i’m fairly new to anodizing niobium and only use the techique as called for with a project. I don’t do this every day as you likely do in your shop. Thus, my TSP and etching solutions may be ”older” than yours at times. I can always make up fresh TSP solution in small quantities and ensure I’m always using fresh solution, but the etchant is more problematic. The way it is packaged I have to make up the whole 1/2 gallon of the 2X concentrate all at one time. (I would make smaller lots if I could divide the dry powder accurately, better yet buy it in smaller units.) I then make up only 1 cup of the 1:1 dilution for a hot etch. After use I always decant it into a heavy polystyrene (?) lidded container for reuse later. I toss this diluted solution if I notice any color tint.
Sorry for the long intro ... Because my use is so limited at times, these soultions tend to ”age in place”. Are there any tell-tales to look out for as an early indicator that it is time to use fresher reagents, or explain what I’m seeing when things go differently than expected?