The water in my area is hard and we frequently get limescale buildup around our taps. According to my municipal water source, we get 10.1 °dH (or 180 ppm CaCO3). I'd like to remove the carbonate hardness from the water I use for my plants. Supposedly it's better for them.
I don't want to shell out for a hydrogen ion exchange softener (or deal with installing one, for that matter). So I'll just add a weak acid the good old fashioned way. I'm using citric acid because it's easy to get the powder and it doesn't smell like acetic acid does.
Since I don't feel like trying to remember my high school chemistry lessons, I opted to copy others' work.
From this online calculator, I got that I need 3.231 dry oz citric acid to neutralize 100 gallons of water down to 2.5 ppm CaCO3.
Right now my plant collection is small and I only want to mix up 2 L of water at a time. Math time:
(.03231 oz / 1 gal) x (1 gal / 3.785 L) x (28.35 g / 1 oz) x (1 tsp citric acid / 3.8 g) x 2 = 0.127 tsp citric acid per 2 L.
That's right about 1/8 tsp per 2 L.
Now to test it. Everything is at around 24°C room temperature.
Initial pH (2 L tap water + 2 mL Wuxal cactus fertilizer): 7.25
pH after adding 1/8 tsp citric acid: 5.5
My hardness measuring kit says at this point there's still about 7 dH left. Yikes. Not even close.
I also repeated the experiment with a slightly different method of calculation and got a similar predicted value of the amount of acid needed. And similar results. Clearly something isn't working as intended.
Tried letting the water sit for a few more hours. No change.
Tried adding another 1/8 tsp for 1/4 tsp total and let it sit a while again. Hardness kit said 6 °dH and pH is now at 5.0.
Clearly there's some buffering going on because the pH didn't change nearly as much.
An additional 1/8 tsp (total: 3/8 tsp) appears to have entirely neutralized the alkalinity. The pH is somewhere below 4.5 -- my test strips don't go down past that.
Letting the bucket of water sit a couple days showed no effect on pH. The pH ought to have gone up due to the loss of CO2, formed after the carbonate ion reacted with the H+ ion, to the atmosphere. What gives?
In the end, the solution turned out to be so acidic that filling up to 3 L of total solution didn't budge the pH above 4.5. This makes absolutely zero sense to me as 1/2 tsp acid in 2 L ought to be the same ratio as 3/8 tsp in 3 L. Sigh. Probably some buffering nonsense, although adding tap water should have added more alkalinity too. I don't want to risk watering my plants with a solution of unknown pH considering citric acid can drag the pH all the way down to 2-3.
Anyways, I think I'm going to chalk this up to a semi-successful experiment for now and do 2 mL of fertilizer + 1/4 tsp of citric acid in 2 L of water from now on. Neutralizing some alkalinity is better than nothing, and I'm certain a pH of 5.0 won't harm my plants.
(Have I contemplated just using rainwater? Yes, but I have no good way of gathering enough to actually be viable. Have I considered distilled water? Yes, but I'm lazy and don't want to carry the giant jugs home from the grocery unless I'm growing seedlings. Have I thought about a Brita filter? The small ones I'm aware of are all sodium/potassium ion exchange filters, and I don't want to think about the effect of that on my plants and whether I need to adjust the fertilizer. So obviously, banging my head against the wall while trying to remember high school chemistry is the correct approach.)
