As the question states, I am interested in growing alkaliphile bacteria. I have seen many media use KOH, mainly to neutralize, and was wondering if it is possible to change this to Ca(OH)2?

Is there a reason KOH is used instead of any other alkali solution to neutralize media?

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    $\begingroup$ Calcium is a bad choice as it will form precipitates with either phosphate ions as well as with carbonate or CO2. $\endgroup$
    – Chris
    Feb 25, 2021 at 10:41

1 Answer 1


As @Chris said, $\ce{K+}$ salts are soluble in water whereas plenty of $\ce{Ca^{+2}}$ salts such as $\ce{CaSO4}$ and $\ce{Ca3(PO4)2}$ are insoluble in the aqueous medium. Therefore it leads to the precipitation of phosphate-containing proteins and sulfate-containing metabolites. In addition, $\ce{Ca^{+2}}$ forms coordination complexes with calcium-binding proteins which participate in discrete signaling pathways. Interfering with this system could lead to subtle and irreproducible results. $\ce{K+}$ ion mostly participates in creating a voltage gradient across the membrane, nothing more.

Moreover, ionic strengths should be considered. $\ce{Ca^{+2}}$ is a divalent cation. A solution of $\ce{KOH}$ and a solution of $\ce{Ca(OH)2}$ with the same pH would definitely have different ionic strengths. Therefore despite achieving the same pH of the medium, ionic strengths vary depending on the salt used. Abnormal ionic strengths affect the structure of proteins in many ways. In general, it has been proven that changes in $\ce{K+}$ concentration lead to less severe outcomes compared to $\ce{Ca^{+2}}$.

EDIT: $\ce{Ca(OH)2}$ forms precipitates in an aqueous solution because it is barely soluble in water ($K_{sp}$ $\pu{=5.5 \times 10^{-6}}$). The pH of a saturated solution of $\ce{Ca(OH)2}$ is 12.347. To achieve other pHs one must add strong acid or base (a standard $\ce{KOH}$ solution perhaps) or use dilution; so as you can see, introducing $\ce{Ca(OH)2}$ just leads to unnecessary complexity, best to be avoided. Also, $\ce{Ca(OH)2}$ leads to the formation of $\ce{CaCO3}$ in the presence of atmospheric $\ce{CO2}$, making it even a less appropriate material for the preparation of the medium. Calculations using $K_{sp}$ shows a saturated $\ce{Ca(OH)2}$ solution has pH = 12.347 and [$\ce{Ca^{+2}}$] = 0.01111

  • $\begingroup$ For my research I am interested in using bacteria to precipitate CaCO3 and at the same time prevent the formations of soluble salts like potassium and sodium sulphates/chlorides. As such, my approach was to try replace them with Ca-containing buffers and salts. Would there be any other Ca-containing salt/buffers that would be better suited? I am new to media preparations so still trying to understand how everything plays a role in bacterial growth. Ideally, I will find alkaliphiles that are low-sodium/potassium demanding. $\endgroup$ Feb 26, 2021 at 12:00
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    $\begingroup$ @Franco Grosso If you need to create an environment with a determined Ca+2 ion concentration and little Na+ and K+, you’ve got to consider some items. First Many famous culture mediums contain NaCl and using it is most likely inevitable even in your research. See [1] [2] [3] [4]. As you know a 15mM NaCl, 150mM KCl solution renders the cytosolic environment perfectly while a 150mM NaCl, 15mM KCl represents extracellular medium nicely [5]. Therefore K+, Na+, Cl- ions should exist in your system at least in trace amounts for successful cell culture. $\endgroup$ Feb 26, 2021 at 15:29
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    $\begingroup$ @Franco Grosso, Still, you can use R-NH2/ (R-NH3+ Cl-) buffers if you insist on lowering Na+ and/or K+ ions demand. Examples of such organic compounds are CAPS, PIPES, TRIS, TAPS, glycylglycine [6]. Subsequently, adjust pH by adding HCl. Here arises another problem. Chemicals mentioned above as well as Phosphoric acid, carbonic acid buffers have the potential to chelate your Ca+2. Therefore, it is best to avoid them. Look in some papers for the exact protocols and the best buffer systems. CAPS can be appropriate. Its pKa is 10.4 and can be used to grow alkali bacteria. $\endgroup$ Feb 26, 2021 at 15:31
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    $\begingroup$ @Franco Grosso, But still search for better results. Avoid making mixtures of buffers. Inject Calcium ion by simply adding CaCl2(aq) to the system. Remove the excess amounts by EDTA (forming stable EDTA-Ca complexes). In this way, you can adjust your calcium ion properly. A somewhat related paper about similar calculations on Mg+2 ion.[7] The precise ingredients of your culture depend on your research topic but you get the general idea. [8] I hope that I’ve understood your question correctly :) good luck! [1] : microbenotes.com/chocolate-agar $\endgroup$ Feb 26, 2021 at 15:35
  • $\begingroup$ [2] : microbenotes.com/new-york-city-agar [3] : microbenotes.com/sorbitol-macconkey-agar [4] : microbenotes.com/category/culture-media [5] : Molecular Cell Biology of Lodish 8th edition page 496 Figure 11-18 [6] : Wilson and Walker’s Principles and Techniques of Biochemistry and Molecular Biology 8th edition Chapter 0 [7] pubmed.ncbi.nlm.nih.gov/11772 [8] : sciencedirect.com/science/article/pii/S0091679X08611152 $\endgroup$ Feb 26, 2021 at 15:35

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