Sciencemadness Discussion Board

Potassium Hydrogen Sulfate Preparation - KHSO4

gravityzero - 20-7-2016 at 12:05

It amazes me sometimes what appears to be a simple chemical is often difficult to prepare or even purchase.

I was looking over a photography developer synthesis where it called for potassium bisulfate.
Now it could probably be substituted for sodium bisulfate, which is sold as a pool chemical, but still I wonder about it's preparation.

It is my understanding that potassium metabisulfite will reduce to potassium bisulfite when placed in water.

K2O5S2 + H2O = KHSO3 + More Water

Then would it be possible to react the solution with sulfuric acid to produce potassium bisulfate?

KHSO3 + H2SO4 = KHSO4 + H2O + SO2

I assume it could then be boiled down to crystallize potassium bisulfate, but maybe not.
Any suggestions are more than welcomed and appreciated.

Orenousername - 20-7-2016 at 12:32

What's wrong with reacting sulfuric acid with KOH or K2CO3 in stoichiometric amounts?

gravityzero - 20-7-2016 at 12:47

Well KOH + H2SO4 will do the trick. I would rather do this with potassium carbonate if it will work. I've never tried, but haven't found much information to confirm it.

Ashot - 20-7-2016 at 12:54

Hi, it is actually quite easy to prepare. All you need to do is to neutralize a sulphuric acid with potassium base, According to Wikipedia hydroxide should work well in this reaction. It is important to be sure that both solutions of sulphuric acid and potassium hydroxide are the equal molar amount.
For example, if you have 100ml of 18 molar solution of sulphuric acid or 96% concentration(maximum available for public in the UK) you will need to dissolve 101 grams of potassium hydroxide in water (or you can add it dry to the acid but this would be too dangerous) and slowly neutralize the acid with base.
According to Wikipedia potassium bisulfate decomposes at 300c so if you evaporate water using the hot water bath method, you should end up with reasonably pure crystals of KHSO4

I believe it is my duty to warn you that this process is potentially dangerous, so please read MSDS for every chemical.

gravityzero - 20-7-2016 at 13:12

Cool. I really appreciate the information and assistance. I think I will try this through a complete OTC synthesis.

First I might try to produce the KOH from a divided cell electrolysis of potassium chloride (salt substitute). I want to try the flowerpot method. ;)

Then I could probably use the dilute KOH produced in a reaction with sulfuric acid. Adding the acid dropwise until the solution is around PH1.
This should ensure a fairly complete reaction I would assume.

Then boil down the mixture to get the potassium bisulfate. Sounds like a fun project I'd like to try.

Ashot - 20-7-2016 at 13:44

Thit should be a very interesting process. I read about the industrial production of KOH by electrolysis of KCl and it sounds like they are using some very complicated processes involving a mercury metal. Really I can see why simple electrolysis won't do the job. However, the purity of your final product may be low if you don't use chemicals in equal molarity.

AJKOER - 30-7-2016 at 17:01

2HIO3 + 5 HSO3{-} = 5 HSO4{-} + H2O + I2

Saw the above reaction, which if valid, means one can avoid using H2SO4 and start with HSO3-.

As, per Wikipedia on HIO3:

I2 + 6 H2O + 5 Cl2 ⇌ 2 HIO3 + 10 HCl

It appears one only needs iodine (a problem, but is recycled) and a source of chlorine.

A better approach, acidify the metasulfite to produce SO2. Then, add water and chlorine, or better just HOCl as:

H2SO3 + HOCl = H2SO4 + HCl (See, for example, Eq 11.19 at )

and heating largely removes the more volatile HCl. Or, add more HOCl as:

HCl + HOCl = Cl2 + H2O

and heating drives off the chlorine gas. Neutralize with NaHCO3 to form the NaHSO4.

The hypochlorous acid can be prepared by adding CO2 to aqueous Ca(ClO)2 and allowing the CaCO3 to settle. Or, add CO2 to Mg(ClO)2 (prepared from 2 NaOCl + MgSO4, and freeze out the Na2SO4) and heat to form the insoluble MgCO3.

An alternate approach, acidify aqueous Na2SO4 (from MgSO4 + 2 NaOH, filter out the Mg(OH)2 ) with HCl. Unfortunately, this introduces a chloride impurity in the NaHSO4.

[Edited on 31-7-2016 by AJKOER]

ave369 - 2-8-2016 at 20:00

It is produced in ample amounts as a side product when you distill nitric acid from saltpeter using Glauber's classic reaction. Just wash out the crystalline goat from the reaction flask and re-dry it, and you've got a lotta KHSO4. If there's still some sulfuric acid left in your flask, use ethanol to wash the crystals. It will remove the acid, but it won't dissolve much KHSO4.

[Edited on 3-8-2016 by ave369]