Ultrapure Water Applications – Reasons for Water Sample Changes

Biological effects: The metabolism of bacteria, algae, and other organisms consumes certain components in a water sample, producing new components and altering the properties of others. Biological effects can influence the content and concentration of analyzable parameters such as dissolved oxygen, carbon dioxide, nitrogenous compounds, phosphorus, and silicon.

Chemical effects: Chemical reactions can occur between the components of a water sample, altering the content and properties of some components. For example, dissolved oxygen or oxygen in the air can oxidize ferrous iron and sulfides; polymers may depolymerize; and monomeric compounds may polymerize.

Physical effects: Light, temperature, static or vibration conditions, exposure or sealing, and container materials all affect the properties of a water sample. For example, increased temperature or strong vibration can cause substances such as oxygen, cyanide, and mercury to volatilize; prolonged static storage can cause precipitation of Al(OH)3, CaCO3, and Mg3(PO4)2. The inner walls of some containers can irreversibly adsorb or absorb organic matter or metallic compounds.

The extent to which water samples change during storage depends primarily on the type of water and its chemical and biological properties. It also depends on factors such as storage conditions, container material, transportation, and climate change.

It must be emphasized that these changes are often very rapid. Samples may change noticeably within a short period, therefore, necessary protective measures must be taken in all circumstances, and analysis should be performed as quickly as possible.

Protective measures are effective in reducing the degree or slowing the rate of change, but currently, no protective measure can completely inhibit these changes. Furthermore, the protective effect varies depending on the type of water. Drinking water is easily stored because it is not sensitive to biological or chemical effects. General protective measures are effective for surface water and groundwater, but not for wastewater. The preservation effect also varies depending on the location of the sample or the nature of the wastewater. For example, the preservation effect differs between urban sewage and wastewater treatment plant water, as well as between wastewater from biological treatment plants and untreated sewage.

Because of the varying properties of the components in samples, some analytical procedures require individual sampling, while others require on-site analysis, and still others allow for long-term sample preservation.

Due to the differences in sampling locations and sample composition, no absolute rule has yet been found that applies to all situations.

In all cases, the storage method should be compatible with the analytical technique used. This standard specifies the most generally applicable techniques.