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Calcium Sulfate (gypsum) CaSo4.2H2O

Hanuman

Hanuman

Member
Joined
4 Jan 2019
Messages
2,504
Location
Thailand
This is a question for the chemists or those who have anchored experienced in chemistry.

I am experiencing so odd situation. I bought some Calcium Sulfate CaSo4.2H2O as can be seen in picture below.
IMG_3794.jpg
I have been having some trouble defining the exact dose for my tank. I assumed my tank was somewhere around 80/100 L when taking into account all hardscape and substrate. Ok so I went to Rotala and figured I would need around 6.87g of CaSo4.2H2O to reach 20ppm ca + 16ppm S for 80L of water. So far so good. Issue comes when I add that amount in my tank after a water change, ppm is way higher so I though maybe I had over estimated the amount of water and so I have slowly been reducing the amount of CaSo4.2H2O added to reach 36ppm (20Ca+16S). I am now dosing 4.8g. I usually measure ppm around an hour after doing an RO water change and adding the CaSo4.2H2O and before adding any other ferts. Problem is that for that amount of CaSo4.2H2O it would mean my tank is 55L and that's definitely not possible.

So I made a control test.

Took 1L of RO water in a Pyrex beaker added 0.86g (should have added 0.086g to reach 20Ca+16S but I messed up the unit. No problem though, I adapted to the situation).
That resulted in ~ 630ppm of CaSo4.2H2O without the whole thing even being dissolved, so I thought what is happening here?!! The result should have been ~360ppm at most if what Rotala says is correct.

IMG_3795.jpg

So one of three things is happening here.
  • Either I was sold something else than CaSo4.2H2O. If that is the case not sure what it is but it hasn't killed any thing yet in the tanks; or
  • The Rotala calculator is providing erroneous results; or
  • I was given some super charged CaSo4.2H2O which I am unaware of. 😎
Any information would be appreciated.

Thank you.

notes:
1. I used 2 scales to double check weights. The small one you see there and a 0.001g precision scale. So no, it's not that my scale is off.
2. I did measure the RO water TDS in the beaker prior adding CaSo4.2H2O. It was at 6ppm. So not that too.
 
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Hi all,
Hanuman said:
That resulted in ~ 630ppm of CaSo4.2H2O without the whole thing even being dissolved, so I thought what is happening here?!!
Click to expand...
It is the conductivity to ppm TDS estimation issue, some ions are better conductors of electricity than others. You can't really use the conductivity meter (set to ppm TDS) in the way you have.

The 0.64 conversion factor (64 ppm TDS ~ 100 microS) is for water where the predominant salt is calcium carbonate (Ca++ and HCO3-), if you have water where the major salt is sodium chloride (NaCl) the conversion factor is 0.5 etc.

cheers Darrel
 
dw1305 said:
It is the conductivity to ppm TDS estimation issue, some ions are better conductors of electricity than others. You can't really use the conductivity meter (set to ppm TDS) in the way you have.
Click to expand...
So what are you saying here? That I am actually getting 360ppm of CaSo4.2H2O but my TDS meter because of the conversion factor is showing 630ppm?
 
dw1305 said:
Hi all, I would do exactly that.

What are the dimensions of the tank?

cheers Darrel
Click to expand...

It’s a 90x45x45cm. Has some hardscape wood/rocks and a rather signficant amount of substrate. Can’t tell how many litres as it was rescaped twice and didn’t keep track of substrate quantities. The back/left is heavily banked.
dfe83875cd079b3018d851723d210c4a.jpg
 
dw1305 said:
Hi all, So potentially ~ 180 litres of water.
cheers Darrel
Click to expand...
Actually more like 166L if you take into account the glass thickness of 10mm.

dw1305 said:
It will be at least 100 litres of water because the interstices in the soil will now all be saturated.
Click to expand...
That was my very first assessment but then came the TDS readings. I will up my CaSo4.2H2O for next water change.

Note aside, is there any meter that can actually give a real reading of what I am adding? What you said earlier about the conversion factor just made me think that all these readings I have been making for the past year on my tank are actually garbage or at least I have been mislead, or is that not? My intention is to keep my TDS below 200ppm as I have loads of Bucephalandra in there and I am not sure they will like if TDS goes too high. That's what I was told my the farm who sold me the plants.
 
Hanuman said:
is there any meter that can actually give a real reading of what I am adding?
Click to expand...

IMO - none (unless we talking mass spectrometer) its just a indication of whats in there as a whole, once its in the mix the way it interacts with whats already present will vary depending on whats already present 😵. Hard water is complex to start with, throw in some AS, rocks, plants and livestock and the order of complexity shoots though the roof. My TDS is .... well I don't take it any more I just accept it and get on with it. Think TDS readings do have there place as Darrels changes his water only when it hits a certain TDS, as being eco friendly and doing his bit for planet earth is the right thing to do, or hes lazy and only does it when necessary!

Having been to Green Aqua and seen their tanks it does make me think starting with RO water remineralise than adding ferts helps control lots as when we start with RO the water has nothing till you add it, thereby avoiding some of the complex interactions of hard tap water

I do think some of the issues I see/have in my tank are all because of the hard water eg 141ppm Ca but most of my Bucc are acceptable.

Maybe some answers/theories will materialize when CERN reboots the Large Hadron Collider, or maybe it will just give us a new set of questions :arghh:
 
Hi all,
Zeus. said:
or hes lazy and only does it when necessary!
Click to expand...
I'm pretty lazy and I definitely follow the path of least resistance.
Zeus. said:
changes his water only when it hits a certain TDS,
Click to expand...
I do and I don't, I actually change about 10% of the water every day. In the summer I might change a bit more, because I usually water the garden pots/house plants with water from the tank. If I've already changed the water (and used the waste water), I just draw some more from the water butt and use that to replace the water that I take out of the tank (for the garden). Usually I don't measure the conductivity, I just watch the plants.

Over time, I've found that if the conductivity gets outside of the 80 - 140 microS range that I need to either add a bit more tap (~650 microS) or a bit more DI (~2 microS).

I just tend to watch the Ramshorn snail shells now. If I have some big, relatively red, snail shells the water has probably got harder and risen above 140 microS, and if I only have very small chalk white ones the water is softer and I'm probably below 80 microS.

Just tried now and the tank was 114 microS. I'll get a photo of the floating plants & snails, and post that in a minute.

cheers Darrel
 
Hi all,
Here the top view of the tank, the <"Magnolia grandiflora"> leaf is for the <"Copella to spawn on">.

Snail_tank_july2020crop.JPG

Looking at that, and bearing in mind the conductivity reading, the plants would <"probably benefit from a feed">, but it isn't absolutely imperative yet.

You can also see the Ramshorn snail is not long for this world, and they never get much bigger than that in these tanks. In the tap water they could reach twice as large and still be red in the older shell whorls.

cheers Darrel
 
dw1305 said:
Hi all, I'm pretty lazy and I definitely follow the path of least resistance.I do and I don't, I actually change about 10% of the water every day. In the summer I might change a bit more, because I usually water the garden pots/house plants with water from the tank. If I've already changed the water (and used the waste water), I just draw some more from the water butt and use that to replace the water that I take out of the tank (for the garden). Usually I don't measure the conductivity, I just watch the plants.

Over time, I've found that if the conductivity gets outside of the 80 - 140 microS range that I need to either add a bit more tap (~650 microS) or a bit more DI (~2 microS).

I just tend to watch the Ramshorn snail shells now. If I have some big, relatively red, snail shells the water has probably got harder and risen above 140 microS, and if I only have very small chalk white ones the water is softer and I'm probably below 80 microS.

Just tried now and the tank was 114 microS. I'll get a photo of the floating plants & snails, and post that in a minute.

cheers Darrel
Click to expand...
Just took a reading of my water with my <handheld COM-80 HM Digital meter>. I switched modes to mS. It shows 0.3mS (I think that's milliS) so switched to µS (microS) I get 250µS.
My tap water is at: 489µS and my RO at 13µS.
 
Hanuman said:
my RO at 13µS
Click to expand...

are your using a deionising filter/resin? The RO unit I have does have a deionising filter/resin and the meter reads 0.0µS anything above 5µS is the first indication of the deionising resin becoming exhausted.
 
Zeus. said:
are your using a deionising filter/resin? The RO unit I have does have a deionising filter/resin and the meter reads 0.0µS anything above 5µS is the first indication of the deionising resin becoming exhausted.
Click to expand...
My filter is a 5 stage filter. It has the following filters: sediment, carbon, resin, RO membrane, post carbon. It has been reading around ~9ppm since the day I got the filter, that is since December 2018. It's now reading ~13ppm. Didn't measure in µS at that time. Manufacturer told me to change the membrane and post carbon every ~ 2 years, and others 3, every years. I changed lower 3 filters namely sediment, carbon and resin less than a year ago as indicated by my beautiful hand writing.

Picture below for those who like pictures.

IMG_2203 2.jpg
 
Hi all,
Hanuman said:
Just took a reading of my water with my <handheld COM-80 HM Digital meter>. I switched modes to mS. It shows 0.3mS (I think that's milliS) so switched to µS (microS) I get 250µS. My tap water is at: 489µS and my RO at 13µS.
Click to expand...
I agree with @Zeus. the RO conductivity is a bit high, I'd just monitor it. Assuming, that it doesn't keep creeping up you are probably OK, because you've changed the filters etc.

In terms of the tank water conductivity, assuming that it doesn't continually rise, I think that sounds reasonable as a datum value. You don't need <"many ions to raise the conductivity">. I'm aiming for minimal plant growth (I want <"some active growth, but that's it">) , so I only add the nutrients that are absolutely necessary, and some times I may dip below that.

cheers Darrel
 
dw1305 said:
Hi all, I agree with @Zeus. the RO conductivity is a bit high, I'd just monitor it. Assuming, that it doesn't keep creeping up you are probably OK, because you've changed the filters etc.

In terms of the tank water conductivity, assuming that it doesn't continually rise, I think that sounds reasonable as a datum value. You don't need <"many ions to raise the conductivity">. I'm aiming for minimal plant growth (I want <"some active growth, but that's it">) , so I only add the nutrients that are absolutely necessary, and some times I may dip below that.

cheers Darrel
Click to expand...
Could it be the sodium chloride of the resin?
 
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