Thursday, 12 April 2012

the sprite experiment

currently involving myself in research has been somewhat taxing. but, to be honest, that's only because i spend all my 'free' time doing unnecessary things like reading and playing games and watching movies and sleeping. in any case, i have had a significant amount of time, in between waiting for bacterial cultures to grow and ultracentrifuge machines to do their thing, to reflect on my career path so far (or, as with most things in my case, the lack thereof). one thing that came to me recently is an old experiment design that i had for a high school project. it was a chemistry project, with the intention of exposing us to experimental design and method, and i have to say, it was an enlightening experience at the time. where most of the experiments were handed down to us and 'science' was a mere following steps and instructions on a drab piece of photocopied paper, the actual opportunity to understand scientific approach was much welcome. to some of us.

anyway, here is the experiment that i conjured. i cannot say if i think back of it as a good or terrible one - i guess it would be an okay experiment. my teacher at the time thought it was terrible, though!

objective: to determine the amount of H2CO3 and H3PO4 in a bottle of sprite.

materials: bottle of sprite (i think it's the 375ml bottle. i can't really remember), run of the mill lab apparatus e.g. beakers and pipettes and retort stands, universal indicator and pH meter!

introduction: the 'fizzy' drink, sprite is listed to have carbonic and phosphoric acid as its two major components of acid (with some minor ones, unlisted), which are responsible for the 'fizzy' taste. texture? something like that. carbonic acid exists in a state of equilibrium between the dissolved acid and gaseous carbon dioxide:

H2CO3 <-> H+ + HCO3- <-> 2H+ + CO3(2-) <-> H2O + CO2

phosphoric acid is not so effervescent, and we assume that it remains dissolved (along with the minor acids).

as most kids will know (and attest to, having pranked their peers), shaking a bottle of carbonated drinks vigorously causes it to become somewhat volatile. the best condition to hand a can of coke to your friend, watch them open it and get drenched in sweet syrupy silliness. this is an effect of the vaporisation of carbonic acid to carbon dioxide. in the closed space of a can or bottle, the gas, which takes up more volume for the same number of moles (or molecules) of liquid, causes an increased pressure to build up (since PV = nRT, where P is pressure, V is volume, n is number of moles, R is the universal gas constant 8.314 J/K/mol and T is temperature; increasing n causes an increase in P, all other variables being constant).

- a sample of the sprite is tested for pH value using the universal indicator and calibrated pH meter.
- one 'shake' of the capped bottle is performed, and the pH reading is taken again.
- the process is repeated for 10 'shakes', after which the bottle is shaked vigorously for 5 mins (lies) and a pH reading taken.
- the data are plotted on a graph with each point corresponding to a pH of each number of subsequent shakes (1, 2, 3, ...) and the final point is extrapolated to a point of infinite shakes.

so i remember the results were amazingly consistent. multiple runs gave a linear graph that tapered off at 'infinity'. and from this, we could estimate the concentrations of carbonic and phosphoric acid (plus non-specific acids).

this was a major flaw: how quantifiable is 'a shake'.

ok and other flaws as well. anyway. that was high school experimental design. at its worst. man, times were funny back then.

1 comment:

etc said...

i like sprite. and seven up. basically any lemonade taste. or citrus taste. actually, i'm not sure what about it makes me like it. but i like sprite.