The Significance of pH🧪 Flashcards by Heather McAinsh

Where are Buffers important?

In living systems

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State 2 functions of buffers in the body?

-Maintain pH of the body within normal pH ranges
-Ensure enzymes and other cellular processes function optimally

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What are the normal pH values of the following substances in the body?

Blood?
Urine?
Gastric Fluid?
Saliva?

Blood- 7.4
Urine- 6.0 (range 4.4-8.0)
Gastric Fluid- 2.0
Saliva- 6.2- 7.2

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Name the two major inorganic buffers in the body and their function

Phosphate- Buffer Reserve in Skeleton
Bicarbonate- Major Buffer in Human Body

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Name an organic buffer in the body

Macromolecules i.e. amino acid side chais in proteins- can also act as buffers

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Name an organic buffer in the body

Macromolecules i.e. amino acid side chais in proteins- can also act as buffers

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Many molecules in biological Systems are what?
And what are their ionic properties Influenced by?

Many molecules in biological systems are weak acids and/or weak bases

Their ionic properties are influenced by their hydrogen ion concentration [H+]

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How would you calculate the pH of a STRONG ACID?

pH= -log[H+]

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What happens when acids dissociate?

They release [H+] ions

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What is an acid?

A substance that will dissociate in water to produce [H+] ions

-PROTON DONATOR

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What is a base?

A substance which in water tends to combine with [H+] ions

-PROTON ACCEPTOR

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Write an equation to demonstrate the dissociation of acids

A <——–> H+ + B-
acid conjugate base

In water;

HA + H20 ———> H30+ + A-
acid water con. acid con.base

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A ———> H+ + B-

This equilibrium reaction always involves what?

A conjugate acid-base pair, comprising a hydrogen ion (acid) donor and an acceptor (base)

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For the dissociation constant of an acid, how is the forward reaction balanced?

By the reverse reaction

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A <——–> H+ + B-
Describe the concentrations of A, H+ and B- when an equilibrium is established?

The ratio of the concentrations of H+ and B- to A is constant when an equilibrium is established

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What equation is used to calculate the acid dissociation constant?

For A <——–> H+ + B-

Ka= [H+][B-]
————
[A]

A <——–> H+ + B-

Describe what happens to the value of K as the strength of the acid increases

As the strength of the acid increases, the higher the degree of dissociation so the value of K increases as the reaction is pushed to the right, producing more conjugate base

What is the Henderson- Hasselbalch Equation?

How would you calculate pKa if given the value of Ka

pKa = -logKa

Describe the relationship between acid strength and Ka

Directly Proportional
-As the strength of the acid increases so does the value of Ka
-The larger the value of Ka the smaller the pH

Describe the relationship between acid strength and pKa

-Inversely Proportional
-As the strength of the acid increases the value of pKa decreases
-The larger the value of pKa the higher the pH

What are indicators?

Weak acids or bases which have a distinct colour difference between their ionised form (In-) and their unionised form (HIn).

e.g. HIn- colourless
In- - red

The degree of dissociation and pH of indicators are related by what equation?

pH= pKa + log [In-]
——
[HIn]

Why is very little indicator added to solutions?

So that the pH of the solution is unaffected

How is the ratio of [In-]/[HIn] determined?

By the pH

What determines the colour of the indicator?

The ratio of [In-]/[HIn]

How can you measure the pH of a solution based on the colour of the indicator solution?

When either the ionised, or unionised form of the indicator is coloured, the depth of colour for the given amount of indicator can be used to measure pH

What is an electrical method of measuring pH?

-pH meter

Describe what a pH meter is

-Measurement made using a glass electrode -Glass electrode- thin walled glass bulb containing a platinum electrode immersed in 1M Hal

What do buffer solutions do?

Resist changes in pH when acid or base is added

What do buffer solutions consist of?

A conjugate acid-base pair of either a weak acid or weak base

Buffering systems in the body play a vital role in what?

Maintaining pH homeostasis

What does buffering in the oral cavity limit?

pH changes caused by acid production by plaque bacteria

Using the Henderson Hasselbalch Equation; pH = pKa- log [Acid] -------- [Base] When is optimum buffering capacity?

-It can be shown that buffering is most effective when [Acid]= [Base] - [A]=[B] so log(1)=0 hence pH=pKa -Buffers operate optimally at their pKa ie when HALF DISSOCIATED (as [A]=[B]) -At optimum buffering capacity pH=pKa

Describe how plaque bacteria results in dental caries

-Plaque Bacteria ferment dietary carbohydrates to produce acid -This causes plaque pH to fall -Leading to increased solubility of Dental Enamel -Which if not reversed leads to Dental Caries

Describe how Saliva prevents Dental Caries

-Saliva acts as a buffer and neutralises the acid produced by plaque bacteria and thereby prevents dental caries (if buffering is adequate)

Where is bicarbonate an important buffer?

Extremely important buffer in body fluids such as blood

What proteins does saliva contain?

Unusual Proteins that are histadine rich and are known as histatins; these too are important buffers in saliva

What is Histadine?

An amino acid which is very good at acting as a buffer