The Determination Of Blood Potassium Levels Biology Essay

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To determine the unknown concentration of potassium ions in the given blood samples by using ion selective electrode.

Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor, and which involve electron transfer between the electrode and the electrolyte or species in solution.

Electrochemical techniques are used to study the electrochemical properties of different materials and to test the applicability of new materials in batteries.

Electrochemistry deals with cell potential as well as energy of chemical reactions.

The energy of a chemical system drives the charges to move, and the driving force give rise to the cell potential of a system called galvanic cell. All these relationships are tied together in the concept of Nearnst equation.

The Nernst equation is used to calculate the voltage of an electrochemical cell or to find the concentration of one of the components of the cell.

The Nernst Equation

Ecell = E0cell - (RT/nF)lnQ

Ecell = cell potential under nonstandard conditions (V)

E0cell = cell potential under standard conditions

R = gas constant, which is 8.31 (volt-coulomb)/(mol-K)

T = temperature (K)

n = number of moles of electrons exchanged in the electrochemical reaction (mol)

F = Faraday's constant, 96500 coulombs/mol

Q = reaction quotient, which is the equilibrium expression with initial concentrations rather than equilibrium concentrations

Sometimes it is helpful to express the Nernst equation differently:

Ecell = E0cell - (2.303*RT/nF)logQ

at 298K, Ecell = E0cell - (0.0591 V/n)log Q

The Nernst equation works only in dilute ionic solutions

Ions of opposite charge tend to associate into loosely-bound ion pairs in more concentrated solutions, thus reducing the number of ions that are free to donate or accept electrons at an electrode.

So Nernest equation cannot predict the half cell potential of the solution whose ionic strength is more than 10-3 M.


At a high pH, the potential of the electrode was affected by other cations in addition to H+ ions, mainly in the case of sodium ions.

So there is increase in the sodium ion response and decrease in the hydrogen response. By this, the concentration of sodium ions in the solution can be determined.

Electrodes are also been developed for the determination of potassium, ammonium, silver ions etc.

All these electrodes are called as Ion selective electrodes. Ion selective electrodes are classified into 4 groups according to the type of membrane employed. They are

Glass electrodes

Solid state electrodes

Heterogeneous membrane electrodes

Liquid ion exchange membrane electrodes.


-These are cation selective only.

-These are used for the determination of potassium, ammonium and silver ions in addition to hydrogen and sodium ions.


-The membrane consists of a single crystal or a compacted disc of the active material.

-For example, a solid state electrode selective to fluoride ions employs a membrane of lanthanum fluoride [LaF3]

-There are also electrodes for the determination of Cl-, Br-, I-, Ag+, Cu2+, CN- ions.


-These are similar to solid state electrodes but differ in having the active material dispersed in an inert matrix.

-Electrodes are available for the measurement of Cl-, Br-, S2-, Ag+ inos.


-The internal reference solution and the experimental solubility are separated by an organic liquid of low water solubility.

-The ion of interest is incorporated in the larger molecules which are dissolved in the organic phase.

-The most important of these electrodes is the calcium electrode in which the ion exchanger is the calcium salt of didecylphosphoric acid dissolved in di-n-acetylphenylphosphonate.

-There are also other electrodes which are available for the determination of ions Clo4-, No3-, BF4- etc.


Standard solutions of potassium ions with concentrations of 1.0,2.0,4.0,6.0,8.0,10.0 mM were prepared as given in SMA0020.

Blood samples were prepared in the same manner by adding TISAB to make ionic strength for the ion selective electrode.

Switch on the mains power to the multimeter.

Voltage readings were to be noted down which were appeared in the voltmeter.

The electrodes should be washed and dried before each measurement.

Triplicate measurements were to be taken for each sample.

Graph 1 : Shows a Schematic Representation of Two Point Calibration Curve for 1 and 10 ppm against Potential( E), mV

Calculations :

From Two Point Calibration Curve:

Unknown Blood Sample 1

From the graph , Y = m x + c is equal to Y = 61.3 x + 196.6

Y = 253.75

From the above equation,

253.75 = 61.3 x + 196.6

253.75 - 196.6 = 61.3 x

57.15 = 61.3 x

x = 0.9323

Log x = 0.9323 ( x = log ai )

x = anti-log 0.9323

x = 8.55 mM

Unknown Blood Sample 2

From the graph , Y = m x + c is equal to Y = 61.3 x + 196.6

Y = 240.12

From the above equation,

240.12 = 61.3 x + 196.6

240.12 - 196.6 = 61.3 x

43.52 = 61.3 x

x = 0.7099

Log x = 0.7099 ( x = log ai )

x = anti-log 0.7099

x = 5.12 Mm

Graph 2: Shows a Schematic Representation Calibration Curve and Potential (E), mV

Calculations :

From Full Calibration Curve :

Unknown Blood Sample 1

From the graph, Y = m x+c is equal to Y = 61.126 x +196.9

Y = 253.75

From the above equation,

253.75 = 61.126 x + 196.9

253.75 - 196.9 = 61.126 x

56.85 = 61.126 x

X = 0.928

Log x = 0.928 ( x = log ai )

X = anti- log 0.928

X = 8.4 mM

Unknown Blood Sample 2

From the graph , Y = m x + c is equal to Y = 61.126 x + 196.9

Y = 240.12

From the above equation,

240.12 = 61.126 x + 196.9

240.12 - 196.9 = 61.126 x

43.22 = 61.126 x

X = 0.707

Log x = 0.707 ( x = log ai )

X = anti-log 0.707

X = 5.09 mM


8.55 - 8.4 = 0.15

5.12 - 5.09 = 0.03


All the solutions were at a constant ionic strength ,


E = Constant + 0.059 log ( k+ )

From this equation a calibration curve is plotted between E and log(k+) and from this graph unknown potassium levels can be determined.


Normal human blood potassium levels are 3.5-5.5 mM / lit.


For unknown blood sample-1, have more potassium levels in blood ( 8.40-8.55 ).

So sample-1 person was suffering from hyperkalemia.

For unknown blood sample-2, have normal potassium levels in blood (5.09-5.12 )


Normal levels of sodium ions in blood are 135-146 mM/lit

Variation in sodium levels can be estimated by using ion selective electrode i.e Sodium sensitive electrodes such as precipitate electrode because the order of selectivity is Ag+>H+>Na+>>,

Li+>.........Ca2+ and these are generally used for anion measurement, but they are generally slow responding and subjected to poisoning.


From this experiment i concluded that given unknown blood sample-1 is having hyperkalemia and the unknown blood sample-2 is having normal levels of potassium ions.

These potassium levels vary in humans depending upon various factors.

The aim of the practical had been met by determining the potassium levels in unknown blood samples and it had been found that sample-1 is having hyperkalemia.