Standard Methods; 4500-H⁺ A, 4500-H⁺ B

Although this is one of the simplest tests, measurement of pH is one of the most important and most frequently used tests in wastewater analysis. The definition of pH is the negative logarithm of the hydrogen ion activity. In most instances that are dealt with in wastewater treatment, it is safe to assume that the activity coefficient will be 1, therefore the pH will be the negative logarithm of the hydrogen ion concentration.

pH is shorthand for expressing the negative logarithm of the hydrogen ion concentration, [H⁺], and is the measurement of hydrogen [H⁺] and hydroxyl [OH^-] ions that are present. A sample having a equal number of each is considered neutral, or pH 7.00. When a sample has a higher amount of [H⁺] than [OH^-], it is considered acidic - the pH is lower than 7.00, or between 0.00 and 6.99. Some examples of acids are nitric acid (HNO₃) and sulfuric acid (H₂SO₄). When samples have a lower amount of [H⁺] than [OH^-], the sample is basic - the pH is higher than 7.00 or between 7.01 and 14.00. Some examples of bases are potassium hydroxide (KOH) and sodium hydroxide (NaOH).

The pH notation of 0.00 - 14.00 was first introduced by a Danish biochemist Søren Sørensen. Before this, acid and basic solutions were noted in concentrations of [H⁺] and [OH^-] in a solution. For example 0.010 M HNO₃ (M is shorthand for the molarity of a solution) or 0.010 M NaOH. These are frequently written in scientific notation as 1.0 X 10^-2, meaning that the one is two (2) places behind (-) the decimal point.

Acidic pH relates to the molarity by the following equation:

pH = - log [H⁺]

If a solution of hydrochloric acid is 1.0 X 10^-2 M, then the pH of the solution is:

pH = - log [1.0 X 10^-2]

pH = -(-2) or 2

Basic solution pH also relates to the molarity using a different equation:

pH = 14 - (-log [OH^-])

If a sodium hydroxide solution is 1.52 X 10^-4 M, then the pH of the solution is:

pH = 14 - (-log [1.52 X 10^-4] )

pH = 14 - (-(-3.82))

pH = 14 - (3.82)

pH = 10.18

A rise in pH from 6.00 to 7.00 is really a ten-fold change in the potential of hydrogen. The molarity of the solution at 6.00 is 10^-6 M hydrogen, where the 7.00 solution is 10^-7 M hydrogen.

Taking a pH reading today has been greatly simplified by the use of a pH electrode and meter. The pH meter measures milliVolts (mV) and changes this into a pH reading by using the Nernst equation:

Where:

E = the potential in milliVolts developed between the sensing and reference electrodes

E₀ = varies with the choice of electrodes, temperature, and atmospheric pressure.

R = gas constant = 8.31451*

T = Temperature in degrees Kelvin (Kelvin = 273.15 + ° C)

n = charge on the ion

F = Faraday constant = 96,485*

a_i = the activity of the ion to which the electrode is responding

*These constants are related in terms of units, the discussion of which is beyond the scope of this manual.

The Nernst equation shows that it is very important that the temperature be taken into consideration when measuring the pH of a sample. This is often accomplished by using automatic temperature compensation (ATC). ATC is either built into a pH electrode or is a separate electrode placed into the sample along with the pH electrode. Temperature can also be compensated for manually if ATC options are not available. The change in pH values of some common buffers as related to temperature is listed in Table 1.

• pH meter accurate and reproducible to at least 0.1 pH unit with a range of 0 to 14. Many units are accurate and reproducible to 0.01 pH units.
• pH probe, there are several types:
1. pH probe – has a probe which contains an electrode for measuring pH and which also requires the use of reference and ATC electrodes.
2. Combination electrode which can be of two types:
1. pH and reference electrode, which requires use of an ATC electrode.
2. pH, reference, and ATC electrode in one unit.
3. Electrodes also come in different styles:
1. Those where the electrodes are contained in a probe with liquid internal filling solution. The user must fill probe with the solution and maintain a certain level in the probe. Generally this level is one inch above the bottom of the electrode bulb to one inch below the bottom of the fill hole of the probe. The level of the filling solution should be above the top of the sample being analyzed.
2. Those where the electrodes are contained in a probe with gel inside the probe body. These probes have the internal filling solution in gel form. They need very little maintenance, however as the probe is used, the filling gel is weakened. The gel cannot be replaced. Therefore these probes must be replaced every six months to one year, depending on frequency of use and care between readings.
• Beakers: Approximately 100 mL in size.
• Magnetic stirplate
• Teflon-coated magnetic stirbar
• Magnetic stirbar retriever
• Magnetic stirrer

1. Standardized Calibration Buffers at pH = 4.00, pH = 7.00, and pH = 10.00 are available commercially or can be made in-house with buffer tablets, buffer powders or reagent chemicals. The laboratory practices committee recommends that buffer solutions be purchased. It is generally recommended that properly stored stock buffers be used within the three month period after they are prepared.

2. Storage Solution for the electrode(s). Note that the electrode in the different probes require different storage solutions. Never store a probe in laboratory grade water. The electrolyte leaches into the water, shortening the life of the electrode. The solution should be slightly acidic to allow contaminants to be replaced by hydrogen ions.

1. Ag/AgCl Electrode ® 1 gram potassium chloride / 100 mL of pH 7 buffer. An indicatore such as bromocresol green may be added to this solution to differentiate it visually from the unmodified pH 7 buffer. Note that silver chloride is more soluble in solutions with high concentrations of chloride.
2. Calomel and double junction reference electrodes ® 1:10 dilution of filling solution.
3. 4 M KCl (potassium chloride). Weigh 29.82 grams of KCl. Place into 100 mL volumetric flask that is half filled with laboratory grade water. When chemical dissolves, fill flask to mark with laboratory grade water.
4. pH 4.00 buffer solution.
5. Commercially prepared electrode storage solution.
6. For overnight or weekend storage, a combination electrode probe can also be stored with the protective cap on the end of the probe and the filling hole covered. This will prevent the electrodes from drying out.
7. Manufacturer’s recommended storage solution.

3. Distilled water or other laboratory grade water. For rinsing off probe between samples or making standard buffers. Do not use this for storage of the probe.

There is no holding time or preservative for pH samples. Run sample as soon as possible after collection of the grab sample. Samples taken for regulatory purposes must be grabs.

Recorded data format

The following must be recorded on the data sheet:

1. Sample identification (source, name, and date of collection)
2. Analyst
3. pH data reported to nearest 0.01 pH unit
4. Description of unusual sample characteristics

The pH meter must be calibrated prior to analysis of each set of samples. A set is defined as any group of samples that can be analyzed consecutively without interruption.

Use a two-point calibration to calibrate pH meters. (For most analyses use pH 7 buffer as the isopotential or starting point.)

The pH meter should be calibrated across the range where the pH of the sample is expected to fall. It is acceptable to measure pH values up to 8.50 when the pH meter has been calibrated over the range of 7 to 4. Likewise, it is acceptable to measure pH values down to 5.50 when the pH meter has been calibrated over the range of 7 to 10.

Record all maintenance performed on the probe, such as adding filling solution or cleaning the junction.

Run duplicate analysis on one sample, the best choice being obtaining a second sample at the time of sampling.

If the sample pH falls outside the acceptable range for the calibration, the pH meter should be recalibrated to encompass the appropriate range.

The calibration of the probe and meter can be checked using a third buffer solution. Record the pH reading and buffer value on a bench sheet. For example – 7.00 and 10.00 were used to calibrate the meter. Use a buffer with value between 7.00 and 10.00 to check meter readings, or use the 4.00 standard buffer.

Recoveries of pH check samples should be between ± 2 standard deviations.

Recoveries of pH check samples must be between ± 3 standard deviations, or the pH meter calibration procedure will need to be repeated before proceeding with the analysis of samples.

Recoveries should be reviewed on a yearly basis and any changes documented in the check sample logbook.

 

Calibration

Always follow the manufacturer’s instructions for pH meter calibration. If instructions are not available, use the following steps. It should be noted that individual instruments might require additional steps to be properly calibrated.

1. Inspect the pH probe to see that it is properly filled (See apparatus section for specifics), that the electrode bulb is intact, and that the probe is properly connected to the meter. Uncover filling hole in the side of the probe.
2. Choose fresh buffers, either 4.00 and 7.00, or 7.00 and 10.00 buffers — whichever buffers will bracket the expected pH of your sample.
1. Example: Expected pH of the sample is 9.00, the correct buffer bracket for calibration would be buffers 7.00 and 10.00.
3. Remove the probe from storage solution. Rinse with distilled water and blot dry with a soft tissue.
4. Pour fresh pH 7.00 buffer into a beaker. Place a stir bar in the beaker.
5. Place the beaker on a magnetic stirrer set to stir at a slow uniform rate.
6. Submerge the probe in the buffer until the tip is covered. Allow the meter to stabilize. Adjust instrument to read 7.00 ±0.05 pH units on the scale.
7. Remove probe from 7.00 buffer and rinse probe with distilled water.
8. Place the beaker containing the second calibrating buffer (pH 4 or 10) on a magnetic stirrer and set to stir on a slow uniform rate.
9. Submerge the probe in the buffer until the tip is covered. Allow the meter to stabilize. Adjust instrument to read the buffer value, ± 0.05 pH units on the scale.
10. Remove pH probe from second calibrating buffer and rinse with distilled water.
11. Record the date, time and initials of analyst performing the calibration in the calibration log book. Record the buffer values used to standardize the meter along with the temperature of the buffers. If the meter displays the slope, record this on the benchsheet too.
12. OPTIONAL: Check the operation of the probe and meter using a third buffer. Record this value in the calibration log book also.

Sample Analysis

The following steps are general in nature to cover all the equipment generally used in laboratories.

1. Collect a representative sample in a clean, dry sample container
2. Place container on magnetic stirrer. Stir at a slow uniform rate.
3. Submerge the probe in the sample.
4. Allow meter to stabilize. Record reading, to the nearest 0.1 or 0.01 pH unit, on the benchsheet.
5. Rinse probe with laboratory grade water and repeat above steps until all samples are read.
6. Place the probe back in the pH buffer 7.00 to check for drift. The meter should read 6.90 to 7.10 pH units. If not, check for a faulty electrode.
7. Record the pH of the sample to the nearest 0.1 or 0.01 pH unit. Record this information in the calibration log book.
8. Rinse probe with distilled water and blot dry.
9. Store in manufacturer’s suggested pH storage solution.

The pH meter gives direct reading of pH. No calculations are needed.

• Inspect the probe and electrodes weekly for scratches, cracks, salt crystal build–up, or membrane/junction deposits.
• Rinse off any salt build–up with distilled water
• Remove membrane junction deposits by soaking the junction in distilled water or holding the junction under running distilled water.
• When slow sluggish response persists, empty the probe completely of internal filling solution. Rinse out with distilled water and fill with fresh internal filling solution.
• Clear a clogged junction by applying suction to the tip or by boiling tip in distilled water until the electrolyte flows freely when suction is applied to tip or pressure is applied to the fill hole. If possible, replace the junction when necessary.

Low slope

• Electrodes may be faulty.
• Electrodes may not be properly conditioned – either the probe has not set long enough after filling solution was added or wrong type of storage solution was used.
• Buffers or glassware may be contaminated.
• Technician has not properly rinsed the probe between buffers or is using improper stirring techniques.

High Slope

• Using buffers in incorrect order.
• Buffer temperatures have not been properly corrected.
• Buffers or glassware may be contaminated.
• Technician has not properly rinsed the probe between buffers or is using improper stirring techniques.

Drift

• Membrane is dirty or junction is clogged.
• Probe and electrodes may be defective.
• Wrong filling solution may have been used.
• Buffer temperatures have not been properly corrected.

• Know safety procedures for pH analysis
• Know how to fill out raw data sheets
• Understand procedures used to perform pH analyses
• Understand equipment used to perform pH analyses

pH meter, pH electrode, How to calibrate, General use and care of electrode

• Know where to find equipment / supplies

pH meter, pH electrode, pH buffer solutions

• Understand quality control procedures

• Prepare raw data sheets
• Calibrate pH meter and probe
• Collect sample
• Check pH of sample(s) as soon after collection as possible
• Record raw data in the log book