Taylor R-0870 test powder (crystals) - hard to measure for anyone?

[waterbear]

The amount of DPD is not critcal. You are doing a titration. It works like this:
1) DPD is added to the sample. If any free chlorine (or free bromine if you are testing bromine) is present it oxidizes the DPD and forms what is known as Wurster Dye which is a magenta colored molecule. If there is a really high level of chlorine prenent then this Wurster Dye is futher oxidize to a colorless imine (the bleachout that is seen with DPD) adding more DPD power will shift this chemical reation to reform the Wuster Base (only possible with FAS-DPD testing and not with DPD testing because with DPD testing we are looking at how deep the pink color is, i.e. the AMOUNT of Wurster Base formed in our sample. With FAS-DPD testingi we just need it as an indicator so we know when we have neutraized all the FC in the sample)
2)we then titrate with a reducing agent (Ferric Ammonium Sulfate--FAS) which 'neutalizes' the chorine in the sample. THIS is the quantitative part of the test. The amount of FAS used is important, not the amount of DPD. Once all the FC is neutralized and gone then there is non left to react with the DPD and the Wurster Dye is gone also--the sample turns colorless)
3) since this test is specific for hypochorous acid and hypochlorite ion (free chlorine or the equivalent acid and ion for bromine or iodine ) we can then determine the amount of other oxidizer present (usually our chloramines or bromamines but also this test will show MPS and ozone present) by adding a potassium iodide solution. This is easily oxidized by all the above oxidizing agents (chloramines, bromamines, MPS, ozone) and forms the iodine equivalents to hypochlorous acid and hypochlorite ions (Free Iodine) that once again form Wurster Dye and turn our sample pink again. If there is no oxidizer present the sample stays clear and we have no CC (or MPS or Ozone interference). If it turns pink we titrate again to reduce the free iodine back into iodide ions and the sample turns from pink to colorless and we have our CC reading.

So you see, the amount of DPD is not critical. You just need enough to determine if there is FC (or Free Bromine or Free Iodine) present in the sample and then you convert that into either chloride, bromide, or iodide by titrating witth a reducing agent (FAS). When all the FC (or other free halogen) is all converted there is no longer any left to react with the DPD and form Wurster Dye and the sample is colorless again. The amount of FAS needed to do this is what measures your FC (or Free Bromine)

To determine the CC you convert it into Free Iodine and titrate again. You do this by adding potassium iodide (which is very easily oxidzed into free iodine). The FAS reduces the free iodine back into iodide and, once again, the amount of FAS needed determines the concentration. The DPD is only a means to create the Wurster Dye, which is our indicator. The AMOUNT of Wurster dyd created is determined by the amount of Free halogen present and not by the amount of DPD, which is added in excess (even 1 scoop is excess as long as the pink color forms!)
Hope this explains it.

(Yeah, I'm the other chemistry geek in the forum!)