Desinfecção no destacamento: um litro que talvez não chegue
A field kitchen at a detachment must disinfect a 5 000 L water tank and prepare 20 L of 0,5 % (m/v) hypochlorite solution for washing surfaces. The medical annexe requires a free-chlorine dose of 2 mg/L in the tank, with a minimum contact time of 30 minutes. The only stock available is one 1 L bottle labelled «hipoclorito de sódio 12 % (m/v)», which has been stored for six months in an unshaded store where daytime temperatures reach 35 °C; sodium hypochlorite is known to lose strength under those conditions. The detachment's chlorine test kit is broken. The duty officer must decide whether to proceed with the disinfection today or request resupply.
1. Calculate the volume of stock needed for the tank dose and for the 20 L of 0,5 % solution, and compare with the 1 L available.
Read the worked answer
Stock concentration: 12 % m/v = 12 g per 100 mL = 120 g/L of product.
Tank (5 000 L at 2 mg/L): 5 000 × 2 mg = 10 000 mg = 10 g. Volume = 10 ÷ 120 = 0,0833 L ≈ 83 mL.
Surface solution (20 L at 0,5 % m/v): 20 000 mL × 0,005 = 100 g. Volume = 100 ÷ 120 ≈ 833 mL.
Total ≈ 916 mL of a 1 000 mL bottle - a spare of about 84 mL, i.e. 8 %.
That spare is smaller than the error of a field measuring jug, the rinse water for the drum, and any second wash. On paper it fits; in practice it does not.
2. The storage conditions suggest the stock may have lost up to half its strength. How does this change the plan, and what would you do without a test kit?
Read the worked answer
If the effective strength has fallen to 6 % (60 g/L):
- Tank: 10 ÷ 60 ≈ 167 mL
- Surfaces: 100 ÷ 60 ≈ 1 667 mL
- Total ≈ 1 834 mL - nearly twice the bottle.
So the plan collapses: both tasks cannot be done with the stock on hand.
Without a test kit the honest position is that the dose is unknown. The tempting fix - «double it, since it has weakened» - is unsafe: if the label is still accurate, doubling gives 4 mg/L, above the residual limit for drinking water.
Judgement: do not guess on drinking water. Options in order of preference: (i) request resupply and a working test kit; (ii) if water is needed today, boil it and reserve the hypochlorite for surfaces, where an approximate strength is tolerable; (iii) never treat an untested tank as potable on the strength of a smell.
3. If only one fresh bottle can be flown in tomorrow, how would you allocate it, and what would you record?
Read the worked answer
Allocate to the drinking-water tank, not the surfaces. The tank dose needs only about 83 mL, so one fresh 1 L bottle covers it many times over and leaves stock for a verified surface batch. Washing floors and tables is a cleanliness measure; potable water is a health measure. When a scarce resource must be rationed, the priority is the one whose failure harms people fastest.
Record in the detachment log: the date and time of treatment, tank volume, product label and batch, volume added, calculated dose, contact time observed, and the fact that no residual test was possible. Note also the storage history of the expired bottle and the request to replace it.
That record matters more than the arithmetic: if illness appears later, the log is what lets a medical officer reconstruct the dose - and it is what prevents a second officer repeating the same guess.