When testing acid and ph with a meter, how much

[olusteebus]

must do you use?

 

[ibglowin]

Did you purchase a kit of sorts? The amount can vary depending on the strength of NaOH.

 

[ibglowin]

Yep, so there are a number of formulas out there with different sample sizes and some kits come with 0.1N NaOH, others come with 0.2N NaOH others come with 0.01N NaOH……

I use the Morewine Kit. It calls for a 5ml sample then you add ~50ml distilled H20 to the sample and titrate to pH 8.2 using 0.1N NaOH

The formula for TA is simply (ml NaOH used) X 1.5 = g/L TA

 

[BernardSmith]

Trying to wrap my head around the relationship (or otherwise) of the sample size to the quantity of NaOH needed to neutralize the TA. If TA is a measure of the % of acid in the solution then why wouldn't the addition of distilled water create a problem (because now the % of acid in the solution is far less? ). What is it that I am not seeing? And why does the sample size (in this case 5 ml but in other examples , the sample to be drawn is 15 ml ) make no difference to the amount of NaOH needed to hit the sweet spot of a pH of 8.2. If the sample size was 5 gallons would the same 6 or 7 ml of NaOH be enough to bring about the sought after change in the pH?

 

[ibglowin]

The addition of distilled water actually does alter the result but it is pretty small error. If you want a "perfect" titration you should adjust your distilled water to the endpoint of pH 8.2 first. You will quickly see that it doesn't take much NaOH to get you there.

Sample size matters! LOL If you use a larger sample size you will need more NaOH to reach the endpoint. If you use a stronger solution of NaOH it will take less to reach endpoint all other things being equal.

Here is the generic formula that allows you to use any size sample with any strength of NaOH and figure out the TA:

TA as tartaric acid (g/100 ml) = (V) (N) (75) (100)/(1000) (v)

V = ml of sodium hydroxide solution used for titration
N = Normality of sodium hydroxide solution
v = sample volume (ml)

 

[Rocco]

My kit says I can add anywhere from 20-50cc of distilled water for reds to allow a more clear detection of the end point. For white it doesn't say to add distilled water. In both cases it asks for 10cc of wine. I basically just use the 10cc of wine and use the pH meter to titrate to 8.2 rather than color change. If the directions call for 20-50 for red oy for better visualization and nothing for whites(color change easier to see) I'm assuming the volume of distilled water doesn't matter as its assuming a fairly neutral pH.

I think I'm ok not diluting it--that way the water doesn't affect the result. If it's not needed for white, it shouldnt be for red. Basically the amount of wine and strength of the NAOH are the two most important variables.

Please correct me if I'm not thinking about this correctly.

 

[ibglowin]

The water does a couple of things. If your using endpoint detection with an indicator then diluting the must helps you to visually see the color change. If your using a pH meter then you need to have enough sample to completely immerse the probe tip in/under the solution. Adding water here helps do just that. You could use a large volume of sample as well to cover the probe tip but then you are just wasting NaOH trying to get to the endpoint and introducing more errors into the analysis by refilling the syringe. Ideally you want to use a small(er) sample size and a strong enough titrant to reach endpoint without the need to refill the syringe.

 

[BernardSmith]

Thanks Mike. That formula must have been used to under-gird the instructions I had for testing TA - which called for a 15 ml sample and then advised that the total amount of 01 normal NaOH (in ml) used to reach a pH of 8.2 was equivalent to the TA of the wine. Hence my question about the volume of the sample, if some samples called for are 5 ml and others are 10 and my own was 15...

 

[ibglowin]

There is a whole bunch of variations on the formula but they each use different sample sizes and Normalities so you can't mix and match and come up with the right answer. I know there is a formula for a certain Normality of NaOH and a certain sample size in that if used everything basically cancels out of the equation and you are left with TA = (vol) NAOH used to reach endpoint (8.2)