Neutralization calculators

[sbowns]

Hi there,

I have a 0.145 M solution of sulphuric acid. I would like to adjust the pH of my solution to 3 through the addition of 50% sodium hydroxide (caustic soda) in order to precipate my iron impurities. My knowledge of acid-base chemistry is extremely rusty. Can anyone help me in determining how much sodium hydroxide will be required to make this pH adjustment?

 

[kenvlach]

What quantity of solution?

Have [H+] = 0.290 N, since 2 H+ per mole sulfuric
For pH 3, want [H+] = 0.001 N,
so need to reduce sulfuric from 0.145 to 0.0005 M, i.e., neutralize 0.1445 M.
Takes 2 NaOH to neutralize 1 mole of H2SO4.

So, desired reaction: 0.1445 moles sulfuric + 0.2890 moles sodium hydroxide = 0.1445 sodium sulfate + 0.2890 water

The FW of NaOH is 40, so need 80 grams of 50 wt% solution for 1 mole NaOH. The SG of 50 wt% NaOH solution is ~1.53 g/mL, so use about 15.1 mLs of 50 wt% NaOH per 1 L of 0.145 M sulphuric acid.

But, to be precise, the new total volume will be ~1.0151 L, which will slightly dilute the H+ below the desired 0.001 N.

Try adding (1000/1015.1) x 15.1 mLs = 14.875 mLs of the 50 wt% NaOH.

Actually, this was Just a preliminary calculation to get an idea of the quantities involve. The NaOH composition is more like 49-51 wt%, and probably don't know its exact SG. So, you carefully add a smaller amount of NaOH while thoroughly stirring, and make final additions via pH. Remember that pH is a log scale, so it will take smaller & smaller additions of NaOH to get the same [Δ]pH. Also, unless the volume of sulfuric solution is very large, you need to use more dilute NaOH to reach the desired pH precisely. In titrating acids, usually use 1 N NaOH (~4 wt%), and final pH adustments are made dropwise (~0.05 mL).

 

[sbowns]

Thank you very much for your help! We are using very large quanities of sulfuric solution (20,000L), so I suppose that means we can use more concentrated NaOH. We have the specs for the 50% NaOH (sp. grav = 1.52, pH = 12).

Anything else we need to know?

Thanks again, your help is much appreciated!

Sheena

 

[kenvlach]

Two comments.
1) A pH of 3 will only reduce [Fe+3] to 20 ppm. Raising the pH to 4 will reduce it to 1 ppm. See chart below

-- from

http://www.finishing.com/Library/metalpH.html

2) Some 50% NaOH solution (technical grade) is quite high in salt, maybe 0.5% NaCl. If undesirable, ask supplier for analytics.

 

[sbowns]

I was hoping you could help me with another question. I'm not entirely sure how you determined 15.1 mL of NaOH needed to be added using the specific gravity. Is it possible for you to show me this calculation?

Thanks

 

[sbowns]

I figured it out! You can ignore the last post.

Thanks,
Sheena