Using the Rainbow 2n1 Swimming Pool Test Kit

Testing Swimming Pool Water with OTO (Orthotolidine)

Sales reps for DPD test kits have widely promoted the idea that OTO is carcinogenic. Maybe so, maybe not. But unless you test your cupped hand, or by squirting drops into the pool (some old time pool services actually do so), it's a non-issue. Adding 10 drops (about 5 ml) of 0.1% OTO to a 20,000 gallon home pool results in an OTO level of 70 parts per trillion. Spilling a little OTO into the pool is not something that keeps me awake at night.
You may hear about flash testing with OTO - especially if you are a service guy - and wonder. OTO reacts almost instantaneously with free chlorine, but takes a little longer to react with the various types of combined chlorine. Some people have tried to take TWO readings from an OTO test, one for free chlorine and one for total chlorine. If you are able to read the kit within 2 seconds of adding OTO, and again in 5 minutes, the difference between the two readings will give you an estimate of your combined chlorine. The first reading is your free chlorine; the second, your combined chlorine.
You may hear that DPD is more accurate than OTO, which is true, sort of. More specifically, some DPD kits can be read more accurately than the most OTO kits. On the other hand, but it's usually easier to goof up the reading with a DPD kit. In particular, most DPD tests are completely unreliable when chlorine levels are above 10 ppm; the DPD-FAS chlorine tests, such as the Taylor K-2006, are the only exceptions I know about. Use OTO first, then use DPD if you need a more accurate reading.
Oh yeah. Hydrochloric acid . . . The solution in the test kits is actually OTO dissolved in 3.5% hydrochloric acid. Don't panic. It's the same kind of acid that's in your stomach, and not lots stronger, either. But, don't drink it, either. Occasionally check out www.poolsolutions.com/testkits.html. New information will periodically be posted (or linked to) from that page.