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.