While trying to get my new pool CYA up to spec, I have already used one bottle of R-0013 from my TF100 test kit. (A mistake on my first try didn't help.)
Does anyone know what is the chemical in the bottle?
My CYA is now up to 28, so I hope to only test once more before closing.

It is essentially melamine but I wouldn't attempt a substitute......I doubt you'll get an accurate result.

If you have one more bottle, that should be enough for 3 more tests and you really have no reason to test it 'til next Spring, IMO.

Absolutely right... but if lances4803 or someone else reading this post has reason to test often, refills are available:

TF-100 Refills
TF R-0013-2
Cyanuric Acid Reagent (2) 2 oz bottles
120ml / 4 oz $7.50 [$1.88 oz]

Taylor CYA Refills
Taylor R-0013A
Cyanuric Acid Reagent
22ml / .75 oz $2.85 [$3.80 oz]

Taylor R-0013C
Cyanuric Acid Reagent
60ml / 2 oz $4.20 [$2.10 oz]

Taylor R-0013E
Cyanuric Acid Reagent
480ml / 16 oz $9.15 [57¢ oz]

The 16 oz container can be used to refill smaller bottles and can be had for as low as $7.00 on other sites. Here, for example.

I hoped Taylor had supplies, and that isn't a bad price for a refill.
Hey, I'm just excited to get this pool going right!! (Of course, it was finished just in time for fall, and 9" of rain in the last week.)