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This is for a liquid detergent that I am developing - when I dissolve boric acid (doesn't fully dissolve though) and then mix it with dissolved sodium borate( also doesn't fully dissolve in water), settling occurs after sometime. How do firstly dissolve these chemicals and how do I prevent the settling of these after sometime at room temperature?

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  • $\begingroup$ Do you respect solubility of substances? $\endgroup$
    – Poutnik
    Commented Mar 3, 2023 at 10:36
  • $\begingroup$ Please explain...I don't understand $\endgroup$
    – TENT
    Commented Mar 3, 2023 at 13:02
  • $\begingroup$ Explain what you do not understand. Every substance has its solubility limit. If you provide enough water not to reach the limit, the substance can be fully dissolved. Otherwise, it cannot be. // It could be temporarily dissolved at higher temperature, but the raise of solubility with temperature very much depends on the substance. E.g. solubility of NaCl is almost T independent, while for KNO3 it raises progressively steeply. And it would precipitate back when cooled down to original temperature. $\endgroup$
    – Poutnik
    Commented Mar 3, 2023 at 13:17
  • $\begingroup$ So I must increase the amount of water when dissolving? $\endgroup$
    – TENT
    Commented Mar 3, 2023 at 13:47
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    $\begingroup$ You must know the thing from everyday life. Take table salt. AFAIK there can be dissolved about 33 g in 100 mL of water. If you provided less than 100 mL for 33 g of salt, it will not fully dissolve, no matter how much you stir. // At 20 deg C, there can be dissolved 4.72 g boric acid/100 mL, or 2.5 g sodium tetraborate decahydrate(borax)/100 mL // See en.wikipedia.org/wiki/Solubility_table $\endgroup$
    – Poutnik
    Commented Mar 3, 2023 at 15:01

1 Answer 1

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Before you go further with boric acid and sodium borate, you should know the characteristic properties of them first. Boric acid has the molecular formula $\ce{H3BO3}$ and has a triangular planner structure: $\ce{B(OH)3}$ with the central boron having only six electrons (not fulfilling Octet rule and hence a Lewis acid). According to this article:

Its physical appearance is white or almost white crystalline powder or can call it colorless. It is soluble in water and in ethanol (96 per cent), and freely soluble in boiling water to give acidic solution. For example, 3.3% aqueous solution of boric acid shows $\mathrm{pH}$ of $3.8$$4.8$. Boric acid is also soluble in glycerol (85 per cent).

Thus, for instance, you may use heat to dissolve boric acid well. Yet, you may need to know this: $\mathrm{p}K_\mathrm{a}$ of boric acid is $8.92$$9.24$ that is sensitive to temperature, ionic strength, and concentration (Ref.1). Accordingly,

The $\mathrm{p}K_\mathrm{a}$ varies with concentration because of polymerization above $\pu{0.02 M}$. Boric acid reacts reversibly with alcohols, especially 1,2-diols including carbohydrates, with carboxylic acids, thiols, and amines. These esters/adducts, are also Lewis acids with lower $\mathrm{p}K_\mathrm{a}$ values. Boric acid can stabilize some materials while catalyzing the degradation of others.

Thus, the settling you experience must be due to the polymerization of boric acid solution:

Polymerization in boric acid

Polymerization in boric acid-2

Finally, boric acid and/or borate can easily undergoes esterification with alcohol functions (Ref.2). For example,

esterification with alcohol


References:

  1. Antonio Lopalco, Angela A. Lopedota, Valentino Laquintana, Nunzio Denora, Valentino J. Stella, "Boric Acid, a Lewis Acid With Unique and Unusual Properties: Formulation Implications," Journal of Pharmaceutical Sciences 2020, 109(8), 2375-2386 (ODI: https://doi.org/10.1016/j.xphs.2020.04.015).
  2. Supachok Tanpichai, Farin Phoothong, and Anyaporn Boonmahitthisud, "Superabsorbent cellulose-based hydrogels cross-liked with borax," Scientific Reports 2022, 12, 8920 (12 pages) (ODI: https://doi.org/10.1038/s41598-022-12688-2).
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  • $\begingroup$ Thank you for enlightening me. $\endgroup$
    – TENT
    Commented Mar 6, 2023 at 7:04

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