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How to check the bacterial contamination of water by testing for sulphide ions? What is the mechanism of action of this test? The query has been assigned to me as part of my chemistry project at school. Aparna firstname.lastname@example.org
The sulphide ion test for bacterial contamination is good. In surface waters, hydrogen sulphide is formed under oxygen deficient conditions. Hydrogen sulphide is also produced from the decomposition of sulphur containing organic compounds. The concentration of sulphide ion becomes significant only at pH 10 or above.
Under acidic conditions, the concentration of hydrogen sulphide predominates. Hydrogen sulphide is a weak acid, which ionizes to yield hydrosulphide (HS_-) and sulphide (S2) ions. Hydrogen sulphide is extremely toxic to fish. Concentrations of total sulphide as low as 0.01 mg/L make the water unfit for uninhibited fish culture. Hydrogen sulphide also reduces the aesthetics value of the water body due to foul color. The bacterial contamination can be tested using H2S strip .
The water to be tested is to be filled in the bottle having H2S strip, and keep it in the incubator ;at 35degree centigrade for 14 to 16 hours. If the strip becomes black then the water is not fit for human consumption as it is contaminated and bacteriologically not potable. This is the simple and quick method as it needs no sterilization.
Department of Mines and Geology
The presence of pathogenic bacteria in drinking water is due to human or animal waste contaminating the water source. These can lead to various gastro-intestinal diseases such as typhoid, cholera. It is difficult & expensive to test for these disease causing bacteria routinely in a water source. Hence, the tests are carried out for the presence of indicator organisms, which are always present in faeces in large numbers.
Their presence shows that drinking water can also have pathogenic bacteria. One group of such indicator organisms are sulphide producing bacteria. These bacteria are present in faeces and their presence indicates that drinking water may have pathogens also.
The principle of this test is the reduction of the sulphate (which is a component of the growth medium) to sulphide by sulphide forming bacteria and the reaction of sulphide with iron to form iron sulphide, which is insoluble and black in colour. The formation of black colour is taken as an indication of bacterial contamination of drinking water.
this is great, school teacher explaining water and its quality etc. In my earlier answers on drinking water quality had given some basic concept on water-quality which all can remember easily. I had explained C A T S which reads for:
C- clean and clear,
A- Alkaline about 7.5
T- taste appealing and pleasant
S- sterile, as being ascertained for your case usingH2S strip-test.
While doing this test to explain students,
1. pl do the test with good quality water and observe no discoluration even after say 12 hours,
2. also have some drain water sample and seen that the colur changes within an hour.
3. any others having medium contamination becomes discolured between 1 to12 hrs.
pl do a complete instructional cum demo exercise.
These concepts form activity exercise under EVS-Environmental Studies lessons.
In senior classes the demo can be dealt with remediation of water using simple Aquaguard and with UV etc.
all the best, with wellwishes from VigyanVijay,
Er. Ajit Seshadri,Head- Environment, The Vigyan Vijay Foundation
Sources and origin of fuels. 20 pgs project!
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