Refractometry Aim The aim of this experiment is to determine the ethanol/water compos itions of 3 unknown samples which are A, B and C provided. Introduction In this experiment I am going use a process called refractometrto determine the 3 unknown samples compositions !using known sample such as "#$ ethanol / %#$ water & w /'(, 3#$ ethanol / )#$ water & w /'( , *#$ ethanol / *#$ water & w /'( , )#$ ethanol / 3#$ water & w /'( , %#$ ethanol / "#$ water & w /'( and water, these are mknown composition solutions. The experiment involves a refractometer and water !ath in which !oth will !e switched on from the start of the experiment. The instrument will !e cali!rated to +* o C using -Cgrade water in other words pure water. This cali!ration will onl!e re0uired once, unless the machine is switched off. The known samples which I have mentioned a!ove will all !e inserted into the instrument. The instrument will then scan the li0uid and give me a refractive index figure measured in degrees centigrade . I will do each experiment three times and then work out an average1 this will increase the relia!ilitof mresults. I will repeat this procedure for the unknown samples as well. I will then plot a graph on the axis refractive index units in degrees centigrade against on the x axis concentration of ethanol and water and then draw a !est line fit and determine the concentration compo sition of the unknown samples using the graph. Hypothesis to be tested The hpothesis of this experiment is to determine the ethanol/water composition of 3 unknown samp les &A, B and C( provided using an instrument ca lled a refractometer. Bplotting a graph refrac tive index vs. etha nol/ water this will let me dete rmine the compositions of the 3 unknown samples. Experimental Plan The o!2ectives of the hpothesis will !e achieved !carring out the refractometer experiment as accuratelas possi!le . I will tradding the sample to the prism as accuratelas possi!le and adding the same amount of drops for each sample I used and !cleaning the prism after I have used it. I will use a different pipette for each sample of the concentration of ethanol and water to avoid conta mination and to minimise the error. The graph will !e done to maximum precision ena!ling me to achieve the accurate results that I want at the end of the experiment. I will repeat the sample testing 3 times so that I can use the average figure when I plot mgraph. 'hen the instrument is scanning I will wait for the value of the 1
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". Cali!rate the prism on the automatic refractometer ! using -C water.
+. 4sing a pipette add five drops of -C water to the prism. 5hut the prism lid gentl
and let the instrument scan the water so it is cali!rated.
3. Clean the prism glass ! using the paper towel provided.
6. 7ow using the samples provided such as the first one which is "#$ ethanol / %#$ water
&w/'(, add * drops of this sample using the pipette to the prism ! lifting the prism lid
and placing the drops of sample onto the prism surface. A refractive index value will
appear on the instrument read out screen. Alwas use a fresh, clean pipette for each
sample to prevent an errors caused ! contamination.
*. 8entl lower the prism lid.
9. Allow the refractive index value to sta!ili:e for at least +#3# seconds !efore taking
the reading.
). ;nce the reading is taken the sample can !e removed from the prism ! lifting the
prism lid. 'ipe !oth the prism and the prism lid with a paper towel provided. The
instrument is now read for the next sample. epeat this techni0ue for each sample 3
times and use fresh samples to increase relia!ilit, and use the average at the end of
the experiment to plot the graph and find the unknown compositions.
<. At the end of the experiment ensure that the used pipettes are cleaned and washed.
Hazards
=thanol if drunken in large amounts can cause drowsiness, headaches, nausea etc. =thanol is
an irritant of respirator tracts which !rings a!out the same effects as a!ove. >ou can !e
expose to ethanol if inhaled it is an irritant to the skin ! repeated contact with the chemical.
If ethanol comes into contact with our ees, flush our ees immediatel with water for "*
minutes, and seek medical attention if irritation occurs. If ethanol is accidentl inhaled oringested, go to a place with fresh air, and seek medical attention if anthing unusual appears
or if an smptoms start to appear. Also if ethanol comes into contact with our skin, wash
the place of contact.
=thanol is also highl flamma!le, so if there is a small fire or one that is increasing leave the
scene and call the person in charge to handle the situation.
Precaution and how to minimize the risk
• Alwas wear a la!, coat and safet glasses. ave a pair of heav gloves availa!le.
• Before starting the experiment alwas make sure that the apparatus is clean and
properl set up.
• All accidents such as chemical spillage or !reakages should !e reported immediatel
to the supervisor
• ?o not leave !ags and coats near the gangwas or the !enches
•=ating and drinking is prohi!ited
• 5moking is not allowed
• @now where our escapes are.
In an Emergency for all apparatus
" In the event of a solvent fire !riefl attempt to extinguish the fire with anextinguisher. If the situation worsens immediatel evacuate the la!, close the door,
and seek immediate assistance.
+ In the event of solvent spillage open the windows to aid evaporation. If a spillage isserious, immediatel evacuate the la!, and seek assistance from the la! technician, or
A graph showing the refractive index &oC( vs. ethanol &$( concentration is drawn, pleasecheck last page.
Calculations
To find out the refractive index for the ethanol/water concentration I had to work outthe average of each sample.
For example for the 30%ethanol/70%water the value calculated was .3!. This was
calculated "# adding up all the three values in the test and dividing it "# three to work
out an average. $g .3!&0 ' .3!&& ' .3!&()/3* .3!
Discussions and interpretation
A refractometer measures the light which is !ent when it is travelling from air into a sample
and this is how the refractive index is measured. ooking at m results it shows that "#$ethanol had a refractive index of ".36, the results increase ! #.#" for the 3#$, and *#$
ethanol compositions, whereas after this the graph !ecomes stationar for *#$, )#$ and
%#$ compositions.
As a result when I cali!rated m results for the unknown concentration of ethanol m
solution A was exactl "#$water/%#$ethanol, solution B was 3#$ethanol/)#$water and
finall solution C was *#$ethanol/*#$water, e.g ".36, ".3* and ".39, m results also
produced no anomalies so I have no reason not to !elieve that the unknown solution were
exactl the same as m first three known concentrations, since the fitted perfectl on m
line of !est fit. There is also an increase in #."after each sample which matches m theor I
also !elieve m results are relia!le since I done the experiment three times for each
composition.
In m opinion I do not think I could have improved m results an more, since the were
ver good, although I do !elieve that there could have !een different since when I read the
reading of the refractometer we had to round up the last digit, so there is a possi!ilit thatrounding errors could have occurred, even though m results are good.