Measuring Dissolved Oxygen (DO)

The concentration of dissolved oxygen in water can be measured using two methods.  The easiest method, which requires no calculations, is using a dissolved oxygen meter.  In contrast, the modified Winkler dissolved oxygen determination requires several calculations which will be outlined below.

Dissolved Oxygen Meter

A DO meter consists of a probe filled with potassium chloride and covered with a semi-permeable membrane, a wire carrying electricity, and two electrodes.  When the probe is placed in the water, the potassium chloride attracts oxygen, just as hemoglobin does in our blood, and pulls the oxygen out of the water and into the probe.  The semi-permeable membrane allows oxygen to pass into the probe but does not allow water in.  

The probe is connected to a wire through which DC electricity flows (the red dashes in the drawing above.)  There is a gold and a silver electrode on the wire, one on either side of the probe.  You can think of these electrodes as devices which measure the amount of electricity passing through the wire at that point.  

Where the probe joins the wire, oxygen mingles with the electricity.  Oxygen is not very ionized, meaning that it does not have a negative charge as electricity does, so the oxygen dilutes the current at the electrode beyond the probe.  The meter measures the difference in current between the two electrodes and uses that difference to determine the amount of oxygen in the water.  

When using a DO meter, the meter must be properly cared for.  The membrane must be kept wet and have potassium chloride solution within it at all times.  The electrode should also be maintained.  The electrode assembly can be cleaned by dipping the electrode tip into ammonium hydroxide.  

The meter must also be calibrated before use to ensure that the reading is accurate.  The meter can be calibrated using water with a known concentration of DO at at set temperature.  This can be done by filling a biological oxygen demand (BOD) bottle halfway full of distilled water.  The water is shaken vigorously to saturate the water with air. Then the probe is placed in the bottle and the reading is adjusted to the known concentration of DO in the bottle. Alternatively, the meter can be calibrated using the Winkler method, which involves adding chemicals to the water and forming a precipitate.  

Modified Winkler Dissolved Oxygen Determination

In the modified Winkler method, various chemicals are added to the water sample.  Then PAO or Sodium Thiosulfate is titrated into the sample.  

The dissolved oxygen concentration can be calculated based on the amount of titrant added to the sample.  The formula used is:

Calculation for DO

BuretFinal is the reading of the buret after titration and BuretStart is the reading of the buret at the beginning of titration.  N is the normality of the solution used to titrate the sample.

If a 200 mL sample and a 0.025 N titrant are used, then the formula can be simplified to:

DO (mg/L) = Amount of Titrant Used (mL)

Example: The operator titrates a 200 mL dissolved oxygen sample.  The buret reading at the start of the titration was 0.0 mL.  At the end of the titration the buret read 7.4 mL. The concentration of the titrating solution was 0.025 N.  What is the D.O. concentration in mg/L?