How to make a stabilized ortho-silicic acid solution with only 3 inputs

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In a previous post, which you can read here, I gave a procedure for the preparation of a stabilized mono-silicic acid using from potassium silicate. The procedure called for the usage of several stabilizing agents, including carnitine and propylene glycol, with phosphoric acid being used as the acidifying agent.

After trying this synthesis myself and talking with other people who tried this process, it seemed clear that the success rate was low and that the process was just too complicated and imprecise for most people to carry out (especially for the patience needed for the addition of the solid potassium silicate). There is a detailed discussion about this procedure, as well as mono-silicic acid synthesis in this forum thread.

Stabilized mono-silicic acid solution created using the procedure below. Note that mono-silicic acid and ortho-silicic acid are the exact same thing, they are two names for the same molecule (H4SiO4). Another molecule with the same nomenclature is ortho-phosphoric acid (H3PO4), which is also called mono-phosphoric acid.

Given these issues, I decided to look for a potentially easier synthesis starting from cheaper, more readily available materials, avoiding the use of Propylene Glycol (which concerned some people) and trying to simplify the steps involved.

The procedure I came up with simplifies the process by relying on the interaction of silicic acid with sorbitol as a stabilizing agent. This stabilization process is well documented in the literature (see here) and is caused by the formation of highly stable polyolate complexes between mono-silicic acid and molecules like sorbitol. These complexes form because molecules like sorbitol have adjacent hydroxy groups in what we call a threo configuration. These do not exist in sugars like glucose or sucrose, reason why these do not work for this process.

The raw inputs you will need are as followed

  1. A potassium silicate with a high K/Si ratio, such as AgSil 16H. You can also use a liquid potassium silicate, such as Grotek Pro-silicate.
  2. Sulfuric Acid (>90%)
  3. Sorbitol
  4. Distilled water.

If using AgSil16H follow this process first. In a 1000mL beaker, add 70g of AgSil16H and 450mL of distilled water. Stir – ideally with magnetic stirring – until the silicate has all dissolved. This will be the silicate solution.

This is now the procedure to prepare the stabilized ortho-silicic acid solution (700mL):

  1. In a 1000mL beaker, add 500mL of distilled water and a magnetic stirrer.
  2. Weigh 200g of Sorbitol and add them to the water.
  3. Start the magnetic stirring.
  4. After the sorbitol has completely dissolved, during a period of 30 seconds add 100mL of the silicate solution (either as prepared above or a commercial silicate equivalent to the Grotek suggestions above (around 7.5% Si as SiO2)).
  5. Stir the silicate and sorbitol solution for 10 minutes.
  6. Add 10mL of >90% sulfuric acid and stir for 5 minutes. The pH should now be lower than 2.
  7. The solution can now be stored.

The above process creates a stable mono-silicic acid solution that has an Si concentration of around 1% of Si as SiO2 and around 0.6% K as K2O. Used at 8mL gal it should provide around 20ppm of Si As SiO2 and 10 ppm of K.

A previous version of this procedure used 50mL of 80-85% phosphoric acid. However, phosphoric acid seems to generate solutions that are unstable after 1-2 weeks of preparation. Solutions prepared per the above process have been confirmed to be stable for at least 1 month.

Did you try it? How were your results? Let us know in the comments below!

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10 Comments

  • Ted
    July 9, 2022 @ 3:14 pm

    Thanks for all you do!

    Why do you choose phosphoric acid? Do you see any issue using sulfuric acid instead?

    Have you done stability testing by leaving the solution in a closed bottle or covered beaker for days/weeks to see when it begins to gel or precipitate?

    Have you sent a sample for the 5-day test to find the % of PAS (silicic acid)?

    How long do you anticipate the stabilized silicic acid will last (before converting to unavailable SiO2) once diluted into nutrient soliton with other ions and pH of 5.5-5.8?

    Are you saying 8 mL/gal provides 20 ppm Si as 42.78 ppm SiO2? Or it provides 20 ppm SiO2 (9.35 ppm Si)?

    Have you seen this from Bruce Bugbee: “pH 11.3 Enhances the Solubility of Potassium Silicate for Liquid Fertilizer”

    • admin
      July 10, 2022 @ 5:44 am

      Thanks for commenting. Let me answer your questions:

      1. I cannot buy sulfuric acid where I live, highly concentrated phosphoric is the only acid I can get. This is the main reason why I used phosphoric, you can try sulfuric and let us know if you see any difference in results.

      2. I can tell you it does not gel or precipitate anything after 2 weeks in a closed bottle (which is when I made the initial experiment), but I cannot tell you stability beyond that at this moment.

      3. No, but I’ll be going to Bruce Bugbee’s lab in early August and we’re going to be trying some experiments to quantify available silicon.

      4. It should last for days, as the polyolate complexes formed with sorbitol are really stable. However I hope to also test available Si in nutrient solution in Bruce’s lab and compare this to Si derived from a traditional potassium silicate solution.

      5. It provides 20 ppm of Si as SiO2, so around 9.35ppm of Si as elemental Si.

      6. Yes, I have read this. Concentrated silicate solutions are more stable when pH > 11. This is well known in the literature and is common practice when preparing concentrated potassium silicate from solid silicates, although no one had published an explicit paper on this exact matter in hydroponics.

      • Ted
        July 10, 2022 @ 8:20 pm

        Thanks for your answers!

        1. I will prepare this solution next week, using sulfuric acid, and let you know the result. Should I reduce pH to a specific value? Or is anywhere between 1.6-1.9 good enough? I will then send the sample for silicic acid analysis and I will update this post with my results.

        3. Very cool you’re going to his lab. I hope you’ll post a blog article about the trip and test results; I am very interested. Are you testing the stabilized silicic acid solution from this blog article, the other solution you wrote about (using propylene glycol), or a different solution?

        4. “I hope to also test available Si in nutrient solution in Bruce’s lab and compare this to Si derived from a traditional potassium silicate solution” That is VERY interesting indeed! I hope you replicate Bruce’s research on calcium compleation by Si with stabilized silicic acid, but with >150 ppm Ca and 5.5-6.0 pH. Are you and Bruce developing a new method to test silicic acid in nutrient solution? (I don’t believe the 5-day test was designed for nutrient solution analysis.)

        6. Why did you choose to make a solution of 20 ppm SiO2 when the max solubility of Si below pH 8 is around 47.74 ppm (102.13 ppm SiO2), and most published research finds 60-100 ppm SiO2 provides improved resuts? I know you have written about using around 5 ppm Si as SiO2 (2.3375 ppm Si) from potassium silicate due to concerns of stability and complexing Ca and other ions.

        7. Have you found research on sorbitol uptake by roots and translocation throughout the plant?

        8. Here’s a new episode from Cannabis Cultivation and Science podcast with Dr. Wendy Zellner. She has been researching silicon as a plant nutrient and its effects on plants’ inherent stress resistance and tolerance for decades. It’s an interesting interview, but the podcast interviewer missed many opportunities to ask better questions because he doesn’t have the necessary understanding of silicon and silicates. He’s an organic grower and is slightly biased against conventional nutrients. She does touch on stabilized silicic acid products but he asked poorly framed questions, so her response was inadequate: https://www.kisorganics.com/blogs/podcast/episode-98-silicon-the-essential-nutrient-with-dr-wendy-zellner

        I am emailing her this week with some questions and posits about stabilized silicic acid use with soilless media. I think it would be an excellent idea to reach out to her before going to Bruce’s lab! I believe she is a wealth of info and helped design the 5-day PAS test. She’s not a chemist, but I think it would behoove you to describe your plans with Bruce and get her input:

        P.S. The Cannabis Cultivation and Science podcast is amazing if you aren’t aware of it. The producer interviews exceptional scientists with diverse backgrounds and scientific expertise, including Dr. Bugbee, Dr. Justice, Dr. Sabeh, Dr. Barrett, and Dr. Cockson. I think it’s the best cannabis cultivation and botany podcast. However, except for Dr. Peters (from J.R. Peters), he hasn’t interviewed a chemist specializing in nutrient formulation. I think you would be a fantastic guest. So I’m going to email him and suggest he asks to interview you

        • admin
          July 11, 2022 @ 1:18 am

          Thanks for your reply! Here are my answers to your questions:

          1. Anything in the 1-2 range is fine.

          3. I will be testing the sorbitol stabilized one.

          4. We will test Si using a standardized colorimetric method used at his lab for available soluble Si. The 5-day test is irrelevant for liquid products that provide instantly available Si.

          6. Just because it was a nice round value, you can multiply those values to get whatever concentration you desire. You can also try to make the silicic acid more concentrated, although I have not tried this. The max possible concentration using this method is likely around 2x-3x the one I prepared, based on limitations from sorbitol solubility.

          7. No. However from research of other polyols in plants, I wouldn’t expect uptake of sorbitol to exceed more than around 0.1% of what’s in solution. Sorbitol is also a significant product of plant photosynthesis so it is already widely present inside the plant.

          8. I did listen to that podcast, I agree with your assessments. I haven’t met Dr.Zellner yet, but we do have some friends in common, so I will try to get an introduction.

          About the podcast, I would be happy to participate in that podcast if he decided to invite me over.

  • Nick
    July 12, 2022 @ 6:10 pm

    What is the need for the sorbitol addition?
    Are there studies showing a benefit from foliar vs fertigation additions?

    • admin
      July 13, 2022 @ 5:38 am

      Sorbitol needs to be added to stabilize the Silicon because monosilicic acid is unstable, even at acidic pH it will polymerize with time. Furthermore, the sorbitol-monosilicic acid complex will keep the monosilicic acid stable when it is added to a hydroponic solution, avoiding the polymerization it would suffer otherwise (at the pH of hydroponic solutions).

      About foliar vs root applications, yes, multiple studies. So far, it is evident that root applications are better because since Silicon is immobile it will be fixed and will need to be reapplied frequently as a foliar if used in this manner. Also bear in mind many Si containing products, especially those that people advise applying as foliars, will often contain plant hormones and other chemicals.

  • sterl
    July 15, 2022 @ 1:34 pm

    Hello Daniel,

    You had suggested wollastonite to me about a year ago. So my question is wouldnt it be easier and cheaper to purchase a 50# bag of wollastonite and get the same effects plus a bump in Ca.

    • admin
      July 16, 2022 @ 2:01 pm

      Thanks for commenting! Wollastonite can be a great option to get Si into tissue. However not all media can be amended – think Rockwool for example – and stabilized mono-silicic acids might have some benefits beyond mono-silicic acid formed from basic silicates without stabilization (although this is still being researched). Silicon from stabilized mono-silicic acid can also be more stable in recirculating systems.

  • Thomas
    August 7, 2022 @ 1:40 pm

    I’m sorry this is a basic question, but here goes. I use Agsil 16 in my hydroponic solutions.
    I found this article and now I’m confused.
    1) what is the benefit for plants of putting the Agsil through this process?

    2) Is using Agsil as labeled providing my plants in coco coir they amount of silicon that the labels states?

    I hope those make sense. I appreciate any guidance.

    • admin
      August 7, 2022 @ 6:36 pm

      1. I don’t know if there is any benefit. There is little research on the subject and although there is some evidence towards acidic forms of concentrated Si giving different results than those derived straight from basic silicates, it is too early to say whether they are better or worse.

      2. At doses of 20-50 pm of Si as elemental Si, the use of AgSil 16H will provide enough Si to plants to show substantial benefits. However, this does not mean that all the Si that goes into solution is in forms that can be taken up by plants. Neither is there such a guarantee if you did what is described in this post, however your results might differ between both.

      In essence, this is a method that provides you with an alternative form of silicon you can experiment with, it is not known to be better or worse though.

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