pH

October 6, 2016 10:28 am

pH is the measure of concentration of H+ ions & OH- ions present in the sample which indicates whether the sample is acidic or basic.

Definition

The term pH test refers to the measure of hydrogen ion concentration in the sample. It is defined as the negative log of H+ ions concentration in the sample. The values of pH less than 7 are termed as acidic and values of pH above 7 are termed as basic. When the concentration of H+ and OH-ions are equal then it is termed as neutral pH.

Range

Implications

Acidic water can leach metals from pipes and fixtures, such as copper, lead and zinc. It can also damage metal pipes and cause aesthetic problems, such as a metallic or sour taste, laundry staining or blue-green stains in sinks and drains.

High alkalinity does not pose a health risk, but can cause aesthetic problems, such as an alkali taste to the water that makes coffee taste bitter; scale build-up in plumbing; and lowered efficiency of electric water heaters.

Sample Handling & Preservation

Samples should be analyzed as soon as possible, preferably in the field at the time of sampling.

High purity waters and waters not at equilibrium with the atmosphere (ground waters or lake waters collected at depth) are subject to changes when exposed to the atmosphere, Therefore, the sample containers should be filled completely and kept sealed prior to analysis.

Analysis Procedure: [ IS 3025-11 (1983) ]

1. Follow the manufacturer’s instructions for operation of pH meter.
2. After required warm-up period, standardize the instrument with a buffer solution of pH near that of the sample and check electrode against at least one additional buffer of different pH value.
3. Measure the temperature of the water and if temperature compensation is available in the instruments adjust it accordingly.
4. Rinse and gently wipe the electrodes with solution.
5. Immerse them into the sample beaker or sample stream and stir at a constant rate to provide homogeneity and suspension of solids.
6. Note and record sample pH and temperature However, if there is a continuous drift, take a second reading with the fresh aliquot of sample without stirring and report it as the pH value.

Interference

• Above pH value of 10, high sodium concentrations interfere with the measurement. Corrections for the sodium error may be made by consulting the chart supplied by the manufacturer of electrodes being used. Sodium errors at pH value levels greater than 10 can be reduced or eliminated by using a low sodium error electrode.
• Oil and grease may interfere by coating the pH electrode and causing a sluggish response. These coatings can usually be removed by gentle wiping or detergent washing, followed by distilled water rinsing. An additional treatment with hydrochloric acid (1 percent) may be necessary to remove any remaining film.
• Temperature affects the pH Values in two ways. The first is covered 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. The second source is the change of pH inherent in the sample at various temperatures. This error is sample-dependent and cannot be controlled. Therefore, the temperature at the time of analysis should be reported.

Interpretation

• pH data should always be interpreted in terms of hydrogen ion concentration, which of course, is a measure of the intensity of acid or basic conditions.
• Concept of Hydroxyl-ion concentration is of particular importance in precipitation reactions involving formation of hydroxides.
• Example: Precipitation of Mg2+ in softening of water with lime and in chemical coagulation processes employing iron and aluminum salts.
• pH adjustment by addition of acidic/basic chemicals is an important part of any wastewater treatment system as it allows dissolved waste to be separated from water during the treatment process.
• By chemically adjusting the pH we can remove heavy metals and other toxic metals from water.
• The pH of water can also be used to kill off bacteria in wastewater in addition to the treatment.

Conclusion

This experiment shows the importance of being precise and accurate to attain accepted value of pH of the given sample.

Disclaimer

The details given are for general understanding. References should be taken from respective authorized agencies.