Certification of buffer solutions reference materials using Baucke cell for supporting the quality of pH measurements

Question

1. What is the Baucke cell used for in pH measurement?

2. What equipment was used for potential measurements?

3. How are Pt/Ptblack electrodes prepared?

4. How was the pH value of the buffer solution prepared from oxalate salts determined?

Accepted Answer

The Baucke cell is used for the restandardization of secondary buffer solutions against primary buffer solutions in the range of pH between 3 and 11. It consists of two Pt/H2 electrodes in contact with each other through a vertical sintered glass disk. The potential is measured by these electrodes, and the liquid junction potential is mainly determined by ions other than hydrogen and hydroxyl. The cell is represented as Pt | H2 | buffer P | | buffer S | H2 | Pt, and the half-cell reaction of the electrode is 2H + (aq) + 2e -- H2 (g). It generally takes place on a platinum electrode while the pressure of the hydrogen gas present in the half-cell equals 1 bar. The Baucke cell is used to certify secondary buffer solutions with regard to primary buffer solutions of the same nominal composition, making it a convenient method for calibrating glass electrodes in pH measurements.

Accepted Answer

A digital multimeter, 0.01 mV (34461A, Agilent Technologies), was used for potential measurements. This device provides accurate and precise measurements of electrical potential, which is essential for conducting experiments and research in various fields. The multimeter's high sensitivity and resolution make it suitable for detecting small voltage changes, ensuring reliable data collection. Additionally, the use of a reputable brand like Agilent Technologies ensures the quality and reliability of the equipment, further enhancing the credibility of the research conducted.

Accepted Answer

Pt/Ptblack electrodes are prepared by welding Pt wire to Pt foil, cleaning with acid solution, oxidizing, electroplating with Pt black, and rinsing. The process involves using an oxyhydrogen welding machine, dipping in HNO3 + HCl solution, heating, and ultrapure water washes. A strong oxidizing solution of CrO3 in H2SO4 is used for further cleaning. The electrodes are then dipped in an electroplating solution containing 0.03% Pb(CH3COO)2 and 3% H2PtCl6, and an electric current of 20-25 mA is applied for 30 seconds to coat the Pt foils with Pt black. Finally, the electrodes are rinsed with water until use.

Accepted Answer

The pH value of the buffer solution prepared from oxalate salts was determined by attaining pKa2 equals 3.81, as reported in the handbook of chemistry and physics, 100th edition [16]. This value was used to ensure the buffer solution had the desired pH level. The preparation process involved drying potassium tetraoxalate (dihydrate) and disodium oxalate, followed by the addition of these salts to ultrapure water in a plastic container. The container was then shaken well and left to homogenize the buffer solution overnight.

Accepted Answer

The Baucke cell measures pH stability by restandardizing secondary buffers with primary buffers within a pH range of 3-11 and a difference of +0.02. It involves immersing Pt/H2 electrodes in the half-cell, connecting them to a multimeter, and stabilizing the potential for one hour. The flow rate of hydrogen gas is adjusted at 8.5 mL/min, and the potential is measured at 25°C. This process ensures accurate pH measurements and long-term stability of buffer solutions.

Accepted Answer

Homogeneity was assessed by dividing selected bottles into three portions and measuring pH using a calibrated pH-meter. Results were recorded in Table 2, tested for outliers, and analyzed using ANOVA-single factor. No significant differences were found, indicating homogeneity among buffer solutions. The uncertainty due to material heterogeneity was calculated using ISO Guide 35 equations, resulting in values of 0.00054, 0.0021, and 0.00075 for pH4, pH7, and pH10 respectively.

Accepted Answer

The short-term stability study showed that buffer RMs remain stable under transport conditions. Samples were stored for 4 weeks, with 9 bottles from each batch. Four bottles were stored in a refrigerator at 4°C, while four were stored in an oven at 40°C. One bottle was kept at room temperature (21°C) for 4 weeks. After the storage period, the bottles were conditioned to room temperature before measurements. The cell potential and pH values were measured and recorded. The pH results were plotted against storage time points, showing stability within certified uncertainty limits. The uncertainty in measured pH values was calculated, with values of 0.0069, 0.0016, and 0.004 for pH4, pH7, and pH10 respectively. This indicates that buffer RMs can be transported to customers at room temperature for up to 4 weeks without significant impact on stability.

Accepted Answer

The explicit sources of uncertainty in pH measurement include cell potential (E), temperature (T), and pH of the primary buffer (pH(p)). Implicit sources are electrode stability, signal stability, and difference in buffer solution levels in the half-cell. These sources are incorporated into the Nernst equation as f1, f2, and f3, assuming their pH values equal zero. The uncertainty sources are visualized in a fishbone structure and estimated accordingly.

Accepted Answer

The uncertainty of the cell potential is calculated by considering three contributions: voltammeter calibration, liquid junction potential, and repeatability of measurements. The uncertainty of voltammeter calibration is determined by dividing the maximum error by 3 and adding half of the uncertainty from the calibration certificate. The liquid junction potential uncertainty is estimated as 10% of the measured potential divided by 3. The repeatability of potential measurements is calculated by dividing the standard deviation of the average by the number of measurements. These uncertainties are then combined using equation 9, where sensitivity coefficients are equal to 1, to obtain the combined standard uncertainty of the cell potential.

Accepted Answer

The slope and t-statistic are crucial in long-term stability analysis. The slope represents the trend of pH values over time, indicating whether the pH remains stable or deviates significantly. In the given study, the slope values for pH4, pH7, and pH10 were 0.0001, 0.00009, and 0.0002, respectively. The standard error of each slope was calculated, and the t-statistic was determined by dividing the slope by the standard error. According to ISO guide 35, the t-statistic is compared to the t-table to assess if the slope deviates significantly from zero. In this study, all t-statistics were less than the t-tabulated value, indicating no significant trend in the measured pH values during the long-term stability. This suggests that the pH of the buffer reference materials (RMs) remains stable over the one-year validity period.

Accepted Answer

Yes, the compatibility criterion is fulfilled for secondary and primary buffer solutions. The absolute difference in pH between the secondary and primary buffers meets the criterion in equation 21. This indicates a strong traceability link between the pH of secondary buffer solutions and the pH of primary buffer solutions. The certified pH values and their uncertainties of the three buffer solutions were tested for compatibility with the pH values and uncertainties of the primary buffer solutions using equation 18 [20]. The results in Table 10 show that the absolute difference in pH between the secondary and primary buffers fulfills the criterion in equation 21, establishing a strong traceability link of the pH of secondary buffer solutions to the pH of primary buffer solutions.

Certification of buffer solutions reference materials using Baucke cell for supporting the quality of pH measurements

Chat with Paper

AI Agents for this Paper

Most frequently asked questions

1. What is the Baucke cell used for in pH measurement?

2. What equipment was used for potential measurements?

3. How are Pt/Ptblack electrodes prepared?

4. How was the pH value of the buffer solution prepared from oxalate salts determined?

5. How does the Baucke cell measure pH stability?

6. How was homogeneity assessed in Figure 2?

7. How stable are buffer RMs under transport conditions?

8. What are the explicit and implicit sources of uncertainty in pH measurement?

9. How is the uncertainty of the cell potential calculated?

10. What is the significance of the slope and t-statistic in long-term stability analysis?

11. Is the compatibility criterion fulfilled for secondary and primary buffer solutions?

References

CRC Handbook of Chemistry and Physics

Estimation of uncertainty in routine pH measurement

Gas analysis. Comparison methods for determining and checking the composition of calibration gas mixtures

Evaluation of a Compact Differential Cell for Secondary pH Measurements by a Bilateral Interlaboratory Comparison

Preparation of Secondary pH of Phthalate Buffer Solution Using Differential Potentiometric Cell: Method Validation and Application

Related Papers (5)

A selective fluorescent probe for the detection of Cd2+ in different buffer solutions and water

Controlled Buffer Sharing in Continuous Media Servers

Security Enhancement in IoV using User and Vehicle Certification

Certification of buffer solutions reference materials using Baucke cell for supporting the quality of pH measurements

Development of certified matrix reference materials for quality assurance of screening 134Cs and 137Cs in food