Making up spill disinfectant from a degraded stock bottle
A ward needs 5.0 L of 0.50% w/v sodium hypochlorite solution to disinfect the floor after a body-fluid spill. The store has one 1.0 L bottle labelled '4.0% w/v available chlorine', opened six weeks ago; a chlorine test strip shows it now contains 3.2% w/v available chlorine. There is also an unopened 200 mL bottle of 5.0% w/v sodium hypochlorite with a valid expiry date. The ward has a 1 L measuring cylinder, a 5 L plastic bucket and cold tap water. A housekeeping aide suggests adding the acid toilet cleaner to the bucket afterwards 'to remove the smell and kill germs faster'. The spill-kit instructions require the solution to be made fresh, used within 24 hours, and left in contact for 10 minutes. Two more spills are expected during the shift. The aide has already poured roughly 300 mL of the old stock into the dry bucket and is waiting for instructions.
1. Calculate how much of the opened stock (now 3.2%) is needed for 5.0 L of 0.50% w/v, and decide whether the ward can make the batch. Compare with the figure the label would give.
Read the worked answer
0.50% w/v means 0.50 g per 100 mL, so 5000 mL needs 5000 × 0.50/100 = 25 g of available chlorine. The opened stock actually holds 3.2 g per 100 mL = 0.032 g/mL, so volume = 25/0.032 = 781 mL (about 780 mL). Using the label figure of 4.0% would give 25/0.040 = 625 mL - 156 mL (25%) less chlorine than needed, which is why the test strip, not the label, must drive the calculation.
The 1 L bottle holds enough for this one 780 mL batch, leaving about 220 mL, but not for the two further spills expected. The unopened 200 mL of 5% contains only 10 g of chlorine, well short of 25 g, so it is not a substitute. The safe decision is to make this batch, order fresh stock, and prepare each later batch from a freshly tested bottle rather than stretching the remainder.
2. The aide has already put about 300 mL of stock into the dry bucket. What should be done, and why is adding the acid cleaner dangerous?
Read the worked answer
The 300 mL already in the bucket is an unmeasured start: 300 × 0.032 = 9.6 g of chlorine, not a known fraction of the 25 g required, and the bucket is now contaminated. Discard and restart. Add hypochlorite to water, measure with the cylinder rather than by eye, make up to 5 L, check with a strip that the working solution reads near 0.5%, use within 24 hours and leave 10 minutes' contact.
Mixing with acid is a chemical hazard, not a shortcut: OCl⁻ + Cl⁻ + 2H⁺ → Cl₂ + H₂O. The acid shifts the equilibrium towards chlorine gas, which is why toilet cleaner plus bleach is a recognised cause of inhalation injury. In a ward with respiratory patients, a bucket of this mixture can cause coughing, chest tightness and, in a small room, chemical pneumonitis. Never mix; ventilate and evacuate the area if it happens.
3. Why is 'make fresh, use within 24 hours' more than a bureaucratic rule? Use the chemistry of hypochlorite.
Read the worked answer
Hypochlorite is unstable: it decomposes slowly towards chloride and chlorate, and the rate rises with temperature, light and organic load. The fall from 4.0% to 3.2% in six weeks is a 20% loss, exactly the kind of decay this chemistry predicts. A solution made at 0.5% will drift below the required concentration in the same way, and a dirty bucket, mop or blood residue consumes chlorine quickly (chlorine demand).
The 24-hour rule keeps the concentration within the effective range. In practice: make only what is needed, cover the bucket, keep it away from light and heat, and check with a strip at the start of each session - a '0.5%' label does not guarantee 0.5% at the point of use.