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Does anyone know how to make hard water with a hardness of 200+ ppm in terms of $\ce{CaCO3}$?

I live in Seattle and we have very soft water and we need hard water for testing purposes. Obviously you cannot dissolve $\ce{CaCO3}$ in water. I've read many "recipes" even ones that claimed you can dissolve $\ce{CaCO3}$ with a soda stream pop maker. Best I could do was a 60 ppm.

I've tried baking soda and Epsom salt and was able to make 130 ppm.

I've tried $\ce{HCl}$ to dissolve it in water, but the resulting pH was not suitable and if I tried to raise the pH, the $\ce{CaCO3}$ precipitated back out of the water.

I'm at a loss on a "recipe" that actually works. Am I trying to replicate the possible? Any ideas?

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  • $\begingroup$ You might want to read Sources of hardness on Wikipedia. It's not about just carbonates, you actually want to have soluble bicarbonates (and other anions such as sulfates and chlorides, for completeness) in solution. $\endgroup$ – andselisk Apr 15 at 1:57
  • $\begingroup$ Andselisk,Thank you for taking the time to respond. Yes I've read about sources of hardness. Allow me to explain what I'm attempting to do. I am trying to replicate the formation of hard scale. I have a clear understanding of how to get CaCo3 to precipitate out of solution. (Pressure, pH or raise in temperature) what I'm lacking is a recipe to create a solution that will precipitate CaCo3 in the form of hard scale. Only thing I can think of trying is Calcium hydroxide and bubble Co2 through it. $\endgroup$ – Jason Rusk Apr 15 at 3:41
  • $\begingroup$ What about some calcium bicarbonate rich mineral water ( composition is usually provided ), diluted to desired concentration ? $\endgroup$ – Poutnik Apr 15 at 4:23
  • $\begingroup$ @JasonRusk, what andselisk says is correct, but I'm not sure you understood correctly. "Hard scale" as you call it does not just consist of carbonates (and calcium), it is also composed of magnesium salts and both with chlorides, sulfates, etc. So, you need to make a mixture. What you are referring to or trying to get around sounds like just calcium carbonate solubility. I'm unsure why you are using CaCO3 - are you referring to the equivalent unit 'mg CaCO3/L'? That is not to be taken literally...best to convert that to mg/L Ca and Mg and choose a few different salts from those. $\endgroup$ – prof.kvothe Apr 16 at 12:08
  • $\begingroup$ I think hard scale might occur only slowly. Fast precipitation tends to make smaller crystals and many of them. $\endgroup$ – James Gaidis Apr 16 at 16:41
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When analytical chemists talk about hardness of water as ppm CaCO3, it is just a statement of "titratable" calcium / magnesium by ethylenediamminetetraacetate (EDTA). From your comments, you would like to replicate hard water perhaps for a project. Keep in mind that hardness are of two types temporary and permanent hardness. Permanent hardness can replicated with a soluble calcium salt.

Basically, you need a soluble calcium salt, but most water soluble salts of calcium are hygroscopic and nobody uses them as primary standard. Fortunately, CaCO3 is actually a primary standard. The way a Ca standard is prepared is by dissolving pure CaCO3 is smallest possible amount of acid. If this is a school project, you can take 200 mg solid CaCO3 in a 1 L volumetric flask. Add a very small amount of dropwise HCl until it dissolves. Test the pH and neutralize the acid if needed with NaOH. Dilute with distilled water. This would be titratable 200 ppm CaCO3 in water and would produce "scum" if you add soap solution (not the liquid ones).

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