Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: mehc on April 20, 2014, 06:30:49 AM
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I want to determine the acid value of stearic acid.
the stearic acid is titrated with KOH and calculations are
volume of titrant x molarity of titrant x 56.1 / weight of sample.
If I use NaOH instead of KOH, should I use molecular weight of NaOH in place of 56.1 (molecular weight of KOH)?
There is confusion because the sources (including books and web pages) which says to use either of NaOH or KOH, they mention the same formula with 56.1 value.
Thanks
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The acid value range for stearic acid is 195 - 205 mgKOH/g.
If the value is lower than 195, it will indicate the lower fatty acid content and more non acid impurities in the sample.
If the acid value is high than 205, what will it indicate? Does it show that stearic acid has some dicarboxylic acid impurities, but they are not present in fats.
Please suggest a reason for it.
Thanks
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If I use NaOH instead of KOH, should I use molecular weight of NaOH in place of 56.1 (molecular weight of KOH)?
I think you should be able to figure this one out for yourself. What is 56.1? Is it the formula weight of KOH? And why is that constant there in the calculation? If you draw in the units at each number, is it there to cancel out an important unit? Can you create a theoretical situation on your own, and try it both ways? I mean to say, consider a sample of stearic acid, 1.0000 grams, 100% purity -- how much hydroxide should that require? What happens in each case if you use either formula weights?
You've had enough posts on these forums to be aware of the forum rules, we don't just dump answers here. That's not just for high school kids, we hold all members of this group to that standard.
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If the acid value is high than 205, what will it indicate? Does it show that stearic acid has some dicarboxylic acid impurities, but they are not present in fats.
Some sort of acid impurity, that's for sure. It appears your guess in this case is a good one. See this reference, if you can: http://www.ncbi.nlm.nih.gov/pubmed/10580186 The reference is basically talking about a metabolic process, but the abstract mentions chemical conversion of unsaturated fatty acids into dicarboxylic acids. Yeah, I know, there shouldn't be unsaturated acids in your stearic acid. But that's the whole point of analytical testing -- is this product what it says it is?
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You've had enough posts on these forums to be aware of the forum rules, we don't just dump answers here. That's not just for high school kids, we hold all members of this group to that standard.
I do have tried it in various forms, and just asked for a second suggestion, I didnt demand a magic answer.
I cannot find option on the forum to delete my high school questions to keep your standard. Thanks for guidance about forum rules.