Analysis of Soda Ash
• Hand in: Pre-Lab and Open Acetic Acid spreadsheet
•
Today: Determine the amount of Na2CO3 in a sample of Soda Ash
• Soda Ash is made up of Na2CO3 and other stuff
Composition of Soda
Ash
Na2CO3
Other stuff
• We can use the fact that Na2CO3 is a base that will react with
standardized HCl to calculate % in the original Ash sample:
2 HCl (aq) + Na2CO3 (aq) → 2NaCl (aq) + H2CO3
Analysis of Soda Ash
2 HCl + Na2CO3 → 2NaCl + H2CO3
Is actually a two-step process:
H+ + CO3-2 (aq) →HCO3- (step 1)
And then
H+ + HCO3- → H2CO3 (step 2)
So when you titrate: you will see two plateaus and two inflection points
Today, we will determine the 2nd endpoint to determine how much HCl is
required to completely titrate all the Na2CO3
HCO3- → H+ + CO32H2CO3 → H+ + HCO3-
Ka2 = 4.7 x 10-11
Ka1 = 4.5 x 10-7
Analysis of Soda Ash
So Today:
Set up Excel with active titration graph – use similar
format as last week:
1.
2.
3.
Column A – mL of HCl
Column B is pH
Column C is derivative
Set up Apparatus Similar to last week including:
ring stand, magnetic stirrer, burette holder, burette, pH meter,
magnetic stir bar.
Acid (HCl) goes in burette
soda ash (about .2 grams) goes in beaker (WRITE VIAL # on
data sheet as soon as you begin)
add 3-4 drops of methyl orange indicator
Calibrate the pH meter probe with 7 & 4 buffers
Analysis of Soda Ash
•
•
•
•
•
Use Indicator Methyl Orange [Yellow(5), Red(3)] to help with the
endpoint – see examples for color change to expect
You will ultimately do 5-6 titrations of the soda ash (enough in the vial)
• Trial 0
• Add HCl carefully in order to graph 2 plateaus and find the 2nd
endpoint
• Determine the pH at the endpoint
• Trails 1-4 or 5
• Simply titrate to THAT pH at the 2nd equivalence point
• Record the volume of HCl to reach that pH
May want to do One or Two titrations to find EXACT pH at 2nd
Equivalence Point
At least four titrations to get %Na2CO3
“Save as” with different file names for first titration curve(s)
Analysis of Soda Ash
Soda Ash Titration with 0.103 M HCl
12.00
0.00
-1.00
10.00
-2.00
-3.00
6.00
point that indicates
equiplibrium pH
-4.00
4.00
-5.00
indicates volume of the
endpoint - must find
corresponding pH
2.00
0.00
0.00
2.00
4.00
6.00
8.00
10.00
12.00
mLs of 0.103 M HCl added
-6.00
14.00
16.00
18.00
-7.00
20.00
Derivative
pH, stnadard Units
8.00
Titration
Derivative
Calculations
2 HCl + Na2CO3 → H2CO3 + 2 NaCl
SIMILAR TO PRE-LAB QUESTION #2
.108
 0104
. moles
mol HCl   1 mol Na2 CO3   105.99 g Na2 CO3 
 x
 x
  ? g Na2 CO3
x L HCl x 
1 L HCl   2 mol HCl   1 mol Na2 CO3 

 g Na2 CO3 
 x 100
% Na2 CO3  
 g ash 
Lab Report Due next Lab Period
 Title Page
 Purpose
 Results
- One Titration/Derivative Graph
- Data Sheet (with sample calculations)
- Present/Staple in order data/calculations were
obtained/performed
- Significant Figures and Units
- Precision and Accuracy (the vials have true values you will be graded against)
 Conclusion [ Errors , Improvements ]
 Individual Checked-Pre-labs