Ok, I’m not sure if I’m just over complicating things to myself as I’ve re-read my lab manual several times over, as my instructor had left this as a sub question. And this is in the context of food titrations where I am sure pKa and Ka is not a part of the calculations.
My lab asked me to get Titratable acidity under the assumption that both dissociated free hydrogen ions and undissociated hydrogen ions are completely titrated. From my understanding, I’m not looking for a specific pH, but the TOTAL pH measured regardless of acid being weak or strong. The lab question is asking pH of unlabeled milk powder. Using the classic “just add a few drops of phenolphthalein” which tells me when it stops being clear around ~ 9 pH
What’s driving me insane in constantly second guessing is that we’re supposed to use the C1V1=C2V2 to determine the pH and I probably screwed up getting my values in the lab.
Things I remember with acidity are:
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pH = -Log(n)
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N = n/vol(in L) and M = n/vol (in L).
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pH + pOH = 14
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neutralization means there’s equal amounts of acid-base (or C1V1=C2V2)
The source of my second guessing is that I don’t “have” a dissociation % to calculate off from. And that we were told to record the NaOH used and determine pH from there with equivalent weights in the manual (which I’m also second guessing)
Things I know:
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0.1N of NaOH was used in the Titration.
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Phenolphthalein was used (so lightly hued means aprox 8-9 pH)
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Regular milk is usually around like 6 pH (my expectation)
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Lactic acid (C3H6O3) + NaOH is a 1:1 neutralization relationship
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1N of NaOH/Lactic acid = 1M of NaOH from…
NaOH + C3H6O3 -> NaC3H5O3 + H2O
My problems:
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Titration point where the Phenol stayed pink was 1 mL of NaOH (using a 25 mL buret to drip into the beaker)
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mL of milk was put into 10 mL of water (Distilled, which is presumed to be CO2 free according the the manual)
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Using C1V1 = C2V2
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The Equivalent weights in the manual has them in mg/meq, but the listed example calculation has them in g/eq.
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Example Citric (anhydrous) listed as 64.04 mg/meq, and yet the sample calculation shows it as 64.04 g/eq
Unless I’m doing something wrong. I’m calculating my pH from getting the C1 of milk by:
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C1V1=C2V2 to find Normality/Molarity of the milk. Then using this molarity to get pH as it is assumed to be representative of the total acidity in the milk.
C1*5mL = 0.1N*1.0mL -> (0.1N*1.0mL)/5mL = 0.02N of milk (lactic acid).
Under the assumption of TA 100% disassociates (As I’m calculating based on predominant acid which doesn’t care about disassociation and undissociated acid mixtures).
Plugging this into -Log(n) yields 1.69 pH which seems wrong to me.
Can someone point me in the right direction of what to do? As I also have to repeat this for vinegar
