pH & titatratable acidity ｑｕｅｓｔｉｏｎ

[Blue-Frog]

How does dilution affect these ?
In general of course... what trends can be expected?

 

[ajdelange]

As you dilute solution A with the solution B the properties of the mix transition from those of solution A at 0 dilution to those of solution B when the dilution is infinite (all B). For example, if B is DI water (truly DI with no dissolved CO2) the pH will, as more water is added, move from the pH of solution A towards the pH of DI water (7 at 25 °C) and the titratable acidity from what ever the titratable acidty of solution A is to the titratable acidity of DI water is at the same end point pH (0.002 mEWq/L to pH 8.3 - phenolpthalein end point which is the usual for an acidity measurement). Dilution calculations are carried out in the same way as any of the acid/base computations of interest to brewers i.e. by tracking the proton flows between the initial pH (solution A) and a hypothesized final pH. The final pH is the one at which the protons lost are just equal to the protons gained and once that is know the acidity computation is trivial.

 

[Blue-Frog]

So a smooth transition.

While it may not be strictly true, my assumption was that dilution (w/DI HOH) would appear to make less of a change in pH than TA. (because pH is a logarithmic scale while TA isn't.) The situation I had in mind was reading pH and TA of concentrates compared to their final diluted concentration. What useful information can be derrived from pH and TA readings taken from such samples, rather than from the target concentrations?

(In making a cider sort of drink it was advised to measure those values of the pure juice, but my orginal Rx is for a beverage not made with 100% pure juice)

How would buffers affect TA measurements?

 

[Vale71]

So a smooth transition.

While it may not be strictly true, my assumption was that dilution (w/DI HOH) would appear to make less of a change in pH than TA. (because pH is a logarithmic scale while TA isn't.)

Your assumption is correct. While TA is affected linearly, PH will not be. Both parameters represent a concentration, but the first does have a linear scale whereas the second has a logarithmic scale.

 

[ajdelange]

So a smooth transition.

Yes

While it may not be strictly true, my assumption was that dilution (w/DI HOH) would appear to make less of a change in pH than TA. (because pH is a logarithmic scale while TA isn't.)

Assuming that the dilutions are reasonable then those assumptions are pretty good. The following curve shows what happens when 1 mL of 88% lactic acid is diluted with various volumes of DI water.

pH is log linear (proportional to the log of the volume) up to say 10L/mL above which the pH change becomes steeper with log volume until around 30,000 L at which point the rate of change decreases finally approaching 0 (constant pH = 7) somewhere above 300,000 L/mL.

The P acidity (pH 8.4) is just the P acidity of lactic acid plus the P acidity of water (0.002 mEq/L). Up to about 20,000 L the acidity of the acid dominates and the total acidity is proportional to the concentration of the acid alone. At higher dilution the acidity of the water at first challenges that of the lacrtic acid and then eventually dominates it settling in assymptotically at 0.002 mEq/L.

The situation I had in mind was reading pH and TA of concentrates compared to their final diluted concentration. What useful information can be derrived from pH and TA readings taken from such samples, rather than from the target concentrations?

The mixing of juice concentrates is an acid/base problem no difference in its underlying theory than the mixing of malts. If you know the pH of the concentrate, its TA and the pH to which the juice was titrated in determining the TA you can obtain an estimate of the buffering of the the juice and from that you can estimate the pH of a mixture of juices and/or water. This is simple to do but for some reason it is very difficult to understand so perhaps it would be best if you gave me an example of what you want to do. I can then walk you through the steps and whether you understand it or not at least you will know how to make the calculation.

(In making a cider sort of drink it was advised to measure those values of the pure juice, but my orginal Rx is for a beverage not made with 100% pure juice)

You would measure the pH and TA of whatever it is you are mixing. For the purpose of predicting the pH of a mixture it is preferable to measure the pH to an end point close to the pH you are striving to realize. Better yet is a set of TA readings to several end point pH's as a single pH TA forces us to assume that the buffering curve for the juice is linear and it may well not be.

How would buffers affect TA measurements?

TA is a measure of the buffering of the sample between the pH at which it comes to you and the end point pH so buffers in the juices are automatically compensated for. If you are referring to a buffer you might add those are easily accounted for by calculating the acidity of the buffer (well known for things like sodium bicarbonate, for example) or measuring it. Added buffers effect the pH of a mix by absorbing or contributing protons in the same way that the buffers in the mixed juices do. This is easily accounted for in the calculation.

 

[ajdelange]

Both parameters represent a concentration,

TA is not a concentration. It is the quantity of protons which must be absorbed in bringing a sample to the end point pH at which TA is specified.

Technically, pH is not a concentration (or minus the log thereof). It is minus the log of the activity of hydrogen ions. At the high dilutions the activity coefficient will be very very close to 1 and the pH is, in that region, very very close to minus the log of the concentration. At smaller dilutions the activity coefficient may differ from 1 though we seldom in brewing concern ourselves with that.

 

[Vale71]

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