MKP solution changing colour

[lisabeeren]

hi,

i've mixed up some solutions of macros, which i then combine in different proportions to achieve the NPK ratio i'm after. i start with deionised water, add 500mg ascorbic per L, 400mg potassium sorbate per L, wait until that completely dissolves, before adding MKP (monopotassium phosphate).

this all works, but after a couple of weeks, the solution has gradually changed colour (see attached image).

any idea what reaction might be taking place here?

i have found a few threads where people are complaining about their macros changing colour, but no definitive conclusion. i suppose my contribution here might be narrowing it down to MKP.

with thanks

 

[MichaelJ]

If it changes color etc. after a few weeks, that is definitely a concern.... What exactly is it you are mixing? What Nitrate, Phosphate and Potassium compounds, and in what amounts per volume?

Cheers,
Michael

 

[lisabeeren]

0.5g/L ascorbic acid, 0.4g/L potassium sorbate ... wait for them to completely dissolve, then 166g/L monopotassium phosphate ... that takes a couple of days to dissolve (presumably) because its up near the solubility limit

kind regards

 

[MichaelJ]

0.5g/L ascorbic acid, 0.4g/L potassium sorbate ... wait for them to completely dissolve, then 166g/L monopotassium phosphate ... that takes a couple of days to dissolve (presumably) because its up near the solubility limit

kind regards

The solubility limit for KH2PO4 is 226 g/L in pure water (distilled water) - However, I would try and cut the amount in half using 83 g/L instead of 166 g/L and see where that takes you ( just add twice the ml to reach the same dose when adding the premade solution to the tank) - it should be able to dissolve fairly fast at room temperature (not days).

Cheers,
Michael

 

[lisabeeren]

hi,

do you think diluting the solution will change the water back to clear? ... or are you just trying to help me dissolve my MKP more quickly?

i've been thinking about this, and it's conspicuous to me that the water is turning orange ... which happens to be the same colour as vitamin-C tablets. so i'm wondering if this isn't the ascorbic acid doing something.

however, the question i haven't been able to answer is "why are vitamin-C tablets orange?!" ... they're typically made up of two things; L-ascorbic acid, and sodium ascorbate. L-ascorbic acid is white, and sodium ascorbate is yellow, so i'm not sure how they get orange ... (perhaps its a hydrated form? ... perhaps i'll leave some out and see if it turns orange)

there's lots of adjacent forms to ascorbic acid:

Chemistry of ascorbic acid - Wikipedia

en.wikipedia.org

but i can't find any information about what colour they are. but if there's an adjacent form to ascorbic acid which is orange, that seems like a pretty good lead.

any thoughts?

with thanks

 

[MichaelJ]

the same colour as vitamin-C tablets. so i'm wondering if this isn't the ascorbic acid doing something.

That is what it is… but it’s not supposed to change over meaningful time. I make my micro mixes every 5-6 months or so… ascorbic acid and potassium sorbate (not necessarily ideal, for sure, but the least of my worries..) and it stays same color and clarity throughout the duration. Many questions around here about molding fertilizers or changes of clarity etc comes down to overly saturated mixtures put into unclean containers overly exposure to O2, inadequate storage etc… preservation of “food” is a well understood discipline. Absolutely no magic to it unless you choose to make it so (which is fine…) … or want your fertilizers to survive a slow space cruise to Alpha Centauri

Cheers,
Michael

 

[lisabeeren]

so any guesses about the reaction which is taking place here? sodium/potassium bonding with one of the sites on the ascorbate ion? (there are other charged sites on the ascorbate ion that allow this to happen and still maintain it's solubility?) or is it an equilibrium reaction? (perhaps involving an insoluble product which redissolves before it can precipitate?)

i've diluted the solution 50% (with premixed ascorbic acid, potassium sorbate mix, so as not to swing the pH) and will see if the reaction reverses.

with thanks

 

[MichaelJ]

so any guesses about the reaction which is taking place here? sodium/potassium bonding with one of the sites on the ascorbate ion? (there are other charged sites on the ascorbate ion that allow this to happen and still maintain it's solubility?) or is it an equilibrium reaction? (perhaps involving an insoluble product which redissolves before it can precipitate?)

i've diluted the solution 50% (with premixed ascorbic acid, potassium sorbate mix, so as not to swing the pH) and will see if the reaction reverses.

with thanks

I’m sorry, given your only 4 posts deep here on UKAPS and initially asked a pretty trivial question, I’m trying to understand exactly what it is you want to know? Understand the chemical reactions in depth or just get a clear bottle of potassium phosphate fertilizer that won’t act up or loose it’s potency and get on with it?

Cheers,
Michael

 

[lisabeeren]

yeah, i'm interested in understanding the chemical reactions.

you've suggested (if i understand you correctly) that a very high concentration of MKP has accelerated an otherwise slow reaction. this seems a very reasonable hypothesis, and i'll try it out.

i know some ferts use citric acid rather than ascorbic, and i have wondered if this might be why.

with thanks

 

[lisabeeren]

oh, another interesting riddle here is that it's only occuring with MKP ... i've got a 250g/L MOP (potassium chloride) solution, and a 500g/L magnesium nitrate solution ... and neither of these are exhibiting "oranging".

none of the three solutions differ much from a pH of 5.5 ... so i don't think it's a pH thing.

so i have wondered if the phosphate isn't implicated here.

with thanks

 

[dw1305]

Hi all,

oh, another interesting riddle here is that it's only occuring with MKP ... i've got a 250g/L MOP (potassium chloride) solution, and a 500g/L magnesium nitrate solution ... and neither of these are exhibiting "oranging".

I'm not sure, the simple salts of monovalent elements are <"~all soluble">, so it must be to do with the added carbon (C). Potassium sorbate is (C6H7KO2) and ascorbic acid (C6H8O6), so you still have a very limited range of elements.

i start with deionised water, add 500mg ascorbic per L, 400mg potassium sorbate per L, wait until that completely dissolves, before adding MKP (monopotassium phosphate)...... then 166g/L monopotassium phosphate

You don't need to add potassium sorbate or citric (C6H8O7) / ascorbic acid to any of your macro-element solutions. They are designed to stop microbial growth in micro (trace)-element solutions, the acid just makes the sorbate more effective. I'm pretty sure nothing is going to grow in your extremely salty solution and that not adding them will get around your problem.

In this case, in solution monopotassium phosphate (KH2PO4.H2O) is the <"weak acid"> in the <"phosphate buffer system">, so you definitely don't need to add them to acidify the solution.

this all works, but after a couple of weeks, the solution has gradually changed colour (see attached image).

I actually wouldn't worry, you have a very concentrated solution so any loss of solubility (in the coloured compound) is, pretty much, irrelevant. If you had a precipitate forming that would be different.

cheers Darrel

 

[LMuhlen]

however, the question i haven't been able to answer is "why are vitamin-C tablets orange?!"

Probably dyed orange for marketing purposes, to associate with the fruit orange, known to be a source of vitamin C.

 

[Wookii]

The ascorbic acid is added to lower the pH of the mixture, prodominantly to maintain the chelation of micros. As such you don't need to add it to macro mixtures as Darrel says, since they don't require the acidification.

I have experienced the same discolouration when adding it to macro mixes in the past, and excluding the ascorbic acid fixed it, though no one could identify why the discolouration occurred beyond possible reaction with impurities in the salts.

 

[MichaelJ]

As far as the chemical reaction goes when Ascorbic Acid is oxidized it will turn to Dehydroascorbic acid which is sort of orange/brown. I do believe that compounds we mix up in water for long term usage / storage should be preserved somehow. With monopotassium phosphate, being an acid already, it's probably fine without, but I do not know for sure. Personally, every macro and remineralization compound I add as dry salts straight to my WC water.

Cheers,
Michael