OUR LADY OF FATIMA UNIVERSITY COLLEGE OF PHARMACY

QUALITY CONTROL 2

REFRACTOMETRY POLAROGRAPHY

Topic 11 (Part 2)

ACTIVITY FOR WEEK __

1. Online Lecture Topic: Polarography, Refractometry Duration: __ hours

UNIT OUTCOME:

At the end of this unit, the students are expected to:1. Understand the concept and principles of Refractometry and Polarography2. Describe the instrument used in the official assay of Refractometry and Polarography

UNIT OUTLINE:

à Principle of Refractometry à Parts of Refractometer à Principle of Polarography à Part of instruments used in

Polarography

CHECKLIST

à Read unit outcomes.à Read study guide prior to class attendance.à Read required learning resources; refer to unit terminologies for jargons.à Participate in the discussion board (Canvas).

REFRACTOMETRY

REFRACTOMETRY

vIs a technique that measures how light is refracted when it passes through a given substance (an unknown compound) and the amount by which the light is refracted determined the refracted index

vA refractometer is the instrument used to measure refractive index (“RI”). Although refractometers are best known for measuring liquids, for quick e v a l u a t i o n o f c o n c e n t r a t i o n o f d i s s o l v e d substances.

Index of refraction is a physical constant frequently made use of it in the determination of the identity and purity of drug products.

It may used to determine quantitatively the strength and purity of solutions or the portions in which liquids are mixed.

e.g., the percentage of alcohol in water Determine the concentration of a solute in a solution by comparing

the solution's refractive index to a standard curve.

Refractive index is defined as the ratio of the velocity of light in air to the velocity of light in the medium being measured:

Whenever light changes speed as it crosses a boundary from one medium into another its direction of travel also changes

Snell's law (also known as Snell–Descartes law and the law of refraction) is a formula used to describe the relationship between the angles of incidence and refraction.

SNELL’S LAWV2 V1---- = ----Sinθ2 sinθ1

Refractive index is temperature and wavelength dependent

Refractive index (n) is determined at temp of 20°C but Pharmacopoeial standard temp is 25°C

To adjust for temperatures greater than 20°C, Use the following formula:

n20 = nT + 0.00045(T - 20oC)

Critical angle – the maximum value of refraction The angle of refraction when angle of incidence

equals 90°Refractive index is temperature dependent. Combining refractive index and substance density it is

possible to define a quantity that is temperature independent. It is called specific refraction:

n2 – 1r = ---------- d (n2 +2)

where r - specific refraction, d - density, n - refractive index

Types of refractometer

ABBE REFRACTOMETER

IMMERSION OR DIPPING

REFRACTOMETER

PULFRICH REFRACTOMETER

ABBE REFRACTOMETER

vLight refraction through liquids to determine the amount of dissolved solids in liquids by passing light through a sample and showing the refracted angle on a scale

vRI of the prism should be greater than that of the sample.v In abbe refractometer, the RI can be read directly,

only a few drop of the liquid are needed, and either white or monochromatic light can be used.

vThis refractometer consist, mainly a telescope and two matched right angle prism

ABBE REFRACTOMETER

v The liquid is placed in contact with the prisms.

v Many refractometers are equipped with a thermometer and a means of circulating water through through the refractometer to maintain a given temperature.

v Most of the refractive index measurements reported in the literature are determined at 20 or 25 °C

v Operation consist of placing 1 or 2 drops of the water sample on the prism, closing a glass plate over the sample, then looking through the eyepiece for the reading.

Operating the Bausch & lomb abbe-3l refractometer

Light source in Abbe’s Refractometer

I- Natural lightII- C- Line of hydrogen lamp (653.6 nm)III- D-Line of sodium vapor lamp (589.3nm)IV- F-Line of hydrogen lamp (486.1 nm)V- G-Line of mercury vapor lamps (435.8 nm)

Immersion refractometer

vThe immersion or dipping refractometer is basically an Abbe type refractometer of short range and without an illuminating prism.

vThe dipping refractometer is used primari ly for determining solution concentrations, such as in the sugar industry, pharmaceuticals and milk. It is particularly useful for acids since only the inert prism dips into the solution.

v I m m e r s i o n r e f r a c t o m e t e r s a l w a y s w o r k a t t h e temperature of the l iquid, so they need bui l t in temperature compensation.

Pulfrich refractometervPulfrich refractometer is the design (after Abbe

Refractometer) that became commercially available in 19th century. It is another type of critical angle refractometer,

vThe pulfrich refractometer is actually only as accurate as a standard abbe (one unit in the 4th decimal place) in absolute determination

However, it can give measurements to 2-3 units in the 5 t h place for differential measurements and for dispersion determination.

Intrumentation

Abbe refractometer has the following parts:

Calibration is done using water as standard with n value of 1.3330 at 20° and 1.3325 at 25°C

This is a view of the front of a refractometer.

REFRACTOMETRY

• Refractive index (n) is determined at temp of 20°C but Pharmacopoeial standard temp is 25°C– To adjust for temperatures greater than 20°C, Use the

following formula:

nD20 = nD

T + 0.00045(T - 20oC)

QuestionsIndex of refraction determined is 1.235, what is the

corrected n if the determination was performed using 26.5°C?

nD20 = nD

T + 0.00045(T - 20oC)

Lorenz – Lorentz Equation

R = r x MW n2 – 1

r = ---------- d (n2 +2)

Carbon tetrachloride has a density of 0.916 g/ml at 25°C. What is its specific refraction?

n2 – 1r = ---------- d (n2 +2)

The molecular weight of propionic acid, C2H5COOH, is 74.08 g/mole, its density (at 20oC) is 0.9930 grams per cubic centimeter; its index of refraction is 1.385;.

The molecular weight of monomethyl aniline, C6H5NHCH3, is 107.15 g/mole; its density is 0.987 grams per cubic centimeter; its index of refraction is 1.5704;

The molecular weight of water, H2O, is 18.01528 g/mole; its density is 1.000 grams per cubic centimeter; its index of refraction is 1.333. Calculate its molar refraction(R).

n2 – 1r = ---------- d (n2 +2)

R = r x MW

TABLE OF ATOMIC AND GROUP REFRACTION ATOM R (mL.mol-1)H 1.1

C 2.418

C (C=C) 1.733

C (C=C) 2.398

O (-OH) 1.525

O (-CO-) 2.21

O (-O-) 1.643

Cl 5.967

Br 8.748

I 13.90

3- membered ring 0.71

4- membered ring 0.48

1. Calculate the molar refraction for A. Phenol (C6H5OH)B. Napthalene (C10H8)C. Butanal (C4H8O)

VIDEO LINKS

https://www.youtube.com/watch?v=Ig9LFQwJGVE (Polarography) https://www.youtube.com/watch?v=y55tzg_jW9I (Refractometry)

REFERENCES

à Jenkins, Glenn L, Jenkins Quantitative Pharmaceutical Chemistry, N.Y, N.Y. : Mc Graw, Latest Edition, (Chapter in Chromatography) à Watson, David G., Pharmaceutical Analysis: A Textbook for Pharmacy Students & Pharmaceutical Chemist, Churchill Livingstone, Edinburgh, 2012 (Chapter in Chromatography)

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