## ELECTRODE PREPARATION

The reference electrode reservoir is shipped filled. However, addition of electrolyte solution might be necessary before the electrode can be used. For proper performance, the level should be up to the filling hole.

1. Remove protective cap from immersion end of electrode.

2. Slide down sleeve or remove plug covering filling hole and fill the reference electrode reservoir with electrolyte solution to level of filling hole. Use a 4M potassium chloride solution saturated with silver chloride (Beckman 8262 Reference Electrode Filling Solution). Replace plug.

3. Soak the electrode in pH 4 or pH 7 buffer for several hours so that the pH-sensitive bulb of the glass electrode will respond correctly, and give stable readings.

## ELECTRODE INSTALLATION

1. Connect the cable leads to the pH meter and electrodes as directed in the instruction manual for the pH meter.

2. When using, immerse tip of electrode into buffer or sample so that junction is covered by solution.

3. Continue with pH measurement as directed in the pH meter instruction manual. Always leave the filling hole uncovered during measurements to ensure a uniform flow of electrolyte solution.

## OPERATIONAL PRECAUTIONS AND LIMITATIONS

The following operational precautions and limitations are cited so that the operator may avoid those actions which can damage the meter or electrodes, or adversely affect the validity of a quantitative determination.

1. Read Beckman Instructions 015-081616 before making pH measurements.

2. Care should be taken not to transfer body capacitance charge to the electrodes when blotting electrodes. Blot only, do not wipe. Preferably, rinse electrodes with next sample instead of blotting. Drift of reading will be experienced if charge is transferred. If glass electrode obtains charge, wait for discharge before proceeding.

3. Place instrument in standby when electrode is not in solution.

4. Take care to prevent breakage of pH glass bulb.

5. Periodically, standardize pH measuring system against NBS (National Bureau of Standards) type buffers or appropriate standard.

6. Check list for accurate pH measurements.

a. Standardize electrode close to sample pH value.

b. Maintain sample and buffer at same temperature.

c. Allow three minutes for stabilization for each measurement.

d. Test reference and glass portions of the combination electrode for performance specification.

7. Electrodes should be soaked before first use.

8. Sodium-Ion Correction

Measurements made in media of high pH with GP glass electrodes may be corrected approximately for sodium-ion error introduced by the glass electrode by use of the nomograph in Figure 3. This applies most accurately to new combination electrodes and the deviation will be greater in those which have had long periods of service in high alkaline solutions containing sodium ions. To use the nomograph, locate the point on the center chart corresponding to the temperature of measurement and that concentration of total sodium ions in moles per liter. Place a straight-edge through this point and the value of the pH scale reading. The pH correction to be added to the scale reading will then be the value indicated at the intersection of the straight-edge and the pH correction scale. When making a series of determinations at constant temperature and sodium-concentration, it will be convenient to drive a pin into the chart at the appropriate temperature-sodium-ion reference point so that the straight-edge can be rotated about that point to obtain each correction. Errors due to potassium and most other ions are much smaller than those due to corresponding sodium-ion concentrations, and may generally be neglected even at the highest pH readings.

9. Test electrodes only in high-impedance circuits such as those employed in Beckman pH meters; otherwise the electrodes will be damaged.

Figure 3. Sodium-Ion Correction Nomograph 225-N for GP Glass Electrodes

# SERVICE AND MAINTENANCE

## 1. Rejuvenation

If the pH-sensitive tip of the glass electrode deteriorates with time, one or more of the following may occur: a lag in response; undue sensitivity of meter needle to physical movement of electrode; failure of electrode to check against two buffers; or, in earlier-manufactured pH meters, inability to standardize within the range of the asymmetry potential control.

Rejuvenate glass electrode by momentarily immersing the tip of the combination electrode in 0.1N HCl or alternately immersing tip in acid and alkali (cycling); this also reduces residual sodium-ion effects. If acid treatment or cycling fails to rejuvenate the electrode, immerse tip in 20 percent ammonium bifluoride solution for three minutes, or in 10 percent hydrofluoric acid for fifteen seconds.

Rejuvenate combination electrode as often as necessary. However, frequent rejuvenation will reduce life and may cause immersion tip to crack. After fluoride rejuvenation, rinse electrode in stream of tap water, dip it momentarily in 5N hydrochloric acid to remove fluorides, then rinse it again in tap water. Store rejuvenated electrode in buffer, rinse off solution before using the electrode.

## 2. Maintaining Electrolyte Solution

Electrolyte contact between the reference electrode and the sample depends upon the continuous flow of electrolyte solution through the liquid junction. Although this liquid junction has a low volume-flow rate, check the liquid level frequently when first installed to determine the necessary maintenance schedule to keep the level of the electrolyte just below the fill hole. Use Beckman 8262 Reference Electrode Filling Solution as required.

## 3. Care and Cleaning

Clogging of the liquid junction of an electrode will be indicated by a drifting of the pH meter reading. A clogged junction may be cleared by applying a vacuum to the junction tip or by boiling the immersion tip of the combination electrode in distilled water until the electrolyte again flows freely. The solution will usually flow immediately if the electrode is stored in a beaker of pH 4 buffer solution.

The electrolyte solution has a tendency to creep onto and encrust the outer surface of the electrolyte with crystals. Encrustation does not harm the electrode; however, for maximum measurement accuracy, it is recommended that the outer surface be washed clean. Application of Beckman Desicote® will facilitate cleaning and minimize encrustation, sorbed film effects, and electrical leaks.

## 4. Contamination

If the solution in the combination electrode becomes contaminated, repeatedly flush the reservoir with fresh 8262 Reference Electrode Filling Solution. Following the final flushing, refill with 4M potassium chloride solution saturated with silver chloride (Beckman 8262 Reference Electrode Filling Solution).

## 5. Storage

Keep combination electrodes ready to use by storing them with bulbs immersed in tap water or Beckman pH 4.01 Buffer. Do not store the electrodes with bulbs immersed in basic solutions, these solutions tend to etch the glass.

If the combination electrode is stored dry, place the protective cap, containing pH 4.01 buffer, over the immersion end, fill the reservoir with electrolyte solution, and see that the filling hole is properly covered. After dry storage, it may be necessary to fill the reservoir and allow the tip of the electrode to soak, in order to resume normal flow of the electrolyte solution.

# PERFORMANCE CHARACTERISTICS AND SPECIFICATIONS

In order to verify the performance of the entire system,

three parts of the measuring system may be checked. A periodic check of meter, electrode, and buffer specifications can assure the operator of a correctly performing system. The following specifications are recommended for periodic check of system accuracy.

## PH METER

Performance specifications are given in the individual pH meter instruction manual.

## ELECTRODES

### SPAN

All Beckman electrodes should have a span error of less than ±0.06 pH per unit pH from standardization. This value is provided only as a suggested point to indicate where rejuvenation or replacement of the glass electrode may be necessary. Therefore, when spanning between pH 4.01 and pH 9.18 with National Bureau of Standards (NBS)-type buffers, no greater than ±0.3 pH units error should be noted.

### REPEATABILITY

### RESPONSE

Ten seconds to 98% of final reading with NBS-type buffers.

### REFERENCE POTENTIAL

0.0 mV ±5 millivolts for same type of internals.

## BUFFERS

Checked against NBS Standard ±0.01 pH or as listed on individual buffer.

## CALIBRATION PROCEDURES

The following procedures are for verification of performance specifications as performed by qualified personnel.

## METER

Performance verification is given in the individual pH meter instruction manual.

## ELECTRODE

Specifications and procedures for electrode verification require a proper working pH meter.

### SPAN

NBS-type buffers pH 4.01 (Beckman Part 582511) and pH 9.18 (Beckman Part 3009) at 25°C are used to check the span of the combination electrode. Place instrument in pH function and readout in standby. Rinse and place electrode in pH 4.01 buffer. Activate readout. Standardize to 4.01 pH. Place readout in standby. Rinse electrode with and place electrode in pH 9.18 buffer. Activate readout.

Allow enough time to get a stable reading. Meter should read 9.18 ±0.3 pH. If error is greater than ±0.3 pH, electrode should be rejuvenated or replaced.

### REPEATABILITY

A series of ten pH measurements in a stable NBS-type buffer, e.g., pH 4.01 (Beckman Part 582511) should agree within ±0.03 pH.

### RESPONSE

Place instrument in pH mode and readout in standby. Rinse electrode with and place electrode in 9.18 buffer. Actuate readout and standardize pH meter to 9.18 pH (after reading stabilizes). Rinse electrodes with and place in pH 4.01 buffer. Ninety-eight percent of final reading should be obtained within ten seconds.