Question

what is the principal of presumptive test?

Answer 1

Presumptive tests, in medical and forensic science, analyze a sample and establish one of the following:

The sample is definitely not a certain substance.

The sample probably is the substance

Answer 2

Water to be tested is diluted serially and inoculated in lactose broth, coliforms if  present in water utilize the lactose present in the medium to produce acid and gas.The presence of acid is indicated by color change of the medium and the presence of gas is detected as gas bubbles collected in the inverted durham tube present in the medium. The number of total coliforms is determined by counting the number of tubes giving positive reaction (i.e both color change and gas production) and comparing the pattern of positive results (the number of tubes showing growth at each dilution) with standard statistical tables.
The presumptive test, is a screening test to sample water for the presence of coliform organisms.
If the presumptive test is negative, no further testing is performed, and the water source is considered microbiologically safe. If, however, any tube in the series shows acid and gas, the water is considered unsafe and the confirmed test is performed on the tube displaying a positive reaction.

The method of presumptive test varies for treated and untreated water.
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