Direct Determination of Gas pH and Carbon Dioxide Concentration

Laboratory Experiment pubs.acs.org/jchemeduc

Direct Determination of Gas pH and Carbon Dioxide Concentration with pH Electrodes Qinlan Yang, Jun Long, Hao Zheng, Xianqing Tian, Dean Lei, Xiaoqiong Zhu, Yong Guo, and Dan Xiao* College of Chemistry, Sichuan University, Chengdu 610065, PR China S Supporting Information *

ABSTRACT: A simple apparatus to measure gas pH and carbon dioxide (CO2) concentration was designed for undergraduate students in the analytical chemistry laboratory. Gas passed through a flow meter and a mixer, then reached the homemade measuring chamber and caused the potential change of a pH combination electrode. Poly(ethylene glycol)-20000 aqueous solutions with hygroscopicity and nontoxicity were used as reaction media by forming absorption liquid films around the junction point and the bulb of the pH combination electrode. A laboratory-made voltage follower and a N2000 chromatography data system were used to transform impedance and record data, respectively. Gas samples containing 10−100% CO2 were measured with the apparatus, and fast potential responses for different concentrations of CO2 were obtained. This apparatus is more cost-effective and ecofriendly as compared with other conventional methods. KEYWORDS: Second-Year Undergraduate, Upper-Division Undergraduate, Analytical Chemistry, Laboratory Instruction, Environmental Chemistry, Hands-On Learning/Manipulatives, Gases, pH

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graduate students about detecting gas pH and CO 2 concentration. Liquid films formed in 10% poly(ethylene glycol)-20000 (PEG-20000) aqueous solutions are used to absorb CO2. A measuring chamber, a pH combination electrode, a voltage follower, and a N2000 chromatography data system are employed to monitor the potential change caused by CO2 in this experiment. Students, majoring in environmental science or chemistry, in groups of 2−3 people can complete the experiment within 3 h. The experiment also can be integrated into lessons relating to environmental science or agricultural science.

n recent decades, determination of gas pH has received much attention because it is a significant factor of industrial production. The pH of gas is one of the important parameters for quality evaluation and product control in many fields, such as ammonia/urea-SCR process1 and tobacco industry.2 The general method for the determination of gas pH is to indirectly determine the pH of a solution that absorbed the gas of interest. The determination of specific gases has been reported in undergraduate instruction,3 but determining gas pH received little attention. It is important for students to understand the principle and methods of detecting gas pH. As a significant parameter for quality control, the quantification of CO2 in the gas phase is of great importance in many fields, such as the food and agricultural industry4 and in environment engineering.5 Several methods have been proposed, such as IR,6 fluorescence,7 and electrochemistry.8,9 Among the various proposed sensing methods, sensors based on fluorescence10 or electrochemical11 methods have been developed for their high selectivity and sensitivity. In addition, indirect detection of CO2 was carried out by measuring the potential of carbonic acid formed in water with Severinghaus electrodes12,13 and by pH-sensitive hydrogel-based sensor.14 It is well-known that polyethylene glycols (PEGs) possess good stability and are nontoxic, nonvolatile, eco-friendly, hygroscopic, abundant, and cost-effective. PEGs aqueous solutions are widely used as reaction and reactive extraction media.15 In addition, PEGs can be recycled by removing solvent and other chemical substances. A simple apparatus is described that can be used in an analytical chemistry laboratory to teach second-year under© 2014 American Chemical Society and Division of Chemical Education, Inc.

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EXPERIMENTAL DETAILS

Equipment Apparatus

The instructor constructs the analytical apparatus before the experiment. A schematic of the apparatus is shown in Figure 1 and consists of eight parts: high-pressure gas tanks, a flow meter, a mixer, a measuring chamber, a pH combination electrode, a voltage follower, a N2000 chromatography data system, and a computer. The flow meter is used to control the flow rates of CO2 and N2 gases from high-pressure tanks. The gases are mixed in the mixer and then flow into the measuring chamber where the potential is measured. The measuring chamber is made of poly(methyl methacrylate) and is shown in Figure 2. The pH combination electrode, which is the most important part of the measuring chamber, is connected to the voltage follower, then followed by the N2000 chromatography Published: February 5, 2014 593

dx.doi.org/10.1021/ed300867n | J. Chem. Educ. 2014, 91, 593−596

Journal of Chemical Education

Laboratory Experiment

was stirred for 4 min. The solution was kept static for 3 min to form a liquid film around the junction point and the bulb of the electrode. The pH combination electrode was transferred into the measuring chamber. The concentrations of sample gases were adjusted by changing the mixing ratio of CO2 and N2. CO2 and N2 passed through two independent channels of the flow meter and mixed in the mixer to get gas samples of certain concentrations. The concentration of CO2 gas (xCO2(g), v/v %) was defined as the ratio of the flow rate of CO2 gas to the total flow rate of CO2 and N2 gases. The total flow rate of gas sample was fixed at 30 mL/s. The gas sample flowed into the chamber from the inlet within 10 s at the fixed rate and the potentials of the gases were recorded for 3 min.

Figure 1. The schematic diagram of experimental apparatus. The Ypipe is labeled 1 and the two pistons are labeled 2 and 3. Stopcock, labeled 4, is used to quickly stop ventilating gas before closing the gas flow meter.

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HAZARDS Gases at high pressure are the major hazards in the experiment. Students should operate the gas tanks under the guidance of the instructor. Students should close the high-pressure tank immediately after the experiment. In addition, the presence of CO2 will lead to CO2 poisoning if this experiment is carried out in a closed environment.

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CALCULATIONS The soluble gas dissolved in liquid film when the gas sample flowed into the chamber, leading to a change in hydrogen ion concentration, [H+], and a change in the potential. The resulting pH can be calculated by the following equations (formula deduction procedure is given in Supporting Information): Figure 2. The schematic drawing of the measuring chamber. The chamber is a cylinder with a height and diameter of 65 mm. The two hollow interior cylinders (7 and 16 mm in diameter) join to form the cavity.

ΔE = E − E0 = 0.059(pH0 − pH)

(1)

pH = pH0 − ΔE /0.059

(2)

where E0 (1.71 mV) is the initial potential of liquid film in chamber and pH0 (7.17) is the initial pH of liquid film in chamber. This value is the average of 20 measured data points (see Table S2 in the Supporting Information). The solubility of CO2 in 10% PEG-20000 aqueous solution is essentially independent of molar mass because the effect of hydroxyl can be ignored.16−18 So the resulting [H+] can be calculated by several chemical reactions that happened in this system:

data system. The voltage follower and the N2000 chromatography data system are employed to transform impedance and record data, respectively. (The circuit of the voltage follower is shown in Figure S1 in Supporting Information. The N2000 chromatography system used for data acquisition is not needed to run the experiment. The experimental data can also be collected by other data acquisition modules, such as Labview DAQ pad and microcontroller.)

Kg

CO2 (g) + H 2O(aq) ⇔ CO2 (aq) + H 2O(aq)

Materials

(3)

Ks

CO2 (aq) + H 2O(aq) ⇔ H 2CO3

A.R. grade PEG-20000 (CAS 25322-68-3) was purchased from Chengdu Kelong Chemicals (Sichuan, China). CO2 gas (98.5%) and N2 gas (99.999%) were obtained from Xuyuan Co., Ltd., (Chengdu, China). N2000 chromatography data system (Ver SP1) was purchased from Sagee Technology Co., Ltd. (Hangzhou, China). The pH combination electrode (Orion SA 720) was from Thermo Fisher Scientific, (Waltham, MA, USA). The flow meter (Brooks, 0154) was from Brooks Instrument Co., Ltd., (Hatfield, PA, USA).

K1

H 2CO3 ⇔ H+ + HCO3− K2

HCO3− ⇔ H+ + CO32 −

(4) (5) (6)

Kg, Ks, K1, and K2 are equilibrium constants of reactions 3, 4, 5, and 6, respectively. Thus, CO2 gas dissolved in the liquid film to produce carbonic acid and the [H+] increased. When [H+] is very low, the second ionization step of carbonic acid, eq 6, can be ignored. [H+] is governed by the first ionization step of carbonic acid, eq 5. So [H+] is deduced from eqs 3, 4, and 5.

Gas Measurement

Before measuring the CO2 gas, the pH combination electrode was calibrated by measuring three standard solutions with pH 4.00, 6.86, and 9.18, respectively. The procedure is the same as that of a standard pH measurement except the potentials of the standard solutions were recorded. Before the experiment, the pH combination electrode was dipped into 10% PEG-20000 aqueous solution and the solution 594

[H+] =

K gK sK1PCO2(g)

(7)

[H+] =

K gK sK1P ΘxCO2(g)

(8)




PCO2(g) is the partial pressure of CO2 in sample gas and PΘ is the normal atmosphere. Takahashi et al. also reported a similar relationship between [H+] and the partial pressure of CO2 in sample gas.19 The relation between ΔE and CO2 concentration obtained here was based on the Nernst equation and eq 8 (formula deduction procedure is given in Supporting Information): ΔE = K ′ + 0.0295 × log xCO2(g)

In addition, the theoretical values corresponding to different CO2 concentrations were calculated. The calculation is based on the assumption that liquid film was formed in ideal redistilled water. Namely, pH0, γ, Kg, Ks, K1, and PΘ are 7.00, 1, 3.39 × 10−2 mol/(L atm), 2.6 × 10−3, 4.2 × 10−7 mol/L, and 101.325 kPa. The theoretical line is shown in the inset in Figure 3 (line b). The trends of theoretical line and measured line are similar, indicating that the presented method is feasible. The students found the experiment interesting and exciting, and they took an active part in this experiment. There are several advantages of using this apparatus to detect gas pH and CO2 concentration for lab courses. Through the new and appealing experiment, students gain understanding of the area of gas pH detecting, and they understand the importance of gas pH detecting, which is significant for industrial production. The students measure gas pH and CO2 concentration by potentiometric measurements and can understand the relation between potential difference and hydrogen ion concentration. In this way, they understand the influence of the pH on the potential of pH electrode.

(9)

where K′ = 0.059 pH0 + 0.059 × log γ + 0.0295 × log(KgKsK1PΘ) and γ is the activity coefficient.

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RESULTS AND DISCCUSSION Gas pH and concentrations of CO2 gas were measured by this apparatus. The typical potentiometric curves are shown in Figure 3. The potentials increased rapidly when the gas samples

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SUMMARY A simple apparatus to detect gas pH and CO2 concentration was designed. A pH combination electrode, a voltage follower, and a N2000 chromatography data system are utilized to measure the potential change caused by a sample gas in the measuring chamber. The feasibility of this system was confirmed. This system provides students with practical experience to detect gas pH and CO2 concentration and helps students to learn the importance of measuring gas pH and CO2 concentration. It is appropriate for undergraduate teaching. In some courses, the proposed apparatus can be used to detect other acidic or basic gases, such as ammonia, hydrogen chloride, and sulfur dioxide. In more advanced courses, the methodology can be modified to detect the gas produced by fermentation. In particular, this experiment is also suitable for an environmental chemistry laboratory course.

Figure 3. Response curves of the experimental apparatus for different concentrations of CO2. The baselines were adjusted to zero by N2000 chromatography data system before recording data each time. The inset shows the relation between potential differences (ΔE) and the logarithm of CO2 concentrations: (a) experimental and (b) theoretical.

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Notes for the instructor; feasibility analysis; derivation of eqs 2 and 9; instructions for the students. This material is available via the Internet at http://pubs.acs.org.

flowed into the chamber. The potentials become steady after about 2 min. A linear relation between potential differences (ΔE) and the logarithm of CO2 concentrations is shown in the inset in Figure 3 (line a), and the square of liner correlation coefficient is 0.9764. The measured pH values of gas samples from the experiment are shown in Table 1.

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10 20 30 40 50 60 70 80 90 100

ΔE/V 5.663 6.321 6.989 7.650 8.291 8.632 8.706 8.861 9.228 9.441

× × × × × × × × × ×

10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

Table 1. The Measured Potential Values and Calculated Gas pH Values from the CO2 Concentration of the Sample Gases XCO2(g) (%)

ASSOCIATED CONTENT

S Supporting Information *

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (20927007 and 21075083) and the Scientific and Technological Contract of Sichuan University (09H1383).

pH 6.21 6.10 5.99 5.87 5.76 5.71 5.69 5.67 5.61 5.57

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Direct Determination of Gas pH and Carbon Dioxide Concentration

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