PH MEASUREMENT AND BUFFER PREPARATION Flashcards
measure of how acidic/basic a solution is.
pH
0 to 14.
range
pH < 7
acidic
pH = 7
neutral
pH > 7
basic
potential of hydrogen
ph
measure of the hydrogen ion (H+) concentration in an aqueous solution
pH
expressed as the negative logarithm of the hydrogen-ion concentration.
pH = - log [H+]
pH
potential of Hydroxide
pOH
measure of hydroxide ion (OH-) concentration
pOH
expressed as the negative logarithm of the hydroxide-ion concentration.
pOH = - log [OH-]
pOH
important in organism and their cells because chemical reactions and processes are affected by the hydrogen ion concentration.
control of pH
requires a pH between 7.8 and 8.4
Nemo (Clownfish)
compound that can donate a hydrogen ion.
acid
substance that accepts hydrogen ions.
base
Hydrogen ions
low ph/ many hydrogen ions
ACIDS
(hydroxide ions)
high ph/ many hydroxide ions
ALKALIS
Chemists have tried to define acids and bases in relation to their compositions and molecular structures.
Background
defines acids as substances that produce H+ ions in aqueous solution while bases are substances that produce OH- ions in aqueous solution.
Pronounced as “svaan-tay”
Svante Arrhenius
Mentioned that acids are electron-pair acceptors and bases are electron-pair donor.
However, the two mentioned definitions have limitations. Thus, the most useful and accepted definition of acids and bases nowadays are those proposed by Johannes Bronsted and Thomas Lowry, and it is known as the Bronsted-Lowry theory.
Gilbert N. (G.N.) Lewis
- HCI (Hyrochloric acid) is an acid because it donates a proton making Cl- (Chloride) while water is a base because it accepts a proton making H30+.
- Furthermore, the theory explains that for every acid-base reaction, there is a creation of conjugate acid-base pair.
- In the above example Cl- is the conjugated base of HCl and H30+ is the conjugated acid of water as shown below.
Rationale:
introduced the pH scale that measures the strength of an aqueous acidic or basic solution.
It converts the H+ concentrations to pH using the formula.
pH = - log [H+]
Seren Peter Lauritz Sorensen
(S.P.L.) Sorensen
is “the negative common logarithm of the activity of hydrogen ion in solution
precise definition of pH
The pH of a solution is the negative logarithm of the hydrogen-ion concentration.
pH = -log|H+]
Computation of pH & [H+]
compute for the pH of pure water with [H+] = 1.0 × 10 -7 M (Molarity).
pH = -log|H + ]
= - 10g|1.0 × 10 -7]
= -(-7.00)
pH = 7.00
compute for the pH of pure water with [H+] = 1.0 × 10 -7 M (Molarity).
pH = -log|H + ]
= - 10g|1.0 × 10 -7]
= -(-7.00)
pH = 7.00
Compute for the pH of solution with [H+] = 2.3 × 10-5 M
pH = -log[H + ]
= - 10g|2.3 × 10 -5]
pH = 4.64 (Acidic)
compute for the [H+] if pH is known using this formula:
[H+] = 10^-рН
*Use the 10^x key on your calculator
What is the [H+] of a solution with a pH of 9.14?
[Н+] = 10^-рН
= 10^-9.14
[H+] = 7.24 × 101-10 M
What is the [H+] of a solution with a pH of 5.4?
[H+] = 10^-pH
=101-5.4
[H+] = 3.98 × 101-6 M
The pOH of a solution is the negative logarithm of the hydroxide-ion concentration.
рОН = -log[OH-]
[ОН-] = 10^-рОН
*Use the 10^x key on your calculator
Constant: pH + pOH = 14
Computation of poH
The pOH of a solution is the negative logarithm of the hydroxide-ion concentration.
рОН = -log[OH-]
[ОН-] = 10^-рОН
*Use the 10^x key on your calculator
Constant: pH + pOH = 14
Computation of poH
A solution has a pH of 4. Find the pOH and [OH-].
pH + pOH = 14
рОН = 14 - рН рОН = 14 - 4
рОН = 10
[ОН-] = 10^-рОН
= 10^-10
[OH-] =1 × 10^-10 M
Find the pOH and [OH-] of a solution with a pH of 8.4.
pH + pOH = 14
рОН = 14 - рн рОН = 14 - 8.4
рОн = 5.6
[ОН-] = 10^-рОН
= 10^-5.6
[OH-] = 2.5 × 10^-6 M
If pH > 7 = Basic
, <7 = Acidic
If pOH >7 = Acidic
, <7 = Basic
7 = Neutral
Interpretation of Results
prevent changes in pH.
Buffers
resist changes in the pH even when acids or bases are added.
Buffers
are a mixture of a weak acid or alkali and one of its salts. Ex: acetic acid + sodium acetate
Buffers
The ability of buffers to resist large changes in pH is governed by
Le Chatellier’s principle.
A principle of equilibrium shift due to changes in buffer conditions.
La Chatellier’s Principle
In our blood, carbonic acid is the most important buffer.
This solution maintains our blood pH to facilitate transport of oxygen from the lungs to the cells.
Buffers in Human Blood
normally slightly basic (7.35 to 7.45)
pH of blood
simple test to check if a substance is acidic or basic using a litmus paper.
Litmus Test
types of litmus paper available that can be used to identify acids and bases
red litmus paper and blue litmus paper
turns red for acidic pH
Blue litmus paper
turns blue for basic pH
Red litmus paper
No color change
neutral pH
impregnated with organic compounds that change their color at different pH values.
The color shown by the paper is then compared with a color standard usually provided by the manufacturer.
Indicator Paper
impregnated with organic compounds that change their color at different pH values.
The color shown by the paper is then compared with a color standard usually provided by the manufacturer.
Indicator Paper
should be calibrated first before being operating the device. The standard procedure for calibrating a pH meter is to calibrate it at three different pHs (pH 7, pH 4, and pH 10).
After calibration, all that needs to be done is to insert the electrodes of the pH meter into the solution to be tested and read the pH flashed on the screen.
pH Meter
an indicator that can be used to determine the pH value of a solution
pH PAPER
an indicator that is used to determine the acidity or the basicity of a solution
LITMUS PAPER
Can give the exact pH value of a solution
pH PAPER
Cannot give the exact pH value of a solution
LITMUS PAPER
Give more precise results
pH PAPER
Give a less precise indication
LITMUS PAPER
Can give a range of colors
pH PAPER
Can give only two colors
LITMUS PAPER
