Precipitation Titration: Mohr's & Volhard's Method // HSC Chemistry
Summary
TLDRThis video script delves into precipitation titrations, a volumetric analysis technique for determining the concentration of substances. It explains two methods: Moore's method, which uses silver nitrate and potassium chromate to precipitate chloride ions and identify the endpoint with a red silver chromate precipitate, and Volhard's method, a back titration technique suitable for acidic solutions, using sodium thiocyanate to indicate endpoint with a red iron thiocyanate complex. Both methods are illustrated with calculation examples to demonstrate their application in analyzing chloride ion concentrations.
Takeaways
- 🧪 Precipitation titration is a quantitative technique used to analyze the concentration of a substance by measuring volume, also known as volumetric analysis.
- 📚 It is recommended to watch the titration video in the acid-base module before understanding precipitation titration.
- 🌀 Precipitation titration uses the formation of a precipitate with a unique color to indicate the end point, unlike conventional titration.
- 🔍 Two common methods discussed are Moore's method and Volhard's method, both used for quantifying chloride ions in a solution.
- 💧 Moore's method involves adding silver nitrate to a solution containing chloride ions and potassium chromate, resulting in the formation of silver chloride and silver chromate precipitates.
- 🔴 The red precipitate of silver chromate is used as an end point indicator in Moore's method.
- ⚠️ Moore's method cannot be used in acidic solutions as chromate ions are protonated, preventing precipitation with silver ions.
- 🔄 Volhard's method is a back titration technique that involves initial precipitation followed by filtration and titration against sodium thiocyanate.
- 📈 The number of moles of chloride ions can be determined by the stoichiometric ratio in the precipitation reaction, which is one-to-one for silver and chloride ions.
- 📊 Volhard's method is preferred for acidic solutions as hydrogen ions do not interfere with the titration process.
- 📝 Both methods involve calculations to determine the concentration of chloride ions in the solution, using the moles of silver ions reacted and the volume of the solution analyzed.
Q & A
What is precipitation titration?
-Precipitation titration is a quantitative analytical technique used to determine the concentration of a particular substance by measuring the volume of a solution, often referred to as volumetric analysis.
How does the end point of a precipitation titration differ from a conventional titration?
-In precipitation titration, the end point is indicated by the formation of a precipitate with a unique color, rather than a color change in an indicator as in conventional titrations.
What are the two common methods of precipitation titration mentioned in the script?
-The two common methods of precipitation titration mentioned are Moore's method and Volhard's method.
What is the purpose of Moore's method in a precipitation titration?
-Moore's method is used to quantify the concentration of chloride ions in a given solution by using silver nitrate and observing the formation of a silver chloride precipitate.
What is the role of potassium chromate in Moore's method?
-In Moore's method, potassium chromate acts as an indicator. When all chloride ions are precipitated, the excess silver ions react with chromate ions to form a distinct red precipitate of silver chromate, signaling the end point of the titration.
How is the concentration of chloride ions calculated in Moore's method?
-The concentration of chloride ions is determined by the number of moles of silver ions that reacted, which is calculated by multiplying the concentration of silver nitrate by the titration volume recorded when the endpoint is reached.
Why is Moore's method not suitable for acidic solutions?
-Moore's method is not suitable for acidic solutions because in acidic environments, chromate ions are protonated to form chromic acid, which cannot precipitate with silver ions.
What is the main difference between Volhard's method and Moore's method?
-The main difference is that Volhard's method is a back titration technique. It starts with the precipitation of silver chloride, followed by filtration and titration of the filtrate against sodium thiocyanate, whereas Moore's method does not involve filtration.
How does the indicator work in Volhard's method?
-In Volhard's method, iron(III) nitrate is added as an indicator. When excess thiocyanate ions react with iron(III) ions, they form a blood-red complex of iron thiocyanate, marking the end point of the titration.
Why is Volhard's method preferred over Moore's method for acidic solutions?
-Volhard's method is preferred for acidic solutions because the presence of hydrogen ions does not interfere with any steps of the method, unlike Moore's method where acidic conditions affect the indicator.
Can you provide an example calculation for determining the concentration of chloride ions using Volhard's method?
-Yes, by using the given example, the concentration of chloride ions is calculated by first determining the moles of silver ions that reacted in the titration with sodium thiocyanate, then subtracting the moles of excess silver ions from the initial moles of silver ions, and finally dividing by the volume of the chloride ion solution to find the concentration in moles per liter.
Outlines
🧪 Precipitation Titration Techniques
This paragraph introduces precipitation titration, a quantitative analysis technique that determines the concentration of a substance by measuring volume, hence its name 'volumetric analysis.' It involves both precipitation and titration and is often used to analyze chloride ions using silver nitrate, with potassium chromate serving as an indicator. The method, known as Moore's method, forms a white precipitate of silver chloride and uses the formation of a red precipitate of silver chromate to signal the endpoint. Key takeaways include the preferential reaction of silver ions with chloride ions and the stoichiometric ratio between chloride and silver ions, which allows for the calculation of the concentration of chloride ions in the solution.
🔍 Bohr's Method for Chloride Ion Analysis
The second paragraph delves into Bohr's method, another precipitation titration technique used to measure chloride ion concentration. It begins with the formation of silver chloride precipitate and continues with filtration to separate the precipitate from the filtrate. The filtrate, containing excess silver ions, is then titrated against sodium thiocyanate with iron(III) nitrate as an indicator, forming a blood-red complex of iron thiocyanate to mark the endpoint. Unlike Moore's method, Bohr's method is a back titration and is advantageous for acidic solutions as it is not affected by the presence of hydrogen ions. The paragraph includes a calculation example to demonstrate how to determine the concentration of chloride ions, emphasizing the one-to-one reaction ratio between silver and chloride ions and the steps to calculate the initial and excess moles of silver ions.
Mindmap
Keywords
💡Precipitation Titration
💡Volumetric Analysis
💡End Point
💡Indicator
💡Stoichiometric Ratio
💡Moore's Method
💡Volhard's Method
💡Back Titration
💡Concentration
💡Moles
Highlights
Precipitation titration is a quantitative technique used to analyze the concentration of a substance by measuring volume.
Precipitation titration involves both precipitation and titration, with a unique color precipitate indicating the end point.
Moore's method is introduced for quantifying the concentration of chloride ions using silver nitrate and potassium chromate.
In Moore's method, silver ions react with chloride ions to form a white precipitate of silver chloride.
Excess silver ions react with chromate ions to form a red precipitate, signaling the end of titration.
The stoichiometric ratio in the precipitation reaction between silver and chloride ions is one-to-one.
The number of moles of chloride ions can be calculated from the moles of silver ions due to their one-to-one reaction ratio.
A calculation example demonstrates how to determine the concentration of chloride ions using Moore's method.
Moore's method cannot be used in acidic solutions as chromate ions are protonated, preventing precipitation with silver ions.
Voges-Proskauer method is presented as an alternative for measuring chloride ion concentration in acidic solutions.
Voges-Proskauer method involves back titration starting with silver chloride precipitation followed by filtration.
In Voges-Proskauer, excess silver ions are titrated with sodium thiocyanate, and the reaction is indicated by a blood red complex.
The major difference between Moore's and Voges-Proskauer methods is the use of back titration in the latter.
Voges-Proskauer method is preferred for acidic solutions as hydrogen ions do not interfere with the titration steps.
A calculation example illustrates the process of determining chloride ion concentration using Voges-Proskauer method.
The concentration of chloride ions can be calculated by dividing the moles of chloride by the volume of the solution.
Both Moore's and Voges-Proskauer methods are essential for analyzing chloride ion concentrations in various conditions.
Transcripts
hey everybody in this video we'll be
discussing precipitation titrations
precipitation titration is a
quantitative technique that analyzes the
concentration of a particular substance
by measuring volume
this is why it is often referred to as a
volumetric analysis
as suggested by the name this analytical
technique involves
both precipitation and titration
it is recommended that you watch the
video on titration
in the acid-base module before this
video unlike conventional titration
precipitation titration uses a formation
of a precipitate
usually with a unique color to indicate
the end point
of the titration in this video we'll use
two common methods
of precipitation titration to
demonstrate key features you need to
know
for this analytical technique the
techniques are
morse method and beauhard's method
moore's method
aims to quantify the concentration of
chloride ions in a given solution
the method involves dispensing a
solution of silver nitrate
from the buret into the conical flask
which contains the chlorine ions that we
want to analyze the conical flask also
contains
some amount of potassium chromate which
is the indicator of this titration
when the stockpot of the buret is opened
silver ions react with chloride ions
to form a white precipitate that is
silver chloride
this is seen in the conical flask in the
second picture in the middle
it is important to add silver nitrate in
excess
so that all chloride ions in the
solution are precipitated
when all chloride ions are reacted the
excess amount of silver ions
added will start to react with the
chromate ions
that was also present in the conical
flask this reaction produces another
precipitate
but this time the precipitate has a
distinct red color
as you can see in the third picture as
the red precipitate is easily observable
when it is formed the stopcock of the
buret is closed to stop the addition of
silver nitrate
the formation of silver chromate
precipitate is used as an end point
of this precipitation titration method
two key concepts to take away from
moore's method
are that the silver ions only react with
the chroma ions
when all the chloride ions in the
solution
have already precipitated and this is
only made possible
because silver ions preferentially
precipitate with chloride ions
over chromate ions secondly
the number of moles of chloride ions in
the solution
equals the number of moles of silver
ions due to the one-to-one
stoichiometric ratio
in their precipitation reaction the
number of moles of silver ions
is calculated by multiplying its
concentration
by the title volume that's recorded in
the buret when the endpoint
is observed let's reinforce the concepts
in moore's method by looking at a
calculation example
excess amount of 0.05 multiple liter
solution
of silver nitrates is added to 50
milliliters of a solution
containing chloride ions some amount of
potassium chromate is added and uses
indicator and the end point of titration
is marked by the formation of silver
chromate as we saw earlier
the average title volume after five
repeats is 18.5 milliliters
and using these information we are asked
to calculate the concentration
of chloride expressed in moles per liter
so first of all the amount of chloride
ions
equals to the number of moles of silver
ions that reacted
and this is due to one to one ratio the
number of silver ions
is equal to its concentration 0.05
multiplied by the title volume so that's
18.5 milliliters
divided by a thousand to get the liters
this gives us
9.25 times 10 to the minus 4 moles
and as we said earlier this is also the
number of moles of chloride ions
we can then find the concentration of
chlorine ions by dividing the number of
moles
by the volume of the solution that we
analyzed and this was
50 milliliters this gives us 0.0185
mole per liter moore's method relies on
the formation of silver chromate
precipitates as a marker of the
endpoints
therefore a disadvantage of this method
is that it cannot be used if the
solution containing the chlorine ions
is acidic this is because in acidic
environments
chromate ions are protonated to produce
chromic acid which can no longer
precipitate
with silver ions the next method we'll
discuss
is very hard's method like moore's
method voja's method
also aims to measure the concentration
of chloride ions in a given solution by
producing
precipitates first excess silver nitrate
is added to the chloride solution
to produce silver chloride which is a
white precipitate shown in the first
picture
when all chloride ions are reacted
filtration is performed
to filter the precipitate and the
filtrate
is titrated against sodium thiocyanates
which is placed in the buret some amount
of iron
iii nitrate solution is also added to
the conical flask as the indicator of
this titration
when thiocyanate ions are added to
excess
silver ions from the previous reaction
they produce a white precipitate
silver thiocyanate now when all
excess silver ions have precipitated in
this reaction
the excess thiocyanate ions will then
react
with the iron three plus ions that was
present in a conjugal flask
this will form a metal complex with a
distinct
blood red color iron thiocyanate
this is shown in the third picture the
formation of this
red complex marks the end point of
voja's method
a major difference between bohart's and
moore's method
is that this is an example of a back
titration
the method starts with precipitation
between silver and chloride ions
followed by filtration to obtain the
filtrate which contains
excess silver ions this is then titrated
without cyanate
and the titan volume is recorded
titration with sodium thiocyanate
helps determine the number of moles of
silver ions in excess
the number of moles of chloride ions
equals to the number of moles of silver
ions that reacted
in the precipitation reaction now this
number
can be calculated by first finding the
total amount of silver ions in the very
beginning and this is usually done by
multiplying the concentration of
solution
by the volume added to the chloride ion
solution
once we find this we can subtract the
number of moles of silver ions in excess
from the titration with sodium
thiocyanate to finally give us
the number of silver ions reacted then
since this is the same number as the
number of moles of chloride
we can then use the number to calculate
the concentration by dividing by the
volume
of the chlorine ion solution voja's
method is preferred over moore's method
when determining chloride ion solutions
when they are acidic
this is because the presence of hydrogen
ions
does not interfere with any steps of
voha's method
let's look at voja's method using a
calculation example
50 milliliters of a solution containing
unknown concentration
of chloride ions is added to 50
milliliters of a solution
of 7 nitrate and this solution here has
a concentration of 0.05
moles per liter this reaction produces a
precipitate
and it is filtered with the filtrate
being titrated
against a standard solution of sodium
thiocyanate
the reaction between the thiocyanate ion
and the silver ion
that is in excess from the precipitation
produces a white precipitate
of the silver thiocyanate and the
average tighter volume
after three repetitions is exactly 25
milliliters
calculate the concentration of chlorine
ions in the solution in moles per liter
now we can start by finding a number of
moles of thousand ions
that reacted in the titration by
multiplying
its concentration by the volume this
gives us
0.0005 moles
since the silver ion and our cyanide
ions react in a one to one ratio
this is also equal to the number of
moles of silver ion
that was in excess from the
precipitation we can then use the volume
of silver nitrate and its concentration
in the very beginning of the experiment
to find the number of moles of silver
ions
initially this is by multiplying its
concentration again
by the volume which gives us 2.5
times 10 to the minus 3 moles the number
of moles of silver
ion that reacted with the chloride is
equal to
2.5 times 10 to minus 3 so the number of
moles of the beginning
by the number of moles that was in
excess
which gives us two times ten to minus
three moles
now we know that silver ions and
chloride ions also react in a one-to-one
ratio
so the number of moles of chloride that
was in the solution
is also 2 times 10 to minus 3
moles we can then find the concentration
of chloride by dividing
this number by the volume and this gives
us
0.0400 mole per liter
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