MrXeric wrote: ↑Sat Feb 04, 2023 11:55 pm
I'm not a chemist, just familiar with the basic principles at the undergraduate level, but calcium bicarbonate exists only as aqueous solution in the form of [Ca]2+ and [HCO3]- ions in water, in equilibrium with dissolved CO2 and carbonate ions. Similar to aqueous sodium bicarbonate, the acid part in acetic acid will react with the bicarbonate (and carbonate) to produce CO2 and water, leaving you with calcium and acetate in ionic form that will crystallize when dry as calcium acetate.
jerrytheplater wrote: ↑Sun Feb 05, 2023 1:07 amCalcium bicarbonate and Calcium Carbonate will react very similarly with acetic acid. Both will produce Calcium acetate. If you want a balanced equation, I could probably do it, but you really don't need it in my opinion. I doubt you are calculating concentrations etc.
I'm glad you guys chimed in because I didn't get an answer from Quora yet.
As MrXeric said, calcium bicarbonate exists only as an aqueous solution, and I was interested by what Jerry just said -- "Calcium bicarbonate and
Calcium Carbonate will react very similarly with acetic acid" [my emphasis]. When hard water evaporates, water vapor and CO2 are released into the atmosphere, which leaves only calcium carbonate (CaCO3) as a solid in the pot. If you've ever seen white stuff around the outside of a porous terracotta pot, that's the visible CaCO3 product of evaporation in hard water. Problem is that calcium carbonate is insoluble, "locking up" calcium which makes it unavailable to the plant. But when the acetic acid in vinegar reacts with calcium bicarbonate and carbonate in water, we get calcium acetate which is highly soluble -- this makes calcium
available to the plant when you water. Because there's no such thing as too much calcium (at least for cacti), calcium acetate gives us and them an additional source of calcium they can use -- a source denied to them by insoluble CaCO3.
Before we continue, I'll admit that I made a bad assumption regarding alkalinity in the pumice component of my hydroponic pumice and granite gravel mix. From the Wikipedia article on pumice (
https://en.wikipedia.org/wiki/Pumice):
"With regard to chemical properties pumice is pH neutral, it is not acidic or alkaline."
Okay, that's settled -- since I know that the hydroponic mix is in fact pH-neutral, all I'm concerned about now is dealing with the hardness of my tap water. After seeing it with a pH fluctuating between 7.5 and 8 for 8 years, the water had been showing a pH of 7.0 for the last 2. I tested again last month, and now it's showing 7.3. Since the hardness is back, I'll have to start periodically testing the tap water again. If it tests at 7.0, there's no need to acidify. Anything above 7, and I'll have to get a pH reading as I determine how much vinegar I should add. Rule of thumb here in SoCal -- tap water pH is lower in spring and fall than it is in winter and summer, so I'll have to keep that in mind for periodic testing.
When acidified water is applied to the plant, pH rebounds up as the vinegar-bicarbonate/carbonate reaction slowly wears off. If the rebound hits a pH of 7.0 and stays there, neutralization is complete. However, if the rebound climbs back up above 7, neutralization is only partial -- and evaporation of the remaining water leaves calcium carbonate behind in the pot. Ideally we would like to get complete neutralization of bicarbonate and carbonate by the time the potting medium dries out from top to bottom, but here's where acidification does have its limits:
- pH_nutrient_ availability_ chart04.jpg (83.12 KiB) Viewed 1350 times
On the lower end of the pH scale, the red boxes show where nutrient leaching causes a significant reduction of N, P, K, S, Ca, and Mg available to the plant. I'm including Mo because we don't want too much leaching of the Sodium molybdate supplement before it gets into the plant for Nitrate-ammonium conversion. In order to maximize availability of all nutrients, I'd suggest that acidification of hard water in a watering solution should be no lower than 6.0. If you're adding dilute fertilizer and any fert supplements you may be using, check the watering solution's pH
before you add the vinegar -- a
final pH of 6.0 for the watering solution is your target.
The issue of pH rebound levels over time is important, so I'll run a test that's a lot simpler than the procedure I described in my February 3 post:
1. Fill a 1-gallon jug halfway up with tap water. Add 1/2 tsp. Dyna-Gro 7-7-7, 5 fluid ounce Potassium sulfate stock solution, 1/4 tsp. TPS CalMag, and 2 tsp. Sodium molybdate stock solution. Fill the jug all the way up to the fill line. Test the pH and write down the number.
2. Add 1/8 tsp. 5% white vinegar, cap the jug, shake well, then pour some of the water out into a jam jar and test the pH. This does take some patience because I'll have to "sneak up" on the target pH of 6.0. Pour the water back into the jug, continue adding vinegar in 1/8 tsp. increments, and test the sample of each increment until I hit the target. I'll write down the total amount of the vinegar I added.
3. Pour some of the watering solution into a large container and put a lid on it to prevent evaporation during the test.
4. Wait for 7 days, take off the container's lid, and test the pH of the water. Write the number down.
After 7 days of rebound, I'll know if neutralization is complete or partial. If it's complete at the 7-day mark, pH between 6.5 and 7 is quite acceptable to the cacti. Whether or not rebound creeps up any higher will depend entirely on what I get when the test is done. Can't come to any conclusions until I get the results, so we'll have to see what happens.
Although I may be in the minority, I'm by no means the only one growing cacti in a soil-less mix. Unfortunately the only soil-less mix I have direct experience with is the pH-neutral hydroponic mix, so growers using other soil-less mixes will need to test the pH of their mixes before they determine if acidifying hard water is required. If I remember correctly, I think Jerry came up with a good testing procedure, but it's buried in with a lot of other posts, so lotsa luck trying to find it again. I'm sure we can come up with something that'll work. For people growing cacti in soil-based mixes, use a pH soil tester.
Going back to the acidification "how to" guide I posted in 2019, there's much I didn't know then, so I think it'll need to be seriously revamped. In the meantime...
The word "acetate" sticks in my mind because one of our veteran members by the name of iann used to bring it up. While I was about to end here, I went to Advanced search, typed in "acetate" and "iann", and found the following item for you:
iann wrote: ↑Mon Dec 06, 2010 4:08 pm
There's very little difference between the deposits left by acetic acid or citric acid. Calcium acetate is highly soluble and calcium citrate is soluble enough that it shouldn't matter at the concentrations you'll be using. The carbonate byproducts are exactly the same. I'm not sure about the speed of the reactions, but it is likely to depend far more on the pH than on the exact acid being used.
This is
really good -- if your collection is too large to make vinegar feasible, citric acid is stronger but safe for home use. And guess what? Calcium citrate is another source of calcium available to cacti.