Chemistry of water -Part 13-

In the previous article, we talked about collecting water samples before analysis, where we touched on the steps that must be followed before collecting samples, in addition to the materials used to manufacture sampling bottles and how to choose the appropriate material according to the type of analysis to be conducted. Today, we will discuss the method of collecting water from various sources, and then we will discuss the methods of preservation and storage of water samples.

  • The method of collecting the sample depends on the source of the water that we want to analyze. If we want to analyze tap water, for example, we must let the water run for ten minutes. And we take small samples from several points and mix them together to get the typical sample if the water source is a lake; on the other hand, it is crucial to collect several samples throughout time and space and to process them independently. As for open water basins, closed water tanks, and river water, the flask is immersed in the water medium away from the sides, and at a certain distance from the surface, while avoiding stirring.

Preservation and Storage of Water Samples:

Depending on the circumstances, removing a water sample from its natural environment may causes more or less substantial alterations. On the time scale we are working on, some can be regarded as stable, but others, like temperature, conductivity, pH, and dissolved gases, as well as nitrates and sulfates, change rapidly. Therefore, when the analysis requires leaving the water sample for a certain period of time before performing it, certain substances are added to the sample to preserve it.

The time period between taking the sample and conducting the analysis depends on the type of the required analysis. Some analyses are conducted directly, and some need to leave the sample for a certain period of time. As that, changes in the solution can also be caused by contact with air and decompression.

Conductivity changes whenever chemical balances are altered, which affects the relative proportions of dissolved elements (The overall conductivity of a solution is influenced by all ionic species). Also, the equilibrium constants of the elements in suspension change in response to changes in temperature; numerous chemical processes take place in order to establish new equilibria at the changing ambient temperature, these processes can cause salt to precipitate and encourage gases dissolution; a low temperature, around 4°C, however, stops the reactions from moving forward. As the water temperature rises, the CO2 in solution tends to escape even more; consequently, the pH may alter as a result of carbonate precipitation; and the activity of microorganisms can lower nitrate and sulfate levels.

The basic requirements for storing water samples pending examination are shown in the following table:

[Created by using Word Microsoft]

  • Note:
    Aqueous samples intended for the analysis of sodium, potassium, magnesium and calcium do not require any additions or special conditions to be preserved.



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