titrations - standardization

[ece]

I have been doing titrations for grease samples to be analyzed for free fatty acid content.

a little background

I was using store-bought lye and measuring 1 g/L of water as a titrant because I work for a start-up with a limited budget for analytical grade titrants. But what happened was my results were off the last batch of titrant I made up. It turns out that the lye we were using, even though it was stored properly, was slowly absorbing water over time. So when I measured out a gram, it was a gram of KOH and water instead of KOH. So I no longer knew what the concentration was, nor what my results should have been.

So I decided to buy some analytical grade KOH in water premixed, like a 1N solution. My results were fairly consistent, but what we want to do now is make up a known solution of acid and water, like 5% by weight hydrochloric acid in water. Then we will titrate that with our standard titrant and see whether our results match. This will also be used for training our technicians to get better repeatability.

My boss has two acid solutions one is HCl 32-40% in water and the other is supposedly reagent grade H2SO4 but it says it's 90-98% H2SO4 in water and doesn't have the molarity listed on there like a typical reagent grade chemical would. I cross checked with the reagents chemical supplier MSDS, nothing.

So my question is, I want to find an acid reagent that is defined to a molarity to at least two decimal places, doesn't have to be pure obviously but I need to know what the number is, and maybe a certificate with the LOT# saying what it was measured. Where is a good place to get one of these, relatively cheaply?

[Bruce Hamilton]

Sadly, the common ( and probably best ) method is to standardise the acid solution using a solid of know purity.

If you have access to an oven, then sodium carbonate is usually used, if not - then Borax ( sodium tetraborate ) is a good alternative.

Once you've standardised the acid solution, you can then use that to determine the molarity/normality of the KOH solution.

I suspect that water may not be the issue, but the accumulation of carbonate - hence the reason why a different solid ( not xOH ) is used to standardise the acid.

Purchasing ampoules of alkali is fine if you store and use them properly, but I've found having an independent reference compound is good peace-of-mind.

Any introductory analytical text, and lots of web resources, will discuss titration standardisation principles and techniques.

If you are following standard methods ( eg AOAC, OCS, etc. ) their manuals will define how to standardise.

Bruce Hamilton

[ece]

we purchased a .1N KOH in water solution, seemed to work fine. some reason why i need a solid KOH? doesn't it take on water from the atmosphere?

[Bruce Hamilton]

Sorry, I wasn't clear. solid KOH not only grabs water, it converts to carbonate on exposse to air, which will also screw up the nominal concentartion. It is totally unsuitable as a reference standard, and is used only after regular standardisation.

No official methods that I'm aware of will allow analysts to standardise acids with KOH solid or solution, you have to use another solid, such as Sodium carbonate or Borax.

The normal alternative is to purchase a prepared acid solution that is traceable to NIST or some other organisation. Most chemical suppliers ( eg Merck ) will sell such standard volumetric acid solutions, certified to 0.1%.

If you want to prepare acid from a concentrate, you have to standardise it. That's because the acid strength can also slowly change, eg HCl can volatilise.

If you purchase standard ampoules to prepare 0.1N KOH, you still have to standardise the acid solutions you prepare to check them, but if you purchase the actual acid concentration you want to use ( no dilutions required ) you can obtain down to 0.1% precision of normality, traceable to NIST. Many will have limited shelf life, once opened.

Most chemical suppliers offer cheaper versions of these solution for schools, so ask the local teaching institutes where they get their supplies.

Bruce Hamilton

[Consumer Products Guy]

I'm going to suggest that purchasing pre-made titrant is actually considerably cheaper than making up a solution and standardizing v. a primary standard (e.g. sodium carbonate for standardizing acids and potassium hydrogen phthalate for standardizing bases). Even at a school, your time is worth something. We purchase pre-made titrants whenever possible, helps efficiency which means lower costs. For free fatty acid titrations we dissolve the sample in about 300 ml of hot neutral alcohol and titrate to phenolphthalein endpoint using 0.500 N NaOH.