@Rick Mathew
I just did a little experiment with this (Saturday, everyone sleeping in makes this easier)
I did the following:
#1
10ml test solution per the Red Sea instructions (watch for the reaction byproduct) = 82 (4.25)
10ml diluted w/ 10 ml RODI(but used 100% test sample for the C1) =39, x2=78 (4.07)
#2
16ml using the procedure as outlined = 100 (5.08)
16 ml sample (10ml test diluted with 10ml RODI) but 100% C1 sample = 46, x2= 92 (4.76)
Then (as I thought of it after I tossed the 10ml samples from #1 above so no comparison)
#3
used the reacted and diluted from #2 above but made the C1 sample 50% RODI/50% test water= 44, x2=88 (4.53)
so all these methods came out pretty close for a one-point curve. the 10ml sample reaction was lower overall Nitrate results as would be expected expected from the 16ml samples so the curve may not work all the way and the methodology would not necessarily support the findings anyhow (really is likely close enough and within accuracy specs for what we are doing as they all fall within 1). It was just a way to kill some time.
reference to the Salifert was reading about 5 as was the Red Sea kit (guessing from the windows of color). So we are seeing validation again.
Now - what if instead of RODI - dilute with test water as you indicated? Is the reaction complete or might it continue?
I think the reaction is completed after the 9 min reaction time...The reason I say that is I have measured the sample over time and it is does not change hardly at all...I did this to see how stable the color was over time...seem to be very stable...another point to keep in mind is the meter variation itself...it is reported to be + - 10ppb + - 4% of reading.... Using the regression formula that represents about + -.5ppm..expected meter error