Precipitation Titration: Mohr's & Volhard's Method // HSC Chemistry

Summary

This 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

00:00

🧪 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.

05:01

🔍 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.

Keywords

💡Precipitation Titration

Precipitation titration is a quantitative analytical technique used to determine the concentration of a substance by measuring the volume of a solution added. It is a type of volumetric analysis and is central to the video's theme, as it is the primary method discussed for analyzing the concentration of chloride ions. The script describes two common methods of precipitation titration, Moore's method and Volhard's method, both of which are used to quantify chloride ions through the formation of a precipitate.

💡Volumetric Analysis

Volumetric analysis refers to a group of analytical techniques that determine the concentration of a substance by measuring the volume of a solution. In the context of the video, precipitation titration is a specific type of volumetric analysis that is used to quantify chloride ions. The script emphasizes the importance of volume measurement in this technique, as it allows for the precise calculation of the substance's concentration.

💡End Point

The end point in titration is the point at which the reaction is considered complete. In the video, the end point is indicated by the formation of a precipitate with a unique color, such as the red color of silver chromate in Moore's method. This visual change is crucial for determining when to stop the addition of the titrant, ensuring accurate results.

💡Indicator

An indicator in a titration is a substance that changes color to signal the end point of the reaction. In the script, potassium chromate is used as an indicator in Moore's method, where it reacts with excess silver ions to form a red precipitate of silver chromate. This change in color is a clear visual cue for the experimenter to stop the titration.

💡Stoichiometric Ratio

The stoichiometric ratio is the fixed ratio in which reactants combine in a chemical reaction. In the video, the one-to-one stoichiometric ratio between silver ions and chloride ions is highlighted, as it is essential for calculating the number of moles of chloride ions present in the solution. This ratio is fundamental to the accuracy of the titration method.

💡Moore's Method

Moore's method is a specific precipitation titration technique used to quantify chloride ions. The script describes how silver nitrate is added to a solution containing chloride ions and potassium chromate, resulting in the formation of a white precipitate of silver chloride. The method is distinguished by the formation of a red precipitate of silver chromate, which serves as the end point of the titration.

💡Volhard's Method

Volhard's method is another precipitation titration technique for determining the concentration of chloride ions. Unlike Moore's method, Volhard's method involves filtration after the initial precipitation of silver chloride, followed by back titration with sodium thiocyanate. The script explains that this method is preferred for acidic solutions, as it is not affected by the presence of hydrogen ions.

💡Back Titration

Back titration is a technique where an excess of one reactant is added, and the remaining amount is titrated after the reaction. In the script, Volhard's method is described as an example of back titration, where excess silver nitrate is added to the chloride solution, followed by filtration and titration of the remaining silver ions with sodium thiocyanate.

💡Concentration

Concentration in the context of the video refers to the amount of a substance present in a given volume of solution, typically expressed in moles per liter (M). The script provides examples of calculating the concentration of chloride ions using the results from both Moore's and Volhard's methods, emphasizing the importance of accurate titration for determining concentration.

💡Moles

Moles are a unit of measurement used in chemistry to express the amount of a substance. The script discusses the calculation of moles of chloride ions and silver ions, which is essential for understanding the chemical reactions taking place during the titration process. The number of moles is calculated based on the concentration and volume of the solutions used in the titration.

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