Application of ultra-pure water in laboratory–change of water sample and selection of water sample container
Application of ultra-pure water in the laboratory-the reason for the change of water sample
Causes of water changes
Biological effect: The metabolism of bacteria, algae and other organisms will consume some components in the water sample, produce some new components, and change the properties of some components. The content and concentration of oxygen, carbon dioxide, nitrogen-containing compounds, phosphorus and silicon are affected.
Chemical action: A chemical reaction may occur between the components of the water sample, thereby changing the content and nature of some components. For example, dissolved oxygen or oxygen in the air can oxidize ferrous iron, sulfide, etc .; polymers may depolymerize; and monomer compounds may also polymerize.
Physical effects: Light, temperature, standing or vibration, exposure or sealing and other storage conditions and container materials will affect the nature of the water sample. Such as temperature increase or strong vibration will make some substances such as oxygen, cyanide and mercury volatilize; long-term standing will cause A1 (OH) 3, CaCO3 and Mg3 (PO4) 2 to precipitate. The inner wall of some containers can irreversibly adsorb or absorb some organic substances or metal compounds.
The extent to which the water sample changes during storage depends mainly on the type of water and the chemical and biological properties of the water sample. It also depends on storage conditions, container materials, transportation and climate change.
It must be emphasized that these changes are often very fast. The samples have obviously changed in a very short time, so the necessary protective measures must be taken in all cases and analyzed as soon as possible.
Protection measures are effective in reducing the degree of change or slowing down the rate of change, but so far all the protection measures have not been able to completely suppress these changes, and for different types of water, the protection effect is also different, drinking water is very Easy to store, because it is not sensitive to biological or chemical effects, general protection measures can effectively store surface water and groundwater, but it is different for wastewater. The preservation effect is different when it is collected from different locations or the nature of wastewater is different. For example, the preservation effect of water collected from urban sewage and sewage treatment plants is different, and the preservation effect of wastewater collected from biochemical treatment plants and untreated sewage is also different.
Due to the different nature of the components in the sample, some analysis projects require separate sampling, some analysis projects require on-site analysis, and some samples can be stored for a long time.
Due to the difference in sampling location and sample composition, so far no absolute guidelines applicable to all occasions and situations have been found.
In each case, the storage method should match the analysis technique used, and this standard specifies the most general applicable technique.
Application of ultra-pure water in the laboratory-water sample preservation measures
Water sample preservation measures
1. Fill the container with water to overflow and seal
In order to avoid the vibration of the sample during transportation, and the interference of oxygen and carbon dioxide in the air on the sample components in the container and the item to be tested, in order to affect the pH, BOD, DO, etc., the water sample should be filled to the overflow and Keep sealed. However, the sample to be frozen cannot be filled with the container, otherwise the container will rupture due to volume expansion after freezing in water.
2. Cold storage
The temperature of the water sample during refrigeration should be lower than the temperature of the water sample during sampling. After the water sample is collected, it should be placed in a refrigerator or ice-water bath and stored in a dark place. It is generally refrigerated at 2 to 5 ℃. Refrigeration is not suitable for long-term storage. The storage time for wastewater is shorter.
3. Frozen (-20 ℃)
Generally, the storage period can be extended, but it is necessary to master the technology of melting and freezing, so that the sample can quickly and evenly restore the original state when it is melted. When the water sample freezes, the volume expands, and plastic containers are generally used.
4. Add protective agent (fixative or preservative)
Adding some chemical reagents can fix some components to be tested in the water sample. The protective agent should be added to the empty bottle in advance, and some can also be added to the water sample immediately after sampling.
The commonly used protective agents are various acids, alkalis and biological inhibitors, and the amount added varies according to needs.
The added protective agent can not interfere with the determination of the components to be tested. If in doubt, the necessary experiments should be done first.
The added protective agent, because its volume affects the initial concentration of the component to be measured, should be considered when calculating the results, but if a sufficiently strong protective agent is added; the dilution effect can be ignored because the added volume is small.
The added protective agent may change the chemical or physical properties of the components in the water, so when selecting the protective agent, the influence on the measurement items must be considered. If the acidification will cause solid dissolution of the colloidal components and suspended on the particulate matter, if the test item is a dissolved substance, it must be acidified and stored after filtration.
For the determination of the fixative added to certain items, a blank test must be done. For example, when measuring trace elements, the amount of the element to be tested must be determined by the fixative. (For example, acids will introduce arsenic, lead, and mercury in non-negligible amounts.)
It must be noted that some protective agents are toxic and harmful, such as mercury chloride (HgCl2), chloroform and acids, etc., and safety protection must be paid attention to during use and storage.
Laboratory ultrapure water application-filtration and centrifugal separation of water samples
Selection of water-containing containers
The choice of the material of the water sample container is the primary issue of sample preservation.
1. Requirements for containers
Select the material of the container must pay attention to the following points:
1.1 The container must not cause new contamination. General glass can dissolve sodium, calcium, magnesium, silicon, boron and other elements when storing water samples. When measuring these items, avoid using glass containers to prevent new pollution.
1.2 The container wall should not absorb or adsorb certain components to be tested. The general glass container adsorbs metal, polyethylene and other plastics to adsorb organic substances, phosphates and oils. It should be considered when selecting the container material.
1.3 The container should not react with certain components to be tested. For example, when measuring fluorine, water samples cannot be stored in glass bottles because the glass reacts with fluoride.
1.4 Dark glass can reduce photosensitization.
Laboratory ultrapure water application-cleaning rules for water sample containers
Container cleaning rules
Determine the method of cleaning the container according to the requirements of the water sample measurement project.
1. Samples for general chemical analysis
When analyzing trace chemical components in surface water or wastewater, new containers that have been thoroughly cleaned are usually used to reduce the possibility of re-contamination. The general cleaning procedure is to wash with water and detergent, then use lead-acid-sulfuric acid lotion, and then rinse it with tap water distilled water. The type of detergent used and the material of the container selected should be determined according to the components to be tested. Phosphate-containing detergents cannot be used for measuring phosphate; chromic acid-sulfuric acid lotion cannot be used for measuring sulfate or chromium. Glass containers and polyethylene containers for heavy metals are usually washed with hydrochloric acid or nitric acid (C = 1 mol / L) and soaked for one to two days and then rinsed with distilled water or deionized water.
2 Samples for determination of pesticides, herbicides, etc.
Brown glass bottles are generally used. Because plastic containers other than polytetrafluoroethylene (PTFT) will obviously interfere with the analysis, after cleaning according to the general rules (ie water and detergent-chromic acid-sulfuric acid washing liquid-distilled water), 180 ℃ in the oven Next 4h drying. After cooling, rinse with purified hexane or petroleum ether several times.
3 Samples for microbiological analysis
Containers, plugs and lids should be sterilized at this temperature and no chemical substances that inhibit biological activity, inactivate or promote biological growth will be released or produced at this temperature.
The glass container should be washed with nitric acid and washed with distilled water according to the general cleaning principles to remove heavy metal or chromate residues.
Before sterilization, sodium thiosulfate (Na2S2O3) can be added to the container to remove the inhibitory effect of residual chlorine on bacteria. (Measured by adding 0.1 mL of 10% Na2S2O3 per 125 mL container.)