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Tap water Hardness Qs

6K views 19 replies 6 participants last post by  KevinC 
#1 ·
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 ?
 
#2 ·
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.
 
#3 ·
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... :icon_redf
 
#4 ·
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.
 
#5 ·
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!
 
#7 ·
Sorry, now I'm confused :icon_conf .

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?? :confused:

Laith
 
#8 ·
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-plants/month.9903/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.
 
#9 ·
OK: Ignore this if you really don't like the chemistry.

ninoboy said:
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-plants/month.9903/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:

web said:
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
 
#11 ·
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.
 
#12 ·
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
 
#13 ·
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.
 
#14 ·
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)
 
#16 ·
Wow, thanks a lot. Now I'm confused with the KH. So the alkalinity in my report is not the exact KH? Sorry if this is a stupid question :icon_redf

The result from my test kit is actually 115ppm although I know it couldn't be very accurate.

Thanks again :proud:
 
#17 ·
I have a hard time to find the definition of 10mg CaO/L = 1ºdH and 1mg CaCO3/L = 1 ppm, but I got it from some german sites (not aqaurium sites :proud: ). The rest I Figure out from a danish chemist site.

Talking about GH and KH there is a danish standard DS and also DIN (deutsche industri norm) defining GH as Ca + Mg no more no less.
With KH it is not so clear but mainly with ph < 8,32 KH = HCO3
 
#18 ·
Jacan, That is very nice that you found the info on a scientific site. I wish we were on the Danish system since it seems to be the simplest! :proud: . I know what you mean about aquarium sites! Quite frankly, I find these sites trying to explain chemistry very poor. Before I get into most articles I'm always finding very basic chemistry principals left out or simply misunderstood. Do you still have the addresses of those chemistry sites?

I know exactly what the Danish site is talking about when they say, pH < 8.32, KH = HCO3. :icon_bigg . The carbonate buffer system is as so:

H2CO3 <----> HCO3- + H+ <----> CO3-- + H+

What dictates the concentrations of each of the three carbonates is pH. At a pH of 6.35, the concentrations of H2CO3 to HCO3 are equal. This is called the half equivalence point. 6.35 is also called the pKa of H2CO3. At a pH of 10.3, the concentrations of HCO3 and CO3 equal. 10.3 is another half equivalence point and the pKa of HCO3. (pKa is a constant used for a chemistry buffers equation).

The median value between 10.3 and 6.35 is 8.32. This is where the pH of 8.32 came from on the Danish site. At 8.32, HCO3 is by far the primary ion (the ratio of H2CO3/HCO3 and CO3/HCO3 is both ~1/87). If you have water that has a substantial dKH level, you might have realized that the pH always reads ~8.3 after it has been out gassed (unless you have phosphate buffers). Buffers work most efficiently at their pKa values; the carbonic buffer system, having 2 pKa's, stabilizes right in between both at a pH 8.32.

So, we know that our that our KH kits measure both HCO3 and CO3. But now we also know that when the pH < 8.32, nearly 100% of the KH is due to HCO3. This is because the higher you go beyond 8.32, the conc. of HCO3 and CO3 become more equal to each other(10.25), meaning the KH is a composite of both ions. Already at 8.32 the conc of CO3 is small, but dropping the pH further makes it insignificant.

Sorry for the confusing and long explanation. I will provide a graph that displays what I said visually, and maybe some math.
 
#19 ·
http://www.natlex.dk/syrekonst.html
http://www.natlex.dk/koncentration.html
Beregning af molarmasse & koncentration

Hey Rolo
This is the danish site I was referring to.
The first is a datasheet where you can find pKa (=pKs) at 25 ºC which is the most common temperature in aquarium.
H2CO3 pKa=6,37 HCO3 pKa=10,32 CO3
The reason pH 8,32 is used is because the ratio of HCO3 is hundred times more than CO3 log2 = 100 (10,32-2=8,32), not because of the median value.

The other site can be a great help calculating the molarmass
Pb2(SO4)3 = 702.61g/mol press the ”beregn” button
Just type the solids you want and you will get the molarmass

I know the graph you are writing about and the math
http://www.thuisexperimenteren.nl/s...rbondioxide in water equilibrium.doc#B0000006

Actually it is possible to calculate the KH (HCO3) if you now the pH on out gassed water with a pH < 8,32

Calculating [HCO3-] pH = 8 at 25°C.
1,16*10↑-5 is the equilibrium for CO2 at 25°C
pH = 8, less than 8,32
H2CO3(aq) + H2O(l) = HCO3-(aq) + H3O+(aq)
[H3O+] = 10↑-pH
[H3O+] = 10↑-8
Ks = ([H3O+][HCO3-]) / ([H2CO3])
10↑-6,37 = (10↑-8 x [HCO3-]) / 1,16*10↑-5
[HCO3-] = (10↑-6,37 x 1,16*10↑-5) / 10↑-8
[HCO3-] = 4,95*10↑-5
HCO3- = 61,02g/mol x 4,95*10↑-5mol/L
HCO3- = 0,0302g/L = 30,2mg/L

please comment this, because its a calculation I found but I am not sure if it is correct.

Jacob Andersen
 
#20 ·
Jacob:

Two things - first, your last calculation assumes that CO2 is at equilibrium with the atmosphere - since we usually inject CO2, it probably is at a higher concentration.

Second, in a prior post you assumed that 133ppm alkalinity = 133ppm HCO3-. I was under the impression that when alkalinity is reported in ppm it is actually the ppm CaCO3 equivalent (the amount of CaCO3 you would need to dissolve in water to achieve the same alkalinity level). If in your situation it is 100% HCO3-, convert by multiplying by 61/100 (molar mass HCO3-/molar mass CaCO3).

Kevin
 
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