Precipitate in DIY micro mix

[Schrute66]

I created primary and secondary DIY micro solutions as described by @GreggZ here. I'm seeing some solids or a precipitate in the newly created secondary solution which contains Cu, Mo, and Ni. I didn't get any solubility warnings from Rotala Butterfly when working up the amounts. I did also add ascorbic acid (1.0g) and potassium sorbate (0.4g) to the 1L of RODI water I used for the solution. I've mixed it thoroughly and let it sit for 24 hours. I sourced the micros from @Burr740. Is this normal or have I possibly done something wrong with the quantities? Or maybe just impurities I can ignore or filter out? Thanks!

 

[GreggZ]

Not sure what you did but I can tell you I have been using my secondary solution for years now. It's clean and clear after all this time.

So like I said not sure what happened but that is not normal. Maybe try again? I also don't recall if I put anything else in the secondary solution. I don't believe I added anything but the salts but honestly can't be sure.

 

[Schrute66]

Thanks Gregg. I think I'll make a smaller batch and add each compound one at a time. I'll also re-check my math ;-) I'll report back!

 

[Schrute66]

Not sure if anyone will find this interesting, but below is the ChatGPT response to my problem statement. I gave it the amount of each compound I added to my solution. I don't know enough chemistry to accept or reject the assertions or recommendations, but it jives with my plan to re-do my mix one compound at a time. Kind of feels like perhaps the ascorbic acid I added could be the cause. In one of Gregg's posts, he mentions using distilled white vinegar rather than ascorbic acid. I'd hate to trash another batch so if I get all the way to the potassium sorbate without seeing any precipitate maybe I'll just call it good!
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Interesting formulation! Based on your listed components and the appearance of a yellowish-green precipitate, we can start analyzing possible chemical causes. Let’s go step by step.

1. You added the following to RO water:​

• CuSO₄·5H₂O (Copper(II) sulfate pentahydrate) → provides Cu²⁺
• Na₂MoO₄·H₂O (Sodium molybdate monohydrate) → provides MoO₄²⁻
• NiSO₄·6H₂O (Nickel(II) sulfate hexahydrate) → provides Ni²⁺
• Ascorbic acid → reducing agent, acidifies solution
• Potassium sorbate → preservative; mostly inert here unless pH is very low

You used reverse osmosis (RO) water, so we can rule out contamination from calcium/magnesium, unless it was introduced during preparation.

2. A yellowish-green solid suggests a transition metal compound, especially since both Cu²⁺ and Ni²⁺ are colored ions.

• Copper compounds:
  
  • Cu²⁺ is usually blue in solution, but forms green or yellow-green solids with certain ligands.
  • In reducing conditions (from ascorbic acid), Cu(I) can form, which is less soluble and forms yellow or green precipitates, like CuCl or Cu₂O (reddish though).
• Nickel compounds:
  
  • Ni²⁺ is green, and nickel hydroxide or basic salts can precipitate as green solids.
• Molybdates:
  
  • These can form yellow solids, especially under acidic or reducing conditions.

3. Let’s consider what might be precipitating out:​

A.

• Ascorbic acid lowers the pH.
• At low pH, molybdate (MoO₄²⁻) can form polymolybdates or Mo(OH)₃/MoO₂, some of which are yellow solids.
• Ni²⁺ and Cu²⁺ may form basic salts or hydroxides if the pH rises slightly during storage (e.g., atmospheric CO₂ dissolving).

B.​

• Ascorbic acid is a strong reducing agent.
• It can reduce Cu²⁺ to Cu⁺, leading to precipitation of CuCl (if any chloride is present) or Cu₂O, which is yellow to reddish.
• It might also reduce Mo(VI) (from MoO₄²⁻) to Mo(V) or Mo(IV), forming colored, insoluble species.

C.

• Metal-ascorbate complexes could be forming. Some of these are unstable and could precipitate if the complex decomposes.
  

4. Given:​

• Color: yellowish-green
• Metal ions present: Cu²⁺, Ni²⁺, MoO₄²⁻
• Reducing agent: Ascorbic acid
• Low pH

Most likely culprits:

• A basic copper salt (like Cu(OH)₂ or Cu(OH)Cl)
• A nickel hydroxide or carbonate
• A reduced molybdenum species (e.g., MoO₂ or polymolybdates)
• Or a mixture of precipitated metal salts/complexes

5. You could:​

• Measure pH of your solution.
• Filter and dry the solid, then test its solubility in acid or base.
• Add each ingredient separately to water and look for precipitate formation.
• Try the same mix without ascorbic acid to see if the precipitate forms — that would isolate the redox effects.

 

[Schrute66]

I re-made my concentrated Cu, Mo, Ni micro mix but used 10ml distilled white vinegar (in 1L) to acidify instead of ascorbic acid. No precipitates this time.

 

[Naturescapes_Rocco]

I re-made my concentrated Cu, Mo, Ni micro mix but used 10ml distilled white vinegar (in 1L) to acidify instead of ascorbic acid. No precipitates this time.

Glad you shared this! I was getting some crystalline precipitates in my Micros mix when it was fairly concentrated, and when I used 0.5g Ascorbic Acid per 450mL.

I switched to 0.2g Ascorbic Acid per 450mL, and kept my 0.2g Potassium Sorbate preservative, and it's been perfect without any precipitates
Will be interested in using vinegar in the future as well.