Salts precipitation in my fert mix?

[parotet]

Hi all

In the last batch of liquid ferts I did, I am noticing that the mix is getting more and more cloudy. This is the recipe I used:

16g KNO3
1,5g KH2PO4
15g K2SO4
10 MgSO4
250 ml H2O
(it is the so-called PPS-Pro mix but recalculated for a 250 ml batch)

When I did it, I noticed that it was quite hard to dissolve and I had to shake it really hard (notice that it's +40g of salts in just 250 ml) but the liquid remained clear for weeks. Now that I am finishing the batch (and there has been probably some water evaporation, not really sure about that), the mix is getting cloudy (although I cannot see salts on the bottom, there is no precipitation for the moment).

Is this amount of salts (or any of the salts) close to the precipitation point? The original recipe was for 1 liter but I did nothing but reduce the amounts to 1/4

Jordi

Jordi

 

[parotet]

Ok, I've done a little bit of search by myself. It looks like I'm far from reaching maximum solubility at least for each one of the salts used:

For KNO3 solubility is 36g/100 ml (so 90g for my 250 ml)
For KH2PO4 it is 22g/100 ml (55g for my 250 ml)
For MgSO4 it is 25g/100 ml (62g for my 250 ml)
For K2SO4 it is 11g/100 ml (27g for my 250 ml)

... So this is not the problem. Temperature seems stable enough and in standard range to affect solubility. Therefore my question is: even if the solubility of salts, at least independently for each one, is far from being reached, do the total amount of salts dissolved in these 250 ml can be the problem?

Jordi

 

[GreenNeedle]

2 things

Firstly - When I mix up a solution I leave it on a warm radiator for a day shaking it every now and again which helps.

The other thing is that you are missing out the obvious. If you make the mix with double the water then you just dose twice the amount. or with 4x the water and dose 4x each time.

Your base is the salts. you can dilute it however many times you want and just increase your fertilisation doses by the same multiple.

So if the recipe says add 500ml water and dose 5ml daily, try adding 1 litre of water and dosing 10ml daily

 

[G H Nelson]

Could be the water your using!
I use RO....Not tap water as it makes my mix go cloudy after a few weeks!
hoggie

 

[parotet]

Could be the water your using!
I use RO....Not tap water as it makes my mix go cloudy after a few weeks!
hoggie

Yes, I use tap water GH 26, TDS around 1000 so my water is plenty of other salts. Could it be this? Is it realted with the other salts in the solution or maybe pH?

2 things
Firstly - When I mix up a solution I leave it on a warm radiator for a day shaking it every now and again which helps

So do I... at least 2 deays before and good shaking during these days

The other thing is that you are missing out the obvious. If you make the mix with double the water then you just dose twice the amount. or with 4x the water and dose 4x each time.
Your base is the salts. you can dilute it however many times you want and just increase your fertilisation doses by the same multiple.
So if the recipe says add 500ml water and dose 5ml daily, try adding 1 litre of water and dosing 10ml daily

Yes, this could be the solution but the weird thing is that I use a standard PPS-Pro recipe (see for for example https://sites.google.com/site/aquaticplantfertilizer/home/pps-pro but the same also recommended in every forum and fert shops that sell dry salts) which is a fert mix widely used in the hobby. The only difference is that I scaled down the mix to adjust the recipe to 250 ml (I have re-done my calculation once again and they are ok, just dividing everything by 4). If it works for 1 liter, it should also work for 250 ml, isn't it?

My guess is that other salts from my tap water or pH (?) is what causes this cloudiness (as mentioned no precipitation yet), as I asume that scaling down the mix would have no effect.

Jordi

 

[dw1305]

Hi all,

Could be the water your using! I use RO....Not tap water as it makes my mix go cloudy after a few weeks!

It is almost certainly the tap water, and the haziness is likely to be the virtually insoluble calcium sulphate (CaSO4.2H2O). As you get towards the solubility limit you see the <"common ion effect">. The easiest option is to dilute the stock solution

You can tell whether it is a carbonate/sulphate by using "SuperColey1's" heating suggestion.

Salts are more soluble with increasing temperature, with the exception of carbonates (HCO3- ions). This is because of the solubility of gases and the CO2 ~ HCO3- equilibrium. As the water warms it can hold less gas, and as dissolved gas levels fall the soluble bicarbonate ion (HCO3-), comes out of solution as insoluble carbonate 2CO3. Carbonates with higher valency are less soluble, so normally calcium carbonate (CaCO3) comes out of solution first.

If you cool the stock solution and the haziness goes away it is calcium carbonate, calcium sulphate dihydrate would become more soluble with heating, but it's solubility is so low it won't make any difference.

cheers Darrel

 

[parotet]

It is almost certainly the tap water, and the haziness is likely to be the virtually insoluble calcium sulphate (CaSO4.2H2O). As you get towards the solubility limit you see the <"common ion effect">. The easiest option is to dilute the stock solution

I have diluted the stock solution, now I wil have to use 2x . My water company says that I have 258 ppm of SO4, 155 ppm of CaCO3, 117 ppm Ca++ and 1,000 uS/cm, pH 7.8... I will cool the stock solution to see what happens.

It looks I will have to use RO water for the next batch. I was already using RO water for the trace mix... otherwise I would be losing most of the iron, isn't it?

Jordi

 

[GreenNeedle]

Yep forgot to mention I always use DI water for solutions. Just means you are starting from a base of zero then adding your own stuff in. Your water report may well show high phosphates in your tap which could also be reacting.

On the calcium part my tap water both cold and hot can quite often come out cloudyish from the tap. I have very hard water here in Lincoln being a limestone area (I walk my dog in an old limestone quarry half a mile from my house) and after a 24 hour degas my tap reads 8.4ph