What does it mean to calculate hardness in terms of ppm of calcium carbonate? [closed]








**Closed**. This question needs [details or clarity](/help/closed-questions). It is not currently accepting answers.
 
 






---








**Want to improve this question?** Add details and clarify the problem by [editing this post](/posts/112373/edit).




Closed 4 years ago.








[Improve this question](/posts/112373/edit) 



PPM means per part million or milligram per litre of solution.Calculation of ppm for a given compound say $x \pu{gms}$ of $\ce{Ca(HCO\_3)}$ in 1 liter of water would be done like 


$$ppm= \frac{1000\cdot x}{1\pu{1L}}$$ 


But I do not understand by what's meant by in terms of $\ce{CaCO3}$ is it equivalents or weight? This was asked in an exam and the formula is not covered in my textbook.




It is meant as molar equivalent. $\ce{CaCO3}$ has the molar mass about $\pu{100 g / mol}$, $\ce{Ca(HCO3)2}$ has the molar mass cca $\pu{ 162 g / mol}$. Therefore $\pu{100 g}$ of the former is equivalent to $\pu{ 162 g }$ of the latter. The similar for the concentration expressed in ppm. E.g. if water contains $\pu{1.62 ppm}$ of $\ce{Ca(HCO3)2}$, it is equivalent to $\pu{1 ppm}$ of $\ce{CaCO3}$


 2

Expressing the hardness of water by ppm CaCO3 is nothing but a conventional convenience in water industry. There is a background behind it.


Basically, you titrate hard water with ethylenediamminetetraacetate (EDTA) and the moles of EDTA consumed are *assumed* to be consumed by Ca2+ ions *only*. As you would guess, this is not true because hard water contains many other ions which can form a complex with EDTA especially magnesium is always present. Anions have nothing to do with the calculation of ppm CaCO3. The calculation is rather trivial


moles of EDTA used = moles of Ca2+ (as calcium forms 1:1 complex with EDTA) = moles of CaCO3 and even = moles of Ca(HCO3)2.


One can easily convert moles of CaCO3 into parts per million from the known volume of hard water taken from titration.


 1