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ENVE 201 Environmental Engineering Chemistry 1 . ACIDITY Dr. Aslıhan Kerç. Acidity. Buffered by CO 2 and HCO 3 system. Natural waters Domestic wastewater Industrial Wastes Acidity is the ability of water to neutralize bases .

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ENVE 201 Environmental Engineering Chemistry 1


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acidity
Acidity

Bufferedby

CO2and HCO3system

  • Naturalwaters
  • Domesticwastewater
  • IndustrialWastes
  • Acidity is theability of watertoneutralizebases.
  • Endpointforcarbonicacid stoichiometricendpoint @ pH 8.5
slide3

Allwater < pH 8.5 containacidity.

Referencepoint phenolphthaleinendpoint

8.2 -8.4

  • Acidity of naturalwater is causedby CO2orbystrong mineral acids.
carbonic acid titration curve
Carbonic Acid Titration Curve
  • @ pH 7 considerable CO2 remainsto be neutralized.
  • Carbon dioxide alone will not depress pH below 4
  • For strong acid  neutralization is completed@ pH 4
slide5

Acidity of naturalwaterscausedby :

    • CO2
    • Strong mineral acids

10

9

8

7

6

5

4

3

2

1

Phenolphthaleinendpoint

CO2acidity

Methylorangeendpoint

Mineral Acidity

slide6

Ifthere is H2CO3pHdoesn’tdropbelow 4

  • IfpH < 4  Mineral acidity

CO2

  • Normal component of naturalwaters.
  • Fromatmosphere (Henry’sLaw)
  • Producedfrombiologicaloxidation ( Endproduct of bothaerobicandanaerobicbacterialoxidation)
slide7

Groundwater high in CO2 since it is not free to escape to atmosphere.

30-50 mg /L can be found.

  • For groundwater that do not contain Ca, Mg
  • Ca, Mg neutralize the CO2 through formation of bicarbonates.

CO2 + CaCO3 Ca2+ + 2HCO3

slide8

Mineral Acidity in industrialwastes

(sulfuricacid , salts of sulfuricacid)

  • Conversion of sulfur , sulfides, ironpyritestosulfuricacidandsulfatesbysulfuroxidizingbacteria.

2S+3O2+2H2O bacteria 4H++2SO42-

FeS2+3 ½ O2+H2O bacteria Fe2++2H++2SO42-

slide9

Significance of CO2and Mineral Acidity

Not veryimportantforpublichealth.

CO2 in malt andcarbonatedbeverageshigherthannaturalwaters.

Ifwatercontain mineral acidity

  • unpleasanttaste
  • no consumption

Acidicwaters  corrosivecharacteristics

Corrosivefactor  CO2

in industrial wastes ; mineral acidity

slide10

CO2content is alsoimportant in lime-soda ashsoftening,

  • InbiologicaltreatmentpHmust be maintained (6.0-9.5). Amount of chemicalsarecalculatedbased on acidityvalues.
  • Combustion of fossilfuels Nitrogenoxides +Sulfuroxides
  • Whenmixedwithrain  formssulfuricandnitricacids.
  • Acidrainalsocauseleaching of chemicals (aluminum ) fromsoil.
slide11

Methods of Measurement

  • Measuredwithstandard alkaline solutions.
  • Mineral acids  Titration to pH 3.7 (methyl orange end point)(methyl orange acidity)
  • Titrationtillphenolphthaleinendpoint (pH 8.3)  Mineral acidity + acidityduetoweakacids.
  • Total acidity  (phenolphthalein acidity)
slide12

CO2

  • Specialprecautionsarerequiredforsamplecollection , handling, andanalysis of CO2 .
  • Partialpressure of CO2 in water is greaterthat in theatm.
  • Avoidexposuretoair.
slide13

Analyze at the point of collection (min exposure to air min. temp. change)

  • Collect sample by using submerged tubes, fill completely , leave no air pockets  keep the temp. same.
slide14

TitrationMethod

  • To minimize contactwithair  titrate in a graduatedcylinder.
  • CO2 will be lost due to stirring.
  • Take second sample for titration, add required titrant .
  • Reach final end point slowly.
  • Pinkish color should persist for 30 s

Excess is siphoned

slide15

Standard reagentNaOHmust be free of sodiumcarbonate.

  • Neutralizationrxn:

2 NaOH + CO2  Na2CO3 + H2O

Na2CO3 + CO2 + H2O  2NaHCO3

  • If Na2CO3 is originally present  cause wrong results.
  • For CO2 measurement Na2CO3 sol’n  can be used as standard titrant.
slide16

Calculation from pH and alkalinity

  • Amount of CO2 can be calculatedusingionizationeqn. forcarbonicacid.
  • H2CO3 [H+] +[HCO3-]

KA1 = [H+] *[HCO3-] / [H2CO3]

slide17

[H2CO3] = carbonicacidmolarcontent + freecarbondioxide.

  • CO2 = 99% [H2CO3] ˜= [CO2 ]

Example :

KA1 =4.3*10-7, [H+] =10-7 , [HCO3-] = 4.3*10-3

[CO2 ]= 10-7*4.3*10-3 / (4.3*10-7) = 10-3 mol/L

= 44 mg /L

slide18

Dissolved solidsconcentration must be known.

Require accurate pH measurement

  • Calculationmethod
  • Titrationmethod

for smaller concentration , excessive titration errors

    • IfCO2 > 2 mg/L  usetitrationmethod
slide19

MethylOrangeAcidity

  • IfpH < 4  contain mineral(methylorange)acidity
  • Mineral acidsareneutralized @ pH 3.7
  • Bromphenolblue has sharpercolorchange.
  • E.W CaCO3 = 50
  • N/50 NaOH is used 1mL = 1 mg acidity
  • Resultsarereported as CaCO3
slide20

PhenolphthaleinAcidity

  • Total acidity = mineral acidity + fromweakacids
  • Weakacidsareneutralized at pH 8.3
  • Phenolphthalein or metacresol purple indicatorsare used
  • Titrant: N/50 NaOH
  • Results are expressed as CaCO3.
slide21

Application of Acidity Data

  • CO2 determinationforpublicwatersuppliesconsideredfortheselection of treatmentmethods.
  • Corrosioncontrol
  • Softening
  • Industrial wastes  Mineral acidity must be neutralized before discharge

Acidity = N*Vtitrant*50*1000 / ( mL sample)

E.WCaCO3 = 100/ 2 = 50 g/eq