Question

Answers are in your own words.

1/ The Spectrophotometer

--> Compare and contrast absorbance and transmittance.

--> What is the purpose of the "blank"?

2/ Bacterial Growth and Optical Density:

--> What is the relationship between turbidity and optical density? How might this change after 24 additional hours of incubation?

Answer 1

Absorbance is the amount of light absorbed by a particular species whereas transmittance is the amount of light transmitted by a particular species.

Blank is used to set spectrophotometer at a value of zero. This is because blank does not have any absorbing species and therefore it's absorbance value should be zero. If the absorbance value of blank is not zero, then there is some problem in the spectrophotometer. If this is the case, then we subtract the value of absorbance of blank from our absorbing species.

Turbidity is the opaque appearance of a solution due to the presence of suspended molecules in it. For example a turbid solution of soil or bacteria.

There is a direct relationship between turbidity and optical density. If the turbidity of the solution increases, then the optical density also increases. This is because an increase in turbidity leads to an increase in the number of absorbing species and therefore an increase in the value of optical density.

This will become constant after an incubation of 24 hours because the bacterial culture will reach decline phase of its growth curve because of the accumulation of toxins and Exhaustion of space. There will be dead bacterial cell in the solution and no increase in the number of cells will be seen. But these dead bacterial cells will also absorb light because absorbing molecules are still present in them, but because they will not divide, there will be no further increase in the value of optical density.

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