Monsoon Floor Disinfection: Dilution, Cost and a Contaminated Bucket
You are the attendant in charge of housekeeping at a district office in Bihar during the monsoon. The sanitary supervisor leaves a written instruction: prepare a 0.5% sodium hypochlorite solution for mopping the floors, allowing 4 litres for each of the four toilets, and finish before the office opens at 10:00 a.m. The store issues 500 ml bottles of 5% w/v sodium hypochlorite costing 45 rupees each. You have one 5-litre bucket, a 1-litre measuring jug and a 10 ml measuring spoon. It is 9:05 a.m. A colleague has already poured half a bottle of acid toilet cleaner into the bucket to make it stronger, and a second attendant says he will add the bleach to that same bucket. Mopping one toilet takes eight minutes when the two of you work together, and rinsing out the bucket takes about three minutes.
1. Calculate how much bleach and water are needed, how many bottles must be drawn from the store and at what cost, and how you will measure out the quantities. Show each step.
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Step 1 - total solution required. 4 litres per toilet x 4 toilets = 16 litres of 0.5% solution.
Step 2 - dilution ratio. 5% divided by 0.5% = 10, so the mix is 1 part bleach to 9 parts water (a 1 in 10 dilution).
Step 3 - bleach needed. 16 / 10 = 1.6 litres of 5% bleach; water = 16 - 1.6 = 14.4 litres.
Step 4 - bottles and cost. 1.6 litres = 3.2 bottles of 500 ml, so 4 bottles must be drawn (whole bottles only). Cost = 4 x 45 = 180 rupees. About 0.4 litre of bleach is left over.
Step 5 - measuring. The bucket holds only 5 litres, so mix in four batches of 4 litres: 400 ml bleach + 3.6 litres water each. Measure the 400 ml with the 1-litre jug (fill to the 400 ml mark), not with the 10 ml spoon, which would need 40 separate spoonfuls and invites a counting error in a corrosive chemical.
Step 6 - the contaminated bucket. The acid already in the bucket means the first batch cannot be used at all; the bucket must be emptied and rinsed before mixing, which is why the three-minute rinse is built into the schedule.
2. What must be done about the acid cleaner already in the bucket? Explain the science in two or three sentences that a new attendant would understand.
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Action: Stop the second attendant from adding bleach. Take the bucket outside or to a well-ventilated area, pour out the acid, rinse it thoroughly with plain water, and only then mix the bleach solution. Nobody should stand over the bucket while it is being emptied, and doors and windows should be kept open.
The science, in plain words: Acid toilet cleaner and bleach react to release chlorine gas, the same poisonous gas associated with industrial accidents; even a small amount stings the eyes and throat and can damage the lungs in a closed toilet. The acid also destroys the disinfecting power of the bleach, so the mixture is both more dangerous and less useful than either product alone. Bleach must never be mixed with acid or with ammonia-based cleaners, and a dilute bleach solution should be mixed fresh because chlorine escapes on standing and in sunlight.
3. Show whether the work can be finished by 10:00 a.m. What will you do if the store can issue only two bottles?
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Time budget from 9:05 a.m.:
- Empty and rinse the contaminated bucket: 3 minutes, done by 9:08.
- Mix four batches of 4 litres (about 1.5 minutes each): 6 minutes, done by 9:14.
- Mop four toilets at 8 minutes each with both attendants working together: 32 minutes, done by 9:46.
- Leaves 14 minutes for washing hands, storing the leftover bleach in a covered container away from sunlight, and putting up a floor-wet warning sign before 10:00.
Verdict: the schedule works, but only if the four bottles are obtained immediately; the mixing cannot start otherwise.
If only two bottles are issued: two bottles give 1 litre of bleach, enough for 10 litres of 0.5% solution. I would mop the two most-used toilets with the correct 0.5% solution (4 litres each, 8 litres used) and the remaining two with detergent and plain water, then record the shortfall in writing for the supervisor rather than secretly weakening the dilution for all four. A correctly disinfected half is safer than a wrongly diluted whole, and the written note puts the stock shortage on record.