Oceanography

4. Procedure

The seawater sample for which the dissolved oxygen content is to be determined is slowly siphoned out into a BOD bottle with the help of a rubber tube. When the water sample is allowed to overflow in this BOD bottle the following reagents are added. One ml of manganous solution is added first to the water sample keeping the tip of the pipette just below the surface of the water. In the same way, one ml of alkaline iodide is added and the bottle is carefully stoppered. The bottle is tilted upside down so that Mn(OH₂) reacs with the dissolved oxygen present in the sample to give a slightly brown coloured Mn(OH_3).  Extra care should be taken not to permit the entry of any air bubble inside the BOD bottle. The estimation of dissolved oxygen may be done immediately or in a day or two. If the analysis is not carried out immediately, the mouth of the BOD bottle should be thoroughly sealed with wax and the bottle is kept in a water bath (at room temperature) so as to avoid the impact of increasing temperatures.

At the time of titration, one ml of sulphuric acid is slowly added to the sample and the bottle is stoppered. The bottle is tilted side wise, so that, all the precipitates are dissolved and the solution turns brownish due to the liberarian of iodine.

The thiosulphate solution of the given normality (0.01N) is then taken in the burette and is kept ready for titration. 50ml of sample from the reagent fixed BOD bottle is taken in a conical flask and is titrated against the thiosulphate till a hay yellow colour appears. At this point, one ml of starch is added to this solution and it turns blush indicating the presence of iodine. The titration is continued till the blue colour disappeard. The titration is repeated to get concordant values. The oxygen content of the water samples is calculated using the following formula:

O₂ ml/1 = CF x N x E x 0.698 x 1000 x V _t    

                  -----------------------------------------            

                                         Vs

Where

CF=correction factor value

correction factor value = Total volume of the stoppered bottle

Total volume of stoppered bottle-2

N= Normality of thiosulphate (0.01N)

E=Equivalent weight of O₂ (8)

0.698 = To convert mg/l into ml/l.

1000= To convert the value to 1000ml or 1 litre

V _t =Titre Value

Vs = Volume of sample taken for titration

If the oxygen content is to be calculated in mg/l, then the conversion factor value (0.698) of the above formula should be avoided. In the case of brackishwater and sea water samples, it is customary to express the value of oxygen content in ml/l and in freshwater samples, mg/l or ppm.

The above formula is derived by treating the dissolved oxygen and thiosulphate as reactants. The reaction and the calculation are as given below.

V₁ N₁ (Dissolved oxygen) = V₂N₂ (Thiosulphate)

Since V₁ = Vs, V₂=V _t

N₂ = 0.01 and N₁ has to be calculated.

Thus, N₁ = V _t x 0.01  

                       Vs

Normality x Equivalent weight = Weight in g/l

Thus, Wt of the dissolved oxygen g/l = V _t x 0.01 x 8

                                                                      Vs

Weight of dissolved oxygen (mg/l) = V _t x 0.01x 8 x 1000

                                                                         Vs

In order to convert the dissolved oxygen content from mg/l to ml/l, the above formula is multiplied by 0.698.

Thus,

Dissolved Oxygen (ml/l) = V _t x 0.01x 8 x 1000 x 0.698

                                                            Vs

Correction factor (CF) is included in the above formula to rectify the defect caused by the addition of MnSo₄ and alkaline iodide solution by replacing 2ml of the water sample.

The total volume of the stoppered bottle can be measured by filling the bottle with tap water and then stoppering it to remove the excess water. The water remaining in the bottle is measured by pouring the contents in a measuring cylinder.

The oxygen solubility of water sample is usually calculated by using the standard values if the salinity and temperature of the water samples have already been estimated. For calculating the oxygen solubility at different combinations of salinity temperature refer Table 11 (Vide Annexures).

By calculating the difference between the dissolved oxygen content and the oxygen solubility values, Apparent Oxygen Production (AOP) and Aparent Oxygen Utilization (AOU) can be calculated. These two parameters function as reliable indices of oxygen production due to photosynthesis and oxygen utilization due to community respiration.