4.6 Gravimetric Analysis
Gravimetry:
Gravimetry is a physical method by which an ion or molecule or a compound is isolated by precipitation reaction and then the ion or molecule or compound amount (concentration) will be determined
Gravimetric analysis is a class of lab techniques used to determine the mass or concentration of a substance by measuring a change in mass. The chemical we are trying to quantify is sometimes called the analyte. We might use gravimetric analysis to answer questions such as:
- What is the concentration of the analyte in a solution?
- How pure is our sample?
The sample here could be a solid or in solution. There are two common types of gravimetric analysis. Both involve changing the phase of the analyte to separate it from the rest of a mixture, resulting in a change in mass. You might hear either or both of these methods being called gravimetric analysis as well as the more descriptive names below.
Figure 4.28 Drawing of Alice from Lewis Carroll’s “Alice in Wonderland” holding a brown bottle labeled, “Drink Me.”
Ref: www.khanacademy.org/
It is generally not recommended to drink a mystery liquid! Maybe Alice could have used gravimetric analysis to figure out what is in the bottle. How might she check for the presence of soluble silver salts? Image of Alice from Wikimedia Commons, public domain
Volatilization gravimetry involves separating components of our mixture by heating or chemically decomposing the sample. The heating or chemical decomposition separates out any volatile compounds, which results in a change in mass that we can measure. We will go through a detailed example of volatilization gravimetry in the next section of this article!
Precipitation gravimetry uses a precipitation reaction to separate one or more parts of a solution by incorporating it into a solid. The phase change occurs since the analyte starts in the solution phase and then reacts to form a solid precipitate. The solid can be separated from the liquid components by filtration. The mass of the solid can be used to calculate the amount or concentration of the ionic compounds in solution.
In this article, we will go through an example of using volatilization gravimetric in a chemistry lab setting. We will also discuss some of the things that might go wrong during a gravimetric analysis experiment and how that might affect our results.
Example: Determining the purity of a metal hydrate mixture using volatilization gravimetric
Bad news! We have just been informed by our inept lab assistant, Igor, that he may have accidentally contaminated a bottle of the metal hydrate.
BaCl2⋅2H2O with an unknown amount of KCl. In order to find the purity of BaCl2⋅2H2O of the metal hydrate mixture to remove water from the sample. After heating, the sample has a reduced mass of 9.14 g,
What is the mass percent of BaCl2.2HO in the original mixture?
Gravimetric analysis problems are simply stoichiometry problems with a few extra steps. Hopefully you will remember that in order to do any stoichiom2etric calculations, we need the coefficients from the balanced chemical equation.
[Why don’t we include KCl in the equation?]
Let’s go through the calculation step-by-step.
Step #1: Calculate change in sample mass
We can find the amount of water lost during the heating process by calculating the change in mass for our sample.
Mass of H2O=Initial sample mass−Final sample mass=9.51g−9.14g=0.37g H2O
Step #2. Convert mass of evaporated water to moles
In order to convert the amount of water lost using the mole ratio, we will need to convert the mass of evaporated water to moles. We can do this conversion using the molecular weight of water, 18.02 g/mol
0.37 g H2O * 1 mol H2O = 0.0205 mols
18.02 g H2O
Step #3. Convert moles of water to moles of BaCl2.2H2O
using the mole ratio from the balanced reaction.
0..0205 mols of H2O * 1mol BaCl2. 2H2O
2 mols H2O
= 0.0103 mols of BaCl2.2H2O
Step #4. Convert moles of BaCl2.2H2O to mass in Grams.
Using the molar mass of BaCl2.2H2O:
0.0103 mols of BaCl2.2H2O * 244.47 g
1 mol BaCl2.2H2O
Step #5. Calculate mass percent of BaCl2.2H2O in the original sample
In the original sample the mass percent can be calculated using the ratio of the mass from Step #4 and the original sample mass.
Mass% BaCl2⋅2H2O=2.51gBaCl2⋅2H2O
In 9.51g of mixture×100%=26.4%BaCl2⋅2H2O (No thanks to Igor!)
SUPPLEMENTAL
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Shortcut: We could also combine #2 to #4 into a single calculation (with the caveat
We just successfully used gravimetric analysis to calculate the purity of a mixture, hooray! However, sometimes when you are in lab, things might not go quite so smoothly. Things that could go wrong include:
Stoichiometry errors, such as not balancing the equation for the dehydration
Lab errors, such as not giving the water enough time to evaporate or forgetting to tare a piece of glassware
What would happen to our answer for the above situations?
Situation #1: We forgot to balance the equation
Concept check: What mass of metal hydrate would we calculate in situation #1?
The moral of this story? Double check that all equations are properly balanced!
Situation #2: We ran out of time and not all the water evaporated
Figure 4.29 Anhydrous Copper sulfate crystals on watch glass
Hand holding watchglass with white anhydrous copper(II) sulfate, and hydrated copper(II) sulfate, which appears as a sky blue spot in the middle of the white powder after water was added.
In some cases, the color differs between the metal hydrate and the anhydrous compound. For example, anhydrous copper(II) sulfate is a white solid that turns bright sky blue when it is hydrated. In such cases, you could use the color change as well as the mass to monitor the dehydration process.
In the second situation, we did not fully dehydrate our sample. This could happen for a lot of reasons, unfortunately. For example, we could run out of time, the heat could be set too low, or maybe we just took the sample off the heat before it was done by mistake. How does that affect our calculations?
In this situation, the difference in mass we calculate in Step#1 will be lower than it should be, so we will have correspondingly fewer moles of water in Step #2. That will result in calculating a lower percent mass of BaCl2 compared to fully dehydrating the sample. In the end, we will end up underestimating the purity of the metal hydrate.
Chemists usually try to avoid situation #2 by drying to constant mass. That means monitoring the change in mass during the drying period until you no longer observe any further change in mass (which also depends on the accuracy of your lab balance). When you first start heating your sample, you might expect to see a significant mass decrease as water is lost. As you continue to heat the sample, the change in mass gets smaller since there is less water left in the sample to evaporate. At some point, there won’t be enough water left to make a significant change in mass, so the measured mass will stay approximately constant over multiple measurements. At that point, you can hopefully assume your sample is dry!
Lab tip: Surface area is always a factor when removing volatiles from a sample. Having a higher surface area will increase the rate of evaporation. You can increase the surface area of the sample by spreading your sample as thinly as possible on the heating surface or breaking up any larger chunks of solid, since moisture can get trapped inside the chunks.
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Summary
Gravimetric analysis is a class of lab techniques that uses changes in mass to calculate the amount or concentration of an analyte. One type of gravimetric analysis is called volatilization gravimetry, which measures the change in mass after removing volatile compounds. An example of volatilization gravimetry would be using the change in mass after heating to calculate the amount or purity of a metal hydrate. Some useful tips for gravimetric analysis experiments and calculations are:
- Double check stoichiometry and make sure equations are balanced.
- When removing volatiles from a sample, make sure to dry to constant mass.
- Always tare your glassware!
Another Example: Precipitation Gravimetry
The amount of lead ions Pb2+ present in a water can present a health risk. One has to design an experiment to check the presence of lead ions and then determine its amount in water by gravimetric method using the precipitation reaction.
The figure below explains this type of method:
Ref. 4.30 Steps for precipitation Gravimetry
Reference: http://www.chemcollective.org/chem/ubc/exp01/index.php
The filtration is carried out using vacuum filtration as shown below:
Figure 4.31 Vaccuum Filtration Set up
Example:
A 0.955 g solid mixture containing K2SO4 is dissolved in water and treated with an excess of Ba (NO3)2,
resulting in the precipitation of 0.850 g of BaSO4.
K2SO4 (aq) + Ba(NO3)2(aq) à BaSO4(s) + 2 KNO3(aq)
What is the concentration (mass percent) of K2SO4 in the mixture?
Ba (NO3)2 is the excess reagent
K2SO4 is the limiting reactant
Amount of K2SO4 based on the product BaSO4 =
[0.850 g BaSO4] x [1 mol BaSO4 / 233.38 g BaSO4] x [1 mol K2SO4 / 1 mol BaSO4] x [174.259 g K2SO4 / mol K2SO4] = 0.635 g K2SO4
[Note: molar mass of BaSO4 = 233.38 g / mol]
[Note: molar mass of K2SO4] = 174.259 g / mol]
The % of K2SO4 = [0.635 g K2SO4 / 0.955 g mixture] x 100% = 66.5 %