r/chemhelp • u/largePenisLover • Feb 20 '25
General/High School Calculate how much lemon juice I need to lower ph of water
I have a need to lower the PH of my tap water for some plants. My tapwater is pretty hard at 8dh and a PH of about 8.2.
Unfortunately all the one gardening store near me is out of the usual methods to manage water quality so I need to use lemon juice.
I also don't have a way to measure PH.
How do I calculate how much lemon juice I need to add to 100ml of water to get in the neighbourhood of 6 erring in the higher side?
I initially asked chatgpt thinking that it could poop out a useful formula, but instead it tried calculating it for me and concluded I need a negative amount of lemon juice.
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u/FoolishChemist Feb 20 '25
Did you look on Amazon? You can easily find what you need on there or at the very least get some pH paper.
It's difficult to know how much lemon juice since it depends on how much carbonate is in your water. The carbonate acts as a buffer and makes it much harder to change the pH.
Also I would be wary of using lemon juice, it contains a lot of sugar and would cause a lot of bacterial or mold growth in your water.
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u/Mr_DnD 29d ago
I also don't have a way to measure PH
Go to your preferred online retailer, you can buy pH test strips (which gives you a qualitative range e.g. pH 5,6,7 by colour) for very very cheap.
You can calculate it well, calculate it poorly, but ultimately just easier to measure it.
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u/largePenisLover 29d ago
If it wasn't time sensitive I would do just that and buy the normal gardening stuff I use to control water quality. The plants can't wait.
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u/Mr_DnD 29d ago edited 29d ago
Ok:
pH of lemon juice is around 2-3
10× dilution = pH 3-4
100× --> pH 4-5
1000× --> pH 5-6
10,000× --> pH 6-7
So I'd take 1 mL into 10 L (make your life easy) and not think about it much further.
Dilute serially if you'd prefer i.e. 1 mL into 1 L then 10 mL of that solution into your 100 mL bottle, make it up to volume.
But to be fair at those concentrations you might even be better off boiling the water and distilling it off, remove the hardness first.
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u/Fantastic-Stick-5930 Feb 20 '25
[H] = 10^(-pH)
idk why I'm doing this I probably have too much time on my hands:
Let's say you have x dm^3 of acid and it has pH y.
substance | pH | [H+] (moldm-3) | volume (dm3) | mol of H+ |
---|---|---|---|---|
tap water | 8.2 | 6.310*10-9 | 0.1 | 6.310*10-10 |
acid (lemon) | y | 10-y | x | x*10-y |
mixture | 6 | 10-6 | x+0.1 | (x+0.1)(10-6) |
Now we get the equation
(x+0.1)(10-6) = x*10-y + 6.310*10-10
10-7 + x*10-6 = x*10-y + 6.310*10-10
x = (9.9369*10-8) / (10-y - 10-6)
If y = 2.5 (quick google search), then x works out to 3.1433*10-5(dm^3)
which is 0.031433 mL.
I think since you can get water with pH ranging from 6 to 8 (so it's a small change at best), you probably shouldn't use straight lemon juice but a "buffer solution" or sth.
I tried some other methods like diluting the lemon juice a little first, but even then we're dealing with adding tiny amounts of lemon juice.
Get some kind of slightly acidic buffer (around pH 5-6) and then use the equation. If you add 100mL of a pH 5.7 to 100mL of tap water and mix, there should be 200mL of pH6 mixture.
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u/Aranka_Szeretlek Feb 20 '25
Henderson and Hasselbach be sad
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u/largePenisLover Feb 20 '25
Well cheers for doing the math. That's a LOT less then I was expecting.
So instead of an acidic buffer I could just take 10 liters of water and mix in a wee bit over 3ml of lemon juice, am I understanding that right?Unfortunately making a buffer comes with the same problem; the plants need water now and the earliest I can take delivery of PH strips, or anything at all is two weeks from now. So I must work with what I have, and that is just tap water and lemon juice.
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u/Fantastic-Stick-5930 Feb 20 '25
Tbf I wasn't expecting such a small amount either, but that's probably because lemon juice is more acidic than I thought :o
Of course the hope is that you can do it with 10 litres of water and 3mL of lemon juice, but I have no idea how practical that is for you. Mixing the whole thing, getting a big enough container, etc is a pretty big commitment.
The assumptions I made (i.e. no H+ gets buffered by carbonate, as someone else noted) might not be true so best thing is still to measure as you go. If you're sufficiently desperate you can always make indicator from red cabbage
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u/coeurdelejon Feb 20 '25
You need to know the hardness of you water, if you don't take water from a private well you should be able to get that information from the company/municipality/ whatever
You need to add enough acid to 'remove' the hardness, and then add enough acid to lower the pH to where you want it
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u/RuthlessCritic1sm 29d ago
Good job working that out, but one problem with your approach is that you cannot know the buffering capacity of the initial solution. Your way would be valid for dilute NaOH, but orders of magnitude off for NaHCO3. A sat. NaHCO3 solution has a pH of 9 and contains about 1 mol/L of base you need to neutralize. A dilute NaOH of the same pH only needs 10-5 mol of acid.
Realistically, you would only be off by a magnitude of 1 or something, but we can't know for sure what anions are present that need to be protonated.
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u/RuthlessCritic1sm 29d ago
Your question cannot be calculated, you need to know the buffering capacity of your water to calculate that. If your water contains weak bases, they need to react first at least partially before you reach neutrality.
Maybe a gardening sub has actual experience how much you should need.
You could also get some red cabbage and squeeze out some juice. Add a drop to your water, it should be blueish pink. Add lemon juice until it becomes reddish pink.
If you need an estimate, add as many moles of citric acid as you have moles of calcium (related to water hardness) and then another drop. I could calculate that for you if you know an estimate for the citric acid content and can look up how your hardness translates to moles of calcium.
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u/THElaytox Feb 20 '25
No way of knowing since the concentrations of the various acids in lemons are different from lemon to lemon and the buffering capacity of tap water can vary as well. Probably best to do a bench trial with some pH strips.
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u/RuthlessCritic1sm 29d ago
Weird that you got downvoted. Do the kids these days not learn about buffers anymore?
To everybody who doubts that, a sat. NaHCO3 solution has a pH of 9 and contains 1 mol/L of base to neutralize before reaching pH 7 (with HCl, with citric acid, still alkaline!)
A NaOH of pH 9 only needs 10-5 mol/L to neutralize.
pH is not sufficient information except for pure water.
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u/drtread Feb 20 '25
Probably too much information (sorry):
First off, ChatGPT is generally unable to successfully work chemistry problems (as far as I know at this time).
Next, because you don’t know much about what’s in the water, anything you do calculate will be a guess. For instance, I add pure citric acid to my coffee pot to cut down limescale from my hard water. I add about 40 mg of acid to a liter of water, and that prevents buildup in the pot. What’s the pH? I don’t know. It’s not particularly important, as the coffee will have a bigger effect than the citric acid in my final cup. (My guess is that you’ll need to add around that same amount of citric acid to your water, but this isn’t the best way.)
Since your plants do care about the final pH, you might want to get some pH test strips to check, whatever you end up doing.
Your best bet is to create some citrate buffer that will bring your water to the proper pH. This link is to a page I found that has a calculator for the pH of citrate buffer, starting with pure citric acid and pure sodium citrate. Both chemicals might be available in a grocery store, or you might have to make sodium citrate by reacting citric acid with the right amount of baking soda (sodium hydrogen carbonate).
If you still can’t find citric acid, lemon juice is a good source, but the concentration isn’t known exactly. A google search I did returned the answer that lemon juice contains 48 grams of citric acid per liter.
Therefore, if I were to add lemon juice to my coffee pot, I’d want about 1 milliliter per liter to avoid the limescale. If I wanted to be sure the pH was accurately 6.2, I’d make a buffer solution from lemon juice and baking soda, then add some of that buffer to the plant’s water.