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Testing NaOH purity

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Barquentine

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I suspect that my NaOH hasn't been stored well at some point, so I intend testing it. Give or take 3%, it takes 0.6g NaOH to neutralise 1 gm citric acid, so I'll use a 10g citric acid sample and test how much NaOH it takes to get pH 7. Then I can adjust my soap recipes accordingly.
 

DeeAnna

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Unless you are looking for very rough answers, you may not get the accuracy you want with this method.

First off, you are mixing (if I'm following you correctly) two highly concentrated materials to get your answer. That's going to add a large amount of error to any test you make, because it will take only a tiny bit of acid or base to make a big difference in the answer you get.

Also, NaOH is deliquescent and highly reactive. This means as you expose NaOH to the open air -- whether the NaOH is in solid form or a liquid solution -- it simultaneously absorbs large amounts of water and quickly reacts with CO2. That is why NaOH solutions are notoriously difficult to accurately measure, even in the laboratory.
 

Barquentine

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Probably I'll make a 0.1N solution of each and use phenolphthalein as indicator. I'll rely on my lab scales for accuracy. At least it will give me a real-life situation that I can plug in. At the moment I have no idea. :(
 

The Efficacious Gentleman

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If you really want to be accurate, would the pheno be enough? As far as I'm aware, not only is it subjective in use, but also only shows alkaline. If you are neutral OR acidic, it will be the same result and that is really not going to be helpful
 

DeeAnna

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Effy -- From B's last post, it appears that B knows how to do an acid-base titration of a dilute aqueous solution. Phenolphthalein is often used in the chem lab as an endpoint indicator for this type of titration. The intensity of the pink color is often used by soapers as proof of "too high pH", but the intensity of color is not accurate and is not the point when titrating. In titrating an alkaline solution with an acid, you look for the color SHIFT at which the indicator fades from very slight pink to colorless. That happens at a pH of 8.2 for phenol-p.
 
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DeeAnna

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Yep! You got it, except you have to make an acid solution in such a way that the concentration is reasonably accurately known. Then you can test an unknown -- the NaOH solution -- against this known standard. In the lab, you might buy a standardized acid solution that is pre-mixed with high precision or make it yourself under carefully controlled conditions with the proper chemicals and equipment. A "kitchen chemist" can make an acid solution that is accurate enough for soaping purposes, but the process might drive a serious chemist mad. :)
 

Barquentine

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Exactly. Actually my problem is that I am unsure as to whether my citric acid is anhydrous or monohydrate which leads to an innacuracy of 3% if I get it wrong; so, I think I will buy some standardized 0.1N hydrochloric or sulphuric acid to titrate against. I have methyl orange and high-accuracy test strips available too. I strongly suspect that my NaOH is partly sodium carbonate.
 

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