A.4 Using the Mohr Method
Add to the last three Erlenmeyer flasks, 25.00 mL aliquots of unknown and 1 mL (with a 1 mL transfer pipette) of potassium chromate indicator. Then taking the initial and final readings titration was performed on the unknown to the end points. The end points were determined when the solution turned into a reddish-brown solution.
A.4.1 Determining a Blank for the Mohr Method
Repeat titration substituting about 2 g of powdered CaCO3 and about 50 mL of boiled distilled water for the unknowns. Also add the potassium chromate indicator. Use this titration as the blank and it will be subtracted from the volumes of silver nitrate used in each titration.
Part B: Gravimetric determination of halides
B.1. Preparation of unknown solutions (prepared the week prior)
About 0.2 g of unknowns were weighed, three times and transferred into 3 separate 250 mL beakers, with the addtition of about 150 mL of boiled distilled water. Each beaker was stirred with separate stirring rods until dissolved. About 0.5 mL (10 drops) of concentrated nitric acid, with constant stirring and with the help of my TA and 40.00 mL of approximately 0.1 M silver nitrate were added. With the watch glass covering the solutions, the solutions were heated to about 90 – 95 C to coagulate the precipitate. While heating, we allowed the precipitate to settle. My TA then added drop of silver nitrate to test for completeness. The beakers were then put into my locker to settle for the next week.
B.2 Separating the unknowns
Three crucibles in the filter adapter were obtained (weighed the initial volume of the crucible), then transferred quantitatively separately the three unknowns that have been settling for 1 week, filtering everything in the beakers and washing 3 to 4 times with 0.02 M HNO3. The suction was then disconnected and removed each crucible after filtration.
B.3 Drying of the unknowns
The three crucibles were put into a 600 mL beaker and put into the oven that is set at 115 – 125 C and let them dry for 1 hour. Then cooled for 15 minutes and weighed the final weight. Again, the crucibles were put into the oven for another 15 minutes and cooled for 15 minutes and weighed the crucibles separately, repeat the step of heating and drying again and the final weight was obtained. The final weigh had to be within 0.2-0.3 mg of the previous to make sure that the weigh could be considered constant.
Part C: Potentiometric determination of halides
C.1 Preparation of unknowns for analysis
Approximately 1 g of our unknown (1.0038 g) was weighed and transferred quantitatively to a 250 mL volumetric flask and diluted to mark using distilled water. 50.00 mL aliquots, using a 25.00 mL transfer pipette twice were transferred to 3 different 250 mL beakers and added a spin bar to each.
C.2 Preparation of burette for titration
0.1000 M silver nitrate was obtained from the stock bottle in the class. The burette was rinsed with 2 – 3 mL of silver nitrate transferred the silver nitrate into the 50.00 mL burette filling to past 0.00 mL.
C.3 Setting up the instrument
Upon arrival in class, the instrument was set up. The instrument was turned on and used the selector to mV and waited to the instrument to warm up. The instrument measures the mV of the reaction when adding the titrant. My samples were moved onto the stirrer which then started the stirring of the unknowns with adding the titrant. My electrode was submersed in the solution not touching the stirrer or the side of the beaker.
C.4 Titration
While recording the initial and final volumes on the burette, and took the reading of mV’s after each time the addition of 1 mL were done, until there is a spike in mV, where the intervals were adjusted to every 0.1 mL. This procedure was repeated two more times.
Results:
2.1 Data
Part A
Weighed of Unknown: 1.0073 g
Table 2.11:Fajans Method
Table 2.12:Mohr Method
Part B
Table 2.13 Gravimetric masses
Part C
Table 2.14: Potentiometric Titrations
Weight of unknown: 1.0038 g
2.2 Calulations
Part A – Volumetric Determination
Table 2.21: Calculation summary for Fajan method
Table 2.22: Calculation summary of Mohr method
Part B – Gravimetric Determination
Table 2.23: Calculation summary
Part C - Potentiometric Determination
Table 2.24: Calculation summary
Table 3.1: Final Derived Equations To Obtain Your Results
Calculations
Part A
KCl + NaCl + AgNO3 → AgCl + Na+K+NO3-
For Trial #1 (Mohr)
0.100 M AgNO3 * 16.24 mL = 0.0164 mol AgNO3
1 mol AgNO3 = 1 mol AgCl = 1 mol Cl
0.0164 mol Cl * (35.453 g/mol) Cl = 0.5758 g Cl
% Cl = (0.5758 gl / 1.0073 g) * 100
%Cl = 57.15 % Cl
Part B
KCl + NaCl + AgNO3 → AgCl + Na+K+NO3-
For Trial #3
0.4809 g AgCl * 1 mol AgCl = 0.003355 mol AgCl
143.321 g AgCl
1 mol AgCl = 1 mol Cl
0.003355 mol Cl * 35.453 g Cl = 0.1190 g Cl
1 mol Cl
%Cl = (0.1190 g / 0.2021 g ) * 100 % = 57.41 % Cl
Part C
E ind = 0.799 – 0.0592 log ( 1/ [Ag+] )
1 – 0.496 V (0.496 + 0.244 V) = 0.794 V and 0.03269 L
2 – 0.475 V (0.475 + 0.244 V) = 0.719 V and 0.03233 L
3 – 0.485 V (0.485 + 0.244 V) = 0.729 V and 0.03262 L
Use the potential values as the Eind and solve for the Concentration of Ag+.
[Ag+} * V (L) (0.05 L Aliquot = mol Ag = mol Cl
Discussion
In part B the suspension is heated to dry any water molecules so our samples are not contaminated with water and our value weighed is our true value and we was with Nitric acid because we are titrating with silver nitrate so the nitric acid will not be an interference, whereas water molecules would be.
For the Mohr titration CaCO3 was added to the indicator blank to mimic the presence of Chloride but Calcium is soluble in water therefore it dissolved whereas the chloride in our titration did not. If I had to choose between the two different methods for Volumetric analysis, I would have to choose the Fajans method for many reasons. First off the endpoint color is more clear and there is only a slim chance that you can miss the end-point, therefore compared to Mohr’s method it is more visual, also there is no blank needed for the Fajns method, so less room for error.
In my personal opinion I think that in looking at the two different plots in part C, plotting (delta E/deltaV) vs. Volume is a better method of determining the end-point because it is more visual as in the high spike is the equivalence point. Instead of looking at the plot E vs. V and determining where the equivalence point by looking for the most vertical area of the plot, which has room for a lot of error.
If I was required to select an accurate and precise method for future chloride analysis, I would have to choose the gravimetric method. I would choose this method because it is a method with less room for error. Unfortunately it is very time consuming so to get accurate and precise method that is not too time consuming I would have to then choose the volumetric method.
Conclusion
After using three different methods for determination of a halide, I have found that by volumetric determination the %Cl is 58.62 ± 0.313% with 95% confidence limits of 0.354% for the Fajans method and 57.47 ± 0.434% for the Mohr method with 95% confidence limits of 0.492%. In using the paired t-test, I have found that there is no significant difference between the two methods. For Gravimetric determination I have found that the %Cl is 57.27 ± 0.339% with 95% confidence limits of 0.383%. And for the Potentiometric determination I have found a %Cl of ____________________ with 95% confidence limits of ___________,
I have also plotted E vs. volume (Graph 3.1) and ΔE/ΔV vs. volume (Graph 3.21) for trial #1, and did the preferred method of graphing for trials 2 and 3 (Graph 3.22 and Graph 3.23) for the Potentiometric determination.
Reference
- Chemistry – CHEM 2080 4.0, York University, p. 13-20
