METHOD #: 150.2 Test Method (December 1982)

TITLE: pH, Continuous Monitoring (Electrometric)

ANALYTE: pH

INSTRUMENTATION: pH Meter

1.0 Scope and Application

1.1 This method is applicable to the continuous pH measurement of drinking,

surface, and saline waters, domestic and industrial waste waters.

2.0 Summary of Method

2.1 The pH of a sample is determined electrometrically using a glass electrode

with a reference electrode or a single combination electrode.

3.0 Sample Handling and Preservation

3.1 The composition of the water or waste contacting the measuring electrode

system must be representative of the total flow from the water body.

4.0 Interferences

4.1 The glass electrode, in general, is not subject to solution interferences from

color, turbidity, colloidal matter, oxidants, reductants or high salinity.

4.2 Sodium error at pH levels greater than 10 can be reduced or eliminated by

using a "low sodium error'' electrode.

4.3 Manually inspect the conditions of the electrodes every 30 days for coating by

oily materials or buildup of lime. If oil and grease and/or scale buildup are

not present, this time interval may be extended.

4.3.1 Coatings of oil, grease and very fine solids can impair electrode

response. These can usually be removed by gentle wiping and

detergent washing. The use of flowthrough electrode housings which

provide higher flow velocity helps to prevent the coating action.

4.3.2 Heavy particulate matter such as lime accumulation can be removed by

careful scrubbing or immersion in dilute (1+9) hydrochloric acid.

Continuous monitoring under these conditions benefits from ultrasonic

or other in-line continuous cleaning methods.

4.4 Temperature effects on the electrometric measurement of pH arise from two

sources. The first is caused by the change in electrode output at various

temperatures. This interference can be controlled with instruments having

temperature compensation or by calibrating the electrode-instrument system at

the temperature of the samples. Forbest results, meters having automatic

temperature compensation shouldbe calibrated with solutions within 5°C of

the temperature of thestream to be measured. The second source is the change

of pH inherentin the sample at various temperatures. This error is sample 
National Bureau of Standards Special Pubulication 260 1

dependent and cannot be controlled, it should therefore be noted by reporting

both the pH and temperature at the time of analysis.

5.0 Apparatus

5.1 pH Monitor - A wide variety of instruments are commercially available with

various specifications and optional equipment. For unattended use, the

monitor should be equipped with automatic or fixed temperature

compensation and with a recorder or alarm function.

5.2 Glass electrode - with shielded cable between electrode and monitor unless

preamplification is used.

5.3 Reference electrode - a reference electrode with a constant potential and with

either a visible electrolyte or viscous gel fill.

NOTE 1: Combination electrodes incorporating both measuring and reference

functions are convenient to use and are available with solid, gel-type filling

materials that require minimal maintenance.

5.4 Temperature sensor - for automatic compensator covering general ambient

temperature range.

5.5 Electrode mounting - to hold electrodes; may be flow through (for small

flows), pipe mounted or immersion.

6.0 Reagents

6.1 Primary standard buffer salts are available from the National Bureau of

Standards and should be used in situations where extreme accuracy is

required.

6.1.1 Preparation of reference solutions from these salts require some special

precautions and handling such as low conductivity dilution water, 1

drying ovens, and carbon dioxide free purge gas. These solutions

should be replaced at least once each month.

6.2 Secondary buffers may be prepared from NBS salts or purchased as a solution

from commercial vendors. Use of these commercially available solutions, which

have been validated by comparison to NBS standards, is recommended for

routine operation. These buffers may be retained for at least six months if kept

stoppered.

7.0 Calibration

7.1 Immersion type electrodes easily removed from mounting.

7.1.1 The electrode should be calibrated at a minimum of two points that

bracket the expected pH of the water/waste and are approximately

three pH units or more apart.

7.1.2 Repeat calibration adjustments on successive portions of the two buffer

solutions until readings are within ± 0.05 pH units of the buffer value.

If calibration problems occur. see 4.3.

7.1.3 Because of the wide variety of instruments available, no detailed
operating instructions are provided. Instead, the analyst should refer to

the particular manufacturer's instructions.

7.1.4 Calibration against two buffers should be carried out at least daily. If

the pH of the fluid being measured fluctuates considerably, the

calibration should be carried out more often. Calibration frequencies

may be relaxed if historical data supports a longer period between

calibration.

7.2 Immersion type electrodes not easily removed from mounting.

7.2.1 Collect a grab sample of the flowing material from a point as close to

the electrode as possible. Measure the pH of this grab sample as

quickly as possible with a laboratory - type pH meter. Adjust the

calibration control of the continuous monitor to the reading obtained.

7.2.2 The temperature and condition of the grab sample must remain

constant until its pH has been measured by the laboratory pH meter.

The temperature of the sample should be measured and the

temperature compensator of the laboratory pH meter adjusted.

7.2.3 The laboratory - type pH meter should be calibrated prior to use

against two buffers as outlined in 7.1.

7.2.4 The continuous pH monitoring system should be initially calibrated

against two buffers as outlined in 7.1 before being placed into service.

Recalibration (every 30 days) at two points is recommended if at all

possible to ensure the measuring electrode is in working order. If this is

not possible, the use of electrode testing features for a broken or

malfunctioning electrode should be considered when purchasing the

equipment.

7.2.5 The indirect calibration should be carried out at least once a day. If the

pH of the fluid being measured fluctuates considerably, the calibration

should be carried out more often. Calibration frequencies may be

relaxed if historical data support a longer period between calibration.

7.2.6 If the electrode can be removed from the system, but with difficulty, it

should be directly calibrated as in 7.1 at least once a month.

7.3 Flow-through type electrode easily removed from its mounting.

7.3.1 Calibrate using buffers as in 7.1. The buffers to be used may be the

process stream itself as one buffer, and as a second buffer after

adjustment of pH by addition of an acid or base. This will provide the

larger volumes necessary to calibrate this type electrode.

7.3.2 Since the velocity of sample flow-through a flow through electrode can

produce an offset error in pH reading, the user must have data on hand

to show that the offset is known and compensation has been

accomplished.

7.4 Flow-through type electrode not easily removed from its mounting. 

7.4.1 Calibrate as in 7.2.

7.4.2 Quality control data must be on hand to show the user is aware of

possible sample flow velocity effects.

8.0 Procedure

8.1 Calibrate the monitor and electrode system as outlined in Section 7. 

8.2 Follow the manufacturer's recommendation for operation and installation of

the system.
8.3 In wastewaters, the electrode may require periodic cleaning. After manual

cleaning, the electrode should be calibrated as in 7.1 or 7.2 before returning to

service.

8.4 The electrode must be placed so that the water or waste flowing past the

electrode is representative of the system.

9.0 Calculations

9.1 pH meters read directly in pH units. Reports pH to the nearest 0.1 unit and

temperature to the nearest °C.

10.0 Precision and Accuracy

10.1 Because of the wide variability of equipment and conditions and the

changeable character of the pH of many process waters and wastes, the

precision of this method is probably less than that of Method 150.1; however, a

precision of 0.1 pH unit should be attainable in the range of ph 6.0 to 8.0.

Accuracy data for continuous monitoring equipment are not available at this

time.

Bibliography

1. Annual Book of ASTM Standards, Part 31, "Water" Standard 1293-78, Method D, p.

226 (1981).