Q.83
In a phenol coefficient test for determining the efficacy of a disinfectant X, the maximum effective dilution for X and phenol was found to be 1/450 and 1/90, respectively. Calculate the phenol coefficient for X.
Correct Answer: (B) 5.0
The phenol coefficient measures a disinfectant’s efficacy relative to phenol. It’s calculated by dividing the maximum effective dilution of the test disinfectant (X) by that of phenol: 450/90 = 5.0. A value >1 indicates X is more potent than phenol under test conditions.
Option Analysis
Option (A) 10.0
Incorrect. This would require X’s dilution to be 900 (10×90), but data shows 1/450, yielding 450/90=5.
Option (B) 5.0
Correct. Direct calculation: phenol coefficient = (dilution factor of X) / (dilution factor of phenol) = 450 / 90 = 5.
Option (C) 1.0
Incorrect. Equals phenol’s potency (dilution ratio=1), but 450>90 shows X is 5× stronger.
Option (D) 0.2
Incorrect. Inverse calculation (90/450=0.2) reverses the formula; coefficient uses disinfectant/phenol ratio.
Introduction to Phenol Coefficient Test
The phenol coefficient test evaluates disinfectant strength against phenol standard, vital for GATE Life Sciences microbiology. Here, disinfectant X’s maximum effective dilution (1/450) exceeds phenol’s (1/90), yielding coefficient=5—X is 5× more effective. Master this for exam success.
Phenol Coefficient Formula & Calculation
Phenol coefficient test formula:
Coefficient = (Highest dilution of test disinfectant killing microbes in 10 min, not 5 min) / (Same for phenol)
For Q.83:
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X: 1/450 (dilution factor=450)
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Phenol: 1/90 (dilution factor=90)
450 ÷ 90 = 5.0
Example: If phenol kills at 1/100 and X at 1/500, coefficient=500/100=5 (X superior).
Test Procedure Overview
In phenol coefficient test (Rideal-Walker/AOAC methods):
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Culture Salmonella typhi (or S. aureus).
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Test serial dilutions (e.g., phenol: 1/90-1/115; X: 1/400-1/500).
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Incubate 5/10 min at 20°C; subculture to check sterility.
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Highest dilution killing in 10 min (not 5) determines value.
Interpretation Guide
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>1: Disinfectant better than phenol (e.g., 5.0 here).
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=1: Equal potency.
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<1: Inferior (use cautiously).
Safe use dilution ≈ 20 × coefficient for practical disinfection.
GATE Relevance & Tips
Phenol coefficient test questions test microbiology control of growth. Practice: Recognize dilution notation (1/450=450 parts diluent:1 disinfectant). Common trap: Reversing formula. Links to disinfectants like quaternary ammonium (high coefficients).
