GB2062223A - Measurement of solute concentration using dilution - Google Patents

Abstract

A device for measuring the concentration in solution of a dissolved substance comprises means for monitoring the output of an instrument which measures a physical property of a stream of the solution which property is related to concentration, means for progressively diluting the solution stream, means for detecting the point at which the output of the measuring instrument becomes equal to a preset value, means for measuring the extent of dilution required to reach this point, and means for calculating therefrom the original concentration of the solution. The device is capable of analysing solutions of very high concentration which can not be analysed with conventional instruments.

Description

SPECIFICATION Concentration-measuring device This invention relates to a device for the determination of the concentration in solution of a dissolved substance.

It is known that the concentration of a solution of a dissolved substance can be determined by measuring a physical property of the solution which is attributable to the dissolved substance, and whose magnitude is proportional to the concentration of the substance in the solution. For example, such determinations may be performed by spectrophotometry, refractometry, fluorimetry, acidimetry, basimetry, conductimetry, potentiometry, radiometry, nephelometry, turbidimetry, polarimetry and other physical methods, depending on the nature and properties of the dissolved substance and the solution.

However, in all such methods, the instruments employed to measure these properties can operate only within a specific range of concentrations of solute in the solution. In particular, there has existed a need for instruments capable of measuring concentrations of solutes in solutions higher than those measurable with currently-available instruments. I have now found that such an instrument may be constructed by incorporating an automatic device for dilution of the solution.

Accordingly, this invention provides a device for measuring the concentration in solution of a dissolved substance or substances which comprises: - a means for determining a physical characteristic of the solution related to concentration; - a means for determining whether the output of the first means is equal to a preset value; - a means for supplying an appropriate diluent to the solution being measured at a variable rate; - a means for activating the third means whenever the equivalence condition determined by the second means does not obtain; - a means for continuously measuring the rate of delivery of diluent to the solution being measured; and - a means for relating the outputs of the first, second and fifth means to the concentration of the solution.

According to the invention, an electronic device, described herein as the system controller, is constructed by methods known to those skilled in the art. The system controller is capable of monitoring the output voltage or current of the instrument being used to measure the appropriate physical property of the solution, and capable of detecting the point at which the said output voltage or current becomes equal to a value which may either be preset by the operator or inherent in the construction of the system controller, the said value corresponding to a point within the proportional range of the measuring instrument.

Additionally, the system controller is capable of controlling a dilution device in such a way that the extent of dilution of the sample solution is progressively altered until the equivalence point described above is reached. The system controller is also capable of measuring the extent of dilution required to reach the said equivalence point, and of converting the said measurement to an analogue or digital output to an external recording device. Additionally, the system controller may incorporate circuitry capable of calculating directly the concentration of the solution according to parameters, physical constants or standard solutions supplied by the operator, and capable of converting the result of the said calculation to an analogue or digital output to an external recording device.

The dilution device referred to above may be constructed by methods known to those skilled in the art. The diluent may be any suitable solvent or solvent mixture which neither alters the response of the measuring instrument nor affects the measured physical property of the solute.

In a preferred embodiment of the invention, the system controller monitors the output of an ultraviolet spectrophotometer or absorptimeter equipped with a flow-through sample cell, which is used to examine the effluent solution from a chromatographic or related separation process.

Where such a process makes use of a device for collecting separate fractions of the effluent solution, the system controller may additionally incorporate delay circuitry, triggered by a signal from the fraction collector marking each change of fraction, to ensure that sampling occurs at a point in the collection of each fraction at which the concentration of the solute in the effluent is representative of the whole effluent collected in each fraction.

The linking together of the units comprising the device is illustrated in Figure 1, wherein the solid lines represent tubing carrying the sample solution and diluent, or the mixture thereof, and the broken lines represent electrical connections between the units in the system. In this embodiment of the invention, the output of the spectrophotometerA, which may operate either at a fixed or at a variable wavelength in the ultra-violet or visible ranges of the light spectrum, is analysed by the system controller B in response to a signal from the fraction collector C.The system controller also controls a valve D which permits diluent to mix with the sample solution, a pump Ewhich permits the rate of addition of diluent from a reservoir Fto the sample solution to be varied, a mixing device G as illustrated in Figure 2, which incorporates a directly or magnetically driven stirrer to ensure homogeneity of the sample solution after dilution, and which may be constructed of glass, stainless steel, or other suitably resistant material, and an external recording instrument H which registers the concentration of the solution.

The internal circuitry of the system controller is illustrated in Figure 3. A signal from the fraction collector marking a change of fraction activates a delay circuit A which, after a period of time whose duration may be varied by the operator, activates a comparator B. The comparator compares the voltage or current output of the spectrophotometer with a voltage or current preset by the operator to represent a point within the proportional range of the spectrophotometer.

The mode of operation of the comparator may be adjusted so as to make the device compatible with any spectrophotometer, as follows: (a) Where the output voltage or current of the spectrophotometer increases with increasing absorption of light by the sample, the comparator may be set to respond to a current or voltage greater than the current or voltage preset by the operator.

(b) Where the output voltage or current of the spectrophotometer decreases with increasing absorption of light by the sample, the comparator may be set to respond to a current or voltage less than the current or voltage preset by the operator.

That is to say, whether condition (a) or (b) pertains, the comparator responds to a current or voltage output from the spectrophotometer which indicates that the concentration of solute in the sample solution is greater than that which would result in a current or voltage output from the spectrophotometer equal to the value preset by the operator. If the comparator detects that this condition exists, it activates a ramp generator C and, through a control circuit D, activates the mixing device and the valve to permit dilution of the sample solution. The ramp generator C generates a control voltage which increases linearly with time, the rate of increase being preset by the operator.This control voltage drives a control circuit E which in turn drives the pump motor in such a way that the speed of the pump, and therefore the rate of dilution of the sample solution, increase linearly with time. The sample passing through the spectrophotometer is thus progressively diluted, and the current or voltage output of the spectrophotometer altered, until a point is reached at which this output becomes equal to the value preset by the operator. The comparator detects this condition, and deactivates the ramp generator C, whose control voltage output thus remains fixed at a value proportional to the pump speed, and therefore the degree of dilution of the solution required to reduce the light absorption to the value corresponding to the preset level.The comparator B then activates a trigger circuit F which may be used to activate an external recording device, for example a chart recorder, to record the control voltage output of the ramp generator. The trigger circuit additionally activates an analogue-to-digital convertor G which converts the ramp generator output voltage into binary form for output to a digital printer or data system. The analogue and digital outputs thus recorded are proportional to the concentration of the sampled solution and a series of such recordings may therefore be used directly as a measure of the relative concentrations of solute in a series of fractions.

Where absolute values of concentration are required, a computerHwithin the system controller converts the binary output of the analogue-to-digital convertor G to a value of concentration which may be recorded on a digital printer or data system, or fed to a digital-toanalogue converter/ before being recorded on an analogue recording device, for example a chart recorder. The concentration computer H is controlled by an input circuit J, by means of which the operator supplies the computer H with the necessary information required to calculate the constant by which the output of the analogue-todigital convertor G must be multiplied to produce the true concentration value.In its simplest form, the input circuit J may comprise a numeric keyboard linked to a binary encoder, by means of which the operator can supply the known concentration of a standard solution. The standard solution is then passed through the system, and the comparator is activated by the operator, independently of the delay circuit A. The degree of dilution required to reach the equivalence point is then determined as described above. The computer H may then calculate the ratio of the pump speed required to reach the equivalence point to the concentration of the solution, and store this ratio as a constant for future use.

Alternatively, the operator may supply physical constants of the solute and apparatus, namely: (a) the extinction coefficient of the solute at the wavelength being measured, (b) the flow rate of the sample solution through the system, (c) the path length of the sample cell in the spectrophotometer and (d) a "system constant", previously determined by measurement of a standard solution, which represents the ratio of the control voltage output of the ramp generator C to the resulting delivery rate by the pump of diluent to the mixing device. From these parameters, the computer H can calculate directly the concentration of the solution.

Once the control voltage output of the ramp generator C has been transformed as described above and recorded in suitable form, the comparator resets this control voltage to zero, switching off the pump, deactivates the mixing device and valve, and awaits the next trigger signal from the delay circuit A.

In another preferred embodiment of the invention, the system is used to measure the absolute concentrations of a number of solutions of the same solute. In this embodiment, the measuring instrument may be a flow-through spectrophotometer, colourimeter, fluorimeter, conductimeter, potentiometer, radiometer, nephelometer, turbidimeter or other instrument appropriate to the physical characteristics of the solute and solution. The system is illustrated in Figure 4, and operates in the same way as the system heretofore described, with the following additions and modifications: The system incorporates two identical pumps A and B, driven by identical control circuits within the system controller C, whose internal circuitry is illustrated in Figure 5.

Pump A delivers sample solution to the mixing device, while pump B delivers diluent to the mixing device.

The ramp generators D and E within the system controller are activated by a signal supplied either by the operator or by an external automatic sampling device which may advantageously be employed to enable large numbers of samples to be studied in the absence of an operator. The ramp generators D and E are designed in such a way that, in the absence of a controlling signal from the comparator F, the control voltage outputs to the pump control circuits G and H are both set to half the maximum value, so that, initially, both pumps A and B (in Figure 4) deliver sample and diluent respectively at identical flow rates, equal to half the maximum flow rate, to the mixing device.

The comparator F is held in a deactivated state for a period determined by the delay circuit /, the duration of this period being variable by the operator. This delay period allows residues of the previous sample solution to be flushed out of the system. At the expiry of this period, the comparator is activated.The comparator F is designed to detect which of three possible conditions obtains: Firstly, if the output voltage or current of the measuring instrument indicates that the concentration of the solution passing through the instrument is greater than that corresponding to a voltage or current level preset by the operator, the comparator generates control signals which cause the control voltage output of ramp generator D to decrease linearly with time, and the control voltage output of ramp generator E to increase linearly with time, thus decreasing the rate of delivery of sample solution and increasing the rate of delivery of diluent to the mixing device.

Secondly, if the output voltage or current of the measuring instrument indicates that the concentration of the solution passing through the instrument is less than that corresponding to a voltage or current level preset by the operator, the comparator generates control signals which cause the control voltage output of ramp generator D to increase linearly with time, and the control voltage output of ramp generator E to decrease linearly with time, thus increasing the rate of delivery of sample solution and decreasing the rate of delivery of diluent to the mixing device.

Thirdly, if the output voltage or current of the measuring instrument is equal to the value preset by the operator, the comparator generates control signals which activate the concentration computer J.

These features allow the equivalence point to be determined rapidly, thus increasing sample throughput, and allow fine adjustment of pump flow rates to ensure accurate results, since overshoot of the equivalence point is automatically compensated for.

The concentration computerJ calculates the ratio of mixing of sample and diluent at the equivalence point, by analysing the control voltage outputs of ramp generators D and E, where appropriate after conversion to digital form by analogue-to-digital convertors K and L. From this ratio, and from parameters supplied by the operator through the input circuit M, or obtained and stored in memory by prior evaluation of a standard solution as described in the preceding embodiment, the computer calculates the concentratIon of solute in the sample solution, and provides the result of the calculation as an output, where appropriate after conversion to analogue form by digital-to-analogue convertor N, to an external recording device, digital printer or data system.The computer additionally provides a control signal to an external automatic sampling device, or a visible or audible signal to the operator, after completion of the above sequence of operations, to indicate that the analysis has been completed, or to initiate loading of the next sample.

In both the preceding embodiments of the invention, the system controller may advantageously consist of a microprocessor system as illustrated in Figure 6, comprising a central processor unit A, a random-access memory B of suitable capacity, a suitably preprogrammed read-only-memory C, together with interface devices comprising an analogue-todigital convertor D to digitise the output voltage or current of the measuring instrument, pump-drive circuits E and F, where necessary incorporating digital-to-analogue convertors, mixing-device and valve control circuits G and H, a universal asynchronous receiver and transmitter (U.A.R.T.) /, to interface input and output data and control signals to and from the central processor unit A, a digital-to-analogue convertorJto provide an output to an analogue recording device, and a parameter input keyboard K and keyboard encoder circuit L. Additionally, an output may be provided to a mass data-storage device M, for example a tape recorder or magnetic disc storage system.

Where such method of construction is employed, the functions performed by the circuitry comprising the delay circuits, comparators, ramp generators, trigger circuits and concentration computers described in the foregoing embodiments of the invention, and illustrated in Figure 3 and Figure 5, may all be performed within the microprocessor system by selection of suitable programme instructions in the read-only-memory C. In addition, provision may be made for statistical analysis of results from a number of samples, where this is required, by suitable programming of the read-only memory C.

Claims (4)

1. A Device for measuring the concentration in solution of a dissolved substance or substances which comprises: - a means for determining a physical characteristic of the solution related to concentration; - a means for determining whether the output of the first means is equal to a preset value; - a means for supplying an appropriate diluent to the solution being measured at a variable rate; - a means for activating the third means whenever the equivalence condition determined by the second means does not obtain; - a means for continuously measuring the rate of delivery of diluent to the solution being measured; and - a means for relating the outputs of the first, second and fifth means to the concentration of the solution.

2. A Device as claimed in Claim 1, wherein the physical characteristic of the solution is its absorption of ultra-violet light.

3. A Device as claimed in Claim 2, which is used to examine the effluent solution of a chromatographic separation process.

4. A Device as claimed in Claim 1, which is used to measure the absolute concentrations of a number of solutions of the same solute.