Common Operations and Wet Chemical Methods in Environmental Laboratories - Fundamentals of Environmental Sampling and Analysis - page 145

the wine red complex and changes back to the blue color of the indicator (the end of
titration):

M 2þ þNa 2 EDTA ! 2Na þ þM-EDTAðColorlessÞ ð6 : 8Þ

Hardness is also reported as mg CaCO 3 /L, hence Eq. 6.5 can be used for the
calculation of hardness, where the volume (V) and concentration (N) represent
EDTA. Despite the simplicity of the standard procedure, a skilled analyst should
further understand two potential problems associated with this procedure and the
corresponding means to mitigate the problems. (a) The sharpness of the end point
color change increases with increasing pH. However, Ca 2þ and Mg 2þ will likely
precipitate out at high pH. A satisfied compromise of pH is in the range of 10.00.1.
(b) The presence of other metals will consume EDTA thereby overestimating the
hardness. By adding other complexing agents such as CN and S 2 , such inter-
ference can be eliminated. This is an example of how important it is to understand the
chemistry rather than just following the step-by-step cookbook procedure.

6.3.4 Oxygen Demand in Water and Wastewater
(DO, BOD and COD)

1. Dissolved Oxygen

DO in natural water originates from the molecular oxygen (O 2 ) in the atmosphere.
The atmosphere has 20.95% O 2 by volume of dry air, but DO in water is typically at
the mg/L level due to the low solubility of O 2 . Many aquatic organisms cannot
survive if the DO is depleted below 4 mg/L. In aerobic biological process, DO is
supplied by aeration whereas in anaerobic process DO is harmful to the bacteria and
must be removed. The amount of DO in natural water depends on many physical,
chemical, and biochemical factors-aeration, wind, water flow velocity, algae,
temperature, atmospheric pressure, organic compounds, salt content, bacteria,
animals, and so forth. Dissolved oxygen in polluted water is highly dependent on the
amount and types of pollutants, and the presence of bacteria therefore, it is an
important parameter for the assessment of water quality. Most often, oxygen-
consuming pollutants are organic compounds (protein, sugar, and fatty acids) that
are readily biodegraded by aerobic bacteria, although in some cases inorganic
contaminants in reduced forms will also consume a significant amount of oxygen.
There are two methods for DO analysis: the classical Winkler method
(iodometric method) and the polargraphic membrane electrode method. The
membrane electrode procedure (refer to Chapter 11) is an electrochemical method
based on the rate of diffusion of molecular oxygen across a membrane, which is the
most common method for in situ measurement. The iodometric method is a
titrimetric procedure based on the oxidizing property of DO. This iodometric test is
the most precise and reliable method for DO analysis; in some case, it is used to
calibrate the electrode method.
For the Winker method, dissolved oxygen is ''fixed'' by adding manganese (II)
sulfate immediately after a sample is collected in a 300-mL BOD bottle in the field