[Brewer Gerard]

I grow brewing hops in my back yard and something that seems to elude homebrewers is the ability to determine the alpha acid content of their hops. Ordinarily titration is used but i don't have everything i need for that and was hoping a rough estimate could be achieved using a ph meter.

So here's a brief background. Hops contain a compund called alpha acid that when boiled isomerises and provides the bitterness for making a beer. If it is not known what the alpha acid content is of the hops being added is then the resulting bitterness cannot be determined. When the hops are boiled for 60 minutes all alpha acid should be dissolved in the beer.

So my thinking is, if I boil 1g of hops in 250ml of water for sixty minutes and strain and reduce further to 100ml, test the difference in ph post boil, i can guess the amount of acid compound that has been dissolved into solution. If i give the figures can anybody determine a content for me?

[Borek]

Too many unknowns for a reasonable dependence between amount of acid and pH.

If it is a weak acid (sure it is, although I have no idea what its dissociation constant is) tenfold difference in the concentration will give a pH difference of around 0.5 unit. You can't expect your pH measurement accuracy to be better than 0.1 unit (or even worse). That means you would be able to tell the order of magnitude, but not the exact concentration.

That's assuming solution that contains only one acid - which is definitely not true. Other substances will make the measurement more difficult and they change pH in a difficult to predict way (only making the result worse than we already know it is).

It can be possible to prepare some kind of a calibration curve, but it will make only future measurements easy - at first you would need titration which you can't do.

Edit: http://en.wikipedia.org/wiki/Alpha_acid says it is not a single acid, but a mixture - which only makes the situation worse.

[Arkcon]

Like Borek: said, you have the beginnings of a good plan, but pH is an inadequate assay for a dilute solution of weak organic acids.  The brewing supply store might sell you calibrated sodium hydroxide solution and phenolphthalein indicator.  And that's a big maybe, some home winemakers titrate their grape must to determine acid level.  If you can get a good result assaying the titrateable acid content of commercial hops that have a known aloha acid content, then maybe you can compare the result with your home-grown to determine what general amounts to use -- i.e. your home grown is 1/3 or 1/5 or 1.5 times the content of commercial.  But still, you won't know the absolute flavor profile of the hops.  I don't know how brewers determine that, but I'd suspect HPLC would be the method.  You might get some interesting profiles comparing commercial and home grown hops on TLC.  Which is likewise a pretty advanced analytical procedure that the wine-making supply store might be able to help you with.

[Brewer Gerard]

So I tried the ph test last night and 1g of hops in 100 ml of water makes no notable difference on a meter of one decimal place of resolution. The sample read the same as what's coming out of the tap. I have the phenolphthalein and NaOH just no pure IPA for making indicator. When i get set up I'll post back with my progress. Could i expect with a greater amount of hops that a titration test could yield some repeatable results?

[Borek]

In what form is your phenolphthaleine? It can be dissolved in IPA or ethanol, but from what I remember even if you have it as a dry powder, you can still add it to the titrated solution (just a very, very small pinch) and observe color changes.

[Brewer Gerard]

Yup. It's powdered alright. I'll mix it up tonight. I think this is going to be a trial and error exercise. Does the IPA need to be very pure or would i get away with 70%~?

[Brewer Gerard]

No matter. I've just managed  get some 99.7% stuff

[billnotgatez]

For a second I was not sure
IPA the kind of beer
or
IPA Isopropyl alcohol

 

[Borek]

IPA purity doesn't matter much - you will use just a drop anyway. In general - the more concentrated the better, as it will work as a solvent.

[Brewer Gerard]

Ok, mixed up a batch of 1% phenol last night and did a test with some NaOH and vinegar. I cal'd my ph meter too. Will try to perform this test at the weekend and see how it goes

[Brewer Gerard]

For a second I was not sure
IPA the kind of beer
or
IPA Isopropyl alcohol

 

Bit confusing given the subject matter alright

[Brewer Gerard]

Ok so i tried the procedure recommended on brewing sites that can be found here . The variety hop i used has a relatively high alpha acid content of 10.9% (amarillo) and i used double the concentration recommended, 2g in 100ml!

I boiled the 2g for 1 hour and mixed with my reagent (10ml of 1% NaOH) in lots of 5ml till the whole 100ml was spent. No colour change! On further investigation my Ph meter indicated an alkalinity of over 11.

If i'm using double the concentration of a high alpha acid hop and cant reproduce whats detailed, how can this procedure be credible?

 

[Borek]

My bet is that there is something wrong with your phenolphthaleine, does it change color when added directly to the NaOH solution?

10.9% of 2 g is 0.22 g, or 6.07×10^-4 moles (assuming humulone, C₂₁H₃₀O₅, molar mass 362 g/mol).

1% NaOH is around 0.25 mol/L.

Your first 5 mL of the NaOH solution contained 1.25×10^-3 mol of NaOH - twice the amount needed to neutralize the acid. Actually it is not clear to me how strong acid it is and whether it should be treated as mono-, di- or even triprotic - but in the worst case 7.5 mL of the solution should be about enough.

[Brewer Gerard]

I put 0.5g of 96% NaOH in 50ml of water. I decant 10ml to a seperate container and add a drop of phenol and the solution turns purple. I then start adding back the supposed acid solution (It has reduced in pH by 0.1) in 5ml amounsts until it's all gone, 100ml. No colour change. I add 5ml of vinegar solution clears immediately.

Unfortunately I have no education in chemistry worth speaking about. The phenol does indicate, not sure at what pH but should that matter?

[Borek]

Have you measured initial pH of your hop brew?

Phenophthaleine changes its color between pH 8 and 10 (approx).

No colour change! On further investigation my Ph meter indicated an alkalinity of over 11.

Which is a contradiction - either your pH meter doesn't work, or your indicator doesn't work. You can't have a pH 11 solution with phenolphthaleine present and not being pink (unless the solution contains some other chemicals that react with the indicator; which is rather unlikely).