Phy Phar Prelim Notes

7/26/2019 Phy Phar Prelim Notes

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PHYSICAL PHARMACY LABORATORY: PRELIM

EXPERIMENT 1: SPECIFIC GRAVITY DETERMINATION OF LIQUIDS

The specific gravity of a liquid of known weight and volume may be calculated by the equation:

If 54.96 mL of an oil weighs 52.78 g, what is the specific gravity of the oil?

54.96 m of water weighs 54.96 g

PYCNOMETERor !pecific "ravity #ottle

$pycnometeris a special glass bottle used to determine specific gravity.

%ycnometers are generally available for laboratory use in volumes ranging from &

m to 5' m.%ycnometers have fitted glass stoppers with a capillary opening to

allow trapped air and e(cess fluid to escape. !ome pycnometers have thermometers

affi(ed in order to relate the specific gravity) as determined) with temperature.

*n using a pycnometer) it is first weighed empty and then weighed again when

filled to capacity with water. The weight of the water is calculated by difference.

!ince & g of water equals & m) the e(act volume of the pycnometer becomes known. Then) when any other

liquid subsequently is placed in the pycnometer) it is of e!al vol!meto the water) and its specific gravity may

be determined.

" 5# mL pycnometer is fo!n$ to weigh %2# g when empty, %7% g when fille$ with water, an$ %6# g when

fille$ with an !n&nown li!i$. 'alc!late the specific gravity of the !n&nown li!i$.

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HYDROMETER

$ hydrometer is an instrument used to measure the density of a liquid as compared to that of water.

+ydrometers usually consist of a calibrated glass tube ending in a weighted glass sphere that makes the tube

stand upright when placed in a liquid.

The greater the density) the tighter or closer the molecules are packed inside the substance. Therefore) the greater the density , specific gravity of a liquid the higher a hydrometer will be buoyed by

it.

-ill your hydrometer ar about / with the liquid you wish to test. *nsert the hydrometer slowly. Do not drop it

in 0ow give it a spin with your thumb and inde( finger) this will dislodge any bubbles that may have formed.

1nce the hydrometer comes to a rest) observe the plane of the liquid surface. 2our eye must be !ori"ont#$to

this plane. The point at which this line cuts the hydrometer scale is your reading.

T2%3! 1- +213T3:

Sp%&i'i& Gr#(it)hydrometers can be used for almost any liquid. !pecific "ravity is a dimensionless unit

defined as the ratio of density of the material to the density of water. *f the density of the substance of

interest and the reference substance 7water8 are known in the same units 7e.g.) both in g,cm or lb,ft8)

then the specific gravity of the substance is equal to its density divided by that of the reference substance

7water & g,cm8

*#+,%hydrometers are calibrated to measure specific gravity on evenly spaced scales; one scale is for

liquids heavier than water) and the other is for liquids lighter than water. These two scales) one for liquids lighter than water and one for liquids heavier than water) were

developed by the -renchchemist$ntoine #aum. *t is widely used in industrial

chemistry)pharmacology)sugar refiningand other industries. The two scales are e(pressed below

as:

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for liquids lighter than water and

for liquids heavier than water and

0ote: any literature sources present the above equations with the specific gravity reference

temperatures being ?'@A) which ignores the small difference between specific gravities at 6' @- and

?' @A.

*ri- .*X/hydrometer is for determining the percentage of weight by sucrose. 1ne degree #ri( is & gram of

sucrose in &'' grams of solution and represents the strength of the solution as percentage by weight 7B

w,w8 7strictly speaking) by mass8. *f the solution contains dissolved solids other than pure sucrose) then the

@#( only appro(imates the dissolved solid content. The @#( is traditionally used in the wine) sugar) fruit

uice) and honey industries.

*t is e(pressed as:

and

MOHR0ESTPHAL *ALANCE

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The principle of the balance is based on the known buoyancy of a reference glass body.

The beam of the balance is balanced with the plummet 7glass cylinder hanging on a thin platinum wire

attached to a hook on the beam8 in air using the adustable screws on the foot.

Chen adusted) the inde( pointer on the end of the beam lines up with the point on the frame.

The plummet is then completely immersed in the unknown liquid) and the system is rebalanced) using a

series of riders on the nine equally spaced notches on the beam) thus specifying the value of the added

mass for each decimal place.

This gives the buoyant force of the liquid relative to water) and hence the density) which may be

obtained to three decimal places.

.

EXPERIMENT 2: MELTING POINT DETERMINATION

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SU*STANCE SPECIFIC GRAVITY

Alove oil &.'>D&.'6' 7?5oA8

3thyl alcohol 0ot above '.>&6 7&5.56oA8

iquid petrolatum '.>6'D'.9'5

!yrup &.&

"lycerin 0ot below &.?49

ot with adjust!"t s#$!ws

!ta% &$a! with adjusta'%! h!i(ht

a%a"#! '!a with "ot#h!s

u!t )(%ass #*%i"d!$ with thi" P%ati"u

,EI-HTS:

.( 1 //01.( 1 //01.( 1 //01. . //01

SPECIFIC GRAVITY READING:

ot#h "u'!$ (i!s th!"u!$i#a% a%u!

,!i(hts i"di#at!s th! 5%a#!!"to& th! "u!$i#a% a%u!


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The temperature at which a solid melts and becomes a liquid is the melting point. !ince this requires that

the intermolecular forces that hold the solid together have to be overcome) the temperature at which melting

occurs will depend on the structure of the molecule involved D an e(ample of the relationship between structure

and properties. +ence) different compounds tend to have different melting points.

$ pure) nonionic) crystalline organic compound usually has a sharp and characteristic melting point7usually '.5D&.'oA range8. $ mi(ture of very small amounts of miscible impurities will produce a depression of

the melting point and an increase in the melting point range. Aonsequently) the melting point of a compound is

a criterion for purity as well as for identification.

The melting point of an organic solid can be determined by introducing a tiny amount into a small capillary

tube) attaching this to the stem of a thermometer centred in a heating bath) heating the bath slowly) and

observing the temperatures at which melting begins and is complete. %ure samples usually have sharp melting

points) for e(ample &49.5D&5'oA or &>9D&9'oA; impure samples of the same compounds melt at lower

temperatures and over a wider range) for e(ample &45D&4>oA or &>6D&>9oA.

M%$tin3 R#n3%4

$lthough there should be a single temperature at which a pure solid and a liquid are in equilibrium)most samples appear to melt over a small temperature range. This happens because) with capillary or block

melting points) the temperature of the bath or block rises a little during the time it takes the sample to melt.

The presence of impurities in the sample can also cause the sample to melt over a range of temperatures.

Thus) the Emelting pointF will usually be reported as a melting range, the temperatures between which the

sample melted.

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T!% M%$tin3 Point #4 # Crit%rion o' P+rit)

$ dilute solution of a liquid begins to freeGe at a temperature somewhat lower than the freeGing point

of the pure liquid. The presence of an impurity causes a reduction of the freeGing point of the sample. $s

the pure solvent crystalliGes from solution) the concentration of the impurity must increase and the freeGing

point of the solution must fall. Thus) a sharp melting point 7actually) a melting range of less than about &@A8

is often taken as evidence that the sample is fairly pure) and a wide melting range is evidence that it is not

pure.

T!% M%$tin3 Point #4 # M%#n4 o' Id%nti'ition #nd C!#r#&t%ri"#tion

*f two samples have different melting points) their molecules must differ either in structure or in

configuration. They must be either structural isomers or diastereomers. *f the melting points of two samples

are the same) the structures of their molecules must be the same) although they might have enantiomeric

configurations. These statements apply only to pure substances) and do not take into account the fact that

some substances can e(ist in different crystalline forms that have different melting points.

Mi-t+r% M%$tin3 Point4

i(tures of different substances generally melt over a range of temperatures) and melting is usually

complete at a temperature that is below the melting point of at least one of the components. Thus) the

nonidentity of two substances of the same melting point can often be established by determining that the

melting point of a mi(tures of the two is $epresse$. *f each individual sample melts HsharplyH 7and at the

same temperature) of course8) and if an intimate mi(ture of the two) made by rubbing appro(imately equal

amounts together) melts over a wide range) the two substances are not the same.Isually) however) you wish to establish the identity rather than the nonidentity of two samples) so it is

unfortunate that the converse is not always true: the absence of a depression of the melting point or of a

wide melting range of the mi(ture is not certain evidence that the two substances are identical in molecular

structure and configuration.

M%$tin3 Point #nd Mo$%&+$#r Str+&t+r%

!ystematic variations of melting point with changes in structure are not as obvious or predictable as are

the variations in boiling point.

Mo$%&+$#r %i3!t

$lthough melting points do generally increase with increasing molecular weight) the first members of

homologous series 7compounds differing by only a A+?8 often have melting points that are considerably

different from what would be e(pected on the basis of the behavior of the higher homology *n some

homologous series of straightDchain aliphatic compounds) melting points alternate: the melting point of

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successive members of the series is higher or lower than that of the previous member) depending on

whether the number of carbon atoms is even or odd. !ometimes) as with the normal alkanes) the melting

points of successive members of the series always increase) but by a larger or smaller amount) depending

upon whether the number of carbons is even or odd 5

Po$#rit)

$s with boiling points) compounds with polar functional groups generally have higher melting points

than compounds with nonpolar functional groups. *n contrast to the case with boiling points) highly

branched or cyclic molecules 7relatively symmetrical molecules8 tend to have higher melting points than

their straightDchain isomers. The combined effects of branching or the presence of rings) then) are to reduce

the range of temperature over which the liquid can e(ist at a vapor pressure of less than =6' Torr. *n

e(treme cases) a liquid range does not e(ist at a vapor pressure of less than =6' Torr; at atmospheric

pressure) the substance will sublime without melting. +e(achloroethane and perfluoroDcyclohe(ane behave

in this way.

EXPERIMENT 6: REFRACTIVE INDEX DETERMINATION OF VOLATILE OILS

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*ntroduction:

$ r%'r#&to,%t%rmeasures the e(tent to which light is bent 7i.e. refracted8 when it moves from air into a sample

and is typically used to determine the ind%- o' r%'r#&tion7aka r%'r#&ti(% ind%-or n8 of a liquid sample.

The refractive inde( is a unitless number) between &.''' and &.=''' for most compounds) and is normallydetermined to five digit precision. !ince the inde( of refraction depends on both the temperature of the sample

and the wavelength of light used these are both indicated when reporting the refractive inde(:

The italiciGed ndenotes refractive inde() the superscript indicates the temperature in degrees Aelsius) and the

subscript denotes the wavelength of light 7in this case the indicates the sodium line at 5>9 nm8.

The refractive inde( is commonly determined as part of the characteriGation of liquid samples) in much the

same way that melting points are routinely obtained to characteriGe solid compounds. *t is also commonly used

to:

+elp identify or confirm the identity of a sample by comparing its refractive inde( to known values.

$ssess the purity of a sample by comparing its refractive inde( to the value for the pure substance.

etermine the concentration of a solute in a solution by comparing the solutionJs refractive inde( to a

standard curve.

R%'r#&ti(% ind%-

The speed of electromagnetic waves in vacuum) c?)99=9?45>K&'> m,s) is one of the most important

constants in physics. $ human eye is able to detect electromagnetic waves in a range from 6' nm 7violet

color8 to =5' nm 7red color8. *t is called a visible range of light.

Chen light waves travel through a medium 7optical medium8) its electric part interacts with the electrons

of that medium) causing them to vibrate. The electrons of the medium thus become radiating light waves as the

secondary sources. +owever) the speed of new waves) v) changes accordingly to the optical properties of the

particular medium. *t is always smaller than the speed of light in vacuum) vLc. $ll materials are characteriGed

by their ability to slow down the light waves) known as optical refractive inde( n

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Fig. 1. Refraction of light


The refractive inde( is a unitless parameter) equal to & for a vacuum and larger than & for any other

material 7e.g. n&. for water8. The speed of light in air is only slightly less than c) resulting into the refractive

inde( of &.'''. Typically) it is truncated to &. The difference between a light speed in different media results

into the change of direction along which the light propagates) refraction 7-ig. &8. efraction occurs when the

light passes from one medium to a medium with a different inde( of refraction) e(cept the light that approaches

the boundary between the two media perpendicularly. $ccordingly to the properties of an optical medium) some

portion of light approaching the interface at an incident angle a is reflected back to the first medium while the

rest propagates into the other medium at an angle of refraction b. The angles of incident) reflection and

refraction are defined as angles between the particular ray and the interface normal 7see -ig. &8.

Not%7 t!#t t!% r%'$%&tion #n3$% i4 #$8#)4 %9+#$ to t!% in&id%nt #n3$%5

1n the other hand) the refractive angle is determined by the !nellJs law

where n& is the refractive inde( of medium & and n? is the refractive inde( of medium ?.

*t is possible to define an optical density for the media of different refractive indices.

edium $ has a higher optical density than medium #) if its

refractive inde( is higher than that of medium #.

$ccording to the !nellJs law) the light ray is Hbending towards

the normalH 7bLa8) if it enters the medium with a higher optical

density 7-ig. &8. Chen it enters the medium with a lower optical

density) it is Hbending away from the normalH 7bMa8.

efractive inde( can be measured by the

refractometer. Ce will use the double prism system called

the $bbeJs refractometer) shown in -ig. ?. *t consists of the two

optical prisms 7illuminating and refracting8 with the thin layer of

a liquid sample between them. The measuring prism is made of a glass with a high refractive inde( 7n?M&)=58)

which allows this refractometer to measure refractive indices up to n&L&.=5. The light enters the refractometer

from the left side of the illuminating prism at many different angles. The bottom part of this prism 7$#J8 is rough)

i.e. it consists of many small areas oriented in different directions. $s such) this surface can be imagined as a

source shining the light into all directions. %art of this light passes through the sample into the refracting prism)

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where the biggest possible angle of incident) ama() corresponds to the ray that propagates from point $ to

point # 7-ig. ?8. $ccording to the !nellJs law) the refraction of this ray is then described by the ma(imum angle

of refraction bma(. $ll other rays enter the refracting prism at smaller angles and thus end up to the left of point

A.

Aonsequently) detector located at the bottom of the refracting prism detects the illuminated region to the left of

point A and a dark region to the right of this point. !ince the ma(imum angle) ama() and the refractive inde( of

the refracting prism) n?) are known constants) it is straightforward to determine the refractive inde( of a

measured liquid) n&. The interface between an illuminated and dark region 7position of point A8 changes as a

function of angle bma() which is different for samples with different refractive indices n&. The simple readout

from the scale of refractometer then provides the refractive inde( directly) or it can be readily determined using

a conversion table.

Fig. 2.The schematic of the Abbe's refractometer.

The refraction inde( depends on the wavelength of light) because the speed of light waves depends on

their wavelength. ight of different colors 7different wavelengths8 is bending at different angles even if it comes

at the same angle of incident7dispersion8. $s a result) the white light) that comprises all the wavelengths)

produces a rainbow after passing through the optical prism 7or droplets of moisture in the atmosphere8.

+owever) despite the beauty of a rainbow) this is an unwanted effect in refractive inde( determination. *t

causes the smearing of an interface between the illuminated and dark regions in the $bbeJs refractometer. To

increase the precision of a measurement) it is therefore preferable to use a monochromatic light 7light of a

single wavelength8. The most commonly used source is sodium light of a wavelength equal to 5>9 nm.

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The refractive inde( depends also on the density of the measured sample) which is affected by its

temperature. Typically) refractive inde( decreases with the decreasing density 7increasing temperature8. The

measurement of a refractive inde( is therefore reported together with the temperature and the wavelength of

light used. !ymbol ?' n then represents the refractive inde( measured at t?' @A using the sodium line

light 7low pressure sodium lamp8.

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Not%:

The speed of light in a substance is slower than in a vacuum since the light is being absorbed

reemitted by the atoms in the sample. !ince the density of a liquid usually decreases with temperature

not surprising that the speed of light in a liquid will normally increase as the temperature incre

Thus) t!% ind%- o' r%'r#&tion nor,#$$) d%&r%#4%4 #4 t!% t%,p%r#t+r% in&r%#4%4 for a liquid. -or many organic liquids the inde( of refraction decreases by appro(imately '.'''5 for every & @A inc

in temperature. +owever for water the variation is only about D'.'''&,@A.


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ABBE REFRACTOMETER


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EXPERIMENT : *UFFER SOLUTIONS

BuffersTheory:

$ buffer is a solution that resists change in p+ upon addition of acid) addition of base) and upon

dilution. #uffer solutions contain significant quantities of both partners of a #ronstedDowry conugate acidD

base pair. To understand how buffers accomplish this) it will be necessary to review some #ronstedDowry

acidDbase chemistry.

$ccording to #ronsted and owry an acid is a proton 7an +N 8 donor. $ base is definedas a substance

that can accept a proton. Chen an acid gives up a proton) a species that can accept a proton) a base) is

formed from the acid. That base is called the conugate base of the acid.

+$ DDDDDDDDDDDDDDDDDDDDDDDM +N N $ D

$cid Aonugate #ase

-or e(ample) acetic acid 7 A+A11+8 loses a proton to form acetate 7A+A11 D 8which will be the

conugate base. The acetate is a potential proton acceptor and) as such)it must be considered to be a base.

The acetate is considered to be the conugate base of acetic acid) and acetic acid is considered to be the

conugate acid of the base) the acetate ion. *n a similar manner sodium bicarbonate 7+A1 D 8 may lose a

proton to become sodium carbonate 7A1D? 8.. !odium bicarbonate is the acid) and sodium carbonate is the

conugate base. -inally) note the sodium dihydrogen phosphate and disodium monohydrogen phosphate

system. The sodium dihydrogen phosphate 7+?%14D 8 is the proton donor) the acid) while its product) disodium

monohydrogen phosphate 7+%14 D?8) is the proton acceptor and hence) the conugate base.

$lthough we frequently represent the proton in water 7aqueous8 solution as +N) actuallythe proton

combines with water to form ions like +1 N ) +51?N ) and +=1N. The main form is the hydronium ion)

+1N.

$cids and bases can be divided into two broad categories) strong and weak. !trong acid lose their

acidic protons virtually &'' B and strong bases accept protons virtually &'' B.Ceak acids and bases lose and

gain protons respectively less than &' B. $ll the acids and bases described above would be considered to be

weak.

To form a buffer one may mi( a weak acid with its conugate base or a weak base with itsconugate

acid. The mi(ture that results will resist any attempt to change the p+ and willact as a buffer. *n this theory we

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will refer to a buffer as a mi(ture of a weak acid and itsconugate base. !uppose we mi( the acetic acid

7A+A11+8 with its conugate base) acetate) 7A+A11 D 8 . There will be present in the mi(ture both an acid

and a base that would tend to react with any added acid or base.

!uppose that a source of +N is added. The conugate base) acetate) will react as follows:

+N N A+A11 D DDDDDDDDDM A+A11+

!uppose that a source of 1+ D is added. The weak acid) acetic acid) will react as

follows:

A+A11+ N 1+ D DDDDDDDM +?1 N A+A11 D

*n either case) the invading species is not allowed to change the p+ of the solution.

1ne must use weak acids in buffers so the conugate bases will have a tendency to react with protons.

*f a strong acid were mi(ed with its conugate base) the conugate base would have no tendency to react with

protons.7 $ strong acid reacts &'' B to lose its protons) so there must be no tendency of the reaction to go inreverse to pick up protons.8This would leave the solution susceptible to attack by protons.

*n this e(periment you will make several mi(tures of weak acids with their conugatebases to create

buffers. 2ou will test these by adding both acid and base to the mi(tures. 2ou will also test distilled water and

single components of the buffers in the absence of their conugates to see how they hold up to the challenge of

added acid and base.

Chen working with a buffer one must be concerned with two maor questions:&8 1ver what p+ range will the buffer work and

?8 Chat is the capacity of this buffer to resist p+ changeO

#uffers do not buffer only at a p+ of =.'. !ome do) but others buffer in the acidic rangeof p+Js) and

others buffer in the basic range of p+Js. The initial p+ of a buffer dependsupon two factors:

&8 the strength of the weal acid or weak base and

?8 the ratio of weak acid to its conugate base.

Aonsidering the first fact) the stronger the weak acid the more acidic will be the p+ of the buffer;

The weaker the weak acid the more basic the initial p+.

$fter the range is determined by the strength of the weak acid component) the actual initial p+ is

determined by the ratio of the weak acid to the conugate base.

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The p+ of the buffer will be a bit more basic if more of the conugate base is present than the weak

acid) and it will be a bit more acidic if more of the weak acid is present than the conugate base.

The buffer capacity depends upon two factors:

&.8 Aoncentration of the buffer

*n general the more concentrated the buffer) the more ingredients areavailable to attack added

+N and 1+ D ions.

?.8 Chat is beingadded to the buffer and how much of each component) acid and conugate base) is available

to react.

!uppose that a buffer had &'' times as much acetic acid as it hadacetate. This buffer could

resist a challenge by base because there would be plenty of acetic acid to react with the base. *t

would not) however) be able to resist an attack by acid) because there would be relatively little

acetate to react with the acid.

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Phy Phar Prelim Notes

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