4 titrations

equation to find the volume of titrant needed to reach the equivalence point

what is the equivalence point

where a stoichiometric amount of titrant has been added so that the acid or base has been neutralised

how can the titration curve be used to find the equivalence point of the titration

the derivative of the curve - ΔpH/ΔV

what are indicators

organic substances that change colour when they are protonated or deprotonated

why do indicators have an equilibrium constant

they exist as an equilibrium between their protonated and deprotonated forms

equilibrium equation for an indicator HInd

equation to find the equilibrium constant for this indicator (+ how is this constant denoted?)

what is the end point of a titration using an indicator

the point where the colour change is detected

what is one problem with using indicators for titrations? indicate this on a titration graph of pH vs volume of NaOH

it can be difficult to identify exactly where the colour change starts - hence the end point and equivalence point are not identical

how is an indicator chosen for a titration

the pK_ind of the indicator should be close to the pH of the equivalence point of the titration

how do the shapes of strong acid vs strong base titrations and strong base vs strong acid titrations differ

strong base vs strong acid will be the inverse titration curve of strong acid vs strong base

show the shape of a titration curve for strong acid vs strong base e.g. HCl vs NaOH

what is the analyte and what is the titrant

HCl is the analyte

NaOH is the titrant

show the equation for the neutralisation reaction between HCl and NaOH in aqueous solution

how can the pH of the equivalence point be calculated

show titration curve for strong base vs strong acid

how is it different + similar to strong acid vs strong base

the neutralisation reaction is the same but the curve is inverted

near to the equivalence point the equation still cancels out to the dissociation of water so the pH at the equivalence point is the same

show titration curve for weak acid vs strong base

describe shape, starting point + equivalence point`

starting pH higher than for strong acid

curve is squashed, equivalence point is no longer at pH 7

write chemical equations for weak acid vs strong base with HCN and NaOH, describe dissociation

analyte? titrant?

how is this equation rewritten for weak acid vs strong base

F_HCN = ?

F_HCN = [HCN] + [CN^-]

label buffer region and mid point

significance of mid point?

how are calculations performed in the buffer region

henderson-hasselbach equation applies in buffer region

midpoint is point at which pH = pKa

during this section of weak acid vs strong base titration, how is pH calculated?

what equation is used and how are the terms determined

calculate pH from pKa of HA dissociation

use H-H equation with

• fraction converted to A^- = (V_OH/V_e)

• fraction of HA remaining = (V_e - V_OH/V_e)

what is present in solution and why? rewrite the dissociation equation accordingly

acid has fully dissociated during neutralisation - all protons have reacted with OH^- to form water - salt remains (A^-)

A^- + H₂O EQM ARROW HA + OH^-

K_b = ? (at equiv)

K_w/K_a

F = ? (at equiv)

(initial conc)(dilution factor)

after equiv, describe mixture

how to find OH conc here?

OH^- dominates

[OH^-] = initial [OH^-] (dilution factor)

how is pH determined after equivalence?

[H⁺] = K_w/[OH^-]

show how the shape of the titration curve changes with

• strength of analyte/titrant

• concentration of analyte/titrant

show the association of a diprotic base

label the equilibrium constants in both directions

show titration curve for diprotic base

show buffer regions and what happens after these

how can pH be calculated before acid is added

use K_b1 to find [OH^-]

how to determine pH at equivalence point 1

all B converted to BH⁺

how can pH be determined at equivalence point 2

use K_a1 to calculate [H⁺]

how to determine pH in the buffering regions

use H-H equation for the dominant reaction occurring, more protonated species goes at bottom

what is a complexometric titration based on

based on complex formation rather than neutralisation

eqm equation + equation for eqm constant (with correct symbol) for metal (M) coordinating to ligand (L)

what is the most common ligand used in complexometric titrations? what is its polyprotism?

EDTA, hexaprotic

initial formula + charge for EDTA

H₆Y²⁺

show steps of EDTA’s dissociation

show EDTA titration curve + label axes with units

describe each region

pM is metal ion concentration

why does EDTA donate all 6 protons

Y^4- forms the most stable complex with metal ions

how is the fraction of Y^4- present in an EDTA titration calculated

Kf = ?

how is this used to consider pH + fractional conc of Y^4-

overall titration reaction for EDTA titration

how to find equivalence volume for EDTA titration

how to calculate pM in region 1?

calculated from excess Mⁿ⁺ concentration

[Mⁿ⁺] = fraction remaining x initial conc x dilution factor

how is pM calculated at V_e

calculated from complex formation equilibrium

how is pM calculated in region 3?

from complex eqm