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Some handy facts about water

_Maq_

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Water hardness​

Introduction
Water hardness provides a very simple indication of whether our water is ‘hard’ – richly mineralized, or ‘soft’ – sparsely mineralized.
Hard water usually forms when rain percolates through base rich rocks and a small proportion of that rock is dissolved and goes into solution as ions. If rain falls onto a catchment that is entirely without base rich rocks the water remains “soft” and sparsely mineralized.
Water hardness does not measure other dissolved solids or the bicarbonate content.

This is a water hardness map for UK tap water.
_England&Wales.jpg


Permanent Hardness
“Permanent” or “General” water hardness refers to the multivalent ion content of the water. In practice these ions are nearly always calcium (Ca) and magnesium (Mg). This is because calcium and magnesium are usually the most abundant cations dissolved in water.
Nothing else contributes to permanent hardness, and when just “water hardness” is mentioned, we are referring to the “permanent hardness”.
Ca and Mg are just two of the six alkaline-earth metals found in group 2 of the periodic table. The other four, beryllium (Be), strontium (Sr), barium (Ba), and radium (Ra), do not make a significant contribution. The same applies for all other metals with multivalent ions (such as iron (Fe), manganese (Mn), aluminum (Al) etc).
There are exceptions, though. The most significant among them is perhaps the water we use for keeping Rift Lake cichlids. This has little calcium, slightly more magnesium and is rich in sodium (Na), which is alkaline but does not contribute to the permanent hardness (it is a monovalent ion (Na+)).

Units
Traditionally we measure water hardness in German degrees (°dGH).
Water processing plants often express hardness in ppm (mg/L) or mmol/L (milimol per litre). Using molar or mg/L values is to be preferred over using derived units like °dGH, but we can convert from one unit to another.
1 mmol/L = 5.61 °dGH
1 °dGH = 0.18 mmol/L
Ca: 1 mg/L = 0.14 °dGH = 0.03 mmol/L = 24.95 µmol/L (µM)
Mg: 1 mg/L = 0.23 °dGH = 0.04 mmol/L = 41.14 µmol/L (µM)
(Identical amounts by weight of Mg and Ca do not contribute equally to water hardness because their relative atomic masses (RAM) differ: RAM Ca = 40.1, Mg = 24.3.)

Alkalinity

Alkalinity is often – inaccurately – called carbonate (or temporary) hardness. In fact, alkalinity or acid neutralizing capacity (ANC) refers to the amount of strong acid needed to change pH from current value to a different (lower) value.
ANC4.5 is used regularly in water processing plants’ reports, and it denotes ANC to reach pH value of 4.5 – which is a point where no bicarbonate ions (HCO3-) remain in water. ANC is not often used in the UK, but is a very useful concept.
The “drop tests”, commonly used by hobbyists, measures ANC4.5, and the result is expressed in German degrees (°dKH).

This may be a bit confusing in two ways:
  1. The units are similar, as well as the obsolete term ‘carbonate hardness’, but alkalinity is not necessarily related to water hardness. A sample may contain lots of dissolved calcium and magnesium and still feature (near-)zero alkalinity. And vice versa, it is easily possible to make a solution with high alkalinity without any calcium or magnesium. - It is only because in practice, higher content of Mg and Ca is often accompanied by higher content of bicarbonates, and these numbers are often provided side by side.
  2. Alkalinity (ANC4.5) does not consist solely of bicarbonates, but also phosphates, silicates, and various organic compounds.
It helps to remember that water hardness refers to cations (Mg2+, Ca2+, a. o.) while alkalinity to anions (HCO3-, H2PO4-, HPO42-, H3SiO4- a. o.).

Like water hardness, alkalinity too is best expressed in mmol/L or mg/L.

alkalinity.png


1 mmol/L = 2.80387 °dKH
1 °dKH = 0.356650 mmol/L
HCO3- : 1 mg/L = 0.016389 mmol/L = 0.0460 °dKH

Carbonate system

Carbonates are of paramount importance in natural waters as well as in our tanks. When carbon dioxide (CO2) dissolves in water it also reacts with water, and following compounds appear: loosely hydrated CO2·H2O (commonly abbreviated as CO2), H+ + HCO3- (bicarbonate), and 2H+ + CO32- (carbonate). The distribution of each compound depends on pH and, to a lesser extent, on temperature.

carbonate_system.png


We can see that in neutral-to-alkaline water, plants are forced to use bicarbonates instead of much-preferred carbon dioxide for photosynthesis. Decades ago, scientists believed that among submerged higher plants, some species are able to use bicarbonates while others are not. Recently the opinion prevails that various species differ in degree of ability to use bicarbonates, and there is no strict boundary either/or. Without exemptions, all higher plants prefer carbon dioxide if available.

If we know pH and alkalinity, we can calculate the CO2 content using following equation:
formula2.jpg

where A stands for alkalinity in German degrees (°dKH) and 𝛠(CO2) stands for CO2 content in mg/L.
Alternatively, you can use the attached chart Tillmans.xlsx.

In planted tanks, we mostly prefer slightly acidic water. There are several reasons for that. First, as we can see, in acidic water larger share of inorganic carbon takes up the form of dissolved CO2. Second, most micronutrients (B, Fe, Mn, Zn, Cu, Ni) and phosphorus are better accessible to plants’ roots in moderately acidic environment.
There is one more reason of which hobbyists are often unaware – bicarbonates can hinder assimilation (in contrast to uptake) of iron (and perhaps other transition metals) inside the plant. So, pH and bicarbonate content (alkalinity) are tightly tied together but have differing effects on plants. Various species differ significantly in their tolerance to both.
This may be of significance when injecting CO2. Addition of CO2 to water lowers its pH but does not change its alkalinity, since the reaction produces the same number of positively contributing species (H+) as negative contributing species (HCO3− and/or CO32−).
Let’s have a look at the following pic (note the alkalinity axis is logarithmic):

co2_injection.png


The blue field encompasses combinations of pH and alkalinity feasible in natural habitats (CO2 content ranging from 0.5 to 4 mg/L). If we increase CO2 content to 10 to 30 mg/L, we enter the red field. We reach pH=6.5, for example, but the alkalinity remains high (from 0.9 to 2.7 °dKH). Such a coincidence is inevitably outside the blue field – the conditions our plants experience in nature.
Is it of practical significance? That remains open to discussion, observation, and research.

Electric conductivity

Electric conductivity is indicative of the quantity of dissolved compounds. As a rule, the more dissolved compounds the higher conductivity. The SI unit of electrical conductivity is S/m (siemens per metre).
1 S/m = 1000 mS/m (mili Siemens) = 10000 µS/cm (micro Siemens).
1 µS/cm = 0.1 mS/m
1 mS/m = 10 µS/cm

Because our water is a “dilute solution” conductivity values are usually shown in micro Siemens (µS) /cm.
Electric conductivity is neither identical, nor linearly related to, total dissolved solids (ppm TDS). Electric conductivity measures the concentration of electrically charged ions (cations and anions) in water. Non-charged dissolved compounds – typically many organic compounds – do not change conductivity.
Total dissolved solids meters (TDS meters) measure electric conductivity and convert it by a constant k: TDS [ppm] = k · EC [µS/cm], where k is either set by the manufacturer or can be adjusted by the user. The conversion factor k usually varies between 0.5 and 0.8. There is no generally accepted value for conversion coefficient. Thus, the deviations among TDS meters are quite significant, and aquarium hobbyists should be discouraged from measuring total dissolved solids.
Measuring TDS is problematic, even in laboratory conditions. When boiling the sample (105 °C), some compounds are lost by evaporation (NH3 etc.), some change their nature (bicarbonates turn to carbonates and even to hydroxides), and some compounds keep water of crystallization (MgSO4·7H2O, etc.). Even more complications occur in the case of organic compounds.

<Many thanks to @dw1305 , @Tim Harrison , @Zeus., @Geoffrey Rea, @Hanuman , @LondonDragon , @X3NiTH for their valuable input and criticism.>
 

Attachments

  • Tillmans.xlsx
    34.8 KB · Views: 158
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Many thanks for tackling a tricky topic, it has so many units used by different countries and many products switch between units in the data on them which I found to be a minefield when adding products to the IFC calculators remineralising sheet.
If folk find the topic hard to follow/understand then your in the same place as most folk.
If folk find any errors in any of the information - Please let us know ;)
 
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Permanent Hardness
“Permanent” or “General” water hardness refers to the multivalent ion content of the water. In practice these ions are nearly always calcium (Ca) and magnesium (Mg). This is because calcium and magnesium are usually the most abundant cations dissolved in water.
Nothing else contributes to permanent hardness, and when just “water hardness” is mentioned, we are referring to the “permanent hardness”.
Permanent hardness is not the same as General hardness which should more accurately be called “Total Hardness” as it was mistranslated from the German “Gesamthärte“ meaning Total hardness. The term “Total” makes more sense as GH is the sum of carbonate and non carbonate hardness. TH=CH+NCH. See attached document for a fuller explanation...
 

Attachments

  • CarbonateHardnessTest.pdf
    54 KB · Views: 185
Hi all,
Thanks, but, ahem, we have water in Scotland... lots if it. To be accurate it's a map for what we, up here, call rUK.
Apologies, that was my fault, not @_Maq_ 's. I was just recognising that it is only a matter of time before Scotland becomes the <"Republic of Alba"> and N. Ireland votes to rejoin the EU by stealth by becoming part of a united island of Ireland.

My wife is always asking why we can't vote for Nicola Sturgeon, so who knows? Possibly even some southern enclaves of Alba may eventually be expunged from the water hardness map.

cheers Darrel
 
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Hi all,
Permanent hardness is not the same as General hardness which should more accurately be called “Total Hardness” as it was mistranslated from the German “Gesamthärte“ meaning Total hardness. The term “Total” makes more sense as GH is the sum of carbonate and non carbonate hardness.
It is just <"a mess isn't it">? I know you've tried to sort it out before in the <"linked thread">, where I think @X3NiTH 's post is where we are at.

@_Maq_ originally offered to write the article because of the inconsistencies he had found in our "water hardness sticky", which @James_C had written a long time ago.

I probably haven't helped, because I've tended to assume that dKH and dGH are different ways of measuring the same thing (the amount of CaCO3, as Ca++ and 2HCO3-) where they are actually measuring two different things, and the dGH = dKH argument is only true in those specific circumstances.

The actual derivation of the hardness units is also fairly strange, and relates the values to their (often hypothetical) calcium oxide (CaO) content: <"Water Hardness">.
dGH - By definition, 1dGH = 10 mg/liter CaO
Atomic Weight Ca = 40, O = 16, CaO = 56
So 10 mg/liter CaO contains 40/56 *10 = 7.143 mg/liter of Ca
By definition ppm Ca is not for elemental calcium but for ppm CaCO3.
Atomic weight CaCO3 = 100
So 7.143 mg/liter of elemental Ca would be expressed as 100/40 * 7.143 = 17.8575 mg/liter(ppm)CaCO3.
1dGH = 17.86 ppm CaCO3 and 7.143 ppm Ca2+.
dKH -
1dKH = 17.86 ppm CaCO3
From above; 1dKH = 17.8575 mg/liter CaCO3. 7.143 mg/liter of this is Ca, the rest ;(17.8575-7.143)= 10.7145mg/liter CO3
1dKH = 10.7145 ppm CO3
For bicarbonate:
CaCO3 forms Ca(HCO3)2 in water at pH less than 10.25 . (Two bicarbonates are formed from each carbonate ion):
CaCO3 + H20 + CO2 ---> Ca(HCO3)2
CO3 mw = 60, HCO3 mw = 61
Therefore 10.7145mg/liter CO3 from CaCO3 (each CO3 carbonate anion forms two HCO3 bicarbonate anions; 61/60*2 *10.7=21.8 mg/liter HCO3

That was why talking about <"milliMols"> (or <"mg / L">) of Ca++ etc. <"makes more sense"> and <"removes all ambiguity">, and that was actually where @_Maq_ started and I suggested we add in "degrees of hardness", and that was because utility companies etc are going to be using all sorts of units, so you need to be able to translate from one value into another. <"Hardness convertor">.

cheers Darrel
 
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Thanks, but, ahem, we have water in Scotland... lots if it. To be accurate it's a map for what we, up here, call rUK.
Hereby I solemnly declare that this part is a work of another person, and I heartily disagree with what I've written. And vice versa.:)
 
As a political solution I did see someone propose we take over rUK for a bit, while certain people calm down and sort themselves out. In terms of water, what a shower.
 
Thus, the deviations among TDS meters are quite significant, and aquarium hobbyists should be discouraged from measuring total dissolved solids.

Hi @_Maq_ I've been harping on this as well in the past and generally agree with you, but I think we should be more pragmatic about it - especially if your otherwise excellent post becomes a sticky here on UKAPS.

The problem, of course, is that every recommendation (almost every) in this hobby from sellers of livestock, popular articles (and scientific papers even) about the preferred conductivity for our specific livestock species are all more or less specified in TDS - same goes for fertilizer calculators (which sometimes can be problematic as various elements conduct differently at same ppm which at face value can be confusing regardless of whether you measure TDS or uS/cm). Whether TDS makes sense or not sort of depends in my opinion... If you're using a TDS meter to monitor fluctuations in dissolved solids and to make sure you get your remineralization / fertilizer dosing right over time, I think the use of a TDS meter is very valid and should in fact be very much encouraged as a helpful measure of consistency and stability in addition to using your eyes obviously - I mean, how else would you go about this as an average but dedicated hobbyist? Yes, lots of organics won't show up directly, but "conductive" waste minerals from decaying plant matter, fish- and food waste will... Sure, it's not telling us the whole story, but it will give you a hint of a trend; if your TDS is ballooning you can be almost sure your maintenance regime, dosing or what have you is at fault. We are dealing with many known unknowns in this hobby and probably as many unknown unknowns... Dissing the use of TDS meters would be a disservice to the hobby in my opinion... It may not be exactly what you meant, but it could come across as such - and you're obviously an expert (and valuable contributor) so that carries more weight.

Comparing TDS between different tanks only make sense if we are referencing the same conversion factor. However, in the lower range of the uS/cm range say <150 uS/cm and using common conversion factors such as 0.5, 0.64 and 0.7 it won't really make a whole lot of meaningful difference at say 100 uS/cm for our livestock (or plants) what conversion factor is employed; be it 50, 64 or 70 TDS... unless your breeding excruciatingly picky livestock ... its fairly well around the tolerance of consumer grade measurement devices anyway.

When people ask about what TDS meter to get for their planted tropical freshwater tank I always recommend to ideally get a low range device (0-1500 uS/cm or 0-750 ppm TDS(x0.5) ) with temperature compensation and one that will give you an EC reading as well as TDS - and know what conversion factor the device uses if it only reads TDS.

Anyway, this just caught my eyes on the initial read - I hope perhaps the language can be tweaked a little to give merit to monitoring TDS - I personally consider an EC/TDS meter as important as a thermometer!

I also have some reservation on the Alkalinity/pH/Co2 section, but that will have to wait for another time - and there are people around here that are infinitely more knowledgeable about this than me that I believe would raise some finer points and be able to contribute to this part.

Cheers,
Michael
 
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Traditionally we measure water hardness in German degrees (°dGH).
May do on the continent but in the UK the traditional unit is degrees Clark, sometimes degrees French. The last this Sceptered Isle was invaded was by the bl@@dy Normans.

Very good posting. The amount of time and effort shows. Well done.
 
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Hi all,
about the preferred conductivity for our specific livestock species are all more or less specified in TDS - same goes for fertilizer calculators (which sometimes can be problematic as various elements conduct differently at same ppm which at face value can be confusing regardless of whether you measure TDS or uS/cm). Whether TDS makes sense or not sort of depends in my opinion..
I agree in some ways, we can't get away from "ppm TDS" even though conductivity as microS / cm makes much more sense and is what the meter reads, even <"if the display says "ppm TDS">.

cheers Darrel
 
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I can see there's a localization issue here.

When it comes to measuring hardness and alkalinity, German degrees are predominantly used among aquarists in my country, while Clark and French units are virtually unknown. So I stick to German degrees in all my published works, while I personally prefer moles. Not only because molar units make all calculations easier, but also because German degrees are quite big. Once you get rid of CO2 injection, anything above 2 °dKH leads to very high pH, over 8. All my tanks are definitely below that level, with pH varying from 5 to 8, and all of them fall under "very low alkalinity" category.

As for electric conductivity (EC) and total dissolved solids (TDS) question, I can report proudly that this is one of the few points where hobbyists of my country are somehow ahead. In ours, nobody pays attention to TDS and all understand that there are actually no TDS meters, only EC meters. While I appreciate the arguments presented by @MichaelJ and others, using TDS units is still an unnecessary detour from what we measure to something we can only estimate.

All in all, it's largely a matter of custom. There are well-thought reasons for using SI units. Still, people tend to keep old ways. Beside that, if we suddenly begin using "better" units, there would appear trouble with reading all what had been written before that glorious day. I think UK residents have encountered it quite recently with departure from pounds and ounces to kilograms and grams.
 
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