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THF"YISIBLE SPECTRA OF SOFT DRTNKS

THEORYIBACKGROI]ND

Th9 word spectroscopy is used to refer to the broad area of science dealing with the absorption,emission, or scattering of erectromap.etic radiatiou by morecur"s, ionl, atomq, or nucrei.Spcctroscopic techniques are some of the most widely used aualytical methods in the world today.Diffsrent regions of the eleckomap.etic spectrum such as infrared, visible, ultraviolet, or, X-rayradiation can be used to interact with matter. There are maoy importaat inskumental methods ofchemicai analysis that rely on the interactiou betweeu lignt ana matter to probe chemical skucture.

white light is visible light that contains all the colors of the rainbow. If white ught hits a coloredsolutio4 the eieckons in the solution'will absorb some of that ligh! while the rest will be ransmittedthrough tlie solution. It is thi trausmitted light that we see with iur eye". lt" i*fort-t ti La"ot*at}at a red solution does not absorb red lig!! it actually absorbs blue-green light- The solution appearsred because the intensity of the blue-green light in the original white light has been decreased- Thefollorving table shows the relationship between the color Ibsorbed by a solution, and the color weobsewe.

EXPERTMENT 1

Table 1-1: Relatiouship Betq/eea Absorbed and Observed Colour

Absorbed'Wavelength in nrn (Absorbed Colour) ObservedColour

t

400 (violet)

450 (blue)

490 @lue-green)

570 (ye11ow-geen)

580 (yellow)

600 (orange)

650 (red)

Greenishyellow '

Yellow

Red

Violet

Dark blue

Blue

Green

"S\J

$x\s-'(0\\

One of the most convcnient, accurate aad seositive methods for measuring the concenkations of dilute

solutions is by colorimetry or absorption spectrophotomeky- The tecb:rique is based upon the

Ereasurement of thc amount of ligbt energy a solutiou absorbs from a bea"' of light of a certain

wavelength. The wavelength chosen is usually that one at which the absorbalce, of tbe species to be

analyzed, is at a maximum.

The Spectronic 20 inskument you will use can correctly bc called a colorimeter, becausc it measures

the atrsorption of light in the visible spcctrum that wc perccive as color, and the technique used is

said to be colorimekic. Althoug! the instuments that arc used to measurc thc intcraction of various

regions of clectoomapetic radiation with matter differ a great deal in dcsip and operation" they all

coutain thc samc basic components. A schematic diagram of a simple 'nskument that is used to

measure the absorption of visible light is shown in Figurc 1.1.

0etectorSource MonochrematorSample

Figure 1.1: Main Components of an Absorptioa Speckophotometer

A]l the parts of the'nctru6ent work together as follows (FigUre 1.1): Ligtrt from the source Passes'-:-:l:: --te ,-,,.rc:5ilr.ator proCucieg abeam with a siagle eoergy or a narrow baad of energies. The-_:-:t:.:::-i.:=a=I..i.-:a.-;::t).'-ed:rctor.The5:-!leis-,ieaplaceriirti:'leambetx'een:. =cl,:^:l=omaaor asci the delestor. lf some of the iigbt is absorbed by the srtnple, the iatensity of the

beam reachi-ng the detector, I, will be less than t. The detector compares the two iatensities and

reports the result as either percent transmittance (hD or absorbance (A). These terms are defaed tobe:

Equation (1)

(the fraction of I" that gets through the samples is called Trancnittan6.;,. -.'.".-\

A: - log T 7 Jog (U t): 2 -log (o/" T) Equation (2)

f--fle amount of iight absorbed is dependeut on how well the substance absorbs ligh! the path length ofthe light, and the concenhation. These parameters are combined into a mathematical relationshiplmown as follows:

.rllA: ebC y Equation (3)

/.Lr equation 3, e (epsilon) b the molar absorpfivity,6 is the path length, and c is the concentration. Themolar absorptivity is a characteristic of the absorbing species; it tells us how much light is absorbed ata n4r{iqq-rl7r'qrarrelenafh $i1qc fhe rrn.ifc nn a 2rp fimil:alltr

-nlec ncr lifer Qf e-d f-he n3fh lensf}' ic

usually given in certimeters, the rroits otr s are most commonly expressed as lvfl sm-t to make Adimensionless- Equation 3 is known as the Beer-Lambert Law, s1 si'nply Beer's Law.It is importantbecause it shows that absorbance is directly proportional to the concenkation of the absorbiag species.

f

However, if the concent-ation is described in units other than moles per liter, then e is replaced withthe symbol a or absorptivity, and Beer's I-aw is written as follows:

Equation (4)

The picture below is a representation of a Siectronic 20-D colorimeter which is the instrument youwill be using in this experimeul Use of the inskument is easy and results are excellent provided youfollow all directions carefully-

Sanrple Gomparbnent

Orr/Off and 0 % Tr:ans.mission adjustment

Figure 1.2: A Diagram Showing the Components of a Speckonic 20 Spectrophotometer

T*'o t1'pes of aaal.vsis caa be dooe with the visible absorption measurements:

lj:3,

the

the

I

2-

Qualitative aaalysis: To determine the wavelengtfu of maximum absorbanceabsorption spectmm as well as absorptivity value of a specific molecule.

Quaatitative aualysis: to determine. the concentration ef 31 nnknown solutioustqn dard calibrafr on curve.

from

from

OBJECTf\TES

1. To determine the origin of the colour of a soft drink sar.'ple from its absorplion speckum-2. To determine the waveleugth at maximum absorbance $'-J for a soft drink 5rmple-3. To determine the udcnown concentration of a soft drink from the calibration curve.

APPARATUS

Beaker

Pipet

Gla-ss rod

\blumeiric flaslq 50 mT

Dropper [1/l - *^ lg^

1 N o/o Transm ission adjusbnen t

Retort stand

CHT"I\,flCATS

A can of soft drink (blue, ted, grea'yellow or or:lnge in colour)

PROCEDI]RE,

In the fust part of this expcrim*t Y1u wi-ll be inkoduced 16 1fos fuadarnentals of spectroscopy' You

will fi,"t learn how to properly use a Spectro*"iO ;.,*""*eot and then you will use the instrumeot to

find the wavelength (t) at which absorbanqJrignt uv a soft Drink 'olotioo

has a maximum valuc

(1-_). The "uro.puolp""t ,rm of a soft Drhk i, ott"iora by mcasuring the absorbance of a sample

of the Soft Drink "t airo"ot *ur"l*gtu"^ usin; t . spectrophotometer. A plot of absorbaacc vs'

wavelength rryill show You ?'*'

Inthesecondpart,youwilllearnhgwalawofsPectoscopy,termedBeer-I-ambert,sl-aworalsoknownasBcer'sLaw,canbeusedtodeterminethcconccntrationofan'unknownsolution-Acalibration curvc will be preparcd by -"*;; 'h" "f'o'u*""

of differe8t dilutions of Soft Drhk at a

single wavel"rr* ;;;il fte absorlrocJof the unlcnown solution can then be measured at the

same wavelength *d compared to the calibratio[ curse to deterEitre its conceltration'

A.Preparationof.Standard,solutionsofSoftDrink(KnownConcentrations)l. Pour soft drink into a beaker aod stir to remove thc carbonation-

2.Pipet5.00rnT,ofthesoftdrilkintoa50.0'.lvolrrmeticflaslcanddilutetothemarkwithdistilled #ater. Cover ald shake to rnake a homogeneous solution' Store ia 2 5 rall beaker'

3. Repeat step 2 using 10 -T ' 15 mT '

20 mr aud 25 mL of soft dri-nk-

B. Operation of the Spectronic 20 and Determination of ]'-,,

Thcspectronic 20 is a moderately e:pensive piecc of scieutific equipment and should be Treated

witl all due care and resPect'

Instrument: Thermo Spectronic/Geuesys 20

OP erating Instructions

1. Turn on the spectonic 20 and wait for the imtrument to warm up (minimrrm 15 minutes)'

2. Set ttre wavelength to 600 nrn-

3. Adjust 0% transmittan ce (h T") (Adjustinc dark current - uothing should be in the sample

comPartment'

i. obtain a cuvette. The cuvette may look like an 9t9"ty test tube, butit is made of special

trigh quatity glass aad i" -rrJi* expensive! Clean the tube a:rd rinse it with distilted

water_ anrl. ther fill f.lre firhe ehnrrt Yr r.rrtt of tl-,'l.arrk' sot,trir-'r' /1ha 'fl2n1q' iq ']icilll''l 'r':+:: i::

thisexperiment).Carefullywipeawayanysolutionandfingerprintsfromtheoutsideof. the tube using a KimwiPe'

1c

5. Adjust 0 absorbance aad 100 % transmittatrce with the cuvette 6sntzining g6s'!!:nk' ia thes.mple holder. Remove the the cuvette and set it aside without emptyiag the distilled water.

6. Cleaa alother cuvetie and rinse it with a small arnount of the 'stendard' soft drink whoseabsorbalce is to be measured (i.e- use any one of the diluted Soft Drink solutions in Part A).Then fill it about % fiil with solution, wipe it with a Kimwipe and place it in the sampleholder with hash marks aliped. Read aad record the absorbalce.

1. Remove the cuvette, close the top aud change the wavelength to a setting which is 20 nm

lower.

8. Reset 0 % transmittarce if it has chaaged (sample comparhent must be empry).

9. Tnsert the cuvette of distilled water and reset the 100 %T- Remove the cuvette-

iO. for"rt the cuvette coutaining the same Soft Drink solutiou you used in Step (6).

i 1. Read the absorbaace aad record the rearlings in Table 12.

12. Repeat Steps 8 through 11 until 360 nm, taking absorbance readi:rgs at each 20 nm interval.

t a r )sing a graph paper, plot the absorption spectrum of your scft dril* and determire the )*.c.: Q'[ote: Remember to bring a graph papt to the lab!)

Preparation of 'Unknown' Soft Drhk $ample

1 . Pour some of the soft drink ilto a beaker and stir to remove {he carbonation-

l. ?:'.: =e s,i i-:-k witlout measuri-ng tie volume iato a 50 lT volr'.'aekic flask a:id diluteo rle rca:t *ith distilied wrater- Stoppcr aod shake the flask to homogeaize the soiution. Makesore the colour of the rmkova soiutiou you prepared is not darker than 1fos most concentratedstandard-sol:rtiou-

3. Put the 'unknowu srmple' into a cuvette until it is about % firll_

(Note: Alternately, the 'ilnknown samplz' may be assigned to you fu the lab instructor.Refer to instructions proided by the lab instnrctor during the laboratory bnefng-)

QuantitativeA-nalysis Of The Soft Drink Solution

1. set the Spectronic 20 to the wavelength maximum (1"*) obtained from part B.

2. Set 0 and 100%T as given in the procedure above.

3. Measwe and record the absorbance of each 5 'standard' soft drilk solution.

4. Measure and record the absorbance of the 'uuktrowr' soft drink solution-

C.

D.

E. Cleaning Up

l. Since you have ouly used soft ddnk in this experinen! the waste catr be poured down the

drain.

Clean and dry all cuvettes- @O NOT use a tcst tubc brush on thc cuvettes; it will scratch

them.)

DATA ANALYSIS

1. Plot the absorption spectrum of thc soft drink ou a graph paper. Your graph should Ell the entire

page and use accurate narkings. aI pphs should contain a title and appropriate labels on the

axes. As cxample is showu below. Include the diagram indicating the wavele,lgth regions and the

correspouding colors of visiblc lig[t. Usc thc following diagram to erplain the origin of the color

of your soft drink in the discussion-

4SmrnranreEngth (nm)

grccnyel

Figure 1.3: Al Example of an Absorption Spectrum and The Corresponding AbsorbedColours.

Next make a plot of the absorbaace of the diluted soft drink solutions versus their concentrations,

where four opeu circles indicate the measurements of the four hown coucentrations. One singleshaight line is drawn that comes closest to all these circles. Sec below for an exam;1le. Note the

equally spaced divisions, the straight line through thc points and the title oa the top- Now you cao

use this calibrati.on plot to determine thc concentration of your u.known soft drhk solution- Findthe concenhatiou of your unknown by detcrmining where its absorbauce crosses the calibrationIine on your graph- The plot which is made of absorbance vs- concentation- a olot like the qrarrh

below is called a "Beer's Law Plot"-

2.

0)od.Dl<o{)).o

J- t Redorange

6

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l_il

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:516lor unknc\,Vn * 123

20 :o 40 50

Corcerdraticn [96]

Figure 1.4: An Example of a Beer's Iaw Plot"

Note iu ttre example, the student created a fifth point - the one with the square - for the unknownwhieh reads 0.31. Following the line straight down from this point, the concenbation for theunknown # L23 !s determiraf'(for the sxarnple, this was 25 %). Report yow value in your'conclrrsions-.

PRE- LABORATORY QITESTTONS

The follow'ing preparation questions should. be answered before c.sming to lab- They are intended tointroduce you to sev.eral l6gas imFortaaf f6 aspects of the experiments. You must turn il the answers tovour in<tructorbefore you will be allowed to begin the experimenl

l- DEfiEeqpe{troscqpy-

2- What lrpe of electomapetic radiation will be used in this experiment?

3. .- -D"fio",

Transmittance and absortance.b. Give the equation which shows how you can mathematically conyert between the fwo ternos.

4. State Beer's Lawmathematically and defi-ue each term-

5- Define 1"--.

QIIESTIONS

1. 'Based on Beer's Iaw, when the concentration of an aualyte increases, how wil-laffected (i.crease, decrease, no change)?a- Absorbanceb. Transmittance

the following be

2- Why is i1 imFortant to first obtain the absoqption spectrum of the soft drink before paking acalibration curve?

3. What is the purpose of using the'blank' solutioa?

4. What colouds) is/are being absorbed in your soft drink

o-5

Absorbance 0-4

al sOC nm

o-3)

-avalu=fotmy z/ 02unknornn grrye*olut'pn # 123

0_1

10

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DATASHEET EXPERTMENT 1

TI{F', VISIBLE SPECTRA OF SOFT DRTNKS

t\ur Sarah Hanaq Et thlrrrad fftirNrme :

Stude,ntID: 2A0qqlqD5

Date : l9 Ju0e Pt3

Group, 6R t C+s Po)

Nameofsofta;rt f d}l (Stfaf,,fLeyfS) arowof softdrink: Bed

Table 1.2: The wavelength and absorbance of soft drink

L-: 5fO

Waveleueth [nm) Absorbance hyeraqa

500 o'ol6 o'olo o-@7 o .olo

580 o.o bG o.oGb s. oGh o-obb

560 o.no o.t 0q o-lq + o.lq t

540o.rgq 0']q I 0-fq1 0 -fBq

520 o.221 o-9a1 O'7?]o d-33-9

500 o'?to 6-3oB a2oA o.308

480 o-13? o-13+ o'a*r o"f,}7

460 o-152 0-lE3 o.ls3 D.153

440 o.l oJ o.105 O-lo3 6-lo3

420 o'oBo o'qb o -oq5 o'Dot

400 6.018 6 -oAl o-cn-3 o-o?-3

380 o-ob+ o'o+o o-ob5 o'6bb

360 6.oSB o-obo o- os.t o-osg

Table 13: Table of soft drink conceotration (vohune Yo) afi absorbance

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Concentration of ualnowu:

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t-bYo **(o o,{tt)tt)(U,".Ofl/- hbL-

Lecturer's signature,

Iuat

$vaph

flvyla",r"v9

LOu,r.,T &nti 0V\

Sampie calculation forthe concert-atioa of solution 2:

A 9-ao

*-{, * L c*'*Y

-o+L[ >q'* c

b*[ C=-t00cwOr

lo /"

Solutions Concenhation (volume %")€olta-ti 0O

Tu?iiild"^e-

Absorbance

'Ern{ o'1qo o'tr?Ll'

2tom{ l-51+ 0.T +l

3

l5r$/ a.'e.) l'os5

42t)rn-t :'$o 1-330

59.Srn{. 3-?o l-?+o

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