hardness/alkalinity scales, testers, alkalinity

Karlsbad

Karlsbad

Sharkbait
Sep 23, 2003
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OK since I began testing my water I've used these mardel multi test strips, and they indicate total alkalinity/buffering capacity in ppm, and total hardness in ppm.

How do these relate to kH and gH and TDS? The total hardness has a scale of 0(duh) to 425 ppm with the range of 50-120 being moderate, I think this is actually a measure of TDS on the scale that TDS is measured, but I'm not sure, is that right? I also believe that TDS has a ratio to gH, is that right? Is kH carbonate hardness and is that a measure of alkalinity/buffering(I think kH is carbonate hardness but not a measure of alkalinity but you would think one of those would indicate alkalinity)?

Alkalinity is what is necessary to maintain a stable pH, is that correct? A tech support guy at Kent told me that, which kind of shocked me I thought for sure it was hardness that mattered. (I have no qualms about talking to tech support for a half hour about a $10 purchase, does that make me responsible for Dell tech support being in India?)But we call low pH alkaline, so wtf. Did I miss anything? If anyone knows some of the answers here go ahead and please say what you know.

I want to get an electronic hardness tester but the ones at bigals and drsfostersmith only seem to measure TDS, am I missing something in this regard?
 
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Wow! There are a few questions in there! I'll try not to miss any. Grab a coffee, this'll take a while!;)

Okay, so:
KH=alkalinity=buffering capacity
GH=1/2TDS I'm not positive about this. But GH measures total Ca (calcium) and Mg (magnesium) which are most often associated with CO3 (carbonate) so for every Ca or Mg you measure with your GH test, there's 1 ion you're missing.

alternatively, an approximation could be:
TDS = GH + KH

Both of these GH relationships completely fail if you use a water softner, add salt, KCl (potassium chloride), PO4 (phosphate) and there are probably a few others as well, but I think that these are the most likely to make any difference. That should cover a few of the questions.

KH is referred to as carbonate hardness. It's a terribly ineffective term. It completely fails to describe what KH is. The term alkalinity is more proper.

KH or alkalinity is a measure of your buffer strength. Specifically, it measures all the "strong" bases you have in your system. That would mean OH (the basic form of water) and the "conjugate bases" of any "weak acids". Sorry, there's a lot of jargon in there, but I wanted to be as specific as possible.

An acid is any compound that either gives off an H+ or causes water to do so. A "strong" acid is one that completely dissociates in water, so hydrochloric, nitric, sulfuric acids, for example. A "weak" acid dissociates based on the pH of the water and the amount of its conjugate base present.

The term "conjugate" means, in a nutshell, what's left after the dissociation. So, for the carbonic acid system:

H2CO3 (carbonic acid, a weak acid) = H+ + HCO3- (the conjugate base)

The conjugate base of a weak acid is, by definition, a "strong" base. This is a little complicated with things like the carbonate system because there is a further dissociation.
 
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...and we're back for more...

so, to sum up:

KH = alkalinity = (HCO3-) + 2x(CO3--) + 3x(PO4---) + (OH-) - (H+)

Since strong acids, i.e. H+ will neutralize the bases, H+ is subtracted from the equation. Other compounds may also play a role, but these are the most likely to actually be in your tank.

These also reflect the buffering capacity of your tank, since any changes to pH will affect the equilibrium of the first three compounds in the equation.

Alkalinity is the buffer, not hardness. Specifically, hardness is a measure of Ca and Mg, which aren't directly affected by pH.

High pH is alkaline, not low pH. Low pH is acidic.
 
No, I think your post answered the question pretty thoroughly. (I confess I get lost at the part with the equations...). I just know that part of the struggle understanding this stuff is that people bandy about degrees and ppm when discussing hardness, and if you don't have a good feel for their relationship, it gets pretty confusing...

Jim
 
Whew. I only hope that it wasn't too convoluted. Sometimes I have a hard time making myself clear. :rolleyes:

I've never used degrees. For all that I'm not keen on ppm, it at least has more basis in reality than a degree of hardness, IMO. But maybe it's just the way that I see things. ;)
 
I don't disagree at all with the expansions of what KH and GH are, but I disagree that any combo or measure or multiple of those has any relationship to TDS. TDS includes nitrate, chloride, sulfate, etc., (all the anions which do not register at all on KH scales) plus all the cations which are equally unresponsive on GH such as sodium, potassium, etc., so while TDS includes those, it is in no way a sum or funtion or KH/GH.
 
Ah. I didn't know that nitrates and sulphates were included. So, does TDS mean everything non-H2O in the water then? I don't think that I've come across the term until these boards, so I assumed that "total dissolved solids" meant things like salts. Although now that I think about it, yeah, KNO3 = solid.:rolleyes:

Well, there ya go. No way to tell TDS from standard tests.
 
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