pH question about water for mixing henna

Got it, thanks!  Makes sense. 

I do notice that when I refreshed the lengths of my hair with just my henna/cassia mix and distilled water, the color looks gorgeous, but it's been coming off on my towel.  I do want my henna to be somewhat more "permanent" BUT my henna result color is now copper red, which I love, and I don't want it to get too darker over time, because my hair is delicate, and I can't keep doing mild "bleach baths" every so often to get it copper again. 

The natural lightening methods (tried them ALL) take a zillion applications to work on me, and even then the result is too subtle to keep up with henna's oxidation.   My henna seems to darken up moreso than one would expect, perhaps because my hair is porous, and because my base color is 50% brown / 50% gray (more or less), so my hair seems to "revert" to brown. 

Thanks to this amazingly helpful website/forum, I  have tried so many tips to prevent the henna application from losing it's red hue and going more auburn, including:

~ no heat tools whatsoever

~ rain wash treatments and clarifying shampoos

~ doing root touch ups only, refreshing the color on the lengths only if/when it looks faded (and lightening a bit first) 

~ using Jasmine instead of Twilight, and adding a bit more cassia to the mix (can't add too much, or it makes my grays look too blond).

I am sure that all of this is helping, but noting has worked super well.  So I have also tried adding Copperberry fruit acid to my henna/cassia mix, but I did not like the color result, it gave an artificial looking vivid red on me.  I bought some Nightfall rose,  but hesitate to try it because I know it's slightly ashening, which would be fine on my grayest areas, but my root areas that are still more brown really need the full brghtness of the henna in order to turn copper red.  I don't want to have to mess with two different recipes, one for my grayer areas one for my browner areas... tried that and it was too nerve wracking and tedious.    

I understand that the more acidic a recipe is, the darker it will go over time.  So my best solution (I think) , is to make my recipe only just as acidic as it absolutley needs to be, to bond well to the hair.   Plus (i think) my recipe should contain a good anti-oxidant, right?  So I have been experimenting with a mixture of melissa balm tea and aloe vera juice.  Both very powerful antioxidants.  I can't test the pH of the tea with my pH drops, because the tea is already yellowish.  But I have tested the aloe since it's clear, it tests at a 4 and even when just 1 tablespoon is added to the distilled water, I still get a 4 (then again, the distilled water itself is a 4, so there's that).   

This tea with aloe juice recipe dye releases well in only 4 hours and the color at the root holds well, so I'm thinking that (used as a gloss) it may hold well on the lengths too, and may not darken as much as when I was using amla.      

Any suggestions would be great. Maya