pH notation and Buffer solution, Chemistry tutorial

Definition of pH:

pH can be simply be stated as the potential of hydrogen in a system. It exhibits the extent of acidity and basicity of a system under investigation.

Mathematically, pH is stated by sir Sorensor as the negative logarithm to base 10 of [H+].

That is, pH = - log [H+].

The only source of H+ is as an outcome of dissociation of water in which equivalent concentration of H+ and OH- are products.

H2O ↔ H+ + OH-

Water experiences self ionization termed as autoprotolysis in which, it acts as both acid and base.

H2O + H2O ↔ H3O+ + OH+

The autoprotolysis constant is equilibrium constant, KW, termed as the ionic product of water.

Kw = [H+][OH+]

This is for all time equivalents to 1.0 x 10-4 at 25oC

Taking log of both side, provides

log Kw = log [H+] + log [OH-]

- log Kw = - log [H+] + - log [OH-]

- log Kw = pH + pOH

pKw = pH + pOH

However at 25oC, pH = 7 and pOH= 7 

Therefore, - log Kw = 14.00

That is, log10 = -14 or KW = 10-14

NB: The (-) negative sign is an indication that, concentration encountered is less than 1.0 M.

pKw = pH + pOH = 14.00

pH Scale:

A universal pH scale place the pH of most solution in the range 0.0 - 14.0. This must not provide an illusion that it is not possible to have negative pH (- pH). It simply means that the concentration hydrogen is more than 1M. This is attainable in concentrated solution of strong acid like HCl and H2SO4.

503_pH Scale-Acidity and Alkalinity.jpg

Fig: pH Scale-Acidity and Alkalinity

NB-N= Neutrality

Measurement of pH:

pH is measured by a glass electrode. A glass electrode is the ion-selective electrode. This is extremely sensitive. The pH sensitive portion of the electrode is thin glass membrane which culminates in the slope of a bulb.

Most of the metal cation is positioned in the hydrated gels area of the metal membrane. These metals diffuse to the solution and are substituted by H+ in the solution in ion-exchange equilibrium. The more the H+ in solution, the more H+ ions are bound to the surface of glass.  The operation of glass electrode is symbolized as:

E = Constant + β (0.05915 log AH+ (outside)/AH+ (inside) at 25oC

Here 'β' is the electromotive efficiency = 1.00 or 0.9 8 and constant is termed as asymmetry potential.

pH at Elevated Temperature:

The major factor which finds out the real pH of a system is temperature. At high temperatures in a fluid system, the pH turns out to be high. Not only does pH influence the ionization of water in the body system, it as well changes the pH of a neutral solution to around 7.4, thus influencing the ionization constant of the acid and bases from which the buffer system is derived.

Buffer Solution:

A buffer solution can be stated as a solution which resists changes in pH whenever small amount of acid or base is added or whenever dilution takes place. Buffer solution consists of a mixture of a weak acid and its salt or a weak base and its salt.

Consider the acid HA undergoing dissociation under the equilibrium condition:

HA ↔ H+ + A-

Ka = [H-] [A-]/[HA]

log Ka = log [H-] [A-]/[HA] = log [H+] + log [A+]/HA

- log H+ = log Ka + log [A+]/[HA]

pH = PKa + log [salt]/[Acid]

The above is the Henderson- Hasselbalch equation

Buffer Capacity:

This is simply stated as the maximum amount of an acid or base which can be added to a buffer system devoid of causing a change or appreciable change in the pH of a system.

It will be noted that the buffering procedure for a mixture of a weak acid and its salt can be described as governed by log ratio of salt and acid.

pH = constant + log [A-]/[HA]

Whenever a solution is de-ionized, the ratio of pH remains constant. If a small quantity of strong acid is added, it will join with an equivalent amount of A- to convert it to HA.

The ratio of [A1] [HA] is small; therefore the change in pH is small, whereas if strong base is added it combines with [HA] to form an equivalent amount of A-. Again the change of pH is small.

Tutorsglobe: A way to secure high grade in your curriculum (Online Tutoring)

Expand your confidence, grow study skills and improve your grades.

Since 2009, Tutorsglobe has proactively helped millions of students to get better grades in school, college or university and score well in competitive tests with live, one-on-one online tutoring.

Using an advanced developed tutoring system providing little or no wait time, the students are connected on-demand with a tutor at www.tutorsglobe.com. Students work one-on-one, in real-time with a tutor, communicating and studying using a virtual whiteboard technology.  Scientific and mathematical notation, symbols, geometric figures, graphing and freehand drawing can be rendered quickly and easily in the advanced whiteboard.

Free to know our price and packages for online chemistry tutoring. Chat with us or submit request at info@tutorsglobe.com