Tap water Hardness Qs

[ninoboy]

My tap water report mentions that it contains 72ppm of Ca and 26ppm of Mg. Also mentions that the Hardness is 290ppm and KH or 130. The hardness and alkalinity are very consistent with my test results (300ppm GH and 120ppm KH). My question is :

- How come the hardness is close to 300ppm and my Ca + Mg is around 100ppm total. Is there anything else that GH take into account ?

[malkore]

lots of minerals would contribute to total hardness. I don't know exactly which ones, but I would think copper, zinc, iron...all these would count as hardness. I tried to do a search to see what all contributes to GH. I didnt' get a concrete list, but it does sound like everything but calcium carbonate is what makes up General Hardness.

[Laith]

That's an interesting one.

I had also thought that GH was made up mainly of Ca and Mg. There are other things but as far as I had heard, they had minimum influence on the total GH reading.

Your Ca and Mg together is 98ppm so your GH being 290 is strange.

Or maybe I have it all wrong...

[ninoboy]

That's what I thought also. Here's the link to my water report : http://www.lvvwd.com/html/wq_water_analysis.html

Maybe I read it all wrong. There are 2 column. LVVWD = Las Vegas Valley water district and I'm not sure what the other column is.

My water has been around 300ppm for years (380ppm 3 year ago). I just never read the details until recently.

[Rolo]

Hardness is measured by CaCO3 and MgCO3 (carbonates of Ca and Mg). The readings for Ca and Mg provided by your water facts sheet are not for hardness, thus why you can't simply add their levels. This is also why they gave you a seperate hardness reading.

I did the calculations on some paper to convert the Ca and Mg ppm into CaCO3 and MgCO3 ppm. It came out to 280 ppm total, just shy of the reports 290ppm and your own 300ppm readings. Remember, other things do increase hardness (but CaCO3 and MgCO3 the main players), so is why my calculations is the lowest of all, didn't incorporate any other metals.

All in all - everything seems perfect!

[ninoboy]

Ahh, thanks so much Someone told me before that even you have 100ppm of GH, it doesn't always mean you have enough Mg for your plants. Now I know what he meant.

[Laith]

Sorry, now I'm confused .

We're talking about GH right? I thought carbonates affect the KH reading, not GH... GH does not measure any carbonates (or so I thought until now...).

That's why if you want to increase your GH & KH, you add something like Calcium Carbonate (CaCO3). If you want to increase *only* your GH, you use instead something without carbonates, such as Calcium Chloride (CaCl2) or Magnesium Sulphate (MgSO4).

There are two readings in the water report related to "hardness": the one labeled Hardness, which I assume is the GH. And the one labeled "Alkalinity", which I assume is related to the buffering capacity, or KH of the water...

Or do I *really*, *really* have it all mixed up here??

Laith

[ninoboy]

Well, I tried to look up the calculation method for GH by looking at Ca and Mg for a while and Rolo is right. You can't just add those 2 numbers together. This is the most I could find on the calculation :
http://fins.actwin.com/aquatic-plant.../msg00279.html

That article doesn't really explain the details but if I use his method, and plug in the Mg and Ca level in my water report, I get around 267ppm (I think because the guy rounded up some numbers). It's pretty close then.

[KevinC]

OK: Ignore this if you really don't like the chemistry.

Originally Posted by ninoboy

Well, I tried to look up the calculation method for GH by looking at Ca and Mg for a while and Rolo is right. You can't just add those 2 numbers together. This is the most I could find on the calculation :
http://fins.actwin.com/aquatic-plant.../msg00279.html

That article doesn't really explain the details but if I use his method, and plug in the Mg and Ca level in my water report, I get around 267ppm (I think because the guy rounded up some numbers). It's pretty close then.

From the cited web page above:

Originally Posted by web

1 degree GH = 18 ppm CaCO3 = 18 mg/L CaCO3 = 18 mg/L MgCO3
(I assume its the same for mg/L for Mg.) ???

Now to convert that to mg/L of calcium only we go to the periodic table
and look up the atomic weights as follows:

Ca = 40
CaCO3 = 40 + 12 + 16*3 = 100

Therefor
1 degree GH = (18 mg/L of CaCO3) * (40 units Ca) / (100 units CaCO3)
= 18 * 40 / 100 mg/L of Ca
= 7.2 mg/L for Ca

Similarly
Mg = 24.3
MgCO3 = 84.3

Therefor
1 degree GH = (18 mg/L of MgCO3) * (24.3 units Mg) / (84.3 units CaCO3)
= 18 * 24.3 / 84.3
= 5.2 mg/L for Mg

I have to disagree with the assumption that 18mg/L CaCO3 = 18mg/L MgCO3 - hardness is expressed as ppm CaCO3. Because one atom of Mg (or Fe or Mn for that matter) acts just like one atom of Ca in terms of hardness, we can treat them the same on a MOLE basis (not grams). Correcting the last equation:

1 degree GH = (18mg/L of CaCO3) * (1molMg/1molCaCO3)*(24.3gMg/1molMg)*(1molCaCO3/100gCaCO3)
= 18*24.3/100
= 4.4 mg/L for Mg per degree GH

Now with Ninoboy's original water report numbers: 72ppm Ca, 26ppm Mg:

72ppm Ca * (1degree GH/7.2ppm Ca) = 10 degree GH
26ppm Mg * (1degree GH/4.4ppm Mg) = 5.9 degree GH
= 16degree * 17.9ppm/degree = 285ppm hardness (as CaCO3)

I would assume the other 5ppm comes from things like Fe and Mn.

Kevin

[malkore]

we are talking about GH.

KH is, as I understand it, STRICTLY a measure of CaCO3 (calcium carbonate).
My Kh is 8degrees, my Gh is 11 degrees...so the majority of my Gh is coming from calcium carbonate...yes?

[Rolo]

Ok just to set the record straight

GH is STRICTLY a measure of hardness in CaCO3. (It is also impacted somewhat by MgCO3 a very minimally by other metal like iron.)
KH is STRICTLY a measure of CO3-- and HCO3-, carbonate and bicarbonate ions.

Malkore, you can almost always assume that GH hardness is mostly caused by Ca++ ion. Mg++ makes up a smaller portion.

[KevinC]

Yes, we chemists have made this confusing. I'll try to clear it up a little bit (again, this does get technical):

Definition: Titrating is the process of adding one chemical solution to another until a reaction is completed (this is the same as adding one drop at a time from test kits).

GH is measured by titrating metal ions with EDTA (a chelating agent) at a pH of about 10. An indicator is used which changes color when all the metal ions have been chelated by the EDTA. At a pH of 10, many metal ions are INsoluble and so do NOT react with the EDTA. However, Ca+2, Mg+2, Fe+2, Ba+2, and Sr+2 all will react. By historical convention, the result is expressed as ppm CaCO3 or dGH, so we are basically pretending it is all Ca and it is all bound to CO3-2 (though this is far from true - do NOT use this number to calculate your carbonate level). In saltwater, Mg is the major contributor to hardness. In freshwater Ca is usually the major contributor - as in Ninoboy's water, the Ca/Mg ratio is 2.8:1 (ppm basis). And we all should realize that in planted tanks the Fe concentration is too low to change the GH.

KH is measured by titrating bicarbonate (HCO3-) and carbonate (CO3-2) ions with a strong acid to a pH of about 4.5. An indicator is present which changes color when the pH reaches 4.5. This reading is expressed as milliequivalents/L or as dKH. Of course, if phosphate or other buffering compounds are present they are titrated as well and you are measuring the combined level of all buffering agents.

The net result is the GH tells you how much Ca and Mg is available in your water while KH tells you how well your water will resist pH changes when small amounts of acid or base are added.

Kevin

[ninoboy]

Malkore, I also had the same assumption as you until yesterday. I read a whole bunch of General hardness articles. Most articles from aquarium sites are misleading to us non-chemists.

Kevin & Rolo, Thanks you for clearing that up. As I said above, many articles are actually misleading. I saw some calculations even at the Kribs and someone just added both number together.

So, for us, chem. challenged people, can we assume that the formula is somewhat like :

(ppm Ca / 7.2) + (ppm Mg / 4.4) = dGH

Is that correct?

Man, should put this calculation into the sticky or you guys should write an article on this calculation. All the articles I read were totally unuseful.

[Jacan]

I hope this can be at any help

M(CaCO3) = 100,09g/mol
M(CaO) = 56,08g/mol
M(Ca) = 40,08g/mol
M(Mg) = 24,31g/mol
M(HCO3) = 61,02g/mol

10mg CaO/L = 1ºdH (deutsche härtegrader)
10mg/L x (40,08g/mol / 56,08g/mol) = 7,147mg Ca/L = 1ºdH
10mg/L x (24,31g/mol / 56,08g/mol) = 4,335mg Mg/L = 1ºdH
10mg/L x (2x61,02g/mol / 56,08g/mol) = 21,761mg HCO3/L = 1ºdH

1mg CaCO3/L = 1 ppm
1mg/L x (40,08g/mol / 100,09g/mol) = 0,400mg Ca/L = 1ppm
1mg/L x (24,31g/mol / 100,09g/mol) = 0,243mg Mg/L = 1ppm
1mg/L x (2x61,02g/mol / 100,09g/mol) = 1,219mg HCO3/L = 1ppm

ppm → ºdH
(56,08g/mol / 100,09g/mol) / 10 = 0,056

ºdH → ppm
(100,09g/mol / 56,08g/mol) x 10 = 17,848

ninoboys tap water contains 72ppm Ca plus 26ppm Mg and alkalinity 133ppm (72mg Ca/L, 26mg Mg/L, 133mg HCO3/L )
GH (72mg/L / 0,400) + (26mg/L / 0,243) = 286 ppm
KH 133mg/L / 1,219 = 109 ppm (taken for granted that it is HCO3)

[Rolo]

Thanks alot for this post Jacan. I've been searching everywhere for the definitions of a dH, and exactly how they came up with the 17.8ppm:1dH ratio. Did you write this yourself?