Soil science

1. Soil Reaction (pH)

2. Introduction:-
H+
• *Water (H2O)
OH-
• Concentration of H+ & OH- ion decides the acidity, neutral &
basic or alkaline reactions.
• If H+ ion dominate over OH- : Acidic reactions
• OH- > H+ : Alkaline reactions
• OH- = H+ : Neutral reactions
Acidic: H+ OH- OH- H+ Neutral
Basic: H+ OH-

3. • Pure distilled water neutral reaction at 25 0C (H+ = OH-).
• Most of agricultural soil pH ranges from 4.5 – 8.5. Although
soil having < 4.5 & > 8.5 (but very rare)
• The acid sulphate soil; < 4.5 (find in coastal area, Kerala),
These soils having sulfuring horizon which is also known as
cat clays (clay particles adsorbed with FeS). In India, 20,000
ha of land is acid sulphate soil.
• pH >8.5 (nearly 10) :- Sodic soils, because of high
exchangeable Na. This pH found in black or calcareous soil, ill
drain conditions.

4. • Aqueous system pH scale is 0-14.
• 1 litre of water total ionic strength of H+ & OH-
concentration expressed in moles/liter – at 200C
• Ionic product (Kw) = [H+] [OH].
• The reaction is expressed in terms of pH
• The term pH in introduced by Dutch scientist Sorenson,
during 1909.

5. II. Soil system:-
• H+ ion activity (pH) is due to the activity of H+
ion that originate from dissociation of soluble
acid & those dissociated from soil particles
• Effective concentration of H+ ions: H+ ions
involve in reaction.

6. Classification of soil based on the pH value.
• pH Categories
• <4.5 - Extremely acidic
• 4.5-5.0 - very strongly acidic
• 5.1-5.5 - strongly acidic
• 5.6-6.0 - moderately acidic
• 6.1-6.5 - slightly acidic
• 6.6-7.3 - Neutral
• 7.4-8.0 - Mildly alkaline
• 8.1-9.0 - strongly alkaline
• >9.0- - very strongly alkaline

7. – In soil – water system some of
adsorbed H+ ion dissociated from
surface of soil colloids into soil
solution.
• These dissociated H+ ions give rise to
active & soluble acidic (1).
• But there are many more H+ ion still on
soil colloid, which give rise to potential
acidic or reverse acidic or exchangeable
acidic (2).
• Measuring pH of soil solution, not of
colloids.

8. pH measurement
• W.R.T. pH measurement – 2 methods.
• Colorimetric method – using indicator dyes
• Potentiometric method – using pH meter
1) Colorimetric method:
• In the Colorimetric method, using indicator & are very specific
for pH change [indicators are nothing but organic dyes]
• 1) Advantages:- It is suitable for routine & rapid pH
determination particularly under field condition.

10. 2) Disadvantages:-
• The indicator gives only range of pH not give the exact pH i.e.
3.5 - 5.5
• Colour develop by indicator is masked by soil clay particles.
• Each indicator has got only narrow range of pH
• For Ex –
– phenolphthalein – 8.4 (colour less) - 10.0(pink)
– Methyl blue – pH 3.2 (Red) – 4.6 (yellow)
– Bromocresal green – 3.8 (yellow) – 5.4(Blue)
• So choice of indicator for determination of pH is important.

11. 2. Potentiometric method:-
• It is the most accurate method in determination of soil pH in
aqueous solution.
• Here use electrode which are sensitive to H+ ion concentration.
The type of electrode is as follows
1. Glass electrode
2. Hydrogen electrode
3. Antimony electrode
4. Ion electrode
5. Quinhydrone electrode

12. Glass electrode :-
• It consists of glass bulb at bottom, which is made up of thin glass
membrane & has low melting point & chemically pure
• Composition of glass bulb is:
– 72% SiO2, 6% CaO, 22% Na2O.
• 0.1M HCl saturated with quinhydrone solution & is made to
electrical contact.
• Glass electrode measure pH, when in association with standard
reference electrode ( colomel electrode, 0.246 volt)

14. • A glass bulb containing 0.1M HCl is then dipped in the
beaker containing the solution whose pH is to be determine
• It is not possible to measure in half cell potential alone. Another
half cell potential that is calomel electrode must use to measure
test solution outer & inner.
• In connection with reference electrode when both together are
dipped into test solution.
• Calomel electrode consists of mercuric fluid contact with KCl
solution.

15. • Equation:
• 1) Glass electrode HT reference electrode
or
• 2) pt wire : 0.1M HCI (aH+): unknown solution: KCI
saturated / crystals : Hg2Cl2 (Hg): pt wire

16. II. Reaction [Sorensen 1909] : pH:
• Meaning – puissance de Hydrogen
Definition: It is the negative logarithm of hydrogen ion activity and
pOH as that of hydrogen ion.
Thus,
• pH= - logaH+ = log (1/aH+) or aH+ = 10-pH
• pOH= - logaOH
- = log (1/aoH-) or aOH
- = 10-pOH
• In pure water, pH + pOH = 14 & pH = pOH = 7
• Soil reaction is measured by pH of a suspension of soil in water.
• The concept of pH may be explained with reference to pure
water.
• Pure water is amphoteric, in which H+ & OH- ions are in
equilibrium with undissociated water molecules.

17. H2O + H2O H3O+ + OH-
Apply LMA
aH3O
+ X aOH
- = k’w
aH2O X a H2O
aH3O+ X aOH- = kw
(CH+ X COH-) X (f H+ X f OH-) = kw’
Where: kw = k’ w X (aH2O)2
a – activity;
c- concentration;
f – activity coefficients ( equal to units in pure water)
Thus,
aH+ X aOH- =CH+ X COH- = kw= 10-14
In neutral solution (pure water) – at 250c;
CH+ = COH-
Therefore , CH+ = COH- = (10-14) ½ = 10-7 g/l

18. • The hydrogen (H+) activity of solution can vary from 100
(normal solution of strong acid) to 10-14 (strong alkaline
solution).
• In case of hydroxyl (OH-) ion activity will vary from 10-14
to 100.
• Therefore the product of CH
+ X COH
- is always 10-14.

19. • Equation :
• Pt(H2), (1atm)/ H+X- / KCI (std) /Hg2CI2, KCl /Hg2CI2/ Hg
E(Emf)
• Given by E = Eref – RT ln aH+
• F
• E= E ref + 2.303RT . pH
• F
• pH = F (E - Eref)
• 2.303RT
• pH = (E-E ref ) 0.0592 at 25 0C

20. Equation
• 1) Pt wire : 0.1m HCl (aH+) ; unknown solution : KCL saturated
/ crystal: HgCl2: pt wire
• (Glass electrode reference electrode)
• pH ranges from 0 to 14, because total ionic product of water
itself is 14.
• That is,
• pH + pOH = 14

21. Degree of dissociation of water is very slightly;
• H2O H++OH-;
• H++H2O H3O+
• Ionization constant of water is 1.8 X10-16
• 1 litre of H2O at 25 0C weighs = 997.0 g
• Molecular Wt. of water = 18
• Molar concentration of H2O = 997 = 55.39
• 18
• That is aOH- = aOH- = 55.39 X 1.8 X 10-16
• Therefore, 1 X 10-7 X 1 X10-7 = 1.01X 10-14

22. Derive:
• Dissociation of water :-
• H2O H+ + OH-
• [This dissolution one in 107 molecules of water (per l cr) and gives H+
and OH- at a given time]
• Apply LMA
• keq = CH X COH [keq=1.8 X 10-16 – this value is referred as
ionization
CH2O constant of water]
•
• The concentration of pure water is 55.5 moles /L ( 1000 = 55.5)
• 18
• Let us substitute the values into equation.
• CH X COH =keq
• CH2O
• CH X COH = 1.8X10-16 X 55.5
• =1.00X10-14
• In case of dissolution of H2O keq is referred as kw therefore kw=
1.00X0-14
• kw is called as ion product of water this reaction is used in the
formation of pH + pOH = 14

23. Principles of potentiometer
• “When a glass electrode having glass tube/ bulb at bottom
contain 0.1M HCl is introduce into unknown solution
whose pH is determine, an electrical potential is created
across the glass membrane between H+ ion concentration
present in the solution of glass electrode and H+ ion
concentration of unknown solution, this potential
difference is due to the activity of H+ ion in & out of glass
electrode, this potential difference is given by Nernst
equation”

24. •
• RT In K (Natural logarithm): log10 = 2.303
• nF M
Where
• R- Gas constant – 8.319 Joules/g
• T – Absolute temperature – 298 0K
• n – Valance of ion i.e. H – 1
• F – Faraday constant = 96500 coulombs
• K- H+ ion activity standard solution inside glass electrode
• M – H+ ion activity of unknown solution.
1pH = 0.0591 volts
= 59.10 milli volts

25. II. Factors affecting pH
1. Liquid – junction potential – (EJ): bottom of reference
electrode
2. Asymmetric potential (EAP) : bottom of glass electrode
3. Suspension effect
4. Temperature
5. Salt content, electrolyte concentration
6. Soil water ratio
7. CO2 concentration of atmosphere

26. 1. Liquid – junction potential (EJ):
• It is not at all a problem because we made use with glass
electrode, the reference electrode is fixed, i.e. It is a combine
electrode.
• A) Purpose of KCl: Only due to mobility of ions in test solution
KCI having equal mobility of K+ & CI-
2. Asymmetric potential (EAP): It always occurs at bottom of glass
electrode.
• Fig.

27. • When dip the electrode in soil water system a turbulence is
created in soil solution this turbulence is results an un-equal
distribution of H+ ion in solution (test solution).
• H+ ion come in contact with electrode only measure as pH,
but not at corner or away from the glass electrode is called
asymmetric potential.

28. • Avoid by:
• Some type of potential is develop across the glass membrane
even though H+ ion concentration is same inside the glass
electrode & outside the glass electrode because of due to
turbulence.
• To overcome this asymmetric potential pH meter must be
calibrated with buffers [standards having known pH i.e. 4.0,
7.0 and 9.2 pH]

29. • Suspension Effect:
• Fig.
• To avoid the suspension effect the soil - water mixture must be
stir well just before the electrode is immerse & take the
reading.
• This suspension is due to movement of H+ ion towards
sediments side.
• [pH of the sediment is less than the supernatant. In supernatant
more of OH- & less of H+]
• Clay attract more H+

30. • Effect of temperature on pH
• pH of water at 0 0C – 7.5
• pH of water at 23 0 C – 7.0
• pH of water at 100 0C – 6.0
• Temperature variation affect pH measurement because of
dissociation of water, along with other salts more H+ ions are
produced in compare to OH- ions.
• And further heating results in even dissociation of OH- ion itself
leading to the production of 2H+ ions & only 1 O (oxygen) for
every 1 molecule of water.
• To avoid the temperature effect, the temperature control knob is
given on pH meter, by using this, Adjust the temperature of test
solution to the temperature of the laboratory.

31. • Effect of salts or electrolyte on pH
• Salts / electrolyte when added to the soil replace the adsorbed
H+ ion & this replaced H+ ion concentration in soil solution
increases & decrease the pH. So soluble salts / electrolyte
decreases soil pH.

32. • Effect of water content on soil pH
• In general more dilute soil suspension higher will be soil pH.
• Eg. Soil water ratio of
• ratio pH
– 1:1 – 5.5;
– 1.2.5 - 5.8
– 1:5 – 5.9 &
– 1:10 – 6:4 pH
• Distilled water has H & OH ions.
• Relative concentration of OH ions is more in soil solution than
H+ ion.
• OH- ion, negatively charged, therefore it is not adsorbed on
negatively charged surface (colloidal)
• It is observed that from sticky point to 1:2.0 ratio there is
increase soil pH from 0.2 to 0.5unit.

33. • CO2 concentration of atmosphere:
• Increase the CO2 concentration, there will be decrease in pH.
Because dissolved CO2 in water is directly related to partial
pressure of CO2 in air.
• Pure distilled water when in equilibrium, with CO2
concentration of atmosphere (0.03%) it gives pH value of 5.7
(DW).
• Suppose the concentration of CO2 to 0.3 % - pH of DW is 5.22
• 0.4% - pH of DW is 4.95
• 1.0% - pH of DW is 3.95

34. • Avoid the effect of CO2 concentration
• The soil water suspension must be stir vigorously just before
electrode dip. This vigorous stirring results in equilibration of
soil water suspension with CO2 concentration of atmosphere.

35. III. Precautions to use glass electrode
• It should not be kept for long time in soil suspension particularly in
alkali pH solution because precipitation of salts on glass membrane
takes place [maximum time is 2 minutes is permissible].
• Immediately after pH measurement wash the glass electrode through
distilled water (DW)[pH – alkaline, wash with dilute HCl].
• Always keep electrode suspended freely with distilled H2O.
• If electrode is not used for long time, do not keep it either for long
time, use or run at least once in a month otherwise it become
sluggish.