Water Analysis 2

Spectrometry-principles

It < Io due tolight absorption

Spectrometry-principles

Beer-Lambert law

C

Spectrum of Fe(phen)3

2+ complex

Spectrometry-principles

Additive property of absorbance

A = A1 + A2 + A3 + ….

It is essential to subtract absorbance of a blank from absorbance of samples

Determine the concentration of a sample - calibration curve

Concentration, ppm

Co C1 C2 C3 C4 C5

Ab

sorb

an

ce

Ax

Cx

Linear relationship

Determination of IRON in water by spectrometry

Total iron (Fe3+ and Fe2+) is determined

PRINCIPLE

• Fe3+ is reduced to Fe2+ by hydroxylamine (pH 3.3)

4Fe3+ + NH2OH 4Fe2+ + NO2 + H2O + 4H+

• Formation of Fe(II)-phenantroline complex (pH 3-9)Fe2+ + 3Ph [Fe(Ph)3]2+

redish-orange (510 nm)

Standard / Sample

Buffer

Reagent

Mix, dark place (15min)Measure at 510 nm

Reduce agentAcidtoC

PROCEDURE

Determination of IRON in water by spectrometry

Determination of nitrite - principles

Nitrite (NO2-) react with sulfanilic acid and N-(1-naphthyl)-ethylenediamine to form red azo dye (max

= 540nm)

Sulfanilic acid can be substituded by sulfanilamide!

Determination of nitrite -precautions

- NO2- can be converted to NO3- or NH3 by bacteria -> preserve samples at 2-5 oC for 1-2 days

- NO2- standard solution is not stable -> re-determine the concn. by KMNO4 / oxalic acid

(KMnO4 is standardized by oxalic acid)

Determination of nitrate - principles

- 220 nm: NO3- + organic compounds absorb light- 275 nm: organic compounds absorb light- ANO3- A220nm – 2A275nm

Chemical Oxigen Demand - COD

• An indicator of organic pollutant in water

• The quantity of oxidant consumed is expressed

in terms of its oxygen equivalence (mg O2/L)

Cr2O72- is used as oxidizing agent

In oxidation reaction 1 mole Cr2O72- equal to 1.5 mole O2

Principle

Cr2O72- + Organic matter Cr3+ + H2O + CO2 +

(known amount)

• Excess K2Cr2O7 is determined by titration with Fe2+

• Excess K2Cr2O7 or Cr3+ produced is determined by

spectrophotometry

K2Cr2O7 consumed

1 mole Cr2O7-2 reacts with 6 mole Fe2+

Chemical Oxigen Demand - COD

STRONG oxidizing and toxic chemicals!

- H2SO4 , 150oC (2hrs),

- Ag(SO4): catalyst

- HgSO4 : minimize Cl- interference (HgSO4:Cl– = 10:1)

Samples should be homogenized and analyzed without delay or preserved by acidification to pH ≤ 2

sample

Digestion2hrs, 150oC

Measurement600nm

BaSO4

Source

Particle < 1m

Determination of SULFATE in water by turbidimetry

PRINCIPLE

Determination of SULFATE in water by turbidimetry

Ba2+ + SO42- BaSO4

Acetate buffer

420 nm

(1-40 mg SO42-/L)

Suspended particles must be removed by filtration before analysis!!!

MgCl2, KNO3

Log(I0/IT) = k.b.C

• C: concentration of the scattering particles (w/v),

• b: is the path length

• k: constant that depends on the size, shape of

the scattering particles and the wavelength of

the source radiation.

Determination of SULFATE in water by turbidimetry

It is necessary to maintain a UNIFORM distribution of

particle sizes throughout the sample and between

samples and standards

Procedure

Standard / Sample

Buffer BaCl2 (s)

Mixed

Mixed well (1 min)Measured (420 nm, in < 5 mins)

Determination of SULFATE in water by turbidimetry

Turbidity of water by nephelometry

Turbidimetry

Nephelometry(more sensitive)

Standard : formazin (polymer)

Turbidity of water by nephelometry

5, 50, 500 NTU

Sample storage

Low temp. or adding formaldehyde

H+

Determination of SULFATE in water by turbidimetry