EXPERIMENT 4 : SOLUBILITY OF IONIC SALTS IN SEA WATER Abstract : This experiment is about the solubility of ionic salts in sea water. In this experiment we have standardised the silver nitrate solution using a known chloride solution. In this part, we need to find out the pH of the solution (NaCI + Distilled water) by using pH paper. The titration process involved in this part, which is one mL of 5% K 2CrO4 indicator acted as a catalyst. Then, this experiment needed us to determine the chloride concentration in sea water by analysing of sea water. In this part, the filtration process is required in ordered to obtain aliquot of diluted sea water. We also determined the pH of that solution by using pH paper. In this part the titration process occur as similar as above with the help the same catalyst. Objectives : 1)
To standardise silver nitrate solution using a known chloride solution.
2)
To determine the chloride concentration in sea water.
Introduction : Precipitation Titration Precipitation titration is a titration process that is carried out such that the stoichiometric reaction between two substances produce a slightly soluble salt that precipitates appear in the solution. The example of equation of precipitation titration : AgNO3 (aq) + Cl-(aq)
AgCl(s) + NO3-(aq)
pX = -log c(X) or pX =-log a(X) The logarithmic p notation is not only for the titration, but commonly also for the general expression of solution concentration.
pCl -------------------------------
Equivalence point
Vol of Ag(L) solution added *The graph is referred to the example of equation above Endpoints The equation of solubility product constant for silver chloride, AgCl is : Ksp = [Ag+][Cl-]
At equivalence point, [Ag+] = [Cl-] The Mohr method is applied for the determining of chloride in a sample solution which is the indicator used is the chromate ion. Method : A. Standardisation of the Silver Nitrate Solution 1.
About 0.02 g of dry sodium chloride, NaCl is accurately weighed. This mass is transferred quantitatively into a 250 mL conical flask. 100 mL of distilled water is added approximately to dissolve the salt.
2.
The pH of this solution is checked by using a pH paper. The pH is adjusted to be between 7 and 10 by adding sodium hydroxide, NaOH.
3.
1.0 mL of 5 % potassium chromate, K2CrO4 indicator is added with the solution.
4.
A 50 mL burette is thoroughly cleaned with tap water and it is rinsed a few times with distilled water. Then, the burette is rinsed and filled with the silver nitrate, AgNO 3 solution that wished to standardise. The salt solution is titrated earlier with the AgNO 3 solution until the permanent orage pink colour obtained.
5.
This standardisation procedure is repeated until we have three values that are in good agreement with each other.
B. Analysis of Sea Water 1.
10.0 mL portion of sea water is approximately measured. The first sample is filtered because it is cloudy. The sample then transferred to 100 mL volumetric flask and it is diluted to the mark with distilled water. The dropping pipette is used towards the end in order to avoid exceeding the line.
2.
10.0 mL aliquot of the diluted sea water sample is pipetted into a 250 mL conical flask.
3.
The pH of the solution is ensured between 7 and 10 by using pH paper.
4.
1.0 mL of 5 % potassium chromate, K2CrO4 is added and it is titrated against the standardised silver nitrate, AgNO3 solution.
5.
The titration is repeated for another two samples.
Results and calculation : A. Standardisation of the Silver Nitrate solution weight of sodium chloride used (g) Final volume of silver nitrate (mL) Initial volume of silver nitrate (mL) Volume of silver nitrate (mL) used
1 0.0209
2 0.0227
3 0.0242
10.60
31.00
27.00
0.00
0.00
0.00
10.60
31.00
27.00
1 10.00
2 10.00
3 10.00
27.50
28.00
27.70
0.00
0.00
0.00
27.50
28.00
27.70
B. Determination of the Chloride in Sea Water Volume of aliquot taken (mL) Final volume if silver nitrate (mL) Initial volume of silver nitrate (mL) Volume of silver nitrate (mL) used
Concentration of silver nitrate, AgNO3, NaCl(s) + AgNO3(aq)
NaNO3(aq) + AgCl(aq)
1 mol of NaCl ≈ 1 mol of AgNO3, mol of NaCl ≡ mol of AgNO3 Mol, n1 of AgNO3 = 0.0209 g ÷ 58.44 g/mol = 3.576 x 10-4 mol Mol, n2 of AgNO3 = 0.0227 g ÷ 58.44 g/mol = 3.884 x 10-4 mol Mol, n3 of AgNO3 = 0.0242 g ÷ 58.44 g/mol =4.141 x 10-4 mol
Mass1 of AgNO3 = (3.576 x 10-4 mol) x 169.88 g/mol = 0.0607 g Mass2 of AgNO3 = (3.884 x 10-4 mol) x 169.88 g/mol = 0.0660 g Mass3 of AgNO3 = (4.141 x 10-4) x 169.88 g/mol = 0.0703 g
First [AgNO3] = 0.0607 g ÷ 0.0106 L = 5.73 M Second [AgNO3] = 0.0660 g ÷ 0.0310 L = 2.13 M Third [AgNO3] = 0.0703 g ÷ 0.0270 L = 2.60 M The concentration of silver nitrate, [AgNO3] = 5.73 M, 2.13 M, 2.60 M Mean of [AgNO3] = (5.73 M + 2.13 M + 2.60 M)/ 3 = 3.49 M
AgNO3(aq) + Cl-(aq)
AgCl(s) + NO3- (aq)
1 mmol of Ag+ ≈ 1 mmol of Cl- , mmol of Ag+ ≡ mmol of Cl-
Mol, n1 of Cl- = 27.50 mL x 0.0337 mmol/mL = 0.9268 mmol Mol, n2 of Cl- = 28.00 mL x 0.0125 mmol/mL = 0.3500 mmol Mol, n3 of Cl- = 27.70 mL x 0.0153 mmol/mL = 0.4238 mmol
First molarity of Cl- = 0.9268 mmol ÷ 10.0 mL = 0.0927 M Second molarity of Cl- = 0.3500 mmol ÷ 10.0 Ml = 0.0350 M Third molarity of Cl- = 0.4238 mmol ÷ 10.0 mL = 0.0424 M The molarity of chloride ion, Cl- = 0.0968 M, 0.0350 M, 0.0424 M
Mass = mmol x mg/mmol Mass1 = 0.9268 mmol x 35.45 mg/mmol = 32.86 mg Mass2 = 0.3500 mmol x 35.35 mg/mmol = 12.41 mg Mass3 = 0.4238 mmol x 35.25 mg/mmol = 15.02 mg ppm1 = 32.86 mg ÷ 0.01 L = 3286 ppm2 = 12.41 mg ÷ 0.01 L = 1241 ppm3 = 15.02 mg ÷ 0.01 L = 1502 The ppm value for each trial = 3286, 1241 and 1502
s = √∑(xᵢ - x)2/ n – 1 Xi 10.6 31.0
(xᵢ - x)2 150.55 66.10
27.0
17.06
s = √∑(xᵢ - x)2/ n – 1 = √(150.55 + 66.10 + 17.06)/2 = √233.71/2 = 10.81 N=3 Mean of volume used of AgNO3 = 22.87 ML Degree of freedom = 2 , so t = 4.30 Confidence limit = μ = X ± ts/√N = 22.87 ± 4.30(10.8099)/√3 = 22.87 ± 26.8367 - 3.97 to 49.71 The confidence limit range is between – 3.97 to 49.71. Discussion : In this experiment, we have carried out the Mohr method. This method is about to determine the chloride ion concentration in sea water. For the standardisation of the silver nitrate solution, we have collected the data as our expected data. The expected data showed that the salt solution will turn its colour from yellowish to orange pink colour. As we compared the expected data to our actual data, it gives the positive result when the colour of salt solution change its yellowish colour to orange pink colour with the help of 1 mL of 5% potassium chromate indicator. But we have done a simply mistake when we tried to add more standard NaOH to salt solution in order to obtain the pH between 7 to 10. The excess of standard NaOH in the salt solution may disturb the change in colour of salt solution through titration process. Then, we also analyse the sea water by determine its chloride concentration. In this part, we have determined the pH of the salt solution is the same with the expected data. So, the titration process that has been set up to analyse sea water gives the positive observation in change of salt solution colour, which is from yellowish to orange pink colour. The volume of AgNO3 that used to change the colour of solution was recorded. By using the recorded volume, we have calculated the concentration of chloride of sea water. As the conclusion , our experimental result is compatible with the expected data.
Conclusion : 1) The concentration of standardisation of silver nitrate, [AgNO3] is 5.73 M, 2.13 M, 2.60 M. 2) The concentration of chloride ion, Cl- is sea water for each trials is 0.0968 M, 0.0350 M, 0.0424 M. References : 1) http://www.federica.unina.it/agraria/analytical-chemistry/mohr-method/ 2) HARGIS L.G, Analytical Chemistry , principle and techniques, 1998
Questions : 1) At equivalence point, [Ag+] = [Cl-] [Ag+]2 = Ksp = 1.6 x 10-10 , [Ag+] = 1.26 x 10-5 M 2) At equivalence point, [Ag+]2 = [CrO4-] [Ag+]2 = Ksp = 1.1 x 10-12 , [Ag+] = 1.049 x 10-6 M So , [CrO4-] = 1.1 x 10-12 M 3) The concentration of chromate used is less because to get fastest formation of Ag2CrO4 due to greater concentration of Ag+ toward the minimum concentration required. 4) In this experiment, we need to determine the concentration of substance by using the volume of solution. 5) Adding too much NaOH to neutral the pH between 7 to 10 is the common source of contamination in this titration.
