TITRATION APPARATUS

Abstract: The invention relates to titration apparatus in which titrant is introduced into a main loop in which a sample solution is circulated. The invention is characterized in that the total internal volume of the main loop is at maximum one milliliter.

Description: TITRATION APPARATUS TECHNICAL FIELD The invention relates to titration apparatus in which a titrant is added to a main loop in which a sample solution (titrand) is circulated. BACKGROUND OF THE INVENTION Titration is a wet-chemical analysis method used to determine the quantity and content. In the simplest form of titration, the sample (which together with any solvent or thinner present is referred to as the titrand or first solution) is introduced into an open vessel and mixing is effected with some form of mechanical stirrer or by rotating the vessel by hand. A second solution (titrant) which contains a known quantity of a substance that reacts stochiometrically with the sample to be analyzed, is added in increments from a burette. When an equivalent amount of titrant has been added, an equivalence point (titre point) is reached and the sample content can be calculated. In this case the equivalence point can be determined visually by a change in colour of an added colour indicator. Known titration apparatus, for instance the apparatus described in "Principles of Instrumental Analysis", Douglas A. Skoog, third edition, Philadelphia 1985, distinguish from the aforedescribed apparatus, primarily by different ways of registering the equivalence point and in the degree of automation, but do not fulfil all the requirements that can be placed on modern, chemical quantitative analysis apparatus. Distinct from modern chemical analysis apparatus based on other methods, these known titration apparatus operate with large volumes, i.e. sample volumes and liquid volumes in the order of centiliters. Volumes of such large magnitudes cause serious problems; require large apparatus, large sample and chemical quantities, comprehensive apparatus cleaning operations, automation difficulties, etc.. One reason why apparatus of large volumetric capacities are required for titration is because of the relatively large mechnical stirrers (or agitators) that are used to ensure that the liquid volumes will be mixed effectively in the titration process. Problems concerning large sample volumes can be solved partially by so-called Flow Injection Analysis, in which there is added to a sample a continuous (liquid) flow which contains a substance with which the sample reacts. The amount and therewith the concentration in the sample can then be determined from measurements made in the flow subsequent to intermixing of and reaction between the liquids, and the measurements compared with calibrated measurements. It is also known from DD 277975 to design a titration apparatus so that the titrand will circulate in a loop to which the titrant is added. However, the loop volume is large and the apparatus according to DD 277 975 is not adapted for small system volumes, which is made evident, among other things, by the fact that there is used a mechanical stirrer and that this stirrer also functions as a rentrifugal-type circulation pump. An open centrifugal pump of this kind will not function with small volumes, since the pressure drop in the loop is very large because of the small dimensions concerned . SUMMARY OF THE INVENTION The object of the present invention is to provide a titration apparatus which is constructed so as to require only very small quantities of samples for analysis and which will also fulfil other requirements placed on modern, chemical, quantitative analysis apparatus. Among other things, the apparatus shall be designed for short response and stabilizing times and be highly accurate and have a high degree of reproducibility. The apparatus shall also be designed to enable it to be readily automated, easy to clean by washing with small volumes of cleaning fluid, have relatively low chemical consumption, low operating costs and small dimensions. The apparatus shall also be closed, among other things to facilitate cleaning and to avoid contamination. These and other objects of the invention are achieved with the inventive apparatus having the characteristic features set forth in the characterizing clause of the following Claim 1. When the apparatus is in use, a titrant is introduced into a so-called main loop in which a sample solution (the titrand) circulates. In one preferred embodiment of the invention, the main loop is a closed circuit in which all component parts of the loop are configured with as small volumes as can be achieved with known techniques. In this embodiment, the main loop includes a) a pump unit for circulating a sample and a solvent or a cleaning fluid in said loop, b) valve means for introducing the sample, solvent or cleaning fluid agent into the loop, c) means for adding titrant to the loop, d) means for measuring and recording the titer process, and e) conduits included in the loop for inter-connecting the different aforesaid components. The apparatus will also conveniently include f) an expansion unit that is capable of accommodating at least a volume increase in the loop corresponding to the volume of the added titrant. A titration apparatus having a system volume (internal volume corresponding to the total combined volume of titrand and titrant in the main loop) in the order of microliters in accor¬ dance with the invention, will be subject to completely different requirements than apparatus which according to known techniques have system volumes that are perhaps 100 times as large. This is reflected in the construction of the inventive apparatus and in the apparatus components. For instance, in a main loop where the system volume is smaller than one milliliter, the pressure drops in the various components and in the conduit system between the different components will be very large when circulating the titrand. The pump provided in the main loop is constructed to pump the titrand around the main loop at a rate of flow in excess of 1 ml/min, preferably 2 to 20 ml/min and more preferably 4 to 10 ιτι]/min. The pump is also designed to generate the smallest possible pulsations. Any pump that will generate sufficient pressure and therewith produce a sufficient flow rate and small pulsations can be used, although preferred pumps capable of fulfilling these requirements are multiaction displacement pumps, screw pumps, certain centrifugal pumps or peristaltic pumps. The construction of the apparatus with a closed main loop in which the pump is fitted enables, in one embodiment, external pumps or pressurized media for supplying liquid to the apparatus to be dispensed with, wherein the pump of this embodiment is able to draw liquid into the loop by suction, via the valve unit. According to one embodiment, the valve unit has a position in which sample is introduced into a sample loop which is included in the valve unit and wuich is capable of receiving and accommodating a defined volume of sample, and another position in which the sample loop, with its defined sample content, is connected to the main loop. According to another embodiment, the valve unit has a valve position in which sample, solvent and cleaning agent can be drawn by suction into the main loop with the aid of the pump, and another valve position in which the main loop is closed. In this case, the volume of liquid drawn into the system loop can be defined by the number of working strokes executed by the pump, when the pump is a piston pump. Known means may be used to meter precise volumes of titrant to the analysis apparatus. Because of the strict requirements concerning precise metering of very small volumes, the metering equipment of one preferred embodiment is provided at the outlet of the main loop with a valve which functions to prevent the diffusion of titrant from the metering equipment into the titrand. The measuring cell can be constructed for different known detection methods (measuring methods) for determining the equivalence point, such as photometric measuring methods which include measuring the ultraviolet, infrared and visible range of the spectrum and electrochemical measuring methods including potentiometric, amperometric and conductometric measuring methods, and for titration methods including complexiometric titrations, redox-titrations, potentiometric titrations, acid-based titrations, conductometric titrations, amperometric titrations and photometric titrations. The invention may also include titrations in which the reagent is generated in the main loop, including coulometric titration. In the case of an embodiment for conductometric titration, the measuring cell has a volume of 13 μl. In the preferred embodiment, the flow in the conduits will preferably be laminar. The conduits that interconnect the component parts of the apparatus are in the form of hoses or pipes which are able to withstand the main loop pressures with no appreciable affect on their volume. The conduit system will preferably have inner diameters of 0.2 to 1.5 mm, conveniently 0.2 to 1.0 mm and preferably 0.2 to 0.8 mm. The same dimensions will preferably be used throughout the largest possible part of the conduit system, so as to avoid disturbing fluctuations in the flow image, and the diameter will preferably be small so as to reduce volume. The conduits may be formed from reinforced or non-reinforced hoses of, e.g., PTFE, KELF or PEEK, preferably PEEK, or from metal material such as acid- proof steel or titanium, preferably titanium. In the case of the preferred embodiment, the main loop is closed, among other things to enable the pump in the main loop to be used also to draw sample, solvent or cleaning agent into the system through the aforesaid valve unit. A closed main loop will also enable the apparatus to be cleaned more easily while using less cleaning agent than is otherwise required and also reduces the risk of contamination at the same time. According to one preferred embodiment, in which the main loop is closed during the titration process, there is included an expansion unit which is dimensioned essentially to accommodate a volume that corresponds to the volume of titrant added to the system. The expansion unit may have the form of an elastic membrane in one part of the system, or the form of a plunger-cylinder device wherewith the aforesaid increase in volume is taken-up as the plunger moves in the cylinder, or may have some other suitable form. It has been found that the inventive titration apparatus fulfils the requirements that can be placed on modem, chemical quantitative analytical equipment in a highly satisfactory manner Excellent results have been obtained with tests carried out with sample volumes of down to 2 μl. The following list of advantages are afforded over existing systems: * The ability to work with very small sample quantities * Higher resolution * Retained reproducibility * Lower chemical consumption * Lower operating costs Other characteristic features and aspects of the invention and advantages afforded thereby will be apparent from the following claims and from the following description of two conceivable embodiments. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a flow chart illustrating a first embodiment of an inventive apparatus and shows the apparatus in a phase preceding the titration process. Figure 2 is a flow sheet which illustrates the same apparatus during the actual process of titration. Figure 3 illustrates schematically a measuring unit included in the apparatus. Figure 4 illustrates schematically a measuring electrode in the measuring unit. Figure 5 illustrates schematically the flow conditions at the surface of the measuring electrode. Figure 6 is a flow sheet illustrating a second embodiment of an inventive apparatus and shows the apparatus in a phase that precedes the titration process. Figure 7 illustrates the apparatus of Figure 6 during the actual titration process. DETAILED DESCRIPTION OF TWO PREFERRED EMBODIMENTS OF THE INVENTION Shown in Figure 1 is a system loop 10 which is included in a main loop 1 in which a known volume of sample in solution, i.e. titrand, is intended to circulate during the titration process illustrated in Figure 2. The system loop 10 includes a pump 2, a measuring unit 6, an eight-path valve 3, an expansion unit 7, a three-path valve 9, and conduits 6. The reference numeral 8 identifies a sample loop for enclosing a defined volume of sample. A metering unit is referenced 4. In the preferred embodiment, the pump 2 is a double-acting piston pump having a very small internal volume. The main loop 1 is comprised of the integrated closed main loop that is formed when the system loop 10 and the sample loop 8 are connected together in series, Figure 2. The schematically illustrated valve unit has eight openings 21-28 and is constructed so that certain of these openings can be connected one with another. For instance, during the phase shown in Figure 1 the opening 21 is connected with the opening 22, the opening 23 is connected with the opening 24, the opening 25 is connected with the opening 26 and the opening 27 is connected with the opening 28. In the operational state shown in Figure 2, the opening 24 is instead connected with the opening 25 and the opening 21 is connected with the opening 28. In this operational state, the opening 22 is connected with the opening 23 and the opening 26 is connected with the opening 27. The sample enclosing loop 8 includes a pipe having an inner diameter of 0.3 mm and, together with the dead volume in the valve, a total volume of 13 μl. The metering equipment 4 is constructed to meter in precise volumes of 0.05 μl with each pulse delivered by a suitable control means. The metering equipment is provided with said valve 9 at the outlet of the main loop 1 , said valve functioning to prevent the diffusion of titrant into the titrand. Figure 3 illustrates schematically the measuring unit 5, a measuring electrode 51, an electrode surface 52 and a reference electrode 53. The measuring unit 5 is connected to the conduit system 6. Figure 4 illustrates the principle in accordance with which the preferred embodiment of the measuring unit 5 is constructed, wherein the measuring electrode 51 has a flat electrode surface 52 which is impinged upon by a laminar flow of titrand. In the preferred embodiment of the measuring unit 5, the flow in the conduits 6 upstream of the electrode surface 52 is a laminar flow. Figure 5 illustrates the electrode surface 52 with the laminar flow along the electrode surface. With this embodiment, an interface is formed closely adjacent the electrode surface in which the measured ions/molecules diffuse. In the case of the preferred embodiment, the conduits 6 that interconnect the parts of the main loop are made of titanium and have an inner diameter of 0.5 mm. The expansion unit 7 includes an elastic membrane. The schematically illustrated equipment including the eight-path valve 3 can be used in several different ways. In a first conceivable application the sample to be analyzed is highly concentrated and needs to be diluted or thinned. In this case, the sample loop 8 is filled from a sample supply source (not shown) with sample liquid which enters through the valve opening 23. The sample liquid is led in through the opening 23, out through the opening 24, through the sample loop 8, in through the opening 21 and out through the opening 22 and into a sample drainage vessel. The system loop 10 is washed and filled with solvent introduced through the opening 26, and then through the opening 25, the expansion unit 7, the valve 9, the pump 2 and so on, and is then discharged through the opening 27 into a solvent drainage vessel. The sample loop 8 is now filled with a defined amount of sample liquid and the system loop 10 is filled with solvent. The valve 3 is then rotated so that the opening 21 connects with the opening 28 and the opening 24 connects with the opening 25, in other words so that the sample loop 8 containing the defined amount of sample liquid is connected with the system loop 10, which according to this particular application contains solvent. The two liquids are circulated through the main loop 1 with the aid of the pump 2, therewith thoroughly mixing the liquids together. When the measuring unit 5 registers a stable condition in the main loop 1 in response to contact of the titrand with the measuring electrode 51, Figure 3, and the reference electrode 53, the valve 9 is opened to the metering equipment 4, so that titrant can be added to the system through the medium of the metering equipment. The titrant is added in increments with the aid of the metering equipment 4 in an amount of 0.05 μl with each pulse delivered by a control unit while monitoring the process with the aid of the measuring unit 5. Metering of titrant is continued until the measuring unit 5 has unequivocally detected the equivalence point. The increase in volume caused in the main loop 1 by the addition of titrant from the metering equiment 4 is taken-up by the expansion unit 7. When the titration process has been completed, the eight-path valve 3 is reset to the position shown in Figure 1, therewith enabling the titration process to be repeated if desired. According to the invention, the titration process is controlled, monitored and evaluated by a computer unit (not shown). Another method of using the equipment illustrated schematically in Figures 1 and 2 can be applied when a sample to be analyzed must first undergo a chemical reaction, i.e. a conversion, in order to be titrated. In this case, the sample loop 8 is filled with a reagent, and the system loop 10 is filled with the sample that must first be reacted with the reagent in the sample loop 8 in order to be analyzed. After having filled the sample loop 8 with reagent and the system loop 10 with sample liquid, the valve 3 is moved to the position shown in Figure 2. The two liquids are both circulated around the main loop 1 with the aid of the pump 2 and intermix and react with one another. The valve 9 is then opened and titrant is added to the system, wherein titration is effected on the obtained reaction product. Figure 6 and 7 illustrate schematically equipment in which the earlier described eight-path valve has been replaced with a four-path valve 3'. The main loop 1' is, in other respects, equipped in the same way as the earlier described main loop 1. The system illustrated in Figures 6 and 7 is intended for samples that can be titrated immediately without needing to be diluted or thinned or reacted with a reagent. The sample is introduced through an opening 31 in the valve 3 ' and passes out through the opening 32 and through the main loop 1, and exits into a sample drainage vessel through the openings 33 and 34. After the main loop 1 ' has been filled with sample liquid, the valve 3' is moved to the position shown in Figure 7, i.e. the main loop 1 ' is closed, whereafter the valve 9 is opened and the titration process is commenced with the addition of titrant from the metering equipment 4, as described above with reference to the earlier embodiment.

Claims: CLAIMS 1. Titration apparatus in which a titrant is added to a main loop (1) in which a titrand is intended to circulate, characterized in that the total internal volume of the main loop (1) is from 1 μl to 10 ml, preferably at maximum 1 ml, and more preferably 9 a maximum of 0.5 ml. 2. Apparatus according to Claim 1, characterized in that it includes - a pump unit (2) which functions to draw-in by suction a first liquid and to circulate said liquid in the main loop (1), - a valve unit (3), - means (4) for adding titrant to the main loop, - a measuring unit (5) for registering the titration process, and - conduits (6) included in said main loop and connecting the pump (2) with the valve unit (3). 3. Apparatus according to Claim 1 or Claim 2, characterized in that the main loop (1) is completely closed during the titration process, by which is meant that the loop forms a closed circuit and that its interior has no communication with ambient air. 4. Apparatus according to Claim 3, characterized in that the main loop (1) includes an expansion unit (7) capable of accommodating at least an increase in volume in the main loop corresponding to the volume of the added titrant. 5. Apparatus according to any one of Claims 1-4, characterized in that the main loop includes a system loop (10) (and a sample loop (8)); and in that the valve unit (3) can be switched between a position in which a second liquid is delivered to the sample loop (8), said sample loop being able to receive and accommodate a defined volume of said second liquid, and a second position in which the sample loop (8) and its defined sample quantity are connected with the system loop (10) containing said pump, measuring unit and means for delivering titrant to the main loop (1). 6. Apparatus according to Claim 5, characterized in that in the first position of said valve the system loop (10) can be filled with said first liquid; and in that the main loop formed by the system loop and the sample loop is closed when said valve is in its second position. 7. Apparatus according to any one of Claims 1-6, characterized in that the pump unit (2) is constructed to deliver liquid at a flow rate of at least 1 ml/min, preferably 2 to 20 ml/min and more preferably 4 to 10 ml/min. 8. Apparatus according to any one of Claims 1-7, characterized in that the conduits (6) have an inner diameter of from 0.2 to 1.5 mm, preferably from 0.2 to 1.0 mm, and more preferably from 0.2 to 0.8 mm. 9. Apparatus according to any one of Claims 1-8, characterized in that the measuring unit (5) is provided with means for photometric measurement within the ultraviolet, infrared or visible range of the spectrum, or for electrochemical measuring processes, including potentiometric, amperometric or conductometric measuring processes. 10. Apparatus according to any one of Claims 1-9, characterized in that the apparatus is provided with means for titration methods which include complexiometric titration, redox titration, acid-based titration, potentiometric titration, conductometric titration, amperometric titration and photometric titration. 11. Apparatus according to any one of Claims 1-10, characterized in that the apparatus is constructed for titration methods in which a reagent is generated in the main loop (1), including coulometric titration. 12. Apparatus according to any one of Claims 1-11, characterized in that the means (4) for delivering titrant to the main loop is a metering equipment which at the outlet of the main loop (1) is provided with a valve which functions to prevent diffusion of titrant from the metering equipment (4) into the titrand. 13. Apparatus according to any one of Claims 1-12, characterized in that the valve unit (3) includes at least eight openings which can be mutually connected in pairs; in that a system loop (10) which includes pump, measuring unit, expansion unit and metering equipment when said valve is in a first position, is connected to a first liquid delivery conduit so as to be filled with said first liquid, while when said valve (3) is in said first position a sample loop (8) is connected to a second liquid delivery conduit through which the sample loop is filled with a defined quantity of said second liquid; and in that the valve (3) in its second position connects the system loop (10) with the sample loop (8) such that said loops will be coupled in series to form a closed main loop (1). 14. Apparatus according to any one of Claims 1-12, characterized in that the valve unit (3 ) is a four-path valve.

Inventors: CARLSSON CHRISTER, KAELLBACK PATRIK, CARLSSON, CHRISTER, KAELLBACK, PATRIK
Applicants: CARLSSON CHRISTER, KAELLBACK PATRIK, CARLSSON, CHRISTER, KAELLBACK, PATRIK
Application year: 1995
Main classification: G01N31
IPC codes: G01N1, G01N31