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New Contributor

mixing acid with alkaline

I am making a mix with the following ratios

water 1200 litres

Phosphoric acid 250 litres

potassium hydroxide 360 kgs

When I add the hydroxde to the acid water it give off a violent reaction and heats up.

Initially adding the hydroxide the reaction is muted. but the more I add the more violent the result.

Is there any way of mixing this mix to lessen the reaction?

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5 Replies
Community Manager
Community Manager

Re: mixing acid with alkaline

I think you need to give some context as to what you are trying to acheive with this reaction. Those are some very large amounts.

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New Contributor III

Re: mixing acid with alkaline

Chris Roberts, I'm very glad that you've stopped with the reaction and asked about the heat! s soon as you can, please tell Christine Brennan what this is for and how big your amounts really are-- it looks from your recipe as though you are mixing 2 tons of stuff. That's a lot of stuff to mix and creates new problems. Also please tell us if you are all right, if you wore safety glasses and used gloves. Christine Brennan can give you good advice.

The big problem-- the reason to stop mixing and look for another way-- is that when you mix an acid (H3PO4) and a base (KOH), the OH- from the base combines with the H+ from the acid to make H2O. This is a very favorable reaction and gives off a lot of heat, 57.6 KJoules/(mole H+ or mole OH-, whichever is less). That heats your solution a lot. Don't worry about the calculation but you will raise the temperature of your mix by about 200 deg C. It will boil and you might get spattered with strong acid or strong base. If you are mixing the ratios you gave, this is as true for a pound of mix as for a ton.

Your mix will get a little hotter because water dissolves KOH and makes heat from that, and it dissolves phosphoric acid and makes heat from that. But the big temperature rise comes from  neutralizing acids and bases. Please tell Christine also what kind of container you tried to mix the chemicals in, and what you used to stir the mix.

Best regards,

Mike Dowell

Contributor III

Re: mixing acid with alkaline

Dear Chris,

First of all, I would reiterate Michael's comments and warnings!  It is very good that you ASKED before continuing (I hope).  Mixing acids and bases (even just in water) is ALWAYS DANGEROUS, in any quantity, and SAFETY should be your first concern.  As Michael also noted, your quantities sound like a commercial process.  Unless you developed it yourself, the lack of associated Material Safety Data Sheets (MSDS) and detailed equipment and operating procedures (and SAFETY!) is worrisome.

Secondly, yes, it is possible to make that mixture safely.  But it requires a combination of proper materials and procedures.  Controls for temperature maximums, solution mixing and safety shut-offs (especially stopping further feed of any item into the reactor!) need to be in place.  Training of personnel - even if it is just you - must be done with the MSDS information and the process design to assure proper safeguards of protective clothing, knowledge of the process, and the ability to conduct the mixing reaction safely.

You may need a particular design and material for the mixing container to remove the generated heat more effectively.  You WILL need to control the rate of addition of the compounds to maintain a relatively constant, safe temperature during the entire mixing/reaction process.

If you need specific advice or knowledge from an expert for a commercial or pilot-scale system, you should contract with a consultant with the requisite chemistry knowledge, or a chemical engineer.  We can offer guidelines and cautions such as this in this forum, but it is not really the place for detailed process discussions.

Best regards,

Steven Cooke, MChE, FAIC, CQE

New Contributor II

Re: mixing acid with alkaline

As others have stated, context would be helpful.

The neutralization reaction will generate a lot of heat.  If you are making a buffer, Is there a reason you can't use "solid" potassium phosphate to make the bulk  buffer and then make any final adjustments with phosphoric acid or potassium hydroxide solution?  One can calculate how much mono or di potassium phosphate would be needed etc. to make the molar ratios come out correct.

New Contributor

Re: mixing acid with alkaline

Hi All

Thanks for your replies. I have solved the problem by getting a hopper with a small outlet. Putting the KOH into this hopper and dribbling it into the acid water mix has fixed the violent reaction. I am using a 1800 litre stainless tank with a very good stirrer, that goes from zero to 1500 RPM. The water still heats up but it is ok. I am using all safety equiptment necessary to ensure my safe handling of this mixing.

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