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C-Vitamin

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Ascorbinsyre indhold i C-vitamintabletter

Formål

Forsøgets formål er at bestemme ascorbinsyreindholdet i C-vitamintabletter ved en potentiometrisk syre-base titrering.

Teori:

I dette forsøg titreres en svagsyre med en stærk base. Ascorbinsyren er en organisk syre som sammen med sin korresponderende base (acorbation) har den c-vitamineffekt som et livsnødvendigt vitamin for mennesket. Uden c-vitamin ville vi få skørbug og andre sygdomme.

Når man titrere er man interesseret i en helt nøjagtigt bestemmelse af den volumen af tilsat NaOH ved ækvivalenspunktet. Ækvivalenspunktet er det punkt hvor syremolekylernene har afgivet hydronerne og hvor syren og basen har opnået samme molforhold, dette vil skabe en neutral blanding.

Reaktions skemaet ved denne titrering ser sådan ud:

[pic]

Ved at lave et skema hvor tilsatte antal ml NaOH og pH-værdierne kan man lave en graf der viser ækvivalenspunktet. ækvivalenspunktet ligger det sted på grafen hvor hældningskoefficienten er størst.

Ved hjælp af mængdeberegning er det muligt at beregne antal mol af ascorbinsyre hvorfra vi kan beregne hvor mange mg acrobinsyre der er i hver vitaminpille.

Apparatur:

• pH-meter med elektrode og holder

• 100 mL bægerglas

• magnetomrører og magnet

• burette med stativ og klemme.

Kemikalier:

• C-vitamintabletter,

• ca. 0,1 M NaOH-opløsning

• Puffer-opløsning.

Fremgangsmåde:

1. Først skal pH-metret kalibreres korrekt. Dette gøres ved at nedsænke elektroden i en puffer-opløsning med pH = 4, hvis pH-metret viser mere end 0,02 fra den rigtige pH, skal det kalibreres.

2. Fyld 7 stk c-vitamintabeletter i bægerglasset.

3. Tilsæt 30-40 ml demineraliseret vand

4. stil bægeret på en magnetomrører og kom en magnet i

5. vent til pillerne er opløst

6. PH-elektroden kommes i bægerglasset OBS: magneten må ikke rører ved pH-elektroden!

7. Aflæs pH-meteret
8. Hæld NaOH-opløsningen i buretten og nulstil den.
9. NaOH-opløsningen dryppes nu i opløsningen i bægerglasset. Foretag aflæsning for hver tilsætning af ca.1 mL base indtil lidt før ækvivalenspunktet. Herefter aflæses for hver 0,1 mL tilsat base. Titreringen fortsættes indtil buretten er tom.

Affaldsbehandling: Affald i dette forsøg må hældes i vasken.

Målinger: I nedenstående skema noteres samhørende værdier for tilsat basevolumen og pH i opløsningen.

NaOH-opløsningens nøjagtige koncentration er 0,0998 M

Ascorbinsyre indhold ifølge deklarationen: 80 Mg

Beregninger:

1. Titreringskurven indtegnes og ved hjælp af kurven bestemmes ækvivalenspunktet

V(NaOH)ækvivalenspunkt = 14,5 mL

Beregn den stofmængde NaOH, der er tilsat ved ækvivalenspunktet.

Stofmængdekoncentrationen udregnes ved hjælp af denne formel:
[pic]
men først skal n(NaOH) isoleres – så formlen kommer til at se sådan ud:
[pic]

Derfor kommer udregningen til at se sådan ud:
[pic]

Vi ved nu at stofmængden af NaOH er 0,0014471 og dermed kender vi også stofmængden af ascorbinsyre da forholdet, ved ækvivalenspunktet, mellem NaOH og C6H8O6 er 1:1
Derfor kan vi udlede at der i 7 stk. C-vitamintabletter er 0,001447 mol ascorbinsyre. Derfra kan vi udregne stofmængden i mg af acrobinsyre i en tablet.

Dette gøres ved brug af formlen [pic]
Her er m massen, n er stofmængden, og M er molarmassen
Den molarmasse findes ved at kigge på ascorbinsyrens formel og finde grundstoffernes vægt i det periodiske system. Molarmassen af ascorbinsyre er C6H8O6
[pic]
M kan vi nu benytte til at udregnemassen af ascorbinsyre i tabletterne.
[pic]
For at finde ud af hvor mange mg ascrobinsyre der er i en tablet skal m deles med 7 og ganges med
1000
[pic]

På c-vitaminernes indhold af ascorbinsyre ifølge deklarationen var 80 mg. Dette passer godt med dette resultat, da ca. halvdelen af ascorbinsyren er omdannet til sin korresponderende base C6H7O6- hvilken der så også er ca. 36,4 mg af.

Ordet syreneutralt betyder at en væske hverken er en syre eller en base. Når en syre bliver neutraliseret af en base så bliver syren neutraliseret – altså den bliver syreneutral.

I forsøget kan der være flere fejlkilder. En af dem kan være aflæsningen af ækvivalenspunktet. For at gøre dette mere præcist vil det være det bedste at beregne en forskrift for titreringskurven for derefter at differentiere den. Alt i alt kan vi konkludere at ascorbinsyre indholdet i tabletterne svarer til hvad der står på deklarationen.

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