Use of colorimeter for the determination of the concentration of a solution

Absorbance is important in determining concentration of a substance in a sample through colorimeter analysis. Colorimeter measures the intensity of colour and light transmittance by the sample to achieve the concentration. When a beam of light passes through a coloured solution, the amount of light absorbed depends on the nature of the molecules absorbing the light, their concentration and thickness (path length) of the solution. The ratio of transmitted intensity to original intensity s known as the “transmittance”, T.

Transmittance (T) = I/I₀

I = intensity of the transmitted light

I₀ = intensity of incident light

The Beer- Lambert law states that there is a logarithmic dependence between the transmittance and the absorbance. Therefore the transmittance is expressed in terms of absorbance;

Absorbance (A) = -log₁₀ T

                             = -log₁₀ (I/I₀)

According to this, the absorbance becomes linear with concentration considering;

A = ℰ ℓ C

ℰ = Molar absorbance coefficient

ℓ = path length

C = concentration of the solution

Therefore in dilute solution,

A = -log₁₀ (I/I₀) = ℰ ℓ C

Molar absorbance coefficient indicates the absorbance under a standard set of conditions, i.e. the light travelling 1cm through a solution of 1moldm^-3. In a material with a low absorption coefficient, light is poorly absorbed and vise versa. This depends on the material and on the wave length of the light.

When using the colorimeter the path length i.e. the width of the glass cell is constant. Also the concentration of one solution used at one specific wave length. Therefore ℰ is also constant through the measurements. This shows out clear relationship between the absorbance and the concentration.

A ∝ C as ℓ and ℰ are constant

The glass cell/ container with plane parallel faces are transverse by monochromatic radiation in the colorimeter. If the glass cell is filled with non absorbing solution, there is 100% transmittance; therefore the absorption would be zero.

Colorimeter applies only in relation to the visible region. Also Beer- Lambert law is applicable for 0.800-0.200 absorbances.

In the experiment, firstly the absorbance reading of the colorimeter should be zeroed using distilled water as distilled water is used to prepare the solutions. 

Also before taking the measurements of the absorbance value in each solution, the glass cell should be washed with distilled water in order to prevent interferences to the reading. It is important not to touch the two smooth surfaces of the glass cell and wipe out the additional drops remain on the surfaces of the glass cell, using a tissue. Otherwise the beam of the radiation would be scattered incorrectly and interfere the accuracy of the reading.

When filling the cell, air bubbles should not be remained inside the cell as it would decrease the absorbance value.

When refilling a glass cell with a different solution, small amount of the new solution should be used to rinse the cell before filing as it would give more accurate results.

Spectrophotometer also uses a monochromatic light to pass though a solution and measure its absorbance. The principle of spectrophotometer and colorimeter is same but a colorimeter can only use one wavelength at a time and have a fixed number of wavelengths that can be used. Also they have to be in visible range only.

A spectrophotometer on the other hand can not function like a colorimeter but take a spectrum of a solution across the entire wave spectrum especially in UV – IR. Therefore use of spectrophotometer is beneficial than a colorimeter and useful to determine concentration of unknown solutions.

Applications of conductivity measurements: Calculating the solubility product of a partially soluble salt (Lab report)

Theory

The solubility of poorly soluble salts is expressed as the solubility product. That is the product of the concentration of ions in the solution which are in equilibrium with the solid ion. These concentrations can be determined via conductivity measurements. In this practical PbSO₄ is used.

                                 PbSO₄ ⇌ Pb²⁺ + SO₄^2-

Assume concentration of solid is a constant.

Concentration of Pb²⁺= concentration of SO₄^2- = C

                             \  Ksp = [Pb²⁺_aq] [SO₄^2-_aq]

                                    Ksp = C²

The measurement of the specific conductivity, K of the saturated solution leads to a value of the concentration.

                      i.e.       C = K/ Λ₀

                      As,   Ksp = C²

                               Ksp = (K/ Λ₀)²

 Λ₀ for PbSO₄ = 3.02 × 10^-2 m²Smol^-

Λ₀ = Molar conductivity at infinite dilution

Procedure

About 2g of finely powdered PbSO₄ was measured using the electric balance into a clean, dry watch glass.

It was added to a clean beaker. The watch glass was washed with distilled water and that washed water was also added to the beaker.

To remove impurities it was shaken by adding distilled water. It was kept aside for a while, for the salt to be collected at the bottom of the beaker. Then the upper layer was decanted and repeated this step for several times.

An empty reagent bottle was kept in a water bath at 24⁰C in order to maintain the temperature of the reagent bottle at 25⁰C.

Then the well- washed salt was added to the reagent bottle and added 100mL of distilled water.

After awhile without disturbing the solid, the clear upper solution was decanted into a clean beaker and the conductivity was measured.

Results

K of distilled water at 25⁰C = 4.2 µS/cm

K of PbSO₄ at 25⁰C = 39.2 µS/cm

Calculations

Corrected conductivity of PbSO₄ = (39.2 – 4.2) µS/cm

                                                           = 35.0 µS/cm

                                                                  = 35.0 × 10^-2 Sm^-

C = K/ Λ₀

C = 35.0 × 10^-2 Sm^- / 3.02 × 10^-2 m²Smol^-

C = 0.11589 molm^-3

C = 0.11589 × 10^-3 moldm^-3

PbSO₄ ⇌ Pb²⁺ + SO₄^2-

Ksp = [Pb²⁺_aq] [SO₄^2-_aq]

Ksp = C²

Ksp = (0.11589 × 10^-3 moldm^-3)²

         = 1.3430 × 10^-8 mol² dm^-6

Conclusion

Exact given Ksp at 25⁰C is 1.6 × 10^-8 mol² dm^-6. Practically obtained Ksp at 25⁰C is 1.3430× 10^-8 mol² dm^-6

As the both values are closer we can use conductivity measurements in order to calculated KSP.

Discussion

The precipitate was washed in order to remove impurities that may affect the results. It should be washed several times to obtain a well- washed precipitate. During this process, precipitate can be also removed. Therefore all 2g of the sample wouldn’t be left.

When maintaining the temperature  of 25⁰C, the water bath was kept at a temperature about 24⁰C. this is because we measure the temperature of the water bath assuming the temperature inside the reagent bottle is at 25⁰C. But actually the temperature of the solution inside would be slightly higher than the temperature of the water bath as the glass of the reagent bottle separates both. Therefore the temperature of the water bath was kept at lower temperature to obtain the required temperature for the solution.

Reagent bottle should be shaken time to time when it was kept in the water bath. This is to increase the dissociation and reach the equilibrium. If not, we might have not measured the conductivity at the equilibrium and that will alter the true Ksp value.

The calculated Ksp value is slightly smaller than the exact given Ksp value due to errors occurred during the experiment. Mainly the personal errors. Such as temperature maintaining errors, measuring errors and personal carelessness.

When preparing the solutions, the distilled water was added. Distilled water also contains ions it self. Therefore additional conductivity from the distilled water would be raised in the solution. For this reason the conductivity of the solution is corrected by deducting the conductivity of water. 

Physiology of Synapse:- Excitatory synapses and Inhibitory synapses

Synapse is simply a junction between two neurons. There are two types of synapses in central nervous system.

·         Excitatory synapses

·         Inhibitory synapses

Excitatory synapses

Pre synaptic knob secretes as excitatory substance. (Ex: Acetyl choline, Noradrinalin, Gutamic acid)

Transmission:

Nerve impulse à pre synaptic knob releases acetyl choline (Ach)àAch diffuse across synaptic cleftàAch combine with receptor sites of post synaptic membraneàpermeability of post synaptic membrane changesà permeability to Na⁺ increases and Na⁺ enter the post synaptic knobà Excitatory post synaptic potential (EPSP) is obtainedàthreshold potential is reachedà action potential is formedà distribution of action potential

A combined EPSP depolarize the membrane to a threshold value to generate an action potential is known as summation. Summation can occur in two ways as temporal summation which is obtained by repeat discharge of single pre synaptic knob and spatial summation which is obtained by numerous pre synaptic knobs.

Inhibitory synapses

Pre synaptic knob secretes an inhibitory substance. (Ex: alpha butric acid and glycine)

This inhibitory synapses function in two ways as post synaptic inhibition and pre synaptic inhibition.

Post synaptic inhibition

Nerve impulseà pre synaptic knob release an inhibitory substance (GABA)àGABA diffuse across synaptic cleftà GABA binds with receptor sites on post synaptic membraneà permeability of post synaptic membrane changesà permeability to K⁺ and Cl^- ions increaseà K⁺ leaves and Cl^- ions enteràInhibitory post synaptic potential (IPSP) is obtainedà membrane hyperpolarizedà no action potential initiated

Pre synaptic inhibition

Nerve impulseà pre synaptic knob release inhibitory substancesàhyperpolarize the pre synaptic  excitatory membraneà prevent excitatory substances releasingà no excitation occurs on post synaptic membrane     

Sequence of events take place during action potential in brief

>In resting state, membrane is permeable to K⁺ and relatively impermeable to Na⁺.

>When a stimulus depolarize the membrane into the threshold value the voltage gated Na⁺ channels open, vastly increasing the membrane’s permeability to Na⁺.

>Na⁺ enters the cell across the membrane under the influence of both concentration gradient and the electric gradient.

>Na⁺ makes the membrane more depolarized and that makes more voltage gated sodium channels to open.

>As the membrane potential approaches equilibrium potential of Na⁺ the driving force of Na⁺ is reduced. Therefore less number of Na⁺ reach into the cell.

>After a short time the voltage gated Na⁺ channels are inactivated and stop the taken in of the Na⁺.  Here the membrane potential rise to +45mV.

>As this occurs, the voltage gated K⁺ channels open and greatly increases the permeability to K⁺ ions. Therefore K⁺ leaves the cell under the influence of concentration gradient and electric gradient.

>As K⁺ leaves, the positive charge inside the cell is reduced. After awhile the equilibrium potential of K⁺ is obtained and the membrane is hyperpolarized. Now the membrane potential is -75mV. Hyperpolarization is due to the delay of closing the K⁺ channels compared to Na⁺ channels.

>The voltage gated K⁺ channels close and by then the Na⁺ channels recovered by inactivation.