ASTM E70: Standardizing PH Measurement With Glass Electrodes

Introduction

An accurate pH measurement plays a critical role in many scientific and industrial processes. ASTM E70 provides a standardised method for using glass electrodes to determine pH, thereby improving the precision and reliability of results in various environments. This standard is essential for laboratories, manufacturing facilities and research institutions that require consistent pH measurements.

Summary of the ASTM E70 Standard

1. Scope and Purpose

• Purpose: To provide a standardised method for determining the pH value of aqueous solutions using a glass electrode paired with a suitable pH metre.
• Applications: Commonly used in water analysis, environmental monitoring, chemical manufacturing, pharmaceuticals and food processing.

2. Equipment and Materials

The test requires the following equipment:

1. pH Metre: An instrument capable of measuring pH with a resolution of at least ±0.01 pH units.
2. Glass Electrode: A pH-sensitive electrode used to measure the activity of hydrogen ions. For further details see the electrode information.
3. Reference Electrode: Often combined with the glass electrode into a single probe.
4. Buffer Solutions: Standard solutions with known pH values (e.g. pH 4.00, 7.00 and 10.00) used for calibration.

3. Procedure

1) Calibration of the pH Metre:

• Use at least two standard buffer solutions that cover the anticipated pH range of the sample.
• Adjust the metre so that its readings match the known pH values of the buffers.
• Perform the calibration at the same temperature as the sample, given that pH is temperature-dependent.

2 Sample Preparation:

• Ensure the sample is homogeneous and free from contamination.
• Record the temperature of the solution since it affects the electrode response.

3. Measurement:

• Immerse both the glass electrode and the reference electrode (or combined probe) in the sample.
• Wait until the pH reading stabilises before recording the result.
• Rinse the electrode between measurements with distilled or deionised water to prevent contamination.

4. Accuracy and Precision

• Proper calibration and regular maintenance of the pH metre and electrode are essential for accurate measurements.
• The standard stipulates that pH values should be reported with two decimal places.

5. Limitations

• This method is applicable only to aqueous solutions and is not designed for non-aqueous or highly viscous solutions.
• The presence of certain chemicals, such as organic solvents or high salt concentrations, may affect electrode performance.
• Temperature fluctuations can influence the measurements; consequently, temperature compensation is recommended.

Comparison of pH Measurement Standards

For further information, please refer to Stanford Advanced Materials (SAM).

Frequently Asked Questions

What is ASTM E70 used for?
ASTM E70 standardises the use of glass electrodes for precise pH measurements in various scientific and industrial applications.

Why are glass electrodes preferred for pH measurement?
Glass electrodes deliver accurate measurements over a wide pH range and suit most aqueous solutions, which is why they are preferred for pH determination.

Can ASTM E70 be applied to all types of samples?
Yes, ASTM E70 is versatile and can be applied to a broad range of samples, including liquids in laboratories, industrial processes and environmental monitoring.

What equipment is required for testing according to ASTM E70?
The standard requires a calibrated glass electrode, a pH metre, standard buffer solutions for calibration and temperature control devices to ensure precise measurements.

How does temperature affect pH measurements per ASTM E70?
Temperature influences both the electrode response and the pH value of the solution. Consequently, measurements must be conducted under controlled temperature conditions to maintain precision.