Glossary of Common Terms used in Water Chemistry

Alkalinity

The buffer capacity of water with respect to the addition of acid, i.e. the resistance to change its pH. The main contributors to alkalinity in natural waters are carbonates and bi-carbonates. That is why alkalinity often is expressed in mg/L of calcium carbonate (CaCO₃), though calcium carbonate itself is hardly soluble in water. In many applications a minimum alkalinity is necessary in order to maintain a stable pH, e.g. swimming pools, aquaculture, boiler feed water.

Chlorine

A chemical element with strong oxidising properties. Because of that it is used as a disinfectant in water treatment and water supply. Chlorine gas is very toxic and difficult to handle and only used in high volume commercial applications. In most cases chlorine is applied as sodium or calcium hypochlorite. Chlorine tends to combine with organic and inorganic nitrogen compounds forming chloroamines. This "combined chlorine" is a less powerful disinfectant than "free chlorine".

Colorimetry

→Photometry

Dilution

Most analytical methods are very sensitive and allow the determination of water constituents at concentrations of only a few mg/L and sometimes even much less. If the concentration is higher than the permissible measuring range of a certain method, that method can not be applied directly, but a sample dilution is required before the actual measurement. Such a dilution is usually carried out with demineralised water. When reporting the result, the dilution factor has to be taken in account.

EC

Electrical Conductivity is a measure of the ability of an aqueous solution to carry an electric current. This ability depends on the presence of ions, their concentrations, mobility and valence and also the temperature. Inorganic compounds usually dissociate into ions when dissolved and solutions of those are good conductors. In contrast, organic molecules generally do not form ions when being dissolved, and hence do not contribute to the electrical conductivity. Units of EC are micro-Siemens per centimetre (µS/cm) or milli-Siemens per centimetre (mS/cm). Typical EC readings are <1 µS/cm for pure water, 50 - 1000 µS/cm for drinking water, ca. 50 mS/cm for seawater.

Heavy Metals

Metals of high atomic weight, like iron, copper, cadmium, mercury, lead. Many heavy metals are extremely toxic.

ISE

An Ion Selective Electrode is an electrode with a selective response to a specific ion, with no or only limited response from other ions. A pH-electrode can be considered an ISE selective for hydrogen ions (H⁺). ISEs are relatively dear and require careful maintenance and calibration.

Measuring Range

Chemical test methods and electronic instruments are limited to a certain range, in which a water constituent can be measured accurately. Outside of this range, the response of the method or instrument becomes non-linear, and hence less accurate. While a less sensitive method cannot be used to measure low concentrations, a more sensitive method can always measure high concentrations by sample dilution.

mg/L

milligrams per litre, equivalent to ppm

Mol

Atoms and molecules are extremely small and it is impossible to handle them individually. In order to work with them on an macroscopic level, the "Mol" has been defined. 1 mol equals 6.023 · 10²³. It is a unitless number (in the same way as 1 dozen = 12). While a mol always describes the same number of objects (atoms, ions, molecules), the macroscopic representation of a mol of different objects can differ very much due to the size and weight of the individual object. 1 mol of hydrogen is 1 g whereas 1 mol of uranium is 238 g. This relationship between a mol of a pure substance and its weight is called its molecular weight. Mol specific concentrations are called "molar": 1 M = 1 mol per litre, 1 mM = 1 millimol per litre.

ORP

Oxidation Reduction Potential: The ability of water to oxidise or reduce. The ORP is measured in millivolts (mV), with positive values indicating an oxidising behaviour and negative values indicating a reducing behaviour. Reference point is the "Standard Hydrogen Electrode", by definition having an ORP of zero.

Ozone

Another form of oxygen (O₃). Ozone is extremely reactive and a very powerful oxidant. It is used as a disinfectant in industrial water treatment. Due to its reactivity it cannot be transported and has to be produced on site by electrical discharges.

pH

A logarithmic scale to express hydrogen H⁺ concentrations. Low pH values (e.g. pH = 0) denote acid conditions, high pH values denote alkaline conditions (e.g. pH = 14). A pH value of 7 is regarded as neutral, as this is the pH value of pure water.

Photometry

A method of determining the concentration of a water constituent by adding one or more suitable reagents that form a coloured product with the water constituent in question. The colour intensity is dependent on the constituents' concentration, and hence can be used to determine it. A photometer is required in order to measure the colour and its intensity accurately. The less accurate visual assessment of the colour intensity is called "colorimetry".

ppm

parts per million, equivalent to mg/L

ppt

parts per thousand, equivalent to g/L (grams per litre), sometimes called ppk, 1ppt = 1000 ppm

Redox Potential

→ORP

Reference Electrode

Required for electrochemical measurements like pH, redox or specific ions (ISE). It provides a stable reference point, against which the voltage (in mV) of a working electrode is measured. Typical reference electrodes are the silver/silver chloride electrode and the calomel electrode. Most modern pH and ORP electrodes have a built-in reference electrode. An external reference electrode is necessary when using a "half-cell", like some ion selective electrodes (ISE).

R/O

Reverse Osmosis: Under pressure, water is filtered through a membrane (hollow fibre), which allows water molecules to pass quite easily, while larger atoms and molecules are rejected.

Salinity

1. Another term for →TDS

2. For seawater as defined by the Practical Salinity Scale 1978 with "standard seawater" having a salinity of S = 35.

TDS

Total Dissolved Solids are determined by complete evaporation of the water after filtration and drying of the residue at 180°C. This is a laboratory procedure and not suitable for on-site testing. A simplified method is the measurement of electrical conductivity (EC). Assuming that all dissolved solids are ions that contribute to EC and that the ionic composition does not deviate too much from a reference solution, the EC measurement can be interpreted as TDS. As such a reading is based only on dissolved salts, it is also called Salinity.

Titrimetry

A method of determining the concentration of a water constituent by adding a suitable reagent which reacts with the water constituent in question (titration). The progress of this reaction is usually monitored by an indicator that changes colour when an excess of the reagent is present. The amount of that reagent (titration solution) needed to achieve the colour change is directly proportional the concentration of the water constituent.

Total Hardness

The combined concentration of earth-alkali metals, predominantly magnesium (Mg²⁺) and calcium (Ca²⁺), and some strontium (Sr²⁺). Most natural waters have a more or less high level of hardness. The source of this hardness is limestone dissolved by water rich in carbon dioxide. Total Hardness is expressed in mg/L of calcium carbonate (CaCO₃), though calcium carbonate is not water soluble, but calcium bicarbonate is. Hardness levels range from <50 mg/L (soft) to >500 mg/L (very hard).

Water Softening

In order to remove Total Hardness, water is passed through a (cation-)ion exchanger, which is loaded with sodium (Na⁺) ions. Calcium (Ca²⁺) and magnesium (Mg²⁺) ions are absorbed by the ion exchanger, while Na⁺ ions are released into the water. Every Ca²⁺ or Mg²⁺ ion is replaced by two Na⁺ ions. As Na⁺ ions do not contribute to water hardness, the water is softened. Since the ion exchanger has a limited capacity, it requires regeneration after some time. The frequency of regeneration depends on the overall water hardness and the amount of water being used. Regeneration is carried out with a concentrated sodium chloride NaCl solution (brine). Now the sodium ions displace the Ca²⁺ and Mg²⁺ ions absorbed previously on the ion exchanger. After rinsing a new water softening cycle begins.