are observed. These observations can be qualitatively explained from an examination of the changes in solute-solute interactions upon mixing.