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Calcium Chloride vs. Calcium Sulphate?

MichaelJ

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9 Feb 2021
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2,219
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Minnesota, USA
Hello,

This has being brought up from time to time in remineralizing related threads, and I wanted to hear what the chemistry experts and people with experience and knowledge of the pros and cons have to say on the topic of Calcium Chloride (CaCl2) vs. Calcium Sulphate (CaSO4). My own motivation for using CaSO4 is the lower TDS footprint from CaSO4 compared to CaCl2. I have used both and haven't experienced issues with either (although I only used CaCl2 for a few months last year, but CaSO4 for years - first with Equilibrium and later from individual salts).

When targeting 25 ppm of Ca (a very common situation):

With CaSo4 you get 20 ppm of S (Sulphur) totaling 45 ppm. Solubility ~0.26 g/100ml
With CaCl2 you get 44 ppm of Cl (Chloride) totaling 69 ppm. Solubility ~85 g/100ml

For a practical translation into TDS I ran the tests in 3 liters of RODI water targeting ~25ppm and measured the TDS with my Hanna DIST1:

0.32g of CaSO4 = TDS ~62 ppm.
0.27g of CaCl2 = TDS ~86 ppm.

Both Sulphur and Chloride are essential nutrients for plants. Plants absorb Sulphor as Sulphate (SO4), and is considered a Macro nutrients, but as a weekly target we probably don't need more than perhaps a few ppms of S in the form of SO4 - anyone knows? much, much less so for Chloride given that is generally characterized as trace element.

One thing that often comes up as an advantage for CaCl2 is the solubility vs. CaSO4. CaCl2 is orders of magnitude more soluble than CaSO4, but in my experience the solubility one vs. the other seems to be a non-issue IF you target moderate to low GH and add the dosing to the WC water or tank right away. For instance, at 0.26g/100ml you will have to add 26 grams into 10 liter to get to the limit - thats 605 ppm of Calcium! If you target the aforementioned 25ppm of Ca you will only add ~1g or ~26 times below to solubility limit, to a 10 liter volume. However, if you pre-mix CaSO4 in a smaller dosing bottle for several applications (high concentration) you will likely have trouble dissolving the CaSO4.

I have noticed that no commercial freshwater GH remineralizers (that I am aware of) are using Calcium Chloride. This is not a raw material price consideration as far as I can tell, as Calcium Chloride is generally somewhat cheaper than Calcium Sulphate.

Now, one thing I read recently with the issue of sulfates is that under severe anaerobic conditions at the substrate level the sulphate may convert into H2S (Hydrogen sulfide - a very toxic gas even at <1 ppm) - when or at what SO4 levels that might happen I do not know. I saw a note about this from D. Walstad in at another forum as well, and believe our member @X3NiTH might have experienced this (would be great to hear more about this @X3NiTH).

I don't really consider toxicity at the water column level one vs the other as we are usually not targeting levels high enough for that to be a problem for our livestock (both Sulphate and Chloride are bad in very high concentrations anyway...). But Chloride might be an issue nevertheless... At least around here in Minnesota Chloride run-offs (mostly from road salt) is a problem for natural waterways: Chloride 101.

So the question is, would our plants be more or less "happy" with CaSO4 vs. CaCl2 with respect to uptake, interaction with other chemicals etc? That would be interesting to know.

Cheers,
Michael
 
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jaypeecee

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21 Jan 2015
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Bracknell
Both Sulphur and Chloride are essential nutrients for plants. Plants absorb Sulphor as Sulphate (SO4), and is considered a Macro nutrients, but as a weekly target we probably don't need more than perhaps ~5 ppm's of S, much, much less so for Chloride given that is generally characterized as trace element.
Hi @MichaelJ

May I ask - how important is chloride in freshwater aquatic plants? I ask this because in Diana Walstad's Ecology of the Planted Aquarium, Table VII-2, chloride doesn't get a mention. And, yet in Table VII-1, chloride is considered a required nutrient being involved in osmosis, charge balance and photolysis of water.

Am I overlooking the obvious?

JPC
 

MichaelJ

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Thread starter
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9 Feb 2021
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Location
Minnesota, USA
Hi @MichaelJ

May I ask - how important is chloride in freshwater aquatic plants? I ask this because in Diana Walstad's Ecology of the Planted Aquarium, Table VII-2, chloride doesn't get a mention. And, yet in Table VII-1, chloride is considered a required nutrient being involved in osmosis, charge balance and photolysis of water.

Am I overlooking the obvious?

JPC
Hi @jaypeecee, Good question and that is what I want to find out. Chloride is needed in small quantities and aids in plant metabolism, photosynthesis, osmosis (movement of water in and out of plant cells) and ionic balance within the cell. Many popular blends (including Seachem Comprensive and Tropica) include Chloride, but in tiny amounts - i.e. With Tropical specialized you get 0.6 ppm/week if you follow the direction on their website (6ml/50L weekly). Sulphur on the other hand is often listed as an essential Macro nutrient - how much they actually need seems questionable! Anyone knows?

Cheers,
Michael
 
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Simon Cole

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25 Dec 2018
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Location
Buckingham
Now, one thing I read recently with the issue of sulfates is that under severe anaerobic conditions at the substrate level the sulphate may convert into H2S (Hydrogen sulfide - a very toxic gas even at <1 ppm)
One way of testing this theory might be to dissolve varying quantities of sodium metabisulphite into your aquarium water in order to assess potential for emssions using olfactory resonance as the measurement; the olfactory detection limit for hydrogen sulfide is ≥5 ppb (Hoshika et al. 1993). It's not been absolutely proven that there are not chemical or biological conditions suitable for sulphidogenesis in anaerobic aquarium conditions, but unless your aquarium can emit from the water column as a gas, then I don't see it as that much of a problem... unless it bubbles up from the substrate like those horrible stories you hear of lakes near volcanic springs.

So the question is, would our plants be more or less "happy" with CaSO4 vs. CaCl2 with respect to uptake, interaction with other chemicals etc?
The calcium cation may have a slight tendency to bond to phosphate anions, but that isn't something that would concern me because I doubt that this would have a measured impact, and calcium phosphate probably only precipitates when it goes out of equilibrium which seems rather unlikely in aquarium water.
 
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_Maq_

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23 Jun 2022
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Location
Czech Republic
Hi @MichaelJ

May I ask - how important is chloride in freshwater aquatic plants? I ask this because in Diana Walstad's Ecology of the Planted Aquarium, Table VII-2, chloride doesn't get a mention. And, yet in Table VII-1, chloride is considered a required nutrient being involved in osmosis, charge balance and photolysis of water.

Am I overlooking the obvious?

JPC
There are two levels.
Chlorine is a micronutrient in the sense that very few chlorine is needed for plant's survival and cannot be substituted by anything else.
Actual presence of chlorides in plants is on the level of macronutrients (much more than P, S). Chlorides help maintaining homeostasis in plant's fluids. In this role chlorine is not essential because it can be substituted by nitrate or organic acids, but can be considered beneficial because chlorides are often the best option.
 
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