Lab 3 - Complexometric titration of magnesium

What is hard water and what ions are contained in it?

Hard water is water that contains a high concentration of dissolved metal ions: Mg²⁺, Ca²⁺, Fe³⁺

What is EDTA

ethylenediaminetetraacetic acid: a chelating/complexing agent that grabs onto metal ions and forms a stable complex.

What determines how EDTA will bind to a given metal?

pH

Ratio for EDTA molecule and a metal ion?

1:1

What is an indicator and which indicator was used in this lab?

• Indicator = substance that shows the presence, absence, or concentration of a chemical

• Indicator for this lab: Eriochrome Black T - used to show when you have reached the endpoint of the EDTA titration.

What is EBT?

Eriochrome Black T: a triprotic acid that acts as a coordination ligand, meaning it can bind to metal ions.

What forms do EBT exist in?

• H₃IN

• H₂IN^-

• HIN^2-

• IN^3-

What is purpose of part one of the lab?

Part 1 is using the EDTA titration to determine the hardness of the water sample which means the measure of dissolved mineral ions (Mg²⁺, Ca²⁺, Fe²⁺).

What are the color changes and what do they represent in this lab?

Starting point = red

• Indicator (EBT) binds with Mg²⁺

• Mg²⁺ + In^3- → MgIn^-

  • makes a red color

Ending point = blue

• EDTA is added and forms a more stable complex with Mg²⁺ than the indicator does (EDTA takes Mg²⁺ away from indicator)

  
  

Why do we buffer to a pH of 10?

The pH affects how strongly EDTA binds to the metal ions. The solution is buffered to pH 10 so that EDTA can effectively bind Mg²⁺ and the metal ions can be selectively titrated.

formulas for part 1

moles = Molarity x Volume (L)

• molarity = moles/L

Total # of moles of metal ions in original 100mL sample

• that is asking for the ratio essentially: 1 mole metal : 1 mole of EDTA

What is the purpose of part 2 of the lab?

You are given an unknown sample and use the EDTA titration to calculate the molarity of Mg²⁺ in that sample.

Name 3 experimental errors that a student might make in performing this experiment and the effect these errors would have on the results.

1. Stopping the titration too early - the solution will be purple instead of blue making the concentration of Mg²⁺ to be too low.

2. Incorrectly reading the initial/final buret volume - calculated volume of EDTA used will be incorrect causing the calculated concentration of Mg²⁺ to be too low or high.

3. Overshooting the endpoint & adding too much EDTA - volume of EDTA will be too high and the concentration of Mg²⁺ will also be too high.

Practice problem: A student analyzes a 100.0 mL water sample to determine its concentration of metal ions using an EDTA titration. The water sample is buffered to pH 10 and Eriochrome Black T (EBT) is added.

The buret initially reads 2.00 mL. At the endpoint, the buret reads 7.50 mL. The EDTA solution has a concentration of 0.0100 M.

The reaction occurs in a 1:1 ratio:

1 mol metal ions:1 mol EDTA

Answer the following:

1. What is the purpose of adding EBT?

2. What color indicates the true endpoint?

3. How many mL of EDTA were used?

4. How many moles of EDTA were used?

5. How many moles of metal ions were present in the 100.0 mL sample?

6. What is the molarity of the metal ions in the water sample?

1. What is the purpose of EBT?

EBT is an indicator used to identify the endpoint.

2. What color indicates the true endpoint?

BLUE = endpoint

3. How many mL of EDTA were used?

5.50 mL

4. How many moles of EDTA were used?

$=(0.0100)(0.00550)$ $\boxed{5.50\times10^{-5}\text{ mol EDTA}}$

5. How many moles of metal ions were present?

5.50×10−5 mol metal ions

6. What is the molarity of the metal ions?

5.50×10−4 M​

Select all solutions below that are contact hazards in this lab.

 

Triethanolamine

 

NH₃/NH₄Cl buffer

 

hard water

 

EDTA solution

 

NaOH

Triethanolamine

NH₃/NH₄Cl buffer

Two compounds in this experiment act as coordinating ligands.  Select these ligands from the list below.

 

EDTA

 

magnesium

 

NH₃

 

Eriochrome Black T indicator

EDTA

Eriochrome Black T indicator

Select all of the statements below that are FALSE according to the background of this experiment.

 

Initially, the indicator forms a complex with the Mg²⁺ ion that is wine-red in color.

 

The indicator forms a more stable complex with the magnesium than does the EDTA.

 

At the endpoint, the EDTA will have displaced all of the indicator from the magnesium.

 

The color of the indicator alone in its HIn^2- form is blue.

 

The endpoint of the titration will be the formation of a purple color.

The indicator forms a more stable complex with the magnesium than does the EDTA.

The endpoint of the titration will be the formation of a purple color.