Adding Muriatic Acid to Caustic

[thefost]

So I want to use caustic to clean various things (especially plate chiller) with caustic and want to do it as safely as possible. I'll have all the usual safety gear (pvc gloves, apron, face shield, goggles) but I also want to use Muriatic (hydrochloric) acid to neutralize the caustic when its finished.

I'm having trouble figuring out the math on this one and could use some help.

I start with 1 gallon cool water and add 1/2 cup dry 100% sodium hydroxide (caustic) as recommended on this site.

Lets say I get some liquid 31% HCL (muriatic) from lowes since at $7 per gallon that's the cheapest I seem to be able to find. How much liquid muriatic acid would I have to add to neutralize the caustic?

 

[WayFrae]

If you have a pH meter, just mix up a small sample of the caustic soda and water solution in the same ratio you have listed. So like a pint of water and one tablespoon of caustic soda. Then measure out small amounts of acid and add it to the caustic solution to get the pH to 7. I honestly don't know how much it would take but say it took 2 tablespoons of acid to achieve a pH of 7. Then when you make your gallon of solution you would know that you would use 1 cup of acid to neutralize it.

 

[Brad2287]

I order to do the calculations you will need to know the mass of the NaOH in order to calculate how much HCl.

Balanced Reaction

HCl (aq) + NaOH (aq) ----> NaCl + H2O

You will need the Molar masses.
HCl = 36.46 grams/mol
NaOH = 39.97 grams/mol

Lets assume a half cup weighs 1 pound. This is just as an example.

1 pound = 454 grams.

454 grams NaOH * (1 mol)/(39.97 grams) = 11.36 mols NaOH

For every mol NaOH one mol of HCl will react per the equation above.

11.36 mols * 36.46 grams/mol = 414.13 grams HCl

Due to it being 31%

414.13/0.31= 1335.90 grams 31% HCl solution.

The density of 31% HCl is 1.1543 g/ml at 20 C

1335.90 grams * 1 ml/1.1543 grams = 1.1157 liters

So for every pound of NaOH you would require 1.1157 liters of 31% HCl

You can always titrate to a pH=7 endpoint like stated above.

This does make some assumptions.

Even if your NaOH is labled as 100% it is not 100%. It is very hygroscopic and readily absorbs moisture from the air. It also readily reacts with CO2 from the atmosphere. Most NaOH unless purchased as anhydrous is actually closer to 85% due to the above reactions. If it is anhydrous as soon it is opened it will be compromised.

HCl is also a very annoying acid to work with due to the fuming nature. Be aware of this when storing and handling. Many people have ruined cabinets thinking a tightly closed bottle of HCl will not fume into the areas near the bottle.

HCl and stainless steel are not compatible. You will destroy stainless steel with HCl. I would use phosphoric instead if that is an option.

The reaction above is very exothermic. Be very careful because it is going to get very hot and possibly boil. Generally when these types of titrations are done the reactants are added very slowly to dissipate heat.

 

[tjr3000]

Why do you feel the need to use HCl? Vinegar is likely much cheaper and far safer.


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[Brad2287]

Why do you feel the need to use HCl? Vinegar is likely much cheaper and far safer.


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It would be safer and easier to handle but more expensive most likely. Honestly, handling both NaOH and HCl on a daily basis and having been exposed to both the caustic is more of a safety concern. The fumes of HCl and the fact that it is corrosive to stainless steal is reason enough not to use it in this application.

Using the same math as above.
CH3COOH 60.05 grams per mole
Concentration is ~5%

Acetic acid is monoprotic so it reacts at a 1:1 molar ratio

This means it would require ~13,643 grams of vinegar per pound.
The density is 1.01 g/ml

This means per pound if NaOH you would require 13.50 liters (3.57 gallons) of vinegar which is over 10X more liquid than HCl.

 

[tjr3000]

No arguing the math. I actually remember most of that (not sure how or why...)

I don't know, though, that pH of 7 is a requirement, though, but that is me making an assumption. I'm looking to make the caustic much less, well, caustic. I don't need full neutralization, just enough to bring pH down to a reasonable level for disposal.


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[Brad2287]

No arguing the math. I actually remember most of that (not sure how or why...)

I don't know, though, that pH of 7 is a requirement, though, but that is me making an assumption. I'm looking to make the caustic much less, well, caustic. I don't need full neutralization, just enough to bring pH down to a reasonable level for disposal.


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A pH of 6-8 wold probably be fine. This a situation where pH strips are adequate and a Ph meter is unnecessary. As long as Proper PPE is used most of the risks will be mitigated.
If using HCl i would do this in a bucket in order to minimize exposure to any stainless steel. Using HCl outside will also help with the fuming issue if one lacks a proper fume hood or respirator.
You will also need some way to dissipate heat from the system. The last thing you want is partially neutralized caustic boiling everywhere. Diluting the caustic before hand and adding the acid slowly can help to slow the heating. I would also use ice water to further ensure safety.

 

[thefost]

Thank you very much for the help on this one. I had no idea HCL was corrossive to stainless steel. Add to the fact that the reaction is highly exothermic, and this idea sounds like its pretty much dead

I thought I was being smart here by neutralizing the lye before I have to move it to dump it, but I guess that's not the case. Vinegar would be really expensive so that's a no go. If anyone has an idea to cheaply and safely neutralize caustic used on stainless it would be appreciated, but I'm getting the feeling a simple solution doesn't exist. Looks like I'll have to skip the neutralization step and just be really really careful with the caustic.

 

[tjr3000]

One more thought - baking soda?


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[LordUlrich]

One more thought - baking soda?


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Won't work, the chemistry is all wrong.

If you are trying to dispose of it, you old use the old sanitary engineering adage, dilution is the solution to polution. If you flush with enough water it will become safe.

 

[tjr3000]

Yeah, your right - I went the wrong way with that one.

Reading up on this a bit more, the most recommended solution I've come across is also the simplest - massive dilution. It doesn't necessarily neutralize the solution, but enough water dilutes it to the point it's not a significant issue.


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[Brad2287]

Won't work, the chemistry is all wrong.

If you are trying to dispose of it, you old use the old sanitary engineering adage, dilution is the solution to polution. If you flush with enough water it will become safe.

The chemistry is mostly wrong. Sodium bicarbonate will act as a weak acid in this case.

NaHCO3 + NaOH ---> Na2CO3 + H2O

It will not be as dangerous as sodium hydroxide but sodium carbonate (Soda Ash) is still fairly basic and will cause skin and eye damage.

I do agree copious amounts of water down the drain is your best bet. You are not doing this on an industrial scale so it is not a problem. A lot of the drain cleaner formulations on the market are NaOH based and they are meant to be put down the drain. You can use that as a justification.