Dissolved oxygen (DO)

What is dissolved oxygen (DO)? 

Dissolved oxygen refers to the amount of oxygen level that present in the water or liquid sample. Dissolved oxygen level is very important to the aquatic life in a healthy ecosystem. Too much of oxygen may create algae bloom provided the high nutrient load present in the water, however too low of oxygen amount that present in water will make the fishes suffocating and eventually die. 

How do we measure DO in the water?

METHOD 1: Winkler titration method - conventional method

We can use Winkler titration to measure DO amount in the water sample that has been took and prepared manually from the site into water bottle. The Winkler titration method measures the DO amount in the water sample precisely, this is a method developed by Hungarian analytical chemist named Lajos Winkler in 1888. This method adding chemicals to the water sample to form an acidic solution by creating reaction with the oxygen that present in the water. Then we will add in the neutralizing agent to perform neutralization with titration and the amount of neutralizing agent required indicate the amount of the oxygen level in the water sample. Below is the diagram to show the Winkler titration in step by step. 

Although Winkler titration can deliver highly accurate result, but this method is laborious, time-consuming and highly susceptible to the interference. 

METHOD 2: Using in-situ DO meter

Alternatively, we can use DO meter that can provide instant result instead. A lot of researchers they prefer to use in-situ DO meter because the amount of oxygen in water sample may fluctuate across the time. The amount of oxygen may varies at different water level. The deeper the water level, the lesser the oxygen amount that present. The creation of in-situ DO meter ease the life of a chemist as winkler titration is a tedious procedure to perform DO measurement of water sample. Nowadays, the in-situ DO meter could deliver and perform almost similar result of DO as compared to Winkler titration and hence in-situ DO meter is the first choice of researchers. 

In in-situ DO meter, we do found two major types of technology that present in the market. The first type is called polarographic (aka Clark electrode) method. This is a method invented by Dr Leland Clark in 1956. In 1965, the first biological oxygen monitor is invented and this is a breakthrough technology in medical field. This technology enabled physicians to carry out open-heart surgery for the first time as the real blood oxygen measurements could be recorded during the operation! 

After years of study, another advanced technology called optical or luminescent technology in DO meter has took over the place of polarographic technology. Some researches has chose the DO meter with optical technology due to the easier maintenance and faster warm up time. You may see the differences in between polarographic electrode and optical technology as below. 

Comparison	DO meter with electrode	DO meter with optical
Technology	Polarization technology (old method)	Optical technology (advanced method)
Calibration frequency	Daily	Weekly
Warm up time	10 – 15mins	NO warm up time
Flow dependent	Flow dependent; need to stir manually while using the meter to get actual reading	Flow independent; no need to stir manually while using
Consumables	Membrane cap, filling solution	Membrane cap only
Maintenance	-change the membrance cap -refill the solution in cap -check the solution in cap from time to time -polish the electrode from time to time	- change the membrane cap once the coating of cap wear off; usually can last long up to 12 months or more, depends on usage
Measurement rate	Faster than optical method Estimated 8-25 seconds per sample, but subject to the sample condition	Slower than electrode method Estimated 40 seconds per sample, subject to the sample condition