PH (POWER OF HYDROGEN IONS)

• PH IS THE NEGATIVE LOG OF HYDROGEN IONS CONCENTRATION.
• PH = -LOG [H+]
• THE HYDROGEN ION CONCENTRATION IS USUALLY EXPRESSED IN MOLES PER LITER.

EXPLANATION:-

• PH OF WATER IS 7 AND IT IS CONSIDER TO BE NEUTRAL IF PH IS FALLS DOWN FROM 7 THEN THE COMPOUND WILL CALLED ACID.
• IF THE PH RISE FROM 7 THEN COMPOUND IS CALLED BSIC.

• CALCULATION:-
• FOLLOWING FORMULA CAN BE USED TO DETERMINE THE VALUE OF PH OF DIFFERENT SOLUTION
• FOR ACID:-
• PH = -LOG[H+]
• FOR BASE:- (IT IS –VE LOG OF HYDROXYL ION CONCENTRATION)
• POH= -LOG[OH]
• WHERE AS
• PH + POH = 14

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DETERMINATION OF PH

• PH CAN BE DETERMINE BY :-
• BY USING PH PAPER
• BY USING PH METER
• BY USING INDICATORS

PH PAPER:- THE PROCEDURE OF DETERMINATION OF PH USING PH METER:

DROP OF SAMPLE ON THE PH PAPER USING A CLEAN DROPER. OBSERVE THE CHANG IN COLOR OF PH PAPER. NOW COMPARE THE COLOR OBTAINED ON PH PAPER WITH COLOR SHADES ON STANDARD PH CHART. MAKE A NOTE OF PH VALUE OBTAINED.

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PH METER :-

PLACE YOUR ELECTRODE IN THE APPROPRIATE BUFFER FOR YOUR SAMPLE & BEING READING. PRESS THE MEASURE BUTTON TO BEGIN READING THE PH ONCE YOUR ELECTRODE IS PLACED IN BUFFER. SET THE PH A SECOND TIME. ONCE YOUR READING HAS STABILIZED, SET THE PH METER TO THE VALUE OF BUFFR’S PH BY PRESSING THE MEASURE BUTTON.

PH INDICATOR:-

LITMUS INDICATORS WILL TURN ACID RED, BASES BLUE AND NEUTRAL SUBSTANCES PURPLE. LITMUS CAN ALSO COME ASS A PAPER THAT IS RED AND BLUE. ACID WILL TURN BLUE LITMUS PAPER RED. A BASE WILL TURN RED LITMUS PAPER BLUE.

PH METER

APPLICATIONS

• PH MEASUREMENTS ARE IMPORTANT IN MEDICINE, BIOLOGY, FOOD SCIENCE AND MANY OTHER APPLICATIONS.
• ENZYMES WORK AT SPECIFIC PH , SO THEIR PREPARATIONS MUST BE MAINTAINED AT THAT PH.
• BLOOD HAS A SPECIFIC PH OF 7.3-7.4,
• PH INDICATORS ARE USED IN TITRATIONS IN ANALYTICAL CHEMISTRY AND BIOLOGY TO DETERMINE THE PH OF DIFFERENT SUSBSTANCES.

PH INDICATR

DEFINITION:-

“THE COMPOUNDS (MOSTLY ORGANIC) WHICH CHANGE THEIR COLOUR WHEN PH OF A SOLUTION CHANGES”

USES OF INDICATOR:

• CHECK THE PH OF A SOLUTION.
• INDICATE THE END- POINT I.E. COMPLETION OF THE REACTION.

THEORIES OF INDICATORS

THERE ARE TWO THEORIES ON THE BASIS OF WHICH INDICATORS CHANGE THEIR COLOURS.

1) OSTWALD THEORY

2) BENZENOID QUINONOID THEORY

OSTWALD THEORY

• INDICATORS CHANGE THEIR COLOR BECAUSE OF IONIZATION AND DE IONIZATION.
• IONIZED SUBSTANCES AND DE IONIZED SUBSTANCES HAVE DIFFERENT STRUCTURES, SO THEY SHOW DIFFERENT COLORS IN IONIZED AND DE IONIZED STATE.

• THE IONIZATION OF THE INDICATORS IS LARGELY AFFECTED IN ACIDS AND BASES AS IT IS EITHER A WEAK ACID OR A BASE.
• IF THE INDICATOR IS A WEAK ACID, ITS IONIZATION IS VERY MUCH LOW IN ACIDS DUE TO COMMON H+ IONS WHILE IT IS FAIRLY IONIZED IN ALKALIES.
• SIMILARLY, IF THE INDICATOR IS WEAK BASE, ITS IONIZATION IS LARGE IN ACIDS AND LOW IN BASES DUE TO COMMON OH- IONS.

EXAMPLE:

• PHENOLPHTHALEIN: IT CAN BE REPRESENTED AS HPH.
• IT IONIZE IN SOLUTION TO A SMALL EXTENT AS:

HPH (COLOURLESS) ↔ H+ + PH(PINK)

• THE UNIONIZED/ DEIONIZED FORM HAS DIFFERENT COLOUR THAN THE IONIZED FORM.

BENZENOID QUINONOID THEORY:-

• ACCORDING TO BENZENOID QUINONOID THEORY AN ACID BASE INDICATOR EXIST IN TWO TAUTOMERIC FORMS HAVING DIFFERENT STRUCTURE.
• ONE FORM IS BENZENOID FORM AND THE OTHER IS QUINONOID FORM.
• THE TWO FORMS HAVE DIFFERENT COLORS. THE CHANGE IS DUE TO THE INTERCONVERSION OF ONE TAUTOMERIC FORM INTO OTHER.
• ONE FORM MAINLY EXISTS IN ACIDIC MEDIUM AND THE OTHER FORM EXISTS IN BASIC MEDIUM.
• THUS DURIG TITRATION THE MEDIUM CHANGES FROM ACIDIC TO ALKALINE OR VICE VERSA.

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• THE CHANGE IN PH CONVERTS ONE FORM TO ANOTHER FORM, RESULTING IN CHANG OF COLOR.

EXAMPLE:-

• PHENOLPHTHALEIN HAS BENZENOID FORM IN ACIDIC MEDIUM AND IS COLOURLESS AND QUINONOID FORM IN BASIC SO PINK IN COLOR.

TYPES OF INDICATORS

THE CHOICE OF INDICATORS DEPENDS UPON THE NATURE OF THE REACTION. THERE ARE THREE BASIC TYPES OF INDICATORS .

1. ACID-BASE INDICATOR
2. REDOX INDICATOR
3. PRECIPITATION INDICATOR

FOR QUANTITATIVE ANALYSIS OF METAL CATION THE USE OF COMPKEXOMETRIC INDICATORS IS PREFFERED.

ACID-BASE INDICATORS

THESE ARE THE ORGANIC COMPOUNDS WHICH ARE USED IN ACID BASE TITRATION. ACID-BASE INDICATORS RESPOND TO A CHANGE IN HYDROGEN ION CONCENTRATION. MOST OF THE INDICATOR ARE THEMSELVES WEAK ACIDS OTHER ARE WEAK BASE WHICH RESPOND TO CHANGE HYDROXYL ION CONCENTRATION.

THE MOST COMMON INDICATOR IS FOUND ON “LITMUS PAPER”. IT IS RED BELOW PH 4.5 AND BLUE ABOVE PH 8.2.

COLOR BLUE LITMUS RED LITMUS

ACID TURNS RED STAY SAME

BASE STAY SAME TURNS BLUE

PH RANGE OF INDICATORS

INDICATORS

1. PHENOLPHTHALEIN
2. METHYL ORANGE
3. METHYL RED
4. BROMOTHYMOL BLUE
5. LITMUS

COLOR CHANGE

1. COLORLESS TO PINK
2. RED TO ORANGE
3. RED TO YELLOW
4. YELLOW TO BLUE
5. RED TO BLUE

PH RANGE

1. 8.3 - 10.0
2. 3.1 – 4.4
3. 4.4 – 6.0
4. 6.0 – 7.6
5. 5.0 – 8.0

WHAT IS END POINT?

IN ACID-BASE TITRATION THE BASE SOLUTION IS GRADUALLY ADDED FROM A BURETTE INTO AN ACIDIC SOLUTION IN TITRATION FLASK. WHEN THE AMOUNT OF BASE ADDED NEUTRALIZE TO THE AMOUNT OF BASE ADDED IN FLASK “ EQUIVALENCE POINT OR END POINT” WILL REACHED. THE END POINT OF TITRATION IS SHOWN BY COLOR CHANGES OF AN INDICATOR, PREVIOUSLY ADDED TO THE ACIDIC SOLUTION IN TITRATION FLASK.

REDOX INDICATOR

• OXIDATION + REDUCTION REACTIONS KNOWN AS REDOX REACTION .
• “A REDOX INDICATO IS AN INDICATOR COMPOUND THAT CHANGES COLOR AT SPECIFIC POTENTIAL DIFFERENCES”. OR “CHEMICAL COMPOUND THAT UNDERGOES A COLOR CHANGES AS IT GOES FROM ITS OXIDIZED FORM TO ITS REDUCED FORM”
• REDOX INDICATORS ARE PH INDEPENDENT.

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• THE COLORS THAT INDICATE THE PH OF SOLUTION, AFTER ADDING A UNIVERSAL INDICATOR ARE;

PH RANGE DESCRIPTION COLOUR

< 3 STRONG ACID RED

3-6 WEAK ACID ORANGE OR YELLOW

7. NEUTRAL GREEN

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8-11 WEAK ALKALI BLUE

> 11 STRONG ALKALI VIOLET OR INDIGO

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• A UNIVERSAL INDICATOR IS USUALLY COMPOSED OF
• WATER
• 1-PROPANOL
• PHENOLPHTHALEIN SODIUM SALT
• SODIUM HYDROXIDE
• METHYL RED
• BROMOTHYMOL BLUE MONOSODIUM SALT
• THYMOL BLUE MONOSODIUM SALT

BDH UNIVERSAL INDICATORS

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• A UNIVERSAL INDICATOR IS A PH INDICATOR MADE OF A SOLUTION OF SEVERAL COMPOUNDS THAT EXHIBITS SEVERAL COMPOUNDS THAT EXHIBITS SEVERAL SMOOTH COLOR CHANGES OVER A WIDE RANGE PH VALUE TO INDICATE HE ACIDITY OR ALKALINITY OF SOLUTION.

PRECIPITATION INDICATORS

• THOSE INDICATORS THAT CHANGES THEIR COLOR DURING PRECIPITATION REACTION ARE CALLED PRECIPITATION INDICATORS.
• IT IS ALSO CALLED AS ADSORPTION INDICATOR.
• FOR EXAMPLE;
• PHENOL RED
• DIFFERENT COLOUR IN OXIDIZED AND REDUCED FORM.
• COLOR OF INDICATOR DEPENDS UPON EMF/POTENTIAL DIFFERENCE

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• E.G;
• DIDPHENYL AMINES IS COLOURLESS IN ITS REDUCED FORM.
• WHEN ITS SOLUTION’S EMF CHANGES TO 0.76V , IT GETS OXIDIZED AN ATTAIN VIOLET COLOR.

PKA:-

DEFINITION:-

THE NEGATIVE LOG OF DISSOCIATION OF CONSTANT OF ACID (KA) IS CALLED PKA

PKA = -LOG [KA]

REASON OF PKA INSTEAD OF KA :-

THE VALUE OF KA FOR WEAK ACID IS VERY SMALL NUMBERS USUALLY EXPRESSED IN EXPONENTIAL FORM. SO IT IS CONVENIENT TO CONVERT THEM INTO WHOLE NUMBERS BY TAKING THEIR NEGATIVE LOG.

RELATIONSHIP B/W KA AND PKA :-

PKA ∝ 1/KA

PKA IS INVERSALY PROPORTIONAL TO KA.

LARGER THE VALUE OF PKA; WEAKER WILL BE THE ACID AND VICE VERSA.

THE DISSOCIATION CONSTANT OF ACID IS DEFINED BY:-

KA = [H3O+][A-]/[HA]………..(1)

THE CONJUGATE BASE A- BEHAVE AS A WEAK BASE IN WATER.

A- + H2O = HA + OH-

FOR BASE KB = [HA][OH-]/[A-]……..(2)

MULTIPLY EQUATION 1 & 2

(KA)(KB) = {[H3O+][A-]/[HA]}{[HA][OH-]/[A-]}=[H3O+][OH-]=KW

(KA)(KB) =KW

ON TAKING –VE LOG ON BOTH SIDES

-LOG KA -LOG KB = -LOG KW

PKA +PKB=PKW

FREQUENTLY ASKED QUESTIONS ON PKA

Q1: HOW DO CALCULATE PKA?

ANS: PKA VALUE IS THE –VE BASE -10 LOG OF AN ACID DISSOCIATION CONSTANT (KA) OF A SOLUTION.

PKA = -LOG[KA]

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Q2: HOW DOES PKA CHANGE WITH TEMPERATURE?

ANS: AN ENDOTHERMIC REACTION REQUIRED HEAT DURING THE REACTION. INCREASE THE DISSOCIATION CONSTANT(KA) WITH AN INCREASE IN TEMPERATURE. HIGHER THE VALUE OF KA LOWER THE VALUE OF PKA. HENCE PKA VALU DECREASES WITH INCREASING TEMPERATURE.

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Q3: IS HIGH PKA MORE ACIDIC?

ANS: NO, HIGHER PKA MEANS LOWER ACIDITY. SO THE HIHER THE PKA THE SMALLER KA, AND THIS MEANS A WEAKER ACID . HIGHER PKA INDICATES WEAKER ACID.

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Q4: WHAT IS THE PKA OF WATER AT 25C?

ANS: THE PKA VALUE OF PURE WATER AT 25C IS 14.