CHM417L
PRELIMS – 1ST SEMESTER
EXP.1
POTENTIOMETRIC TITRATION OF PHOSPHORIC ACID IN COLA PRODUCT
I. INTRODUCTION
POTENTIOMETRIC TITRATION
➔ Instrument measures:
◆ Electrical potential (voltage) between two electrodes –
indicator and reference electrode
◆ Measures proton concentration (H⁺) and converts it to
voltage using a pH-sensitive glass electrode
➔ Half cell reaction for Ag/AgCl reaction:
◆ AgCl(s) + e⁻ ⇌ Ag(s) + Cl⁻(aq)
ADVANTAGES
➔ Less affected by colored or turbid solutions
➔ No need for chemical indicators, avoiding errors or interference
➔ Indicator electrodes can be selectively designed for specific ions,
improving selectivity
pH Measurement
II. THEORY
PHOSPHORIC ACID EQUILIBRIA
➔ Based on the concentration of hydrogen ions
H3PO4 is a weak, polyprotic acid, releases more than one proton (H⁺) ➔ Combination pH glass electrode
➔ H2PO4(aq)⇌H2PO4−(aq) + H+ , pK1=2.15 ◆ Tool for measuring pH
◆ First dissociation (one proton is released) ◆ Consist of a thin-walled glass bulb sensitive to hydrogen ion
➔ H2PO4− (aq)⇌HPO4 2− (aq) + H+ , pK2=7.20 conc.
◆ Second dissociation ➔ Calibration
➔ HPO4 2−(aq)⇌PO4 3− (aq) + H+ , pK3=12.35 ◆ The electrode is immersed in solutions of known pH, buffer
◆ Third dissociation solutions
Titration Curve ◆ pH 10 (buffer solutions)
III. REAGENTS AND INSTRUMENT
➔ Can be tracked with a pH meter, producing a titration curve
ROLES OF THE REAGENTS
➔ Buffer regions : slow change in pH
➔ Phosphoric acid sample : analyte (the concentration that you’re
➔ First dissociation (pKa1) : midpoint of first buffer
determining)
➔ Second dissociation (pKa2) : midpoint of second buffer
➔ 0.01M NaOH : titrant (strong base with a known concentration)
➔ Third dissociation (pKa3) : midpoint of third buffer
𝑉 𝑏𝑒𝑓𝑜𝑟𝑒 𝑟𝑖𝑠𝑒 + 𝑉 𝑎𝑓𝑡𝑒𝑟 𝑟𝑖𝑠𝑒
➔ Dried KHP (potassium hydrogen phthalate) : primary standard in
➔ First pKa of buffering solution: 2 NaOH
➔ Second pKa of buffering solution:
𝑓𝑖𝑟𝑠𝑡 𝑒𝑞. − 𝑠𝑒𝑐𝑜𝑛𝑑 𝑒𝑞. ➔ Phenolphthalein indicator : chemical indicator (colorless to pink –
2
endpoint)
Type your initials here | 1
, ➔ pH 4 and pH 10 buffer solutions : to calibrate the pH meter May be applied to different types of reactions
➔ Soft drink: contains a lot of acids (carbonic acid) and phosphoric acid ➔ Acid-Base
INSTRUMENT ◆ A sudden change in conductivity marks the neutralization
Signal Conversion point
➔ Voltage generated by hydrogen ion activity difference across the ➔ Precipitation
glass membrane is read by high-input-impendance voltmeter (zero ◆ Formation or dissolution of an insoluble precipitate affects
current) conductivity
➔ Nernst equation converts voltage to pH ➔ Complexometric
Instrumentation (parts) ◆ Formation or dissolution of complex compounds with specific
➔ Internal solution: Known HCl concentration with Ag/AgCl reference ligands changes the solution’s conductivity
electrode in KCl (have a silver coated wire and then expose it to
some salt and immerse in a stable KCl solution) ➔ Redox
➔ Reference electrode: Ag/AgCl in stable KCl (potential remains ◆ Change in oxidation state and resulting ion formation or
constant) dissociation can be measured
➔ Connection: Salt bridge or porous frit connect the indicator and Advantage & Disadvantage
reference electrode ➔ Advantage : Does not use visual indicator
IV. PROCEDURE/FACTORS ➔ Disadvantage : Your sample must be an ion to react with HCl (hard
TEMPERATURE to apply with complex mixtures)
➔ While doing the current, we must observe the temperature because it II. THEORY
affects the Nernst slope and the electrode potential knowing it allows CONDUCTOMETRY
accurate conversion of the measured voltage to pH The key components of a conductometric measurement set up
➔ Slope of the response ➔ Electrodes
◆ pH7 buffer : when electrode is nernstian ◆ A pair of electrodes (measuring/sensing and reference)
➔ Removing carbonic acid made of non-reactive materials like platinum, gold, or
◆ Heating up the coke sample and let it down cool (indicator: graphite.
no bubbles) ◆ Can create a potential difference to drive an electric current
➔ Detecting carbonic acid in your sample though the solution
◆ None if pH in pH meter is lower ➔ Conductivity cell
EXP.2 ◆ A container that holds the solution being measured and the
electrodes
CONDUCTOMETRIC TITRATION OF ACIDS WITH A STRONG BASE ➔ Conductivity meter
I. INTRODUCTION ◆ The instrument used to measure the current flowing through
CONDUCTOMETRIC TITRATION
the electrodes for a given applied voltage
Technique
Principle
➔ To determine the concentration of a specific substance in a solution
➔ Ohm’s Law
by measuring its electrical conductivity
◆ Higher conductivity, lower electrical resistance
Principle
➔ Conductivity
➔ Conductivity of a solution changes as the reaction between the
◆ Measurement of resistance
analyte and titrant progresses
2
, ◆ Unit: Siemens per meter (S/m)
◆ Resistance (R) : I = V / R V. REAGENTS AND INSTRUMENT
◆ Conductivity (K) : K = 1 / R ROLES OF THE REAGENTS
◆ Two parameters: distance between the two plates of ➔ 0.1 M NaOH : is the titrant. It is a strong base used to titrate and
electrodes and area determine the concentration of the unknown acids
● A (K = L/A) ➔ Potassium Hydrogen Phthalate (KHP) : used to standardize the 0.1
● Cell constant (K), electrodes (L), cross-sectional M NaOH solution
area (A) ➔ Unknown HCl solution : strong acid and the analyte titration with
➔ Cell constant (G) & Conductivity (K) NaOH
◆ Directly proportional : K = (K) (G) ➔ Unknown Acetic Acid solution : an analyte titrated with NaOH to
TITRATION CURVE determine concentration
HCl NaOH INSTRUMENT
➔ Graph going up Conductometer electrode
◆ High concentration of hydroxide ions ➔ Two bold parallel lines: probes (non-reactive materials such as
◆ Addition of OH ions (excess) platinum, gold, or graphite)
◆ Endpoint ➔ Signal used: alternating current electrode (changes direction (back
➔ Graph going down and forth) of a voltage across the electrodes
◆ High concentration of hydrogen ions ◆ 1000 Hz (1 kHz)
◆ Hydroxide neutralized the hydrogen ions (conductive) ◆ Polarization: build-up of ions during direct current flow; it will
◆ Titrating of HCl with NaOH build-up because of a constant direct current drives ions to
➔ Slope react electrode surfaces
◆ HCl has a higher point than arctic acid because arctic has a ◆ Used to prevent polarization, which can distort
weaker acid (weakly ionized) measurement, and to ensure stable and accurate
CONDUCTOMETRIC TITRATION conductivity reading
How it works ◆ Calibration
➔ 1. An analyte solution is placed in a cell with two electrodes ● Used the standard solution, KCl, will define the cell
➔ 2. A titrant of known concentration is slowly added. Reaction constant
between the analyte and titrant occurs, the conductivity of the Electrode build-up
solution changes due to formation of ions ➔ It builds up because a constant direct current drives irons to
➔ 3. A conductivity meter continuously monitors the solution’s accumulate or react at the electrode surfaces farming charge leaders
conductivity, which changes as the reaction alters ion concentration are deposits that hinder iron movement and increase resistance
and mobility ➔ Build-up ions can cause electrolysis (exceed of voltage)
➔ 4. Equivalence point is the point in the titration where the conductivity ➔ Negative side: more electron build-up; positive side: few electron
undergoes a sudden change where the reaction between the analyte build-up
and titrant is complete ➔ Cation: side where more electrons attracts
➔ 5. Plot the conductivity change over the course of titration.
3
PRELIMS – 1ST SEMESTER
EXP.1
POTENTIOMETRIC TITRATION OF PHOSPHORIC ACID IN COLA PRODUCT
I. INTRODUCTION
POTENTIOMETRIC TITRATION
➔ Instrument measures:
◆ Electrical potential (voltage) between two electrodes –
indicator and reference electrode
◆ Measures proton concentration (H⁺) and converts it to
voltage using a pH-sensitive glass electrode
➔ Half cell reaction for Ag/AgCl reaction:
◆ AgCl(s) + e⁻ ⇌ Ag(s) + Cl⁻(aq)
ADVANTAGES
➔ Less affected by colored or turbid solutions
➔ No need for chemical indicators, avoiding errors or interference
➔ Indicator electrodes can be selectively designed for specific ions,
improving selectivity
pH Measurement
II. THEORY
PHOSPHORIC ACID EQUILIBRIA
➔ Based on the concentration of hydrogen ions
H3PO4 is a weak, polyprotic acid, releases more than one proton (H⁺) ➔ Combination pH glass electrode
➔ H2PO4(aq)⇌H2PO4−(aq) + H+ , pK1=2.15 ◆ Tool for measuring pH
◆ First dissociation (one proton is released) ◆ Consist of a thin-walled glass bulb sensitive to hydrogen ion
➔ H2PO4− (aq)⇌HPO4 2− (aq) + H+ , pK2=7.20 conc.
◆ Second dissociation ➔ Calibration
➔ HPO4 2−(aq)⇌PO4 3− (aq) + H+ , pK3=12.35 ◆ The electrode is immersed in solutions of known pH, buffer
◆ Third dissociation solutions
Titration Curve ◆ pH 10 (buffer solutions)
III. REAGENTS AND INSTRUMENT
➔ Can be tracked with a pH meter, producing a titration curve
ROLES OF THE REAGENTS
➔ Buffer regions : slow change in pH
➔ Phosphoric acid sample : analyte (the concentration that you’re
➔ First dissociation (pKa1) : midpoint of first buffer
determining)
➔ Second dissociation (pKa2) : midpoint of second buffer
➔ 0.01M NaOH : titrant (strong base with a known concentration)
➔ Third dissociation (pKa3) : midpoint of third buffer
𝑉 𝑏𝑒𝑓𝑜𝑟𝑒 𝑟𝑖𝑠𝑒 + 𝑉 𝑎𝑓𝑡𝑒𝑟 𝑟𝑖𝑠𝑒
➔ Dried KHP (potassium hydrogen phthalate) : primary standard in
➔ First pKa of buffering solution: 2 NaOH
➔ Second pKa of buffering solution:
𝑓𝑖𝑟𝑠𝑡 𝑒𝑞. − 𝑠𝑒𝑐𝑜𝑛𝑑 𝑒𝑞. ➔ Phenolphthalein indicator : chemical indicator (colorless to pink –
2
endpoint)
Type your initials here | 1
, ➔ pH 4 and pH 10 buffer solutions : to calibrate the pH meter May be applied to different types of reactions
➔ Soft drink: contains a lot of acids (carbonic acid) and phosphoric acid ➔ Acid-Base
INSTRUMENT ◆ A sudden change in conductivity marks the neutralization
Signal Conversion point
➔ Voltage generated by hydrogen ion activity difference across the ➔ Precipitation
glass membrane is read by high-input-impendance voltmeter (zero ◆ Formation or dissolution of an insoluble precipitate affects
current) conductivity
➔ Nernst equation converts voltage to pH ➔ Complexometric
Instrumentation (parts) ◆ Formation or dissolution of complex compounds with specific
➔ Internal solution: Known HCl concentration with Ag/AgCl reference ligands changes the solution’s conductivity
electrode in KCl (have a silver coated wire and then expose it to
some salt and immerse in a stable KCl solution) ➔ Redox
➔ Reference electrode: Ag/AgCl in stable KCl (potential remains ◆ Change in oxidation state and resulting ion formation or
constant) dissociation can be measured
➔ Connection: Salt bridge or porous frit connect the indicator and Advantage & Disadvantage
reference electrode ➔ Advantage : Does not use visual indicator
IV. PROCEDURE/FACTORS ➔ Disadvantage : Your sample must be an ion to react with HCl (hard
TEMPERATURE to apply with complex mixtures)
➔ While doing the current, we must observe the temperature because it II. THEORY
affects the Nernst slope and the electrode potential knowing it allows CONDUCTOMETRY
accurate conversion of the measured voltage to pH The key components of a conductometric measurement set up
➔ Slope of the response ➔ Electrodes
◆ pH7 buffer : when electrode is nernstian ◆ A pair of electrodes (measuring/sensing and reference)
➔ Removing carbonic acid made of non-reactive materials like platinum, gold, or
◆ Heating up the coke sample and let it down cool (indicator: graphite.
no bubbles) ◆ Can create a potential difference to drive an electric current
➔ Detecting carbonic acid in your sample though the solution
◆ None if pH in pH meter is lower ➔ Conductivity cell
EXP.2 ◆ A container that holds the solution being measured and the
electrodes
CONDUCTOMETRIC TITRATION OF ACIDS WITH A STRONG BASE ➔ Conductivity meter
I. INTRODUCTION ◆ The instrument used to measure the current flowing through
CONDUCTOMETRIC TITRATION
the electrodes for a given applied voltage
Technique
Principle
➔ To determine the concentration of a specific substance in a solution
➔ Ohm’s Law
by measuring its electrical conductivity
◆ Higher conductivity, lower electrical resistance
Principle
➔ Conductivity
➔ Conductivity of a solution changes as the reaction between the
◆ Measurement of resistance
analyte and titrant progresses
2
, ◆ Unit: Siemens per meter (S/m)
◆ Resistance (R) : I = V / R V. REAGENTS AND INSTRUMENT
◆ Conductivity (K) : K = 1 / R ROLES OF THE REAGENTS
◆ Two parameters: distance between the two plates of ➔ 0.1 M NaOH : is the titrant. It is a strong base used to titrate and
electrodes and area determine the concentration of the unknown acids
● A (K = L/A) ➔ Potassium Hydrogen Phthalate (KHP) : used to standardize the 0.1
● Cell constant (K), electrodes (L), cross-sectional M NaOH solution
area (A) ➔ Unknown HCl solution : strong acid and the analyte titration with
➔ Cell constant (G) & Conductivity (K) NaOH
◆ Directly proportional : K = (K) (G) ➔ Unknown Acetic Acid solution : an analyte titrated with NaOH to
TITRATION CURVE determine concentration
HCl NaOH INSTRUMENT
➔ Graph going up Conductometer electrode
◆ High concentration of hydroxide ions ➔ Two bold parallel lines: probes (non-reactive materials such as
◆ Addition of OH ions (excess) platinum, gold, or graphite)
◆ Endpoint ➔ Signal used: alternating current electrode (changes direction (back
➔ Graph going down and forth) of a voltage across the electrodes
◆ High concentration of hydrogen ions ◆ 1000 Hz (1 kHz)
◆ Hydroxide neutralized the hydrogen ions (conductive) ◆ Polarization: build-up of ions during direct current flow; it will
◆ Titrating of HCl with NaOH build-up because of a constant direct current drives ions to
➔ Slope react electrode surfaces
◆ HCl has a higher point than arctic acid because arctic has a ◆ Used to prevent polarization, which can distort
weaker acid (weakly ionized) measurement, and to ensure stable and accurate
CONDUCTOMETRIC TITRATION conductivity reading
How it works ◆ Calibration
➔ 1. An analyte solution is placed in a cell with two electrodes ● Used the standard solution, KCl, will define the cell
➔ 2. A titrant of known concentration is slowly added. Reaction constant
between the analyte and titrant occurs, the conductivity of the Electrode build-up
solution changes due to formation of ions ➔ It builds up because a constant direct current drives irons to
➔ 3. A conductivity meter continuously monitors the solution’s accumulate or react at the electrode surfaces farming charge leaders
conductivity, which changes as the reaction alters ion concentration are deposits that hinder iron movement and increase resistance
and mobility ➔ Build-up ions can cause electrolysis (exceed of voltage)
➔ 4. Equivalence point is the point in the titration where the conductivity ➔ Negative side: more electron build-up; positive side: few electron
undergoes a sudden change where the reaction between the analyte build-up
and titrant is complete ➔ Cation: side where more electrons attracts
➔ 5. Plot the conductivity change over the course of titration.
3
