Arrange the following parameters in the order of increasing complexity (simplest first):

Question

Arrange the following parameters in the order of increasing complexity (simplest first):

I. COD

II. Turbidity

III. BOD

IV. Total nitrogen and phosphorus

Choose the correct sequence:

1. A. II-I-III-IV
2. B. II-III-I-IV (Correct answer)
3. C. I-II-III-IV
4. D. III-I-II-IV

Correct Answer

Option B — II-III-I-IV

Detailed Solution & Explanation

The correct answer per the official JKSSB final key is II-III-I-IV.

Key Points

• Ranking the determinations by the apparatus and skill each demands, simplest first:
  • II. Turbidity — an optical reading on a nephelometer, taking seconds and requiring no reagents.
  • III. BOD — needs only an incubator and a DO probe, but demands five days and careful dilution and seeding.
  • I. COD — requires reflux digestion with hot acidic potassium dichromate, a hazardous reagent, followed by titration or colorimetry.
  • IV. Total nitrogen and phosphorus — the most involved, requiring digestion and separate determinations for several nutrient fractions.

Additional Information

• This item was among the ten reviewed after objections, and its key was revised from A (II-I-III-IV) to B (II-III-I-IV) — the change being whether BOD or COD counts as the simpler test.
• The revision is defensible on apparatus rather than time. BOD needs an incubator and an oxygen probe; COD needs a reflux assembly, concentrated sulphuric acid, dichromate and silver sulphate catalyst, all hazardous and requiring more laboratory capability.
• Judged by elapsed time the ranking would reverse, since COD takes about three hours and BOD five days — which is exactly why the question drew objections.
• The practical lesson stands regardless: COD is preferred for process control because of its speed, BOD for regulatory reporting because standards are written in terms of it.