Sodium hypochlorite is the most widely used disinfectant in nursery work and the most widely misused. The chemistry is simple. Almost every failure comes from the arithmetic, the storage or the record — not from the product.
The unit everybody gets wrong
Hypochlorite is sold by available chlorine, expressed as a percentage. Dilutions are specified in parts per million. Those are the same quantity in different clothes, and the conversion is the whole game.
A 12 % solution contains 120 grams of available chlorine per litre.
To make 10 litres of a 1 000 ppm working solution, you need 1 gram per litre, so 10 grams in total.
10 g ÷ 120 g/L = 0.083 litres — 83 millilitres.
That is the entire calculation. Write it on the wall of the chemical store, because the failure mode is not a supervisor who cannot do arithmetic; it is a supervisor who was never given the number and estimates a capful.
The dilutions that actually get used
| Application | Target | From a 12 % concentrate, per 10 L | Contact time |
|---|---|---|---|
| Tool dip, between plants | 1 000 ppm | 83 ml | 30 seconds or more |
| Footbath, refreshed daily | 1 000 ppm | 83 ml | on entry |
| Bench and surface wash | 500 ppm | 42 ml | 5 minutes or more |
| Irrigation line shock | 200 ppm | 17 ml | 60 minutes or more |
| Empty-house terminal clean | 2 000 ppm | 167 ml | 10 minutes or more |
From a 10 % concentrate, multiply each volume by 1.2.
Contact time is not a formality
A dip is not a rinse. Chlorine kills by oxidation, and oxidation takes time. A blade passed through solution and immediately used has been wetted, not disinfected. Thirty seconds is the working minimum for a tool dip, which in practice means two blades in rotation rather than one — one in solution while the other is cutting.
The same logic applies to a footbath that has dried out by mid-morning, and to a bench wiped and immediately loaded.
Your drum is weaker than it says
Hypochlorite decomposes. Heat accelerates it, light accelerates it, and time does it anyway. A drum at 12 % on the day it was made is meaningfully weaker after months in a hot store, and nothing on the outside of the drum tells you by how much.
Three consequences:
Date every drum on receipt. Not the manufacture date if you can help it — the date it arrived, next to the batch code.
Rotate rather than stockpile. Buying a year’s supply to save on freight is a false economy if half of it is out of specification when you use it. We work to a 90-day shelf life at 25 °C or below, stored out of direct light, and we would rather deliver more often than have you dilute from a drum you cannot trust.
Store it cool and dark. A shaded, ventilated store is worth more than any amount of care at the dilution stage.
What an auditor will actually ask for
Not “is the house clean”. They will ask for:
- the written procedure, with a strength and a contact time for each application
- the safety data sheet, current revision
- evidence of the concentration you diluted from — a certificate of analysis or equivalent
- refresh records for footbaths and dips
- evidence that the product is authorised for use in the country of production
If you cannot produce the fifth, the rest does not help you. Anything used as a disinfectant or plant protection product needs to be registered for use in Kenya, and an informal purchase cannot be evidenced.
Two practical safety points
Hypochlorite and acid produce chlorine gas. Never mix it with an acid cleaner, and never store the two where a spill could bring them together.
It also degrades many metals and some plastics. Use compatible containers — high-density polyethylene is standard — and rinse tools after the contact time rather than leaving them to soak all day.
The shortest version
Work in available chlorine, convert once, and print the conversion. Give every application a strength and a time. Date the drums and rotate them. Keep the paperwork as you go, because none of it can be reconstructed later.