Question

1. Is there a relationship between the temperature and the dissolved oxygen concentration? If so, describe
their relation.

2. What is the importance of knowing the amount of dissolved oxygen in water quality?

3. What is the standard acceptable range of dissolved oxygen (DO) set by the government agencies for
water quality?

Answer 1

(a) Yes,there exists a relationship between the amount of any dissolved gases either its oxygen or any other gas on temperature as well as its pressure.

I will consider 3 cases:

(i) If the liquid vappour mixture are ideal,then the mole fraction of any component is dependent on the temperature as well as pressure,which can be find out by Raoult's law.This has been derived by taking into consideration the thermodynamic K values,which you can say is analogous to relative volatility.

(ii) If the gas solubility in the liquid is less,which is the case of dissolved oxygen in water then we find its value with the help of Henry's law. And it can be derived by the same procedure,only the saturation pressure will be replaced by Henry's coefficient in the derivation.Hence,the mole fraction of any gas in liquid depends on its Henry's coefficient and this coefficient is function of temperature itself.

(iii) If the gas liquid mixture shows non-ideal behaviour,then the amount of liquid in equilibrium with vapour will depend not only on the temperature and pressure but also on the activity coefficient of the component in liquid.

The above statement can be understood by a common observation.You might have observed that in summer seasons most of the open drainage system smells bad.This is because with rise in temperature their solubility in water decreases and hence they escape into the surroundings.Thus,we can say that solubility of any gases depends on its temperature.

(b) It is important to know the amount of dissolved oxygen in water because as we require oxygen for breathing,aquatic organism require oxygen for their survival.Also there are lots of organic matter which are present in the waste water and can be harmful.So they require oxygen for their degradation or decomposition into biomass and make the water suitable for drinking or other purposes.If the water is stagnant ,the amount of Dissolved oxygen is very less while for running water it is very high because of aeration.

(c) The acceptable range for the dissolved oxygen in healthy water should be 6.5-8mg/L or little greater than this will also be okay but not less.