RU2167416C2 - Method and device for determining hydrogen ion concentration - Google Patents

Abstract

FIELD: measurement technology. SUBSTANCE: proposed method involves determination of hydrogen ion concentration (pH) by measuring electrical characteristics of medium by steady state potential of signal under measurement corresponding to physical and chemical composition of medium using high-impedance electrodes for the purpose. Signal is recorded by time interval between start of measurement and threshold value attained in each cycle. In the process signal under measurement is shaped from dynamic potential difference across measuring and comparing electrodes of measuring cell due to accumulation of ions across measuring electrode. Cycle initiation is organized upon zeroing signal under measurement as soon as its amplitude rises to its threshold value by end of preceding cycle. Device implementing proposed method has measuring cell, amplifier, calculator, analog-to-digital converter, and switch designed to couple output of measuring cell with amplifier input. Amplifier output is connected through analog-to-digital converter over data bus to calculator built around personal computer which is connected over control bus to control input of switch. EFFECT: enhanced measurement speed, enlarged check-up range at preset metrological characteristics. 2 cl, 4 dwg, 1 tbl

Description

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

 There is a method [see a. from. N 1509719 (USSR), cl. G 01 N 27/30, 09/23/89] pH measurement, where an ion-selective field effect transistor (IPT) is used as a primary converter (PP). It consists in determining the transmission coefficient of IPT from the pH of the test solution. In this case, a dielectric layer is applied to the gate, through which the electric field inside the transistor interacts with the electromagnetic field of the analyzed hydrogen ions in solution. A device implementing this method consists of a measuring cell connected to a current source and reflector, an amplifier connected to the output of the measuring cell, and a recording device connected to the output of the amplifier.

 The disadvantages of this method and device are the low accuracy of measurements and the complexity of manufacturing IPT.

 The known dynamic method [see a. from. N 918839 (USSR), class G 01 N 27/56, 07.04.82], which consists in measuring the potential between electrodes with high internal resistance. To do this, determine the speed and acceleration of the measuring signal from the electrodes, and the results are used to find the pH of the test solution. A device that implements this method includes a series-connected measuring cell, amplifier, calculator, and recording device.

 The disadvantage of these solutions is the low accuracy of pH measurement caused by the error of the minimum discrete inertial signal pH = f (t).

 For the prototype adopted the method [see A.S. N 1599752 (USSR), cl. G 01 N 27/416, 10.15.90], 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 the 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, the input of which is connected to the output of the amplifier, and the outputs 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 the prototype are the low accuracy of the measurements caused by the parametric drift of the measuring electrode, the inertia of the measuring electrode and a narrow measurement range associated with a fixed threshold value.

 The technical task of the method and device is to increase the efficiency and expansion of the control range for a given metrological characteristics.

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 according to the steady-state potential of the measured signal corresponding to the physico-chemical composition of the medium, which is recorded over the time interval from the start of the measurement to the threshold value in each cycle, in contrast from the prototype, the measured signal is formed from the dynamic potential difference between the measuring and comparative electrodes of the measuring cell due to the accumulation ion flow at the measuring electrode, and the beginning of the cycle is organized after zeroing the measured signal at the moment when its amplitude reaches the threshold value at the end of the previous cycle.

 2. In the device for determining the concentration of hydrogen ions, consisting of a measuring cell, amplifier and calculator, in contrast to the prototype, an analog-to-digital converter and a switch are additionally introduced, connecting the output of the measuring cell to the amplifier input, the output of which is through an analog-to-digital converter via the data bus connected to a computer made on the basis of a personal computer, which is connected via a control bus to the control input of the switch.

 The essence of the proposed method is as follows (see Fig. 1).

Determination of the acidity of the medium is carried out by a measuring cell with high-resistance electrodes by the steady-state potential of the measured signal. The measured signal E is determined 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 (Fig. 1A). The steady-state potential E _{pH is} recorded by the time interval τ _i in each measurement cycle from the moment the measured signal is equal to zero until it reaches the threshold value (E ₀ ). The time interval τ _i (Fig. 1c) is represented in the code N _i (Fig. 1d), due to the counting in the measurement cycle of high-frequency pulses F ₀ (Fig. 1b). At the same time, the beginning of a new measurement cycle is organized after zeroing the measured signal at the moment of reaching the amplitude of the threshold value (E = E ₀ ) at the end of the previous cycle.

The accumulation of ions (Fig. 1) in inertial converters of the concentration of hydrogen ions varies exponentially:
E = E _pH • (1-e ^{-τ / T} ),
where: E is the current EMF of the measuring cell; E _pH - the maximum value of the EMF corresponding to the determined pH value; τ is the current measurement time; T is the time constant.

Постоянная времени T определяется предварительно на образцовом (или принятом за образцовый) растворе:

Известно, что код N = F_o•τ, тогда, умножив правую и левую части уравнения (3) на F₀ (с учетом что F₀•T=N_max), получим (см. фиг. 1 г):

отсюда потенциал установившегося режима насыщения:

По установившемуся потенциалу E_pH определяют искомую величину pH исследуемого раствора:

где: pH_и и E_и - координаты изопотенциальной точки электродной системы; S₀ - чувствительность электродной системы при 0^oC; α - температурный коэффициент чувствительности; t - температура исследуемого раствора.For the proposed method (see Fig. 1), taking into account the variable threshold value E _0, equation (1) takes the form:
E _o = E _pH • (1-e- ^{τ / T} ),
hence the time interval for determining the steady state potential of the measured signal:

The time constant T is preliminarily determined on a sample (or adopted as a model) solution:

It is known that the code is N = F _o • τ, then, multiplying the right and left sides of equation (3) by F ₀ (taking into account that F ₀ • T = N _max ), we obtain (see Fig. 1 g):

hence the potential of the steady state saturation:

The steady-state potential E _pH determines the desired pH of the test solution:

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

 In FIG. 2. shows a 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, analog-to-digital converter 4 (ADC), personal computer 5 (PC).

 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 enhance the EMF coming from the measuring cell 1 to the normalized level of the ADC 4.

 ADC 4 converts the normalized signal of cell 1 into a digital code for further processing in the microprocessor of a personal computer 5.

 PC 5 is intended for measuring and converting EMF from measuring cell 1 with subsequent determination of the concentration of hydrogen ions according to a given algorithm.

 The operation of the device is as follows.

, полученное за n измерений интервалов τ, коды которых регистрируется в оперативной памяти микропроцессора ПК 5. С учетом полученного значения E_pH по формуле (5) микропроцессор ПК 5 определяет искомую величину pH исследуемого раствора.The electrodes of cell 1 with high internal resistance are placed in the analyzed liquid. In the initial state, cell 1 is reset, because Switch 2 is open and closes the cell electrodes to zero potential (E = 0). The PC microprocessor 5 closes the switch 2 via the control bus and opens the electrodes from zero potential, as a result of which a dynamic EMF appears in measuring cell 1 (1) and the measurement cycle starts. The value of the dynamic EMF E _i from the measuring cell 1 through the switch 2, the amplifier 3, and the ADC 4 is converted into a digital code N _i via the data bus and goes to the microprocessor of PC 5. The time interval τ _i of the measurement cycle (3) is fixed at the moment of reaching the dynamic EMF represented by code N _{i of the} threshold value N ₀ . After fulfilling this condition, by opening the switch 2 and zeroing the measuring cell 1, the beginning of a new measurement cycle is organized. From the measured interval τ _i and the known value of the threshold E ₀ in accordance with formula (4), the microprocessor PC 5 determines the steady-state potential E _pH . The actual value is the average value

obtained for n measurements of intervals τ, the codes of which are recorded in the random access memory of microprocessor PC 5. Taking into account the obtained value of E _pH according to formula (5), microprocessor PC 5 determines the desired pH value of the test solution.

 Let us prove the effectiveness of the proposed solutions.

Следовательно, быстродействие предлагаемого способа на порядок выше, чем у прототипа.1. Speed
The time τ of one experiment for the prototype method is the sum of the time (k • T) of reaching the steady state potential of the measuring electrode and the time τ _{u of} measuring the stationary potential E _pH (Fig. 3):
τ = k • T + τ _u .
For the proposed method, the experiment time τ ₁ and measurements τ _u equal τ ₁ = τ _u , therefore, the main component of time for the prototype method is the value of k • T:
Δτ = τ-τ ₁ = k • T.
Let τ ₁ = 1 s, coefficient k = 3, and T = 3-30 s, then efficiency:

Therefore, the performance of the proposed method is an order of magnitude higher than that of the prototype.

 2. To expand the dynamic range of control for a given measurement accuracy.

Выразим ΔT через диапазон D, разбитый на n-эталонов:

тогда погрешности измерения для предлагаемого ε₁ и известного ε₂ способов имеют вид:

где i=1,2.The accuracy of measuring the time interval is the ratio of the absolute error ΔT to T _min :

Express ΔT through the range D, divided into n-standards:

then the measurement errors for the proposed ε ₁ and known ε ₂ methods have the form:

where i = 1,2.

определяется из математического описания предлагаемого способа и способа-прототипа (см. фиг.1 и 3) соответственно:

Принимая для упрощения рассуждений, что

и E₀₁=E₀₂, находим соотношение между точностью и диапазонами:

где

=n•E₀ (n - фиксированный коэффициент),

=i•E₀ (i - варьируемый коэффициент).

is determined from the mathematical description of the proposed method and the prototype method (see figures 1 and 3), respectively:

Taking to simplify the argument that

and E ₀₁ = E ₀₂ , we find the relationship between accuracy and ranges:

Where

= n • E ₀ (n is a fixed coefficient),

= i • E ₀ (i is a variable coefficient).

с учетом

диапазон D₁ предлагаемого способа изменяется в пределе:

Отсюда критерий эффективности:

Следовательно, предлагаемый способ при фиксированной точности позволяет расширить диапазон измерений в n раз.a) subject to a fixed error

taking into account

the range D _{1 of the} proposed method varies in the limit:

Hence the performance criterion:

Therefore, the proposed method with fixed accuracy allows you to expand the measurement range n times.

с учетом

погрешность ε₁ предлагаемого способа изменяется в пределе:

отсюда критерий эффективности:

Следовательно, предлагаемый способ при фиксированном диапазоне позволяет повысить точность измерений в n раз.b) subject to a fixed range

taking into account

the error ε _{1 of the} proposed method varies in the limit:

hence the performance criterion:

Therefore, the proposed method with a fixed range allows to increase the measurement accuracy n times.

 Implementation of the proposed method is carried out in a microprocessor pH meter, built on the basis of a personal computer "Sirius" and millivolt-pH meter pH-150.

The experimental results were carried out on a pH titer (the acidity of the latter changed during the experiment, pH 10, 9.6, 7.5) and are presented in table 1 and in FIG. 4. Previously, an experiment was conducted for this solution and the time constant T = 9.7 was obtained. Using the values of T and F ₀ = 60 kHz, the code value N _max = 582000 was found. In FIG. 4 shows three experimental dynamic curves for different pH values (the experiment was carried out at an ambient temperature of 20 ^o C). Table 1 shows the comparative calculations for the steady-state value of the potential according to the mathematical model of the proposed method (analytical curve) and real experimental values (experimental curve). The table shows that the proposed method and microprocessor pH meter with fairly high accuracy allow us to determine the desired value of the steady-state value of the EMF E _pH .

 Thus, the proposed method and microprocessor pH meter, in contrast to the known solutions, can increase the speed by 9 times and expand the dynamic range of control by n times with a fixed measurement accuracy or reduce the measurement error by a factor of n for a given range.

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 by the steady-state potential of the measured signal corresponding to the physico-chemical composition of the medium, which is recorded over the time interval from the start of the measurement to the threshold value in each cycle, characterized in that the measured signal is formed from the dynamic potential difference between the measuring and comparative electrodes of the measuring cell due to the accumulated I ions at the measuring electrode, and the cycle start organize after zeroing of the measured signal upon reaching its amplitude threshold at the end of the previous cycle.  2. A device for determining the concentration of hydrogen ions, consisting of a measuring cell, an amplifier, and a computer, characterized in that an analog-to-digital converter and a switch are added that connects the output of the measuring cell to the amplifier input, the output of which is connected through an analog-to-digital converter via a data bus with a computer made on the basis of a personal computer, which is connected to the control input of the switch via the control bus.

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