pH measurement is usually an essential part of chemical characterisation. Many processes are pH dependent so it is usually important to monitor the pH to ensure continuity. The pH scale is generally accepted to be from 1 to 14 pH units, with 1 being extremely acidic and 14 being extremely alkaline. pH 6.5 to 7.5 is considered neutral. Natural waters are usually slightly alkaline due to the bicarbonates and carbonates from dissolved minerals. Dissolved CO2 can decrease pH.
Sample measurement for pH has a recommended holding time of just 15 minutes according to the Australian Standard. However, AST is still able to provide a NATA accredited result for this test.
Conductivity (Electrical Conductivity or EC) is a solution's ability to conduct electric current. This depends on the ionic composition of the solution and the temperature at which it is measured. Generally, the higher the conductivity the higher amount of dissolved minerals (both positively and negatively charged), however it is also dependent on the mobility of those ions. Large organic molecules do not usually conduct and therefore give a low conductivity reading. Conductivity is measured in micro Siemens per centimetre (µS/cm) and reported at 25ºC. Conductivity is related to total dissolved solids and salinity but is not the same.
Acidity and Alkalinity
Acidity and Alkalinity are the base and acid neutralizing capacities of water and are usually expressed as milligrams of calcium carbonate per litre (mg CaCO3/L). Units of CaCO3 provide an indication of how much CaCO3 it would take to create an equivalent acidity or alkalinity in a solution.
Acidity is a titration with strong standardised base to pH 8.3. It is the sum of all titratable acids. Hydrogen ions present in the sample react during titration. These hydrogen ions are usually present due to hydrolysis or dissociation of other compounds in the water which can come about from industrial or other activity. In unpolluted surface waters the major acidic component is usually from dissolved carbon dioxide.
Alkalinity is a titration with dilute standardised acid to pH 4.5. It is the sum of all titratable bases. In surface waters the most significant contributors to alkalinity are likely to be bicarbonate, carbonate, and hydroxide. Alkalinity is therefore taken as an indication of the concentration of theses ions. Bicarbonate alkalinity, carbonate alkalinity, hydroxide alkalinity, and total carbon dioxide are determined by calculation.
TS, TSS, TDS, Volatile and Fixed Solids
Solids refers to suspended particulate matter or dissolved minerals and is reported in mg/L. Total Suspended Solids (TSS) is the portion retained on a filter. Total Dissolved Solids (TDS) is the mass retained after filtration and evaporation. Total Solids is the sum of both TSS and TDS or the mass retained after evaporation (no filtration). Waters with high suspended or dissolved solids are generally unpalatable, look aesthetically unfavourable or have undesirable effects on equipment or health.
Volatile Solids are the solids lost upon heating at 550 ± 50 °C. Fixed solids is the residue remaining after this process. Volatile solids is equivalent to the Loss on Ignition test on soils. These tests can be used as a rough approximation of organic matter however they do not distinguish between organic substances, inorganic minerals, decomposition or volatilisation. Better characterization of organic matter can be made with organic carbon tests like NPOC, BOD and COD. The volatile/fixed solids test can be applied to the suspended, dissolved or total portions of the solids test.
True and Apparent Colour
Colour is the measurement of light absorbance at 455nm. True Colour is filtered. Apparent colour is not filtered and so measures any suspended material contributing to the colour appearance. Colour is reported in Colour Units (CU) as compared with a standard platinum cobalt complex solution.
Colour in waters occurs naturally from organic matter, colloidal particles like clay, algae, iron and manganese oxides. Industrial wastewaters can contain tannins, lignin, dyes, and other chemicals that can contribute to colour.
Turbidity is an expression of optical property that causes light to be scattered and absorbed. The higher the intensity of scattered light, the higher the turbidity. Turbidity in water is caused by suspended and colloidal matter such as clay, silt, plankton, microscopic organisms and finely divided matter. Turbidity is reported in Nephelometric Turbidity Units (NTU). The clarity of natural waters is an indicator of natural condition and productivity and is important for human consumption and many manufacturing operations.
Hardness is calculated by the sum of the concentration of calcium and magnesium obtained from metals analysis. It is reported as mg equivalent of calcium carbonate per litre (mg equivalent CaCO3/L). Water hardness influences the ability of soap to lather; harder waters are more difficult to lather.
Non carbonate hardness is assumed to only occur when the calculated hardness exceeds the Alkalinity bicarbonate and carbonate, the difference being non-carbonate hardness. When the calculated hardness is equal to or less than the sum of carbonate and bicarbonate alkalinity, all hardness is carbonate hardness and noncarbonated hardness is absent.