Water Sample handling and preservation

Handling and Preservation of Water Samples

Waters are susceptible to change as a result of physical, chemical or biological reactions which may take place between the time of sampling and the analysis. If precautions are not taken, at the time of sampling, changes may occur rendering analytical data unrepresentative.

Changes may occur due to:

consumption of certain constituents by bacteria, algae etc.,
certain compounds being oxidised by the dissolved oxygen in the sample,
precipitation from the liquid, eg. calcium carbonate, aluminium hydroxide,
loss into the vapour phase,
absorption of carbon dioxide from the air, changing the pH value,
adsorption of metals and certain organic compounds on to the container's surface,
depolymerisation of polymerised products and vice versa.

These changes will be affected by the storage temperature, exposure to light, the nature of the container used and the time between sampling and analysis. In adverse conditions, changes can occur in just a few hours.

Fortunately, preservatives are available to prevent these changes. However, it must be borne in mind that methods of preservation are less effective with heavily contaminated samples than with those with light contamination.

General Considerations

Always fill sample containers to the brim and stopper them so that no air is left above the sample.

Use an appropriate container. For example. polyethylene bottles should not be used for hydrocarbons, since adsorption on to the bottle's surface is likely to occur.

Glass containers are suitable for most determinations. Brown bottles should be used since this will reduce photosensitive reactions to a considerable extent.

Containers must be clean. Whilst this may seem obvious, scrupulous cleanliness is important due to the low detection levels now being adopted.

Samples should be kept at a temperature below that at the time of filling. Cooling between 2 degrees and 5 degrees (ie. in melting ice, refrigerator or cool bag with ice packs) is adequate. It is not suitable for long-term storage.

Suspended matter, sediment, algae and other micro-organisms should be removed at the time of sampling by filtration or centrifuging or immediately on receipt at the laboratory. Filtration should not be carried out if the filter is likely to retain one or more of the constituents to be analysed.

Generally filtration is achieved by use of 0.45 micron filter paper.

The following table gives an indication of the sample container and preservatives for a variety of parameters.

Determination	Sample Container	Preservative	Filtration Required?
Acidity or Alkalinity	Plastic or Glass	None	Yes
BOD	Plastic or Glass	None	No
Boron	Plastic	None	Yes
Bromide	Plastic or Glass	None	Yes
Carbon, Total Organic	Plastic or Glass	HCl to pH <2	No
Carbon Dioxide	Plastic or Glass	None	No
COD	Plastic or Glass	H₂SO₄ to pH <2	Yes
Chloride	Glass	None	Yes
Chlorine (residual)	Plastic or glass	None	No
Chlorine dioxide	Plastic or glass	None	No
Chlorophyll	Glass	None	No
Chromium VI	Plastic	None	Yes
Colour	Glass	None	Yes
Conductivity, pH	Glass	None	No
Cyanide, Total	Plastic	NaOH to pH>12	Yes
Dissolved Solids	Glass	None	Yes
Fluoride	Plastic	None	Yes
Halogenated Solvents	Glass	None	No
Hardness	Glass	HNO₃ to pH <2	Yes
Hydrazine	Glass	HCL to 1M	No
Iodine	Glass	None	No
Metals (General)	Plastic	HNO₃ to pH <2	Yes
Nitrogen Ammoniacal	Glass	H₂SO₄ to pH <2	Yes
Nitrogen Nitrate	Glass	None	Yes
Nitrogen Nitrate & Nitrite	Glass	H₂SO₄ to pH <2	Yes
Nitrogen Nitrite	Glass	None	Yes
Nitrogen Organic	Plastic or Glass	H₂SO₄ to pH <2	Yes
Odour	Glass	None	No
Oil and Grease	Glass	H₂SO₄ to pH <2	No
Organo Phosphorus	Glass	None	No
Ozone	Glass	None	No
PAHs	Glass	None	No
PCBs	Glass	None	No
Pesticides	Glass	Ascorbic Acid if residual Chlorine present	No
Phenols	Glass	H₂SO₄ to pH <2	Yes
Phosphate, Sulphate	Glass	None	Yes
Sulphide	Glass	Zinc Acetate & NAOH to pH <12	Yes
Surface Active Agents (Ionic)	Glass	H₂SO₄ to pH <2	No
Surface Active Agents (Non-Ionic)	Glass	40% Formaldehyde to give 1% solution	No