The color that you're getting is odd. Maybe something related to your water is causing interference. Does your water contain manganese or do you use a manganese greensand filter?
Nothing bad in the well water that we know of; no filters on the fill water
Get some bottled water and see if the 0870 causes the same reaction.
distilled water=light purple; Then, add 1 drop of bleach to a quart of the bottled water, mix well and test that to see if the test works.
Not enough distilled water
Also, try an oto chlorine test on your pool water to see what that shows.
Yellow, 2-3 ppm
And try this: Fill test vial to 10 ml with tap water, add 1 drop r-0007 (thiosulfate), swirl for 10 seconds and then add r-0870 to see if it still goes purple.
Purple, cloudy
Then try 10 ml tap water, add 3 drops r-0012 (edta), swirl for 10 seconds then add the 0870 powder to see if you're getting the purple.
Purple, cloudy
Then try 10ml tap water, 5 drops r-0003 (iodide) swirl for 10 seconds and then add the 0870 powder to see if you still get purple.
Purple, cloudy
Then try 10 ml pool water, add 5 drops r-0871, swirl for 10 seconds, add the 0870 powder and then add 0871 until clear.
Purple, did not clear
I know that it's a lot of tests but I don't think that it's a problem with the reagents. The above tests should give a path forward.
The reason for the tests:
The bottled water test is to see if the color is specific to your pool/fill water or if it’s related to the reagent. If no reaction to bottled water, then there is probably something related to your water that is causing interference. Taylor indicates that oxidized manganese may cause positive interference.
Potential Interferences
Since you have a well, I suspect that that is the most likely interference.
1 drop of regular 8.25% bleach in 1 liter of water will raise the fc by about 4.2 ppm. If the powder does not cause a purple reaction in bottled water, then the reagents can be verified by testing a solution with a drop or two of bleach in a liter of bottled water.
The OTO test should give you a good reading.
I suspect that the purple is interfering with your ability to discern a true endpoint. Since you now know that the purple is in there, you might be able to discern the end of the red when only the purple is left.
The thiosulfate test should help determine if the interfering agent is an oxidizer.
The edta is a sequestrant, and can sequester manganese.
The iodide can possibly reduce oxidized manganese. The test would then give only the total chlorine.
