CHEM 01LC Lecture Notes - Lecture 4: Bromocresol Green, Acetic Acid, Acid Dissociation Constant
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CHEM 01LC Experiment 4: Ka of an Indicator
●Experimental purpose:
○Determine the acid dissociation constant of acid acid from the
measured pH of acetic acid-acetate buffer solutions of known
concentration, as well as the acid dissociation constant of bromcresol
green
●Bromcresol Green
○Bromcresol green: commonly used indicator for acid-base titrations
■Used to signal the equivalence point of an acid-base titration
■HIn ⇌ In-
●Turns yellow in acidic solutions
●Turns deep blue in basic solutions
●Remains green in solutions that contain acid and base
forms in approximately equal concentrations
●Experimental Notation
○Acetic acid = HAc = HC2H3O2
○Acetate ion = Ac- = C2H3O2
-
○Bromcresol green (acid form) = HIn
○Bromcresol green (base form) = In-
●Measurements
○Note: all pH measurements should be recorded to ±0.01
●Calculations
○Acid Dissociation Constants:
■KHAc = [HAc]
[H][Ac ]
+−
●pKHAc = pH + log [Ac ]
−
[HAc]
■KHIn = [HIn]
[H][In−]
+
●pKHIn = pH + log [In ]
−
[HIn]
○KHAc
for Acetic Acid
■pKHAc = pH + log moles Ac−
moles H Ac
■Notes
●Exclude outlier values from calculation of the average
●Value calculated will be different from literature value
for KHAc (effect of ionic composition)
●Treat solution as weak base Ac- prior to addition of
solution A (use pH to calculate the base dissociation
constant of Ac-)
○KHIn for Bromcresol Green
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Document Summary
Chem 01lc experiment 4: k a of an indicator. Determine the acid dissociation constant of acid acid from the measured ph of acetic acid-acetate buffer solutions of known concentration, as well as the acid dissociation constant of bromcresol green. Bromcresol green : commonly used indicator for acid-base titrations. Used to signal the equivalence point of an acid-base titration. Remains green in solutions that contain acid and base forms in approximately equal concentrations. Note : all ph measurements should be recorded to 0. 01. Pk hac = ph + log [ac ] . Pk hin = ph + log [in ] . Pk hac = ph + log moles ac moles hac. Exclude outlier values from calculation of the average. Value calculated will be different from literature value for k hac (effect of ionic composition) Treat solution as weak base ac - prior to addition of solution a (use ph to calculate the base dissociation constant of ac - )
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