How can I tell if kalkwasser water is too old

Hello,
I'd like to get opinions on kalkwasser.

I mix approx 5 tsps of kalkwasser a week with ro/di water. It is
stirred in a five gallon bucket with a powerhead (covered). I put the
solution in a 5 gallon kent marine drip (cover on). This solution
lasts a week.
How do I know if the kalkwasser is still good during the last days. Is
ph measurement a good guide, I just measured it (6th day and ph is
12.2 using a pinpoint ph probe)?

thanks in advance,
Albert