Chloride ion on-line detection technology for COD correction

CHEN Fu-qiang， FU Xing

(State Key Laboratory of Precision Measuring Technology and Instruments(Tianjin University), Tianjin 300072， China)

Abstract： Chemical oxygen demand (COD) is an important index to evaluate the water pollution level. The method of potassium dichromate is used as a national standard for determination of COD in China. Chloride is the most common interference in COD determination process. In order to solve the problem, this paper analyzes the effect of chlorine ion on the determination of COD in principle. And then a kind of measurement system is designed based on the structure of nanometer glass probe, which achieves rapid measurement of the concentration of chloride ions within a larger range and provides a new technical solution for improving the accuracy of the COD measurement. After theoretical studies and experimental verification on the distractions in the process of ion current detection, the effects of probe diameter and bias voltage on the system measuring range and sensitivity are discussed.

Key words： chemical oxygen demand (COD); chlorine ion; ionic current; nanometer probe

Chemical oxygen demand (COD) as an important indicator of energy saving[1]and emission reduction was clearly put forward in the “12th Five-Year plan” of China[2]. The definition of COD is the consumption of oxygen when sample is treated under defined conditions[3]. According to the detection principle, the detection methods of COD mainly include potassium dichromate method, permanganate index method, ozone oxidation method, electro catalytic oxidation method, spectral analysis method, correlation coefficient method, biological method, etc. As the national standard method, the potassium dichromate method is the most widely used in COD determination field at present, which is commonly used in the determination of organic pollutants in domestic sewage and industrial waste water[2]. The working principle of potassium dichromate method is that in case of strong acid and heating, potassium dichromate is taken as the amount of consumed oxidant in the treatment of water samples. Approximately 90% of the organic matter present and most of the chloride were oxidized[4]. This method is well reproducible, but is easily affected by chloride that exists in the sample, especially in seawater, leather and industrial wastewater with high content chlorine ions. Therefore, a correction is required for the chloride. As for modification mothed, accurate determination of the content of chloride ions in water and the partial deviation introduced by chlorine ion can make COD determination results close to the real value. At present, the methods of chlorine ion detection in solution environment mainly include ion chromatography, chemical titration, spectrophotometric method, turbidimetric method, ion selective electrode method, etc[5]. These methods have some problems, such as long reaction time, narrow detection range and soon. In this paper, a detection system for chloride ion concentration based on the structure of nanometer glass probe is presented, which can achieve rapid measurement of chloride ion concentration with larger range and establish a COD procedure with reliability and environmental friendliness.

1　Effect of chloride ions on COD determination

In the standard COD determination method, potassium dichromate is added as a strong oxidant to oxidize organic compounds in aqueous sample,and silver sulfate is added as the catalyst for the complete oxidation of the linear chain aliphatic compounds in sulphuric acid environment by continuously heating up to 2 h[6]. The amount of oxidant consumption is the COD value. However, if there is chloride in the sample, the oxidant and catalyst will also be consumed by chloride ions, which will make the result of COD somewhat high[7].

1.1　Reaction of Cl- and potassium dichromate to consume oxidant

The equation of chemical reaction between chloride ion and potassium dichromate is expressed by

(1)

From Eq.(1), it can be demonstrated that 1 mg Cl-ions consume 1.38 mg K2Cr2O7oxidant. The result shows that the effect of chloride ion on the determination of COD can be expressed by

(2)

According to the calculation, 1 mg Cl-per oxide will consume 0.226 mg oxygen in theory, and the actual measurement results indicate that the chlorine ion will cause the positive error of COD value[7].

1.2　 Reaction of Cl- and silver sulfate to consume oxidant

In the process of COD determination using potassium dichromate method, the organic compounds containing hydroxyl will be oxidized to carboxylic acid in the strong acid environment. The further generated fatty acids react with the catalyst Ag2SO4to form the fatty acid silver. Because of the action of silver ions, the carboxyl group breaks easily and then produces CO2and H2O. Finally, the organic compounds are completely oxidized to CO2and H2O. In the above chemical reaction process, Ag2SO4acts as a catalyst to control the oxidation process of organic compounds. Because of the existence of high-concentration chloride ion, Ag2SO4will react with Cl-as

Ag++Cl-=AgCl↓.

(3)

Obviously, Cl-will continue to consume Ag2SO4to generate AgCl precipitation, The reduction of catalyst leads to the reduction of the rate of oxidation reaction[8].

In general, the chloride interference is a serious issue when using potassium dichromate method for COD determination. Various approaches have been developed to solve this problem in the last decades. Initially, the heating method was used to drive off the chlorides before digesting the sample with dichromate[9]. After that, a COD correction method was proposed to determine the chlorides separately via chemical titration[10]. Another consideration was given to the removal of chloride via precipitation and filtration before digestion[11]. In 1951, some people removed chloride by adding mercuric nitrate under the conditions for COD determination, as a result, a 10∶1 ratio of HgSO4to Cl-is generally accepted at present[12]. Because mercury salts could create hazardous waste, a mercury-free method was proposed, which used the sliver nitrate solution to suppress the chloride interference, but the mercury-free technique still suffered from chloride interference in wastewater with high-content chlorine[13-17].

In conclusion, the chloride interference is a unavoidable trouble in standard COD determination, but the chloride removal procedures are cumbersome and not very practical at present. The measurement system based on the structure of nanometre glass probe tries to solve the problem from a new angle of view.

2　Detection principle and system design

2.1　Detection principle

The measurement system detection principle is based on conductance measurement method. Ag and AgCl plated silver wire as two electrodes immerse in the solution environment containing chloride ions (KCl solution is used as the experimental solution). The driving electric field is added between the two electrodes, and the power supply, the electrodes and the solution constitute a circuit. The ions in the solution are moved directionally by the bias voltage, forming an ion current, as show in Fig.1.

Fig.1　Detection principle

The electrochemical principle is that the conductivity of the solution is linearly dependent on the concentration of each component ion, so the measurement results are affected by other anions, such as Br-, I-, and so on. Considering the application put forth in this article mainly aims at the wastewater with high content chlorine, chloride is the main anion whose content is much higher than that of other anions, thus the final measurement can treated as chloride ion content.

It is known that ion current is determined by the number of ions involved in electrochemical reaction process per unit time. The number of ions is mainly affected by temperature, intensity of the driving electric field, resistance value of the electrode, geometrical structure of the probe, the contact area between the electrode wire and the solution, etc. After other conditions are determined, the generated ion current is determined mainly by the content of chloride ions involved in the electrochemical reactions in the solution. Therefore, the ion content of the solution can be accurately measured by detecting the ion current.

The framework of the system is shown in Fig.2, including FPGA controller, ion current detection circuit, signal conditioning circuit, filter circuit, AD module, DA module, USB communication module, etc. The AD module collects the ion current signal and the position of piezoelectric ceramic, and then transfers them to FPGA through SPI protocol. The FPGA performs the PID negative feedback control, the package upload of AD acquisition signal data and the issue of upper computer’s PID control parameters, and then the motion of piezoelectric ceramic displacement platform is driven by high voltage amplification. The AD, DA and PID control modules are written in Verilog language and each module runs independently and without interference. The control process and USB module are written by C language in NIOS soft core. The interactive interface of the upper monitor is written by LabVIEW, which can display data, store data, control FPGA and adjust the parameters in real time.

Fig.2　Black diagram of system structure

2.2　Ion current detection circuit

Usually the ion current is of pA-nA magnitude because the measured signal has large internal resistance and small output signal. Meanwhile, the detection circuit needs shorter time for electrochemical reaction, therefore, the response speed of the system is fast. This paper combines direct resistance feedback method and two-stage amplifyication circuit for ion current detection. The design scheme is shown in Fig.3.

Fig.3　Ion current acquisition circuit

3　Experiment

The schematic diagram of ion migration in the electrolytic pool environment based on nanometer glass probe structure is shown in Fig.1. In the electrolytic pool circuit, the ion current was formed by the directional movement of the carrier-positive negative ions. The process of charge transfer is shown in Fig.3. The nanoprobe structure directly influences the migration of ions. The influence of driving electric field strength and probe tip diameter on ion current was studied by combining electrochemical principle and nano probe structure.

3.1　Experiment of ion current under different bias voltages

The experimental conditions are as follows: room temperature at 25 ℃; glass probe tip diameter of 300 nm, the concentrations of self-matched KCl solutions of 25 000, 37 500, 50 000 mg/L, respectively, instead of the environment of high chlorine wastewater.

After sampling every +15 mV bias voltage each time and recording the system output voltages at three different chloride concentrations, respectively, the relationship between the system output and the bias voltage is shown in Fig.4.

The triangle points line represents the experimental condition when the concentration of chloride ion is 25 000 mg/L, the round points line represents the experimental condition when the concentration of chloride ion is 37 500 mg/L, and the square points line represents the experimental condition when the concentration of chloride ion is 50 000 mg/L.

Fig.4　Relationship between system output voltage and bias voltage

3.2　Experiment of ion current under different tip diameter probes

Experimental conditions are as follows: room temperature at 25 ℃; glass probe tip diameters of 100 nm and 300 nm, the concentrations of self-matched KCl solutions of 1 375, 2 500, 3 750, 5 000 and 7 500 mg/L, respectively.

At the bias voltages of +250 mV and +210 mV, the detection of ionic current with five different concentrations of KCl electrolyte solution was carried out using glass probes with 100 nm and 300 nm diameters, respectively.

The experimental data are shown in Table 1, and the plot based on the experimental data is shown in Fig.5.

Table 1　Different tip diameters experiments

Fig.5　Relationship between system output voltage and tip diameters

Fig.5 shows the relationship between system output voltage and tip diameter. In Fig.5(a), the bias voltage is +250 mV, and the triangle points line represents a glass probe with a tip diameter of 100 nm, while the round points line represents a glass probe with a point diameter of 300 nm. In Fig.5(b), the bias voltage is +210 mV, and the triangle points line represents a glass probe with a tip diameter of 100 nm, while the round points line represents a glass probe with a point diameter of 300 nm.

4　Discussion

4.1　Effect of driving electric field intensity on ion current

In the electrolytic pool circuit consisting of power, electrode and solution, the output voltage and bias voltage of the system should satisfy the following relationships, namely

Vout=IionRprop,

(4)

(5)

whereIionis the ion current,Vbis bias voltage,Rprobeis the resistance of probe,Relectircis the resistance of circuit part,Rliquidis the resistance of the solution part,Voutis the output voltage of conversion circuit, andRpropis conversion amplification ratio of conversion circuit, including I/V conversion coefficient, the secondary magnification, the attenuation coefficient of low-pass filter, etc.

Further more

(6)

According to Eq.(6),VoutandVbshould be linearly proportional in principle. When the electrode position is constant, the ion current increases with the bias voltage. The experimental results are consistent with the theoretical derivation, and the sampling points exhibit a consistent change trend when the bias voltage was gradually increases from +45 to +225 mV. Therefore, the magnitude of the bias voltage does not cause additional effects on the system output voltage.

4.2　Effect of probe tip diameter on ion current

Two conclusions can be summarized from the experiment.

1) The larger the tip diameter of a glass probe, the larger the ion current was collected.

2) The output voltage curve of 100 nm tip diameter and the output voltage curve of 300 nm tip diameter show the trend of separation.

Because the reaction electrode was inserted from the glass probe, the internal fluid of the probe was connected with the solution in the electrolytic pool through the pore diameter of the probe tip, forming the channel of the chloride ion migration. Based on this structure, the process of ions flow is shown in Fig.1, and the object of the glass probe is shown in Fig.6.

Fig.6　Glass probe object (from network)

The actual glass probe tip is long and thin, and the tube body is approximately like a cylinder. The total amount of chloride ions involved in the electrochemical reaction is determined by the tip diameter of the micro glass tube. Eq. (7) shows the relationship of the ion flux and the probe tip diameter as

(7)

whereQis the solution flow,Sis the cross-sectional area (tip diameter), the tip diameter of the probe andvis the velocity.

Assuming that the concentration of the solution isnmol/L, the total amount of ions enteringNwithin a certain period of timetis calculated by

(8)

The above equation shows that the number of ions participating in the reaction is linearly positive with the square of the tip diameter of the probe without change of other conditions.

4.3　System performance

When using different bias voltages and probes, the system has perceptible difference in detection range and sensitivity. System detection is restricted to the saturation and fluctuation range of circuit. Under the conditions of +210 mV bias voltage and 100 nm diameter tip, the detection range can reach 200 000 mg/L. Changing the tip diameter to 300 nm, the detection range is 140 000 mg/L. Under the conditions of +250 mV bias voltage and 100 nm diameter tip, the detection range can reach 250 000 mg/L; Changing the tip diameter to 300 nm, the detection range is 180 000 mg/L. According to the output curves, the system output voltage will change in the range of 50-70 mV when ion concentration change is 1 000 mg/L. Therefore it is straightforward to adjust the system detection range and sensitivity to accommodate extensive testing environment by different combinations of bias voltages and tip diameter probes.

Stability reflects the system state with consistently working conditions, which has a crucial impact on detection data reproducibility and veracity. Recording the drift in the output of the system to embody the system stability is a simple and reliable approach, as shown in Fig.7.

Fig.7　System stability experiment

Experimental conditions are as follows: room temperature at 25 ℃, glass probe tip diameter of 100 nm, bias voltage of +170 mV, standard PBS solution and recording system output voltage every ten minutes.

The experiment results are as follows: system drift can reach 0.07 V with 240 min continuously working condition, and there is approximately 1% fluctuation relative to the system’s full range output, which indicates that the system has excellent stability.

5　Conclusion

This paper presents the scheme of on-line detection system of chloride ion based on nanometer probe structure, designs and constructs a weak ion current detection circuit. By using the micropipette puller to pull the nanometer probes with tip diameters of 100 nm and 300 nm, respectively, a series of self-prepared KCl experiments by means of electrolyte solutions with different concentrations were carried out. The experimental results indicate that the system can realize the rapid detection of chloride ion concentration within a larger range.