Environmental Engineering Reference
In-Depth Information
21. To prepare EDTA solution for hardness measurement, 8.54 g EDTA disodium salt
(C
10
H
14
N
2
O
8
Na
2
2H
2
O, MW¼372.2) was dissolved in 1000 mL DI water. After
calibration, its concentration was measured to be 22.82 mmol/L. A 50-mL water sample
consumed 6.06 mL EDTA solution. (a) What is the actual concentration of EDTA in
mmol/L compared to the measured concentration of 22.82 mmol/L? (b) What is the
hardness of this water sample in mg CaCO
3
/L?
22. Write all chemical reactions involved in the measurement of DO using Winkler
(iodometric) method.
23. Write the chemical reactions for the measurement of (a) COD, and (b) chloride using
Mohr method, (c) residual chlorine using iodometric back titration, and (d) cyanide.
24. During the reflux process of COD measurement, if the solution turns into green color,
what causes this and what is the solution to this problem?
25. Define the difference between chloride, free chlorines, and combined chlorines. How
they differ in their disinfection capacity?
26. Explain what types of interference (positive or negative) will be caused in the presence
of reducing agent or oxidizing agent for DO measurement?
27. List three substances that interfere with the Winkler method, and indicate which
modification would be used to overcome each interference?
28. (a) Explain why the conversion factor in Eq. 6.13 is 8000. (b) If 200 mL sample was
used for dissolved oxygen (DO) measurement, it requires 8.6 mL of 0.0275 N Na
2
S
2
O
3
solution. What is the DO in mg/L?
29. A standard procedure by APHA's method was followed to determine DO in water. The
volume of 0.025 M Na
2
S
2
O
3
consumed was 6.8 mL in titrating 200-mL solution. What is
the DO in mg/L?
30. Glucose is oxidized according to: C
6
H
12
O
6
þ6O
2
! 6CO
2
þ6H
2
O. It can be used as a
standard of COD because its theoretical COD can be calculated from its known
concentration. The oxidation reduction reaction involved in COD measurement is:
6Fe
2þ
þCr
2
O
2
7
þ14H
þ
! 6Fe
3þ
þ2Cr
3þ
þ7H
2
O
(a) What is the theoretical COD value of a solution containing 0.50 g/L glucose
(C
6
H
12
O
6
; MW¼ 180) (Hint: 1 mole glucose consumes 6 moles of O
2
)?
(b) Why the equivalent weight of K
2
Cr
2
O
7
ðMW¼ 294
2Þ is 294.2/6¼49.03?
(c) To prepare 500 mL 1.0 N K
2
Cr
2
O
7
, how many grams of K
2
Cr
2
O
7
are needed?
(d) List
:
three major sources of error in COD measurement. How might
they be
eliminated?
31. Potassium hydrogen phthalate (abbreviated as PHK, HOOCC
6
H
4
COOK, MW¼204.23)
is always used as the COD standard for its measurement. To prepare 1 liter of a PHK
solution containing 500 mg/L COD, how many grams of PHK need to weigh. The
oxidation of PHK is assumed: 2HOOCC
6
H
4
COOKþ15 O
2
! K
2
Oþ16CO
2
þ5H
2
O
(Hints: 2 mole PHK consumes 15 moles of O
2
).
32. Describe the difference between BOD and COD in terms of the chemicals each method
represents.
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