Swiss Federal Institute of Technology in Lausanne
Question
Asked 20 April 2021
How to Prepare 1 M KOH solution with 85% KOH pellets?
Hello,
I need to prepare large quantities of 1M KOH solution (20 Litres) in Deionised water. I have KOH pellets which are around 85% KOH, the rest being water. The pellets are from Reactolab and the details of the composition are in the attachment.
1M KOH = 14 pH.
I'm verifying the pH of the solution using a Thermofisher Orion Star A2216 pH sensor with ATC (automated temperature correction). https://www.thermofisher.com/order/catalog/product/STARA2210#/STARA2210
I tried the method which included Molar Mass/concentration of KOH = 56.11/0.85 which leads to 66g. I reach pH around 13.65 and not 14. The pH sensor has been calibrated with pH 7 and pH 10. I reach pH around 13.93 with 77g of KOH pellets in 1L of Deionised water. Currently, I'm hand stirring the dissolution process.
I also want to know if I need to stir for a longer duration for the dissolution process to stabilize.
Thanks
Suhas
Most recent answer
Duried Alwazeer Thank you very much for the link. It was the first site that I tried. But it uses already available standard solutions while I am using pellets to use the solution.
Carlos Araújo Queiroz Thank you for the detailed answer! I need to understand the usage of the calculation that you have put up better. The information on Carbonation and standardisation is helpful. I do not have access to the links in (IV). I will try to look into the literature on the appropriate titration method. If you have any at your disposal kindly share them.
I also realised that the response of ATC is not as accurate as I expected it to be. The dissolution of KOH pellets in DI water is highly exothermic and is subject to temperature change. Currently, I'm preparing batches of 2 litres and measuring the pH the next day once the solution is in equilibrium with the climate control settings. Later I adjust the pH using trial and error.
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All Answers (5)
Iğdır Üniversitesi
This link could be helpful.
"Molarity Calculator & Normality Calculator for Acids & Bases | Sigma-Aldrich" https://www.sigmaaldrich.com/chemistry/stockroom-reagents/learning-center/technical-library/molarity-calculator.html
Universidade NOVA de Lisboa
I. On pH prediction for (pure) KOH aq. sol.:
The following pH prediction for KOH aq. sol. takes into account the hydrolysis equilibrium of the K+ cation:
The pH of KOH aq. sol. can be predicted from the hydrolysis constant (Kh = 3.2·10-15 M) (*) of K+ for the equilibrium: K+ + 2 H2O ⇌ KOH + H3O+; Kh = [KOH]·[H3O+]/[K+]. We can substitute [KOH] at previous equation based on charge balance and molar balance for potassium, respectively:
[K+] = [OH-] - [H3O+] = Kw/[H3O+] - [H3O+], [KOH] = CKOH - [K+] = CKOH - [OH-] + [H3O+] = CKOH - Kw/[H3O+] + [H3O+], where Kw is autodissociation constant of water (Kw = 1.0·10-14; units omitted) and CKOH is the nominal molar concentration of dissolved KOH (formality, to be precise).
Hence we can write: Kh = {CKOH - Kw/[H3O+] + [H3O+]}·[H3O+]/{Kw/[H3O+] - [H3O+]} = {{[H3O+] + CKOH}·[H3O+] - Kw}·[H3O+] /{Kw - [H3O+]2}.
As long as CKOH > 100·[H3O+] and CKOH > 100·Kh, this simplifies to:
[H3O+]2 - ½2(Kw /CKOH)·[H3O+] ≈ Kh·Kw/CKOH.
This solves as:
[H3O+] ≈ ½(Kw/CKOH){1 + √{1 + 4CKOH·Kh/Kw}}
With CKOH = 1.00·10-1 M:
[H3O+] ≈ ½((1.00·10-14 M2)/(1.00·10-1 M)){1 + √{1 + 4(1.00·10-1 M)·(3.2·10-15 M)/(1.00·10-14 M2)}} = ½·(1.00 + √1.128)·10-13 M = 1.03·10-13 M; pH = - log10{[H3O+]/M} = 13 - log10(1.03) = 12.99
II. By predicting the pH of (pure) KOH aq. sol. as that of a strong base, we simply have:
[OH-] ≈ CKOH; pOH = - log10{[OH-]/M}
With CKOH = 1.00·10-1 M: pOH = 1.00; pH = 14.00 - pOH = 13.00
Note that for most practical purposes we can consider KOH as the strongest base in water.
―
(*) pKa = - log10(Kh) = 14.5: https://www.wou.edu/las/physci/ch412/Table2.htm
Universidade NOVA de Lisboa
III. Please note that the weighted KOH pellets may actually contain more moisture than what might have been accounted for, while both pellets and solution are prone to carbonation when exposed to the atmospheric CO2. Solutions of KOH should be standardized (by titration) for quantitative analytical purposes; so that the actual solution concentration can be determined.
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Universidade NOVA de Lisboa
IV. This standard may help: ASTM E291 - 18, "Standard Test Methods for Chemical Analysis of Caustic Soda and Caustic Potash (Sodium Hydroxide and Potassium Hydroxide)"; https://www.astm.org/Standards/E291.htm
Swiss Federal Institute of Technology in Lausanne
Duried Alwazeer Thank you very much for the link. It was the first site that I tried. But it uses already available standard solutions while I am using pellets to use the solution.
Carlos Araújo Queiroz Thank you for the detailed answer! I need to understand the usage of the calculation that you have put up better. The information on Carbonation and standardisation is helpful. I do not have access to the links in (IV). I will try to look into the literature on the appropriate titration method. If you have any at your disposal kindly share them.
I also realised that the response of ATC is not as accurate as I expected it to be. The dissolution of KOH pellets in DI water is highly exothermic and is subject to temperature change. Currently, I'm preparing batches of 2 litres and measuring the pH the next day once the solution is in equilibrium with the climate control settings. Later I adjust the pH using trial and error.
1 Recommendation
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