How cold is cold? - Mixing Sodium Hydroxide solutions

[Tom Kershaw]

I need to make a 10% Sodium Hydroxide solution for use with a Thiourea toner. As an NaOH solution needs to be prepared using cold water, how cold is cold? ice cubes, 5ºC, 10ºC, 20ºC?

The formula in Tim Rudman's book is for 1000ml of 10% solution.

Tom

 

[jovo]

I recently made up a batch. I put the coldest tap water I could get in an ice cube bath for as long as it took for most of the ice to melt (it was a warm day), and then made up the solution out on the deck so there would be less chance of any fumes. I wore long sleeves, a darkroom vinyl apron, a dust mask, and nitrile gloves. I also wore googles over my glasses. I probably over did it a bit, but everything went well, so I'm certainly not sorry I went to the extra trouble. Good luck.

 

[Mike Wilde]

I make up NaOH stock solutions for making soap. I compute the stock solution on a weight per weight volume. I have found that 40% is about as high as I can go without having the hydroxide re-chrystalize.

So if I want 40% stock solution, about 1000mL, I start with 300g water as ice, 300g water as room temp/ from cold water tap, and then mix the two for a minute to lower the water temp. Then weigh and quickly re-cap the balance of your hydroxide stocks. With 40%, the ice is gone in less than 30 seconds, and the heat generated never gets above 50C. I do not know exactly what final volume of liquid is generated, because I use my hydroxide by weight, not volume.

 

[Robert Hall]

Just like most things, it's not how much, but how fast. If you keep a thermometer in the mix, you will have a better idea how hot it becomes as you _slowly_ add the NaOH to the water.

I wouldn't call it over done John, just good practice.

Cheers,

 

[Kirk Keyes]

Just like most things, it's not how much, but how fast.

That really is the trick. Just take your time and add a little at a time, stir until dissolved and then add a little more NaOH. If it gets hot, place it in a bath of cold water, and let it cool. Then continue adding more NaOH - slowly.

I've made a gallon of 50% NaOH, and it took me over an hour with lots of cold, running water over the container. It got hot, but taking it easy on the addition rate, it can be done. I'll never do that again, as it is just easier (and safer) to buy it premade.

And follow the safety suggestions made by the guys above.