Introduction

pH measurements are made in solutions which contain relatively large amounts of acid or base, or which contain substantial amounts of dissolved salts. Under these conditions, conventional pH electrodes make measurements quickly and precisely. There has been growing interest in making pH measurements in pure water: waters in which the total amount of acid or base is very small.

Recommended Equipment

1. Direct concentrations readout ISE meter. Other suitable meters with millivolt accuracy to 0.1 mV may be used; calibration curves will be required. Some of Orion’s compatible meters are EA 940, 920A, 920Aplus, 720 or 720Aplus.
2. Orion ROSSTM pH Electrode, Orion 82-72 (ROSS Sure-Flow? Combination Electrode with glass body). No reference electrode is required.
3. Volumetric glassware and plastic beakers are recommended for measurements.
4. Magnetic Stirrer and stir bars.

Required Solutions

1. Orion Catalogue No. 910104, pH 4.01 buffer (color coded pink).
2. Orion Catalogue No. 910107, pH 7.00 buffer (color coded yellow).
3. Orion Catalogue No. 810007, Reference Filling Solution, ROSS reference filling solution.
4. Distilled or Deionized water.

Set Up

1. Assemble the combination pH electrode and check the electrode operation   (Slope) as described in the electrode instruction manuals.
2. Buffer 4.01 at 25°C: Use the pH buffer directly, Orion Catalogue No. 910104.
3. Buffer 7.00 at 25°C: Use the pH buffer directly, Orion Catalogue No. 910107.
4. Connect the pH combination electrode to the meter as specified in the meter instruction manual.

Calibration Standard Preparation

1. Pour about 40 ml of the pH buffer 4.01 into a 50 ml beaker. Add a magnetic stir bar to the beaker and cover the beaker with a watch glass.
2. Pour about 40 ml of the pH buffer 7.00 into a 50 ml beaker. Add a magnetic stir bar to the beaker and cover the beaker with a watch glass.
3. These are the standards used in actual calibration.

Sample Preparation

1. Store about 1 liter of fresh water just prior to the analysis into a plastic beaker.
2. Using a graduated cylinder, measure about 40 ml of the sample into a 50 ml beaker. Add a magnetic stir bar to the beaker and cover the beaker with a watch glass.
3. Repeat this procedure three to five times to obtain five readings of the same for consistent results.

Analysis

For instruments with direct concentration readout capability, consult meter manual for correct direct measurement procedures. Brief procedures for all meter types are found in the section How Ion-Selective Electrode Measurements are made.

1. Allow all the buffer solutions and the samples to attain room temperature for precise measurements since the electrodes are temperature sensitive.
2. Calibrate the meter using prepared standards, stirring the standards at a uniform rate.
3. Rinse electrodes with distilled or deionized water between measurements. Shake after rinsing to prevent solution carry-over. Do not wipe or rub the sensing element.
4. Measure samples, stirring at the same uniform rate used for calibrating the standards, and rinsing electrodes in distilled or deionized water between measurements.

Results

SAMPLE NUMBER pH
Sample # 1: 7.165
Sample # 2: 7.193
Sample # 3: 7.201
Sample # 4: 7.198
Sample # 5: 7.205

Mean:   7.1924
Standard Deviation: 0.0159
%CV:   0.2215

Storage

For brief storage periods, soak electrode in pH Electrode Storage Solution, Orion Catalogue No. 910001. If Orion Storage Solution is not available, use about 200 ml of pH 7 buffer to which about 1 gm. KCl has been added, as a temporary substitute. For longer storage periods, refer to the instructions manual provided with the electrode.

Maintenance

1. Inspect the electrode for scratches, cracks, salt crystal build-up, or membrane/junction deposits.
2. Rinse off any salt build-up with distilled water, and remove any membrane/junction deposits by holding junction open under running distilled water.
3. Drain the reference chamber, flush it with fresh ROSS Internal Filling Solution, Orion No. 810007, and refill chamber.