Exp 4.docx

CHM 421 : ANALYTICAL CHEMISTRY

Title: SOLUBILITY OF IONIC SALTS IN SEAWATER

Date of Experiment

: 28th Mac 2019

Date of Submission

: 4th April 2019

Name of Student

: JAMILAH BINTI MOHAMED SALLEH

UiTM ID

: 2017676526

Course/ Group

: AS2455D

LECTURER NAME

: MADAM RUHAINI

EXPERIMENT 4: SOLUBILITY OF IONIC SALTS IN SEA WATER Abstract: This experiment is about the solubility of ionic salts in sea water. This experiment of solubility of ionic salts in seawater was done to determine the chloride concentration in seawater, in the same time, to standardise the silver nitrate 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% K2CrO4 indicator acted as a catalyst. The salt solution was titrated with the silver nitrate solution until the colour of indicator changed. 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. Then, the seawater (in basic condition) was analysed by diluting and titrate it with the standardised silver nitrate solution with the help the same catalyst until the colour of indicator changed.

Objectives: 1)

To standardise silver nitrate solution using a known chloride solution.

2)

To determine the chloride concentration in sea water.

Introduction : Some of the most familiar chemicals are ionic, water soluble salts such as sodium chloride (table salt), potassium nitrate (saltpeter), and magnesium sulfate (Epsom salt). Because of the water solubility and low chemical reactivity of many simple salts, as well as the high terrestrial abundance of their constituent elements, high concentrations of the ions Na+, K+,Ca2+, Mg2+, Cl-, and SO42- have built up in the earth's oceans. They have been leached from mineral deposits by the action of ground water over geological time. Together, these six ions account for 99.6% by weight of the dissolved salt content of the oceans. It is generally believed that life evolved in the oceans, and living organisms reflect this aquatic past both by high overall water content (50% for trees, 65% for people, 99.7% for jelly fish) and by an elaborate biochemistry established in salt solutions. These solutions contain the same ions that were brought from the seas by the first amphibians. Thus, perhaps it is not surprising that there is a striking resemblance between the ionic composition of seawater and that of human blood plasma, one of the solvent systems for human metabolic chemistry. This method involves chemical volumetric analysis. A silver nitrate (AgNO3) solution of known concentration is used to precipitate out the chlorides in a seawater sample. The procedure is complicated by the fact that the dense white precipitate, silver chloride (AgCl), does not settle rapidly. It is impossible to tell when all the chlorides have been removed from the seawater unless an indicator is

used to signal complete precipitation by a visual color change. The indicator in the analysis is chromate ion. When the entire chloride ion is exhausted, the chromate ion reacts with silver ions and produces silver chromate, which is red. The instant a permanent orange tinge appears in the solution (one that doesn’t vanish with mixing), the addition of silver nitrate is stopped. The final solution color should look like that of orange juice.

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 About 0.02 g of dry sodium chloride, NaCl was accurately weighed. This mass was transferred quantitatively into a 250 mL conical flask. 100 mL of distilled water was added approximately to dissolve the salt. The pH of this solution was checked by using a pH paper. The pH was adjusted to be between 7 and 10 by adding sodium hydroxide, NaOH. 1.0 mL of 5 % potassium chromate, K2CrO4 indicator was added with the solution. A 50 mL burette was thoroughly cleaned with tap water and it was rinsed a few times with distilled water. Then, the burette was rinsed and filled with the silver nitrate, AgNO3 solution that wished to standardise. The salt solution was titrated earlier with the AgNO3 solution until the permanent orange pink colour obtained. This standardisation procedure was repeated until we have three values that were in good agreement with each other. B. Analysis of Sea Water 10.0 mL portion of sea water was approximately measured. The first sample was filtered because it is cloudy. The sample then transferred to 100 mL volumetric flask and it was diluted to the mark with distilled water. The dropping pipette was used towards the end in order to avoid exceeding the line. 10.0 mL aliquot of the diluted sea water sample was pipetted into a 250 mL conical flask. The pH of the solution was ensured between 7 and 10 by using pH paper. 1.0 mL of 5 % potassium chromate, K2CrO4 was added and it was titrated against the standardised silver nitrate, AgNO3 solution. The titration was 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

Rough 0.0235

1 0.0217

2 0.0216

3 0.2000

30.50

28.40

29.00

27.00

0.00

0.00

0.00

0.00

30.50

28.40

29.00

27.00

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

Rough 10.00

1 10.00

2 10.00

3 10.00

33.70

33.50

33.50

33.60

0.00

0.00

0.00

0.00

33.70

33.50

33.50

33.60

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 NaCl = 0.0235 g ÷ 58.44 g/mol = 4.0212 x 10-4 mol Mol, n2 of NaCl = 0.0217 g ÷ 58.44 g/mol = 3.7132 x 10-4 mol Mol, n3 of NaCl = 0.0216 g ÷ 58.44 g/mol =3.6961 x 10-4 mol Mol, n4 of NaCl = 0.0200 g ÷ 58.44 g/mol =3.4223 x 10-4 mol

mol of NaCl ≡ mol of AgNO3 Mass1 of AgNO3 = (4.0212 x 10-4 mol) x 169.88 g/mol = 0.0683 g Mass2 of AgNO3 = (3.7132 x 10-4 mol) x 169.88 g/mol = 0.0631 g

Mass3 of AgNO3 = (3.6961 x 10-4 mol) x 169.88 g/mol = 0.0628 g Mass4 of AgNO3 = (3.4223 x 10-4 mol) x 169.88 g/mol = 0.0581 g

First [AgNO3] = 0.0683 g ÷ 0.0305 L = 2.24 M Second [AgNO3] = 0.0631 g ÷ 0.0284 L = 2.22 M Third [AgNO3] = 0.0628 g ÷ 0.0290 L = 2.17 M Fourth [AgNO3] = 0.0581 g ÷ 0.0270 L = 2.15 M

The concentration of silver nitrate, [AgNO3] = 2.24 M, 2.22 M, 2.17 M, 2.15 M Mean of [AgNO3] = (2.24 M + 2.22 M + 2.17 M + 2.15 M)/ 4 = 2.195 M

AgNO3(aq) + Cl-(aq)

AgCl(s) + NO3- (aq)

1 mmol of Ag+ ≈ 1 mmol of Cl- , mmol of Ag+ ≡ mmol of ClMol, n1 of Cl- = 33.70 mL x 0.0132 mmol/mL = 0.4448 mmol Mol, n2 of Cl- = 33.50 mL x 0.0131 mmol/mL = 0.4389 mmol

Mol, n3 of Cl- = 33.50 mL x 0.0127 mmol/mL = 0.4255 mmol Mol, n4 of Cl- = 33.60 mL x 0.0127 mmol/mL = 0.4259 mmol

First molarity of Cl- = 0.4448 mmol ÷ 10.0 mL x 10 = 0.4448 Second molarity of Cl- = 0.4389 mmol ÷ 10.0 Ml = 0.4389 M Third molarity of Cl- = 0.4255 mmol ÷ 10.0 mL = 0.4255 M Fourth molarity of Cl- = 0.4259 mmol ÷ 10.0 mL = 0.4259 M

The molarity of chloride ion, Cl- = 0.4448 M, 0.4389 M, 0.4255 M, 0.4259 M Mass = mmol x mg/mmol Mass1 = 0.4448 mmol x 35.45 mg/mmol = 15.77 mg Mass2 = 0.4389 mmol x 35.45 mg/mmol = 15.56 mg Mass3 = 0.4255 mmol x 35.45 mg/mmol = 15.08 mg Mass4 = 0.4259 mmol x 35.45 mg/mmol = 15.10 mg

ppm1 = 15.77 mg ÷ 0.01 L = 1577 ppm2 = 15.56 mg ÷ 0.01 L = 1556 ppm3 = 15.08 mg ÷ 0.01 L = 1508 Ppm4 = 15.10 mg ÷ 0.01 L = 1510

The ppm value for each trial = 1577, 1556, 1508, 1510

s = √∑(xᵢ - x)2/ n – 1 Xi 30.5 28.4 29.00 27.00

(xᵢ - x)2 729.54 620.51 650.76 552.72

s = √∑(xᵢ - x)2/ n – 1 = √(729.54 + 620.51 + 650.76 + 552.72)/3 = √2553.53/3 = 29.17 N=4 Mean of volume used of AgNO3 = 28.73 mL Degree of freedom = 3 , so t = 2.353 Confidence limit = μ = X ± ts/√N

= 28.73 ± 2.353(29.1749)/√4 = 28.73 ± 34.32 - 5.59 to 63.05 The confidence limit range is between – 5.59 to 63.05 .

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. Then, we also analyse the sea water by determine its chloride concentration. 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. In this experiment, we had calculate the accurate concentration of silver nitrate as it mean concentration of nitrate is 2.195 M. Beside that, we also calculated the concentration each of the silver nitrate which are 2.24 M, 2.22 M, 2.17 M, 2.15 M. Next, we had calculated the the molarity of the chloride and then we had conveted it to ppm. The results of this calculation was stated in the conclusion. This calculation was times 10 as the dilution factor of the seawater is 10x.

Conclusion : In the conclusion, the concentration of standardisation of silver nitrate, [AgNO3] is 2.24 M, 2.22 M, 2.17 M, 2.15 M. The concentration of chloride ion, Cl- in sea water for each trials is 0.4448 M, 0.4389 M, 0.4255 M, 0.4259 M while the ppm value of chloride ion, Cl- in seawater is 1577, 1556, 1508, 1510. The confidence limit range is between – 5.59 to 63.05.

References : 1) http://www.federica.unina.it/agraria/analytical-chemistry/mohr-method/ 2) HARGIS L.G, Analytical Chemistry , principle and techniques, 1998

Questions : 1) For this titration, the concentration of the indicator {CrO42-} is important. In order to give a color change at the equivalence point, Ag2CrO4 should just start to precipitateat the point when the solution is just saturated with AgCl (Ksp = 1.6 x 10-10). Calculate the concentration og Ag+ ions at this point. At equivalence point, [Ag+] = [Cl-] [Ag+]2 = Ksp = 1.6 x 10-10 , [Ag+] = 1.26 x 10-5 M 2) The solubility product constant, Ksp for Ag2CrO4 is 1.1 x 10-12. Determine the concentration of CrO42- necessary in the solution to ensure that Ag2CrO4 will start to precipitate at the equivalence point. 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) For the purpose of carrying out the experiment, the concentration of CrO4 used is less than the value found in Q2. What do you think the reason for this?

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 previous titrations the volume of the solution in the conical flask was not important. Why is it important in this experiment? In this experiment, we need to determine the concentration of substance by using the volume of solution. 5) What do you think minght be a common source of contamination in this titration? Adding too much NaOH to neutral the pH between contamination in this titration.

7 to 10 is the common source of