Experiment 1 Online Tutorial >> Gravimetric Analysis >> Introduction

Experiment 1 - Gravimetric Analysis

Introduction

In this experiment, your goal is to determine the amount of lead present in a water sample and, if lead is present, to determine whether the level of contamination is potentially dangerous. Preliminary preparation of the sample has resulted in a solid of unknown composition. This solid contains lead (Pb²⁺) in the form of a water soluble salt and gravimetric analysis will be used to determine the amount of lead present.

The experimental strategy is shown schematically below.

We start with a solid that contains some lead(II) ions (Pb²⁺, shown as brown dots) along with other chemical species (shown as yellow dots). For the purposes of this analysis, the precise identity of the other chemical species is not important. Our goal is to somehow separate the lead(II) from these other species and then, once isolated, measure the amount of lead(II). We do this through the following 3 steps:

Step 1


Action: We dissolve the solid. We use an acidic solution to prevent formation of solid Pb(OH)₂. (Since the concentration of OH^- in an acidic solution is small, we suppress formation of the Pb(OH)₂.)
Result: All species that were in the solid are now in the solution, including the Pb²⁺ and the other chemical species.

Step 2


Action: We add a solution containing SO₄^2-. We must add enough SO₄^2- to precipitate all of Pb²⁺, we add excess SO₄^2-. Any excess SO₄^2- remains dissolved in the solution and does not interfere with the remainder of the experiment.
Result: A solid that is essentially pure PbSO₄ is formed. The other chemical species remain dissolved in solution.
Note: This step takes advantage of the fact that SO₄^2- forms an insoluble salt with only a few ions, of which Pb²⁺ is the most common. (Ksp at 25°C for PbSO₄ is 1.6 x 10^-8, making it quite insoluble). We therefore assume that the only chemical species that will form a solid when we add SO₄^2- to the solution is Pb²⁺, and so the solid we have after step 2 is pure PbSO₄.

Step 3


Action: Filter the solution and weigh the solid PbSO₄.
Result: From the mass of the PbSO₄, we can determine the amount of Pb²⁺ in our final PbSO₄ solid. Since all of the Pb²⁺ in the original solid ended up in our final PbSO₄, we have determined the amount of Pb²⁺ in the initial sample.

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