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Where I live it's believed that local tap water is really bad for drinking because of the amount of limestone. As a science experiment for school I wanted to approximately calculate the percentage of calcium carbonate in tap water so I can make the conclusion: Is that amount of drinking $\ce{CaCO3}$ actually the problem or something else?

This question might be similar to DK4's question but I'm trying just to calculate not extract, only by using items you can find in a typical house (or in a local shopping center).

If calcium carbonate isn't the issue what is?

Edit: As pointed out in comments, percentage of $\ce{Mg}$ is also applied.

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    $\begingroup$ You can't do a "decent" analysis without some basic chemistry lab equipment. // In general you'd need to consider both Mg and Ca which is known as the water hardness. $\endgroup$
    – MaxW
    Mar 31, 2019 at 21:01
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    $\begingroup$ You can buy / borrow / order online some water hardness analysis kit for hobby aquarists. The kits are using a solution drop counting to determine amounts of the total and carbonate hardness. For your intentions they are precise enough. $\endgroup$
    – Poutnik
    Mar 31, 2019 at 21:25
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    $\begingroup$ See: en.wikipedia.org/wiki/Hard_water $\endgroup$
    – Mathew Mahindaratne
    Mar 31, 2019 at 21:44
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    $\begingroup$ "bad for drinking " = Water that may cause health issues $\endgroup$
    – Spellcaster
    Apr 1, 2019 at 20:11
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    $\begingroup$ Well, eating limestone, while not necessarily palatable, is unlikely to cause health problems, unless you ingest so much that it causes indigestion from over neutralization of stomach acid. On the other hand, because limestone is not actually soluble in water, you'd be hard pressed to drink enough. Technically speaking, water itself may cause health problems. Over consumption of water leads to over dilution of electrolytes. In the extreme case, too much water leads to drowning, which is definitely bad for health. $\endgroup$
    – Zhe
    Apr 2, 2019 at 20:57

2 Answers 2

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  1. Water contains negligible amount of $\ce{CaCO3}$ or $\ce{MgCO3}$, as these are almost insoluble in water.
  2. What you are interested in are bicarbonates $\ce{Ca(HCO3)2 + Mg(HCO3)2}$. Their content is known as the temporary or carbonate water hardness that can be expressed as $\ce{CaCO3}$ equivalent.
  3. The complement to the carbonate hardness is the permanent hardness, formed mostly by $\ce{CaSO4, MgSO4, CaCl2, MgCl2}$. The sum of the temporary and permanent hardness is called the total hardness.
  4. Bicarbonates ($\ce{HCO3-}$) are not threat to health themselves, as they convert to $\ce{CO2}$ in the stomach and are present in blood as the component of the blood $\ce{pH}$ buffer.
  5. What can be a subject of health concern is the total content of $\ce{Ca^2+}$ (and in lesser extent $\ce{Mg^2+}$). It is again expressible as $\ce{CaCO3}$ equivalent.
  6. Calcium/magnesium content is not measurable by resources usually found in households or local shop center.
  7. The best way is to ask the water provider for the data.
  8. The 2nd best way, close to home-like determination, is to obtain the set for aquarists from a pet shop for determination of carbonate and total water hardness. I am a former aquarist, and this was available even in 1980s in the former communist Czechoslovakia.
  9. Determination is done by counting the drops of the colored solutions, mixed with the tested volume of water, until the color changes.
  10. Carbonate (in fact bicarbonate) hardness determination is based of reaction of an acid with bicarbonates: $$\ce{HCO3- + H+ -> H2O + CO2 ^}$$ Drop counting ends when yellow color (methyl orange dye indicator) changes to red. The dye turns red by the first excess of the added acid, as has red color in acidic solutions.
  11. Total hardness is based on reaction of $\ce{Ca^2+ + Mg^2+}$ with ethylendiamintetraacetic acid (EDTA) in alkaline solution, enriched by the addition of the dye indicator Eriochrome black T, $$\ce{Ca^2+ + H2EDTA^2- -> 2 H+ + CaEDTA^2-}$$ The EDTA first reacts with free $\ce{Ca^2+ + Mg^2+}$. The drop count ends when red turns to blue, as EDTA captures $\ce{Ca^2+ + Mg^2+}$ bonded to the Eriochrome black T.
  12. As result, you obtain the carbonate (temporary) and total (temporary + permanent) hardness in 2 possible units: $\pu{mmol/l}$ or $\pu{dGH}$ (the degree of General Hardness). $\pu{1 mmol/l = 5.6 dGH}$.
  13. $\pu{1 mmol/l}=\pu{100 mg/l}= 0.01\%$, as molar mass of $\ce{CaCO3}$ is cca $\pu{100 g/mol}$.
  14. The typical total hardness of drinking water is $\pu{1\!-\! 3 mmol/l}$, i.e. the equivalent of $\pu{100\!-\! 300 mg/l}$, resp. $0.01\!-\! 0.03\%$ of $\ce{CaCO3}$.
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  • $\begingroup$ @Mathew Mahindaratne : Thanks, Mathew, for thorough editing. One objection: Unless American English is mandatory here, colour and centre is proper spelling in British English. $\endgroup$
    – Poutnik
    Apr 8, 2019 at 8:22
  • $\begingroup$ Thanks for the valuable point. I know they are correct but I also confused on what format been preferred. I guess both should be okay, but changed for the consistency. :-) $\endgroup$
    – Mathew Mahindaratne
    Apr 8, 2019 at 15:08
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As you have already understood the detailed chemistry of Hardness in water by above-mentioned answers in great detail, I am going to answer How you can calculate %mass of Hardness in Water in a cheap but crude way by which you can help your school students to find out.

So Basically, Hardness also causes basicity of water and just by simple Acid-Base or EDTA titration of small amount of sample tap water with Acids such as H2SO4 or HCl or EDTA (Ethylenediaminetetraacetic acid) by which you will get the volume of Base present in water which can be converted to Mass units. Though it is a very crude and inaccurate method but you can explain school kids by letting them find the amount the Alkalinity in water.

Here I am attaching a link where you can find the stepwise procedure and calculation to calculate Total hardness in water by using EDTA which you can find in your school chemistry Lab. http://www.uobabylon.edu.iq/eprints/publication_2_2630_250.pdf (Please read the very first procedure in this document)

Please ask for any further clarification or queries. Keep Learning :)

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