RU2423689C2 - Method and device for determining concentration of hydrogen ions - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method of determining concentration of hydrogen ions by using electrodes with high internal resistance to measure electrical parameters of the analysed medium from the steady-state potential of the measured signal, which corresponds to the physical and chemical composition of the medium, which is formed from dynamic potential difference between the measuring and comparison electrodes of a measuring cell and recorded on the time interval between the beginning of measuring and achieving the upper threshold value in each measurement cycle. The beginning of the measurement cycle is marked by the amplitude of the measured signal reaching the level of the lower threshold value after forced discharge when the amplitude reaches the upper threshold value at the end of the previous measurement cycle. Reference medium with standardised electrical parameters which are also recorded on the test time interval from the beginning of measurement until achieving the upper threshold value in each test measurement cycle is introduced. The beginning of the test measurement cycle is marked by the amplitude of the measured signal reaching the level of the lower threshold value after forced discharge when the amplitude reaches the upper threshold value at the end of the previous test measurement cycle. The time constant of the analysed medium is determined from the ratio of time intervals of the analysed and reference media. The discrepancy is minimised through adaptation of the standardised time constant of the analysed medium through successive approximation, and the approximation result is identified as the real value of the steady-state potential of the analysed medium which is proportional to the unknown concentration.

EFFECT: high accuracy by reducing dynamic and systematic error.

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Description

The present invention relates to measuring technique, in particular to measuring the concentration of hydrogen ions pH.

The known method [see A.S. No. 1599752 (USSR), class G01N 27/416, BI No. 15.10.90], which consists in measuring the potential between electrodes with high internal resistance. To do this, the input of the measuring circuit is locked with a bias voltage and the sum of a linearly varying voltage and the measured signal is fed to it, and the measured signal is determined by the time interval from the beginning of the linear voltage change until the sum of the voltages reaches the unlocking value of the circuit. A device that implements this method includes a measuring cell connected to the input of the amplifier, a computer whose input is connected to the output of the amplifier, and the outputs are connected to a counter and a ramp generator, the outputs of the generator and the bias source are connected to the input of the measuring cell.

The disadvantages of these solutions are the low accuracy of the measurements caused by the parametric drift of the measuring electrode, the inertia of the measuring electrode and the narrow measurement range associated with a fixed threshold value.

There is a method for determining the concentration of hydrogen ions [see Patent No. 2164416 (RF) G01N 27/416, BI No. 14 of 05.20.2001] due to the measurement by electrodes with high internal resistance of the electrical parameters of the medium by the steady-state potential of the measured signal corresponding to the physicochemical composition of the medium. The signal is recorded over the time interval from the beginning of the measurement to the achievement of the threshold value in each cycle. In this case, the measured signal is formed from the dynamic potential difference between the measuring and comparative electrodes of the measuring cell due to the accumulation of ions on the measuring electrode. The beginning of the cycle is organized after zeroing the measured signal at the moment of reaching its amplitude threshold value at the end of the previous cycle. The device according to the method consists of a measuring cell, an amplifier and a computer, an analog-to-digital converter and a switch connecting the output of the measuring cell with the input of the amplifier. The output of the amplifier through an analog-to-digital converter via a data bus is connected to a computer made on the basis of a personal computer, which is connected to the control input of the switch via a control bus.

The disadvantages of this method and device are the low accuracy of the measurements due to the residual potential on the measuring electrodes after zeroing.

The prototype adopted a method and device for determining the concentration of hydrogen ions [see patent No. 2316761 (RF), IPC G01N 27/416, BI No. 4 of 02/10/2008] by measuring with electrodes with high internal resistance the electrical parameters of the medium by the steady-state potential of the measured signal, corresponding to the physicochemical composition of the medium, which is formed from dynamic potential difference between the measuring and comparative electrodes of the measuring cell due to the accumulation of ions on the measuring electrode, and is recorded by the time interval from the start of measurement to reaching the upper threshold value in each cycle e measurements. The beginning of the measurement cycle is organized by achieving the amplitude of the measured signal level of the lower threshold value after the forced discharge at the time of reaching the amplitude of the upper threshold value at the end of the previous measurement cycle. A device for determining the concentration of hydrogen ions, consisting of a series-connected measuring cell, a switch and an amplifier, as well as a calculator, a digital-to-analog converter and a comparator, the information input of which is connected to the output of the amplifier, the normalizing input is through a digital-to-analog converter with the output of the calculator, and the output of the comparator is combined with the control input of the switch and the information input of the computer.

The disadvantage of the prototype is the relatively low accuracy of the measurements due to the lack of a standardized measure of the pulse duration, regulated by a model environment with known properties, which leads to a dynamic and methodological error.

The technical task of the method is to increase accuracy by reducing the dynamic and methodological error.

The technical task is achieved in that:

1. In the method for determining the concentration of hydrogen ions by measuring electrodes with high internal resistance of the electrical parameters of the medium under study by the steady-state potential of the measured signal, corresponding to the physicochemical composition of the medium, which is formed from the dynamic potential difference between the measuring and comparative electrodes of the measuring cell due to the accumulation of ions on the measuring electrode, and register the time interval from the start of measurement to reaching the upper threshold value In each measurement cycle, the beginning of the measurement cycle is organized by achieving the amplitude of the measured signal of the lower threshold value level after a forced discharge at the moment of reaching the amplitude of the upper threshold value at the end of the previous measurement cycle, in contrast to the prototype, a model medium with normalized electrical parameters is introduced, and namely, the time constant and steady-state potential, which are also recorded by the test time interval from the start of measurement to reaching the upper pore of the test value in each cycle of the test measurement, the beginning of the test measurement cycle is organized by achieving the amplitude of the measured signal of the lower threshold value level after a forced discharge at the moment of reaching the amplitude of the upper threshold value at the end of the previous test measurement cycle, determine the time constant of the medium under study in relation to time intervals of the studied and reference environments, the residual is minimized by adaptation of the normalized time constant of the studied environment by sequential an approximation whose accuracy is estimated by the error between the potentials determined at each step of approximation until a normalized error is achieved, and the result of the approximation is identified as the actual value of the steady-state potential of the medium under study, proportional to the desired concentration.

2. In the device for determining the concentration of hydrogen ions, consisting of a series-connected measuring cell, switch and amplifier, as well as a computer, in contrast to the prototype, a pulse shaper is introduced that connects the amplifier output to the clock input of the computer, the control bus of which is connected to the control output of the switch.

The essence of the proposed method is illustrated in figure 1.

The determination of the concentration of hydrogen ions is carried out by electrodes with high internal resistance of the electrical parameters of the medium under study by the steady-state potential of the measured signal, corresponding to the physicochemical composition of the medium. The measured signal is determined from the dynamic potential difference U between the measuring and comparative electrodes due to the accumulation of ions on the measuring electrode. The steady-state potential E _{pH is} recorded by the time interval τ in each measurement cycle from the moment the measured signal U is equal to the lower threshold value U ₀₁ until it reaches the upper threshold value U ₀₂ . At the same time, the beginning of a new measurement cycle is organized after a forced discharge at the moment when its amplitude U reaches the upper threshold value U ₀₂ at the end of the previous measurement cycle.

The time interval τ of the measurement cycle (figure 1) is determined by the pulse durations τ ₁ and τ ₂ described by the expressions:

where τ ₁ is the pulse duration for the lower threshold, τ ₂ is the pulse duration for the upper threshold, T is the time constant, E is the maximum EMF value corresponding to the determined pH value.

The time interval τ is found as the difference between the pulse durations for the upper and lower threshold

for the studied environment will be calculated as:

To find informative parameters T and E, a model medium is introduced (Fig. 1) with normalized electrical parameters T ₀ and E ₀ . We compare the time intervals of the studied τ and reference τ ₀ media; for this, we compose a system of equations using formula (3).

We find the ratio of d time intervals of the investigated and reference environments

by which the residual p is estimated as the ratio of time constants T ₀ / T, taking into account the normalized measurement function η

,

,

because d = p · η.

The informative parameter T is determined by successive approximation of the normalized time constant T ₀ with a step ΔT _i to the actual value T ^{* of} the medium under study

in each iteration loop.

.

.

The problem p is minimized by adapting the normalized time constant T of the medium under study by a successive approximation. The accuracy of the approximation p through the ratio d is estimated by the error ε _i between the potentials E _i determined at each step of the i-th approximation until the normalized error is reached:

where the parameter E _{i is} calculated as:

The result of the approximation is identified as the actual value of the steady-state potential E _i = E _{pH of the} test medium, proportional to the desired pH concentration:

,

,

where pH _u and E _u are the coordinates of the isopotential point of the electrode system; S ₀ - sensitivity of the electrode system at 0 ° C; α is the temperature coefficient of sensitivity; t is the temperature of the test solution.

Figure 2 shows the structural diagram of a device for implementing the proposed method.

The structural diagram of a microprocessor pH meter includes: measuring cell 1, switch 2, amplifier 3, pulse shaper 4, personal computer (PC) 5.

As measuring cell 1, standard high-resistance glass pH electrodes are used.

Switch 2 acts as an analog voltage switch and serves to switch the measuring cell 1.

The amplifier 3 is designed to amplify the EMF coming from the measuring cell 1 to the normalized levels U ₀₁ , U ₀₂ of the pulse shaper 4.

The pulse shaper 4 generates a pulse signal determined by the zero and single switching thresholds U ₀ , U ₁ , depending on the EMF level of the measuring cell 1, and also controls the switching of the switch 2 through PC 5.

PC 5 is designed to measure and convert into code N the duration τ of the pulse of the pulse shaper 4 with subsequent determination of the concentration of pH of hydrogen ions according to a given algorithm.

The operation of the device is as follows.

The electrodes of the measuring cell 1 with high internal resistance are placed in the analyzed liquid. In the initial state, the measuring cell 1 has zero potential (Fig.3, a), the switch 2 is open. PC 5 sets the lower measurement threshold U ₀₁ at the normalizing input of the pulse former 4, when the signal level U of this threshold U ₀₁ = U is reached, the measurement cycle starts, and PC 5 sets the upper measurement threshold U ₀₂ . If the signal level is equal to the upper threshold U = U _{02, the} measurement ends, the pulse shaper 4 switches the switch 2 through the PC 5 to the forced discharge position, and the PC 5 generates a lower measurement threshold U ₀₁ at the output. The duration of the discharge is regulated by the switching time of the pulse shaper 4 from a single state U ₁ to zero U ₀ (Fig. 3, b), performed, for example, on a logic element. The measurement cycle is repeated.

Let us prove the metrological effectiveness of the proposed method relative to the prototype according to the methodological and dynamic error.

Based on the found parameters E _i and T ^* we build a simulated curve U * (Fig. 4, 1):

We estimate the dynamic error (figure 5) of the simulated curve from the studied (figure 4, 2) by the formula:

In this method, through the use of a reference medium, the accuracy of measurements is increased, namely, the non-dynamic error is reduced to 10 ^-3 %.

To prove the reduction of the methodological error, we calculate the informative parameter E _{p of the} prototype according to the formula (3), i.e. excluding exemplary environment.

Assume that the time constant T _p without using the reference medium is within

We estimate the methodological error

The data obtained during the calculation of expression (13), we will issue in the form of a table.

Table Methodical error T _p T ^* -ΔT T ^* T ^* + ΔT U _p 0.285 0.341 0,309 γ,% 16 0 9

As a result of the calculations, the spread of the error in determining the acidity for ΔT = 0.1 is 9-16%.

Thus, the introduction of an exemplary environment with normalized electrical parameters in the proposed method, as well as a pulse shaper in the device allow to increase the accuracy of the proposed solutions by 9-16% with respect to the prototype.

Claims (2)

1. The method of determining the concentration of hydrogen ions by measuring electrodes with high internal resistance of the electrical parameters of the medium under study by the steady-state potential of the measured signal corresponding to the physicochemical composition of the medium, which is formed from the dynamic potential difference between the measuring and comparative electrodes of the measuring cell due to the accumulation of ions on measuring electrode, and register the time interval from the start of measurement to reaching the upper threshold value In each measurement cycle, the beginning of the measurement cycle is organized by achieving the amplitude of the measured signal of the lower threshold value level after a forced discharge at the moment of reaching the amplitude of the upper threshold value at the end of the previous measurement cycle, characterized in that a sample medium with normalized electrical parameters is introduced, and namely, the time constant and steady-state potential, which are also recorded by the test time interval from the start of measurement to reaching the upper threshold the value in each cycle of the test measurement, the beginning of the cycle of the test measurement is organized by achieving the amplitude of the measured signal level of the lower threshold value after the forced discharge at the time of reaching the amplitude of the upper threshold value at the end of the previous cycle of the test measurement, determine the time constant of the medium under study in relation to time intervals of the studied and reference environments, the residual is minimized by adaptation of the normalized time constant of the studied environment sequential at lizheniem whose accuracy is evaluated by the error between the potentials determined at each step, until the normalized approximation error and approximation result is identified as actual value of the steady-capacity medium under investigation proportional to the desired concentration. 2. A device for determining the concentration of hydrogen ions, consisting of a series-connected measuring cell, switch and amplifier, as well as a computer, characterized in that a pulse shaper is introduced that connects the amplifier output to the clock input of the computer, the control bus of which is connected to the control output of the switch.

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