[bob58o]

I did not know where to post since I am not a student.  This question is probably a HS/Undergraduate level question.  I have not looked at a chemistry equation in over 15 years.  The answer is not very important either.  I am just looking to settle an argument about hardness and pH of water.

Argument began when I was told that attempting to lower the pH of water from my tap for feeding my garden was useless because the calcium carbonate will always be there and won't go anywhere.  I argued that using citric acid will turn the calcium carbonate into Tricalcium Dicitrate plus Carbon Dioxide and Water.

I tried to work out the chemistry to find out how much lemon juice would be needed to neutralize the calcium carbonate in the water, but I'm not sure that I am understanding how this would work in real life.

I am starting with 1 gallon of tap water.  Assume it has 120 mg/l calcium carbonate.  I think I did the chemistry correctly, but I think I might be missing something.  Will a 1/2 oz of lemon juice in a gallon of water be a strong enough concentration to make calcium citrate or would the concentration need to be much greater?  Does the concentration of the acid solution need to be strong enough to dissolve the calcium carbonate for this reaction to take place? Or are the two unrelated? I just worked on how much to neutralize the calcium carbonate in the water for now, ignoring what calcium carbonate that may already be in the soil.

Forgive the formatting.  I am going to copy and paste my work.  Let me know if I am on the right track or what am I missing.  Thanks.

3 CaCO3 + 2 C6H8O7 > Ca3 (C6H5O7)2 + 6H + 3 CO3

The H and CO3 will combine to form H2CO3.
3 CaCO3 + 2 C6H8O7 > Ca3 (C6H5O7)2 + 3 H2CO3

The Carbonic Acid will disassociate into water and carbon dioxide
3 H2CO3 > 3 CO2 + 3 H2O

3 CaCO3 + 2 C6H8O7 > Ca3 (C6H5O7)2 + 3 CO2 + 3 H2O

So at this point we've turned Calcium Carbonate into Calcium Citrate plus Water and Carbon Dioxide.
Calcium Citrate is a salt that is "sparingly" soluble in water (0.95 g/L at 25C), compared to Calcium Carbonate which is barely soluble (0.013 g /L at 25C). Calcium Citrate is about 73X more soluble in water than Calcium Carbonate.

A gallon of water with 120 mg / L of CaCO3 contains 0.45 grams of CaCO3. That is 0.0045 moles of CaCO3 in a gallon of water. Every 3 moles of CaCO3 needs 2 moles of Citric Acid for the balanced equation. So we need 0.0030 moles of C6H8O7 to react with the calcium carbonate. Lemon juice has 1.44 grams of citric acid per oz. Thats 0.0075 moles per oz.

0.4 oz of lemon juice per gallon of water if the water has 120 mg / L of Calcium Carbonate should make 0.75 grams of Tricalcium Dicitrate which is soulble up to 3.5 grams per gallon of water at 25C. We started with 450 mg of CaCO3 per gallon of water that is only soluble up to about 50 mg per gallon at 25C.

[chenbeier]

Looks roughfly ok, but what is your question?

To decrease the hardness you have to remove die calcium by ionexchange, reversed osmosis or destillation. Now you converted it from Carbonate to Citrate.  Calcium still present.

[Borek]

IOW: hardness is not about CaCO₃ but about Ca²⁺/Mg²⁺. Broadly speaking converting calcium carbonate to other salt doesn't change the water hardness, as the calcium is still there.

[bob58o]

Well I was figuring that since we converted insoluble CaCO3 into a soluble salt, then the Calcium Citrate would be able to be taken up by the plant roots and used for nutrition (which would remove it from the water).  Or perhaps because it is soluble, it could get flushed out of the root zone- as opposed to the the insoluble Calcium Carbonate - which cannot be taken up by the roots and may stay in the soil (root zone of plants) longer.

I don't want to get rid of the Calcium.  My garden needs calcium. I just need it to be able to be used by the plants. I have an RO filter.  I don't use it to water my garden because I want all the minerals in the tap water for the plants.  Plus it would take forever.  The issue is that my soil has a pH of around 7.0 and some plants like tomatoes like a lower pH.  I have been lowering the pH of my water to around 6.2 - 6.4 before watering.  Supposedly an ideal pH for nutrient uptake in soil is around 6.5.  I was told it was a waste of time and energy to lower the pH before watering.

The discussion heads off in a biology or botany direction.  The CaCO3 in the soil in another issue.  It will buffer the water I throw at at.  Plus it is said that micro organisms will deliver nutrients to the roots regardless of pH.

So I guess the real question is what I am doing?  LOL I don't know.  Is there any reason to lower the pH of water (by adding acid) before watering plants in soil?  I thought that converting to a soluble salt would make it harder for the calcium to build up in the soil.  I do not want the pH of the soil to get any higher.  I'd like to lower it.

So what would be the difference (instantaneously at each watering, and long term) between watering straight from the hose with water that has pH of 8.0 and 120 mg/l CaCO3 into soil with pH of 7.0, compared to that same water treated with citric acid and pH lowered to 6.0 then used on soil with pH of 7.0.

Will the soil buffer the water to 7.0 regardless of the input pH?  Can the soil pH go up or down based on what I do, or don't do, to the water I feed it?  Can I add extra citric acid to the water in hope that it will react with the CaCO3 in the soil, so that it gets dissolved, flushed away or replaced with calcium citrate?  Is there even any benefits if that does happen?

Potential benefits I see (the reasoning behind my use of lemon juice that I am trying to justify)...

-Lowers pH of nutrient solution in the soil (at time of watering).
-Lowers the pH of the soil over time.
-Makes minerals/nutrients available to be "drank" by roots by converting to a soluble salt



 I guess I'm looking for a chemist's answer to a "what happens if..."  gardening question.

[billnotgatez]

Of interest is the WIKI article

Calcium carbonate
https://en.wikipedia.org/wiki/Calcium_carbonate

Where it states

Calcium carbonate will react with water that is saturated with carbon dioxide to form the soluble calcium bicarbonate.

There are other passages that talk about Calcium carbonate uses in agriculture and also its activity in acid.

[Borek]

I thought that converting to a soluble salt would make it harder for the calcium to build up in the soil.

My bet is that citrate will get quickly metabolized by the soil microorganisms, and - as there is abundance of CO₂ present - Ca²⁺ will get converted back to CaCO₃.

[chenbeier]

Maybe add nitric acid.  Plants also need nitrogen.

[AWK]

Thread Title -

Calcium Carbonate Plus Citric Acid (Softening and Lowering pH of Tap Water).

For what? Acidification of water does not remove water hardness - it only changes transient (carbonate) hardness into solid (non-carbonate) hardness. Long-term and especially intensive soil cultivation causes its acidification. Only a few plants like to grow in acidic soil. Much more plants only tolerate not very acidic soil.

Assume it has 120 mg / l calcium carbonate.

Tap water only contains this content of calcium carbonate as a result of converting all dissolved calcium salts into carbonates. If this number were true for calcium carbonate, the tap water could only contain 10-15 mg CaCO₃/L. So there is no problem with the availability of calcium from drinking water for plants. In the case of intensive cultivation of certain calcium-loving plants, calcium deficiency may occur - then the soil is additionally fertilized with calcium sulfate. In garden crops, ground dolomite or limestone, possibly calcium nitrate are used, but any liming should be preceded by an analysis of the available calcium content in the soil, and at least checking the soil pH and appropriate (even estimated) calculations.

as opposed to the the insoluble Calcium Carbonate - which cannot be taken up by the roots and may stay in the soil (root zone of plants) longer.

Not true - if there is calcium carbonate in the soil, most plants are great at extracting calcium from the soil (except for intensive crops).

[bob58o]

Thank you.  Those responses helped my understanding.  I was originally confusing hardness with alkalinity.  And I think because it is expressed as "mg/l of CaCO3" I was imagining precipitate in the solution that would get dissolved when when the citric acid was added.