GB2234349A - Microcalorimeters - Google Patents

Abstract

A microcalorimeter has a cell (2) in an insulated metal block (1), a reservoir (11) for a fluid to be fed into the cell, and a magnetically-driven stirrer (8, 13, 14 ,15). Thermal disturbances are minimised, and accuracy increased. Temperature is measured by a Wheatstone wedge arrangement of thermistors (4, 5, 6, 7). The apparatus is calibrated using a heating coil (9). Heats of adsorbtion, dilution, mixing and immersion can be measured. <IMAGE>

Description

1MPROVX TN MIOCA1MEIERS This invention concerns improvements in microcalorimeters, and more particularly it provides a novel microcalorimeter having features permitting increased sensitivity in use.

Various designs of microcalorimeters are known and are available commercially, including flow-type and Calvet-type instruments. A particular use of microcalorimeters is in the study of adsorption mechanisms, which may be done by ascertaining heats of adsorption; heats of mixing, dilution, immersion and the like are also important sources of information for scientists and technologists and for the control of industrial processes and product quality.

The present invention provides a microcalorimeter having a cell for a first fluid, a reservoir for a second fluid, preferably in the form of a tube of inert material such as stainless steel, means for permitting flow of the second fluid into the cell, means for controlling the flow of second fluid into the cell, temperature sensing means adapted to sense the temperature of fluid in the cell, and stirring means for fluid in the cell which stirring means is driven through a magnetic coupling, and said cell, reservoir, flow permitting means and temperature sensing means being retained within a thermally insulated container.

The stirring means may be, for example, a glass stirrer such as a helicoidal stirrer, preferably attached to a drive rod extending cut of the thermally insulating container and carrying a steel or iron member, or a magnet. Desirably, drive means such as an electric motor is used to drive a magnetic member spaced apart from the drive rod member.

Preferably, the magnetic member is adjustable in spacing from the drive rod member so as to vary the applicable torque. The speed of the stirring means is preferably also adjustable. It has been found that other drive arrangements introduce mechanical disturbances which adversely affect temperature sensors such as thermocouples, thermistors or diodes which are operating to detect extremely small changes of temperatures, and thus the sensitivity of the microcalorimeter of the invention can be substantially free from any heat effects caused by changes in stirring speed.

An important feature of the invention in its preferred embodiment is the positioning of the stainless steel tube reservoir within the thermally insulating container, so that the second fluid is in thermal equilibrium with its surroundings, and the need to separately monitor and control the temperature of the second fluid is avoided. The means for controlling the flow of the second fluid may be external of the container, and may be, for example, a precision syringe pump, which is preferably capable of feeding the fluid at a predetermined constant rate, in aliquots of predetermined, but not necessarily equal, size, under the control of an operator direct, or via a computer.

The invention will now be described with reference to the accompanying drawing, which is a schematic cross-section of a microcalorimeter according to the invention.

A thermally insulated metal block, 1, for example of aluminium, contains a stainless steel calorimetric cell, 2, having a useful volume of 30cc i.e.

capable of containing a volume of a first liquid ranging from 15cue to 30cc with or without added adsorbent. The cell is closed by a cover, 3, made from an inert insulating material and rests on the flat bottom of the cavity in the metal block.

The temperature of the liquid in the cell 2 is measured by two matched thermistors, 4 and 5, forming two arms of a Wheatstone bridge circuit.

Reference matched thermistors, 6 and 7, placed in metal block, 1, form the other two arms of the Wheatstone bridge. Any imbalance in the bridge that can be caused by evolution of heat in the contents of the cell, 2, produces a potential difference between the two pairs of thermistors, which is amplified and recorded. The liquid in the cell is stirred by a glass stirrer, 8, so designed as to provide efficient mixing between the liquid in the cell and a second liquid that can be injected into the cell via a stainless steel tube, 10. The second liquid is held in a stainless steel heat exchanger tube, 11, in close contact with the metal block.

The heat exchanger tube is wound on the surface of an aluminium cylinder, 16, which closely fits the cylindrical cavity in metal block, 1, and is connected to the cell cover 3, by a rigid inert tube, 19.

The heat exchanger tube 11, is also directly connected to a tubular preheater coil, 12, where the second liquid is electrically heated to a temperature close to that of the temperature of the first liquid in the cell. The preheater tube is connected to syringe pump, 17, which can displace a desired volume of the second liquid into the first liquid in the calorimetric cell via injection tube, 10, with the liquids in the cell being continually agitated by stirrer, 8. A uniform stirring movement, essential for accurate determination of small quantities of heat of mixing of the two liquids, is ensured by magnetic coupling of the stirrer rod and a stepper motor, 15, with magnets, 13 and 14, separated by an air gap.

The stirring rod is held by rigidly mounted bearings 18, within the tube 19.

The speed of the motor is electronically controlled to give constant speed of rotation.

Heat effects, for example produced by mixing and adsorption on solids dispersed in the first liquid, can be calibrated by a calibration coil, 9, through which known electric currents can be passed for predetermined periods of time according to generally known principles. The leads from thermistors, 4 and 5, the calibration coil and the stirrer rod all pass through cover, 3, fitting the calorimetric cell.

In use, a first liquid may be a knawn solvent or suspension in the solvent and a second liquid may be an aqueous solution, for example of surfactant. By operation of an external pump (syringe pump) a small quantity (e.g. 0.1 cc) is introduced into the suspension in the cell, under continuous stirring. The solution contains a calculated number of surfactant molecules, of which a proportion are adsorbed on the suspended particles. The thermal effect is recorded, and when a steady state is reached, a further introduction of solution is made. Differential or integral heats of adsorption may be calculated. In another series of experiments, heats of dilution may be measured by introducing into a solvent in the cell, a solution from the reservoir or vice versa. Other uses of the microcalorimeter may be chosen according to the scientific or technological requirements.

Claims (6)

1. A microcalorimeter having a cell for containing a first fluid, a reservoir for a second fluid, means for permitting flow of the second fluid into the cell, temperature sensing means adapted to sense the temperature of fluid in the cell and stirring means for fluid in the cell, which stirring means is driven through a magnetic coupling, and said cell, reservoir, flow permitting means and temperature sensing means being retained within a thermally insulated container.

2. A microcalorimeter according to claim 1, wherein said cell is located within a thermally insulated metal block.

3. A microcalorimeter according to claim 1 or 2, wherein said reservoir comprises a tube.

4. A microcalorimeter according to claim 3 as dependent upon claim 2, wherein said tube is mountable in thermal contact with the walls of said insulated metal block.

5. A microcalorimeter according to any one of the preceding claims, wherein said stirring means is driven by a stepper motor magnetically coupled thereto.

6. A microcalorimeter according to claim 1, substantially as hereinbefore described.