KCl and KH2PO4 solubility

[Fuzzy Shaq]

Hi everyone, hope you are all well. I have a problem, hoping for some advice.
I have been using potassium chloride, potassium phosphate and epsom salts as my macro mix (EI). I have been adding epsom salts every water change, since all the salts wouldn't dissolve in a bottle.
However since removing the magnesium sulphate from the mix I am still getting some white precipitation at the bottom.
Could it be the salts not dissolving or fungus? ( I do boil the water and allow it to cool)

 

[dw1305]

Hi all,
Try with rain, or RO water, and I'm guessing your problem will go away.

The Science bit is below.

potassium chloride, potassium phosphate and epsom salts as my macro mix (EI). I have been adding epsom salts every water change, since all the salts wouldn't dissolve in a bottle.
However since removing the magnesium sulphate from the mix I am still getting some white precipitation at the bottom.

All potassium compounds <"are soluble">. When you were adding Epsom Salts (MgSO4.7H2O) (also soluble) to the mix you were getting issues with (tri)-magnesium phosphate (Mg3(PO4)2.8H2O) formation, because the (3)Mg++ and (2)PO4--- ions were combining, and tri-magnesium phosphate isn't a soluble salt.

However since removing the magnesium sulphate from the mix I am still getting some white precipitation at the bottom.
Could it be the salts not dissolving or fungus? ( I do boil the water and allow it to cool)

I'm guessing the issue is the water, and that it is tap water?

The problem is that it still contains <"some calcium ions (Ca++), even after boiling">.

Boiling would remove the dKH, the "temporary hardness" (the bicarbonate (2HCO3- ions)), but it won't change the dGH, the "permanent hardness" the Ca++ ions. The issue then is calcium phosphate (Ca3(PO4)2.xH2O) formation. Because calcium carbonate (CaCO3) isn't very soluble you don't get a lot of precipitate form before all the Ca++ ions have precipitated out as insoluble compounds.

cheers Darrel

 

[Aleman]

One other issue you may have, is the purity of your "salts". I only say this following an issue I had with my macro mix throwing quite a crystalline precipitate ... and I use RODI water. In the end, as I have a 4 channel dosing pump, I split my macro mix into individual solutions ... The magnesium sulfate and di-potassium hydrogen phosphate, dissolved completely with no problem. The potassium nitrate on the other hand 😡 yep, threw a precipitate, white and crystaline, and I'm only talking 11.65g in 1 litre of RODI water, well within the solubility limit. I got my wife to bring some home from work, and that dissolves completely without any precipitate, so something has contaminated my KNO3, which is annoying considering I got it from a aquarium plant food specialist😡

 

[_Maq_]

something has contaminated my KNO3

I've observed the same in KNO3 made for culinary use. I suspect it's oxalic acid. It precipitates in the tank as calcium oxalate. Under certain circumstances, you can make a smoke flowing in your tank when adding oxalic acid solution. For plants it's harmless, for fish - I suspect it might be dangerous, just as for humans. Calcium oxalate is the kidney stone, very sharp crystals.😕

 

[Vsevolod Stakhov]

All potassium compounds <"are soluble">. When you were adding Epsom Salts (MgSO4.7H2O) (also soluble) to the mix you were getting issues with (tri)-magnesium phosphate (Mg3(PO4)2.8H2O) formation, because the (3)Mg++ and (2)PO4--- ions were combining, and tri-magnesium phosphate isn't a soluble salt.

It seems that it is higly dependent on PH.
Here is a solution of Tesco Liquid 7-7-7 in a tap water (btw, there is no difference with RO water in this case) after adding some Epsom Salts:

You can see a flaky white perciptation as expected.
But then I have added some citric acid to the solution before mixing with MgSO4, and there is no perciptation after that whatever amount of MgSO4 I add to the solution:

All I can see are some undissolved epsom salt crystals on the bottom.
However, it might be not directly related to the PH but also because of the chelating properties of the citric acid. But I'm not sure whether Magnesium citrate is stable in low PH.

 

[dw1305]

Hi all,

But then I have added some citric acid to the solution before mixing with MgSO4, and there is no perciptation after that whatever amount of MgSO4 I add to the solution:............However, it might be not directly related to the PH but also because of the chelating properties of the citric acid. But I'm not sure whether Magnesium citrate is stable in low PH.

Unfortunately that is a question I don't know the answer to.

Calcium phosphate complexes are definitely soluble in weak acid, it is what <"your teeth enamel is made out of">, so I'd guess the same applies to magnesium phosphate.

I think tri-magnesium citrate nonahydrate (C12H10Mg3O14.9H2O) isn't very soluble, but I don't know what its solubility limit is and <"Wikipedia doesn't help">.

cheers Darrel

 

[dw1305]

Hi all,

The potassium nitrate on the other hand 😡 yep, threw a precipitate, white and crystaline, and I'm only talking 11.65g in 1 litre of RODI water, well within the solubility limit. I got my wife to bring some home from work, and that dissolves completely without any precipitate, so something has contaminated my KNO3, which is annoying considering I got it from a aquarium plant food specialist

(Nearly) all potassium (K) and nitrate (NO3) compounds are soluble, so you definitely have contamination, with a salt that contains some elements other than those. Does your wife have access to an <"ICP or similar">?

It is the solubility of most monovalent ions (ions that carry a charge of "+" or "-") that makes testing for them with <"colormetric methods problematic">, you need to find a compound <"that ideally is both insoluble and coloured">.

This is <"problematic with potassium (K)">, so test kits may use <"turbidity and a white salt">.

......... Sodium tetraphenylboron reacts with potassium to form a white precipitate...........

cheers Darrel

 

[Vsevolod Stakhov]

Unfortunately that is a question I don't know the answer to.

Calcium phosphate complexes are definitely soluble in weak acid, it is what <"your teeth enamel is made out of">, so I'd guess the same applies to magnesium phosphate.

I have checked that at some extent. Unfortunately, I don't have any strong mineral acid but I have tried Ammonia Sulfate to acidify solution. It didn't help (left to right: control sample, (NH4)2SO4 added, citric acid added):

There is clear floating perciptation in the first two samples. Then I have added some citric acid to the first two samples as well, and the perciptation has immediately dissolved with no remains:

But I doubt that ammonia sulfate solution can really take PH below 5. I wish I had some mineral acid and a precise PH meter 🙂

 

[Vsevolod Stakhov]

This is <"problematic with potassium (K)">, so test kits may use <"turbidity and a white salt">.

Btw, since potassium tests are not really available in the UK, I'm curious if anybody tried DiY Potassium test kit?
The methodic is quite simple - you just need two reagents

• Sodium tetraphenylborate (expensive stuff)
• 0.1N (or 0.1M) solution of hydrochloric acid

To measure K concentration, you use 2 5ml probes of water. First, you add 2 drops of the hydrochloric acid sollution to one probe. Then you add 15mg of the sodium TPB to the same probe, mix and evaluate perciptation. It seems you can buy 5g of Sodium TEB at around £18.90, that is enough for 333 tests. This method is also quite precise, as the shelf life of dry TPB is long and the shelf life of the solution is, on the contrary, quite short.

 

[dw1305]

Hi all,

Btw, since potassium tests are not really available in UK, I'm curious if anybody tried DiY K test kit?

I've never been too concerned about potassium (K) levels. I don't see potassium deficiency as a big problem and I'm not an EI user so water changes are always going to deplete its level in my tanks, if I get <"too ad hoc. with its addition">.

Because of the solubility of the potassium (K+) ion , and its mobile nature in the plant, once you add KNO3 etc you should get a pretty quick growth response. It might be that nitrate (NO3-) was <"Liebig's limiting nutrient">, rather than K+, but as plants need a lot of both of them I'm not too bothered about which it was.

I think the other "problem" is that it is about the easiest ion to test for via <"flame photometry"> "atomic optical emission spectroscopy", or more modern variations on that theme, and if you had a lab. set up you could process thousands of samples in a day.

Same would apply to an <"Ion Selective Electrode">, once you had it set up you could process a lot of samples quickly.

cheers Darrel

 

[Fuzzy Shaq]

Hi all,
Try with rain, or RO water, and I'm guessing your problem will go away.

The Science bit is below.

All potassium compounds <"are soluble">. When you were adding Epsom Salts (MgSO4.7H2O) (also soluble) to the mix you were getting issues with (tri)-magnesium phosphate (Mg3(PO4)2.8H2O) formation, because the (3)Mg++ and (2)PO4--- ions were combining, and tri-magnesium phosphate isn't a soluble salt.

I'm guessing the issue is the water, and that it is tap water?

The problem is that it still contains <"some calcium ions (Ca++), even after boiling">.

Boiling would remove the dKH, the "temporary hardness" (the bicarbonate (2HCO3- ions)), but it won't change the dGH, the "permanent hardness" the Ca++ ions. The issue then is calcium phosphate (Ca3(PO4)2.xH2O) formation. Because calcium carbonate (CaCO3) isn't very soluble you don't get a lot of precipitate form before all the Ca++ ions have precipitated out as insoluble compounds.

cheers Darrel

Thanks for your really helpful advice. I will use some ro water. Would distilled water be okay?
I had a feeling the salts were some how interacting, I'll give the RO water a go( or distilled) and report back if it solves it.
Thanks everyone for your advice

 

[dw1305]

Hi all,

Would distilled water be okay?

Perfect.

cheers Darrel

 

[Aleman]

JBL have a potassium test, using precipitation of some white product ... It's actually reasonably accurate, having made up a range of "Standards" within the test limits.

As for my precipitate, who knows what it is, but as it happens with RODI water it has to be some sort of calcium salt ... I'll be getting some ANALR grade when this runs out.

 

[ScapingScotsman]

Just to add also, having played around with adding npk powders in various orders, if ever I added magnesium last, it would always end up leaving a white clump residue in the bottom of the bottle which only ever dissolved if shook up and added to the tank.
I always add Mg initially first with ro water, warm, allow it to fully dissolve and then add the remaining additions.
I also found, even when just adding twice ei amounts in a litre or 500ml container, which should be fine for dissolution of all salts, I used to still get the same issue.

 

[Aleman]

Just to add also, having played around with adding npk powders in various orders, if ever I added magnesium last, it would always end up leaving a white clump residue in the bottom of the bottle which only ever dissolved if shook up and added to the tank.

Yeah, while magnesium salts are more soluble than calcium ones, they are still less soluble than monovalent ones, sodium, potassium etc

 

[Vsevolod Stakhov]

I've never been too concerned about potassium (K) levels. I don't see potassium deficiency as a big problem

I do not think about potassium defficiency here - I'm curious about the influence of plants decomposition level on the K levels. Last time I did ICP-MS, there was about 43ppm of K in my tanks. Unfortunately, ICP-MS is no longer available in my case, so I'm looking for other options.

t might be that nitrate (NO3-) was <"Liebig's limiting nutrient">, rather than K+, but as plants need a lot of both of them I'm not too bothered about which it was

In my case it is definitely Mn. It is quite interesting that I can even control plants growth rate by changing Mn dose (by defining Mn:Fe ratio in my micro dosing). Algae is also Mn defficient surprisingly (or not so surprisingly).

JBL have a potassium test, using precipitation of some white product

I'm pretty sure they use Sodium TPB as well (another option is sodium cobaltonitrite, but it is less precise and has yellow'ish colouring). The problem is that JBL K test kits are not available neither on amazon nor on ebay. Or they are way too expensive for just a curiosity. On the other hand, it seems you cannot just buy Sodium TPB unless you have a business account with the sellers...

 

[dw1305]

Hi all,

I'll be getting some ANALR grade when this runs out.........having made up a range of "Standards" within the test limits.

It definitely won't have any impurities, but ANLAR (analytical grade) is <"definitely overkill"> and a very expensive option. We use it for the standards, but food or technical grade for everything else.

JBL have a potassium test, using precipitation of some white product ...

It is the one @Vsevolod Stakhov mentions "Sodium tetraphenylboron reacts with potassium to form a white precipitate".

Yeah, while magnesium salts are more soluble than calcium ones, they are still less soluble than monovalent ones, sodium, potassium etc

The <"common ion effect">.

. Last time I did ICP-MS, there was about 43ppm of K in my tanks.

That is a lot.

In my case it is definitely Mn. It is quite interesting that I can even control plants growth rate by changing Mn dose (by defining Mn:Fe ratio in my micro dosing). Algae is also Mn defficient surprisingly (or not so surprisingly).

Manganese (Mn) is in an interesting one. Naturally there would be a reasonable amount in hard water (and plants only need a trace) but it causes a bitter taint in tap water and is often removed during treatment. I assume this is why it is present in <"Chempak Sequestered Iron">.

On the other hand, it seems you cannot just buy Sodium TPB unless you have a business account with the sellers...

It has been phased out of "Borax" powder etc. since 2010 , because of the toxicity of boron (B).

cheers Darrel

 

[Vsevolod Stakhov]

That is a lot.

The main question there is why I had that much Potassium. My assumption is that it was because there were too much of the decomposing plants matter in my tank at that time. Unlike nitrogen, potassium is not very eagerly assimilated by heterotrophs, so it could be the main reason why many people think so much about the potassium levels and a hypothetic potassium overdosing. I have an idea that high levels of potassium actually indicates more likely problems with the plants than problems with the potassium itself 🙂

Manganese (Mn) is in an interesting one. Naturally there would be a reasonable amount in hard water (and plants only need a trace) but it causes a bitter taint in tap water and is often removed during treatment. I assume this is why it is present in <"Chempak Sequestered Iron">.

Our tap water has also only traces of Mn and plenty of Fe (however, I have a strong belief that this Fe is merely colloid iron that is not available for the plants). Furthermore, the soils here are also defficient it terms of Mn. So I think it is the main reason why Mn is added to the Chempak formula. I've even heard the opinion that Fe solely is useless without Mn, as plants use the same transports for these elements. But in general soils have enough of the soluble Mn.

It has been phased out of "Borax" powder etc. since 2010 , because of the toxicity of boron (B).

Well, it is not a problem to buy H3BO3 on ebay and other platforms. I think that Sodium TPB is just an exotic compound that is not 'popular' enough to sell it widely for apparent reasons. And it is also toxic indeed.

 

[ScapingScotsman]

Yep, and even less soluble in boiled tap water than say ro or di water. We run our water through very large ro units at work for anything involving pottasium phosphate etc

 

[Fuzzy Shaq]

Hi all,

Perfect.

cheers Darrel

Hi @dw1305: , I know this is an old post but wanted to check if De-Ionised Water the stuff they use in cars and irons could be used in my macro mix? Or will the lack of ions affect the salts.
Thanks 👍