Experiment 9:

Impurities in

Natural Water

Purpose

To learn some techniques for the purification of natural water.

To learn how to test for the contaminants of natural water (mostly ions).

To learn the technique of softening using an ion-exchange resins.

To learn about the properties of phosphate salts and their role as contaminants of natural water.

To determine quantitatively (by spectrophotometric means), the phosphate content in a number of

water samples and in an unknown.

Contaminants in natural

water

Very little of Earth water is fresh water (less than 3%).

Impurities any substance other the H2O molecule.

Classified into five major categories.

Types of Contaminants

Suspended solids: sand, clay, silt, organic material, (bits of leaves).

Dissolved gases: O2, N2, NH3, H2S, oxides of nitrogen (NOx).

Dissolved ionic substances (salts):

a. Cations: H+, Li+, Na+, Mg2+, Mn2+, Fe3+, Ca2+

b. Anions: Cl, F, OH, NO3, SO4

2, HCO3,

PO43

Types of Contaminants

Dissolved organic substances: vegetables and animal matter and their

decay products.

Microorganisms: bacteria, viruses, protozoa, algae.

Theory and

Procedure

Distillation

Dissolve few crystals of cobalt chloride in water and distill.

Co2+(anhydrous) Co(H2O)62+, 2Cl CoCl4

blue pink blue

- Note the color of the distillate

- Save the distillate; Label A1.

Add concentrated NH3 to water and distill.

- Note the odor of the distillate is the distillate free of

ammonia?

- Test with phenolphthalein

- Save the remainder of the distillate; Label A2.

Minerals in tap water: cations

Hardness in Ca2+ and Mg2+

Complex with EDTA:

M2+ + Y4 MY2

Determine water hardness by titration with EDTA

Report the total [Ca2+, Mg2+] in mol/L (M)

convert to mg CaCO3/L solution.

Minerals in tap water: anions

Chloride ion

Silver ion test:

Ag+ (aq) + Cl (aq) AgCl (s)

test on tap water and distillates A1 and A2

Sulfate ion

Barium ion test:

Ba2+ (aq) + SO42 (aq) BaSO4 (s)

test on tap water and distillates A1 and A2

Carbonate

Evaporation to dryness scale

Ca(HCO3)2 (aq) CaCO3 (s) + H2O (l) + CO2 (g)

Mg(HCO3)2 (aq) MgCO3 (s) + H2O (l) + CO2(g)

At high T, solubility of CO2 decreases: equilibrium driven to the right.

Test for the carbonates: HCl

Test for carbonate

CaCO3 (s) + 2 HCl (aq)

Ca2+ + 2Cl + CO2 (g) + H2O

MgCO3 (s) + 2 HCl (aq)

Mg2+ + 2Cl + CO2 (g) + H2O

Treat the solid with HCl fizzing

Water Softening using ion-

exchange resins

Resins are polymers with cross-linking (connections between long C chains in a polymer).

A resin has active groups in the form of electrically charged sites.

Ions are attracted at these sites, and are replaced by other ions.

Two key factors:

affinity for a specific ion

number of active

sites for exchange

Structure of a resin

Ion-exchange polymeric resin

with cross-linking

Cross-linking agent

Binding sites for cations

Properties of resins

Maximize the number of active sites; significant surface areas are desirable (frequently cast in

the form of porous beads).

Activity (efficiency) depends on:

pH

Temprature

Types of resins

Household type:

The resin captures all metal ions and releases Na+

ions in their place

Na2R + CaCl2 CaR + 2 NaCl

- R represents the negatively charged site of the

cation exchange resin.

DOWEX: made of spherical (round) particles, yielding optimal flow kinetics

highly efficient.

De-ionizing type (both cation and anion exchange resins):

Cations taken and replaced by H+

Anions taken and replaced by OH

H+ + OH H2O

De-ionization less expensive than distillation due to lower energy

requirements.

Water Softening using Deionizing

type resins

Phosphate

Contamination

in Water

Phosphate: A Major

Contaminant*

Phosphate ion

serious contaminant in water (deprives living species from sufficient oxygen).

enters water from agricultural run-off, biological and industrial waste.

used in the manufacture of soap and detergents (major ingredient, 50% in domestic sewage!).

contributes to excess growth of algae Eutrophication.

Quantitative determination

Complexation of PO43 with:

Reaction:

3

6

247

VO vanadate

OMo molybdate

OH3MoO1PVOMoH

H13OMoVO6POH

23

)(POO(OH)VMo

64069

6

247342

427966

Vanadomolybdo

phosphoric acid

Calibration curve

Prepare 5 standard phosphate solutions:

From a stock KH2PO4 solution 3.230103 M,

prepare five standards: 6.460 10-5 M (1:50)

1.292 10-4 M (2:50)

1.938 10-4 M (3:50)

2.584 10-4 M (4:50)

3.230 10-4 M (5:50)

Determine b (M1) from slope

420 nm

Analysis of water samples

Determine the concentration (M) of phosphate, [PO4

3] in three water

samples:

Tap water

Sea water

Unknown

Convert to mg P/L solution (ppm)

Procedure and report

Procedure

Report

Assigned questions: 1, 2 and 4

Useful links (applications)