Titration Flashcards
Why should the iodine solution be titrated as soon as possible once it is prepared ?
Iodine is volatile and will vaporise easily at room temperature, and [explain impact always] this reduces the amount of FA3 needed to complete the titration and reduces the titre value
if not possible, cover all flasks containing iodine
Why should the starch indicator be added only towards the end of the titration when the iodine solution is pale yellow instead of at the start
Iodine forms a blue black complex with starch in which the iodine is trapped within the starch molecules. When starch is added at the start when there is high concentration of iodine, some iodine may remain trapped in the starch even at equivalence point, reducing the concentration of iodine molecules in the solution and the titre volume to be inaccurate less than actual
after titration is complete, a slow return of the blue colour is observed. why is this so ?
this is due to the atmospheric oxidation of I- back to I2 (rxts w starch to form blue black complex)
why is it necessary to dilute FA1 before the titration
to prevent significantly large value of FA2 to titrate it which requires refilling of burette and increasing the uncertainty and percentage error as more measurements are being made
explain how uncertainty arises from burette reading
since each burette reading has +/- uncertainty of 0.05cm3, the initial reading -0.05 uncertainty, and the final reading +0.05 uncertainty, hence total +0.10cm3 uncertainty
why is KMnO4 titrations only can use H2SO4 as acidic medium ? (any why this acidification no need to add using burette / pipette ? - can js transfer using 50cm3 measuring cylinder ?)
Nitric acid is an oxidising agent (oxidise Fe2+ to Fe3+, lower than expected titre value since u tryna get rid of the Fe2+ by titrating w KMnO4) while HCl oxidised by manganate ions to give chlorine and higher than expected titre value (since u titrating w KMnO4 in burette - depends tho) and since H2SO4 is at max oxidising state, it does not rxt with the solution
because added in excess to acidify the solution and precision is not required
how does not transferring all the washings affect the titre value ?
less fa3 needed to titrate the fa2 and thus titre value reduced since less fe2+ ions are present in the fa2 solution and its concentration decreases and requires less MnO4- to reach end point
explain the colour changes observed during the I2/Na2S2O3 titration
- brown to yellow as the concentration of I2 in the solution decreases as it is being reduced to iodide ions
- solution turns from yellow to blue black as the addition of starch reacts with remaining iodine and forms a starch iodine complex
- solution turns from blue black to colourless as all the iodine is reacted, so no more starch iodine complex and left colourless iodide ions
why use KMnO4 in acidic medium ?
in alkaline medium, KMnO4 forms brown ppt of MnO2, which interferes with the detection of the end point of the titration
3 e- + MnO4- + 2 H2O → MnO2 + 4 OH-
Why can an alkaline solution of K2CrO4 not be used (in the presence of Ag+ ions)?
Brown precipitate of AgOH will be produced, reducing Ag+ ions in the solution. More Ag+ will be required, which affects the accurate determination of the end-point of the titration OR colour change will not be distinct.
why does the ROR not change ?
For certain titration between C2O42- ions and MnO4- ions, solution needs to be at least 65°C at the start, but when cold MnO4- is added and temperature decreases, why does the rate of reaction not decrease?
⇒ Production of Mn2+ ions, which act as a catalyst, causes [Mn2+] to increase. Net effect of temperature decrease and concentration of catalyst increase leads to a constant rate of reaction.
Equivalence Point vs. End-point
Equivalence point is the point where the reactants have just reacted with each other according to the stoichiometric ratio given by the balanced equation.
End-point is the point when the reaction is practically complete, when the indicator shows a clear colour change.
Why would adding too much indicator affect the accuracy of the results?
Indicators are weak acids or bases, thus adding too much of it will affect the actual amount of acid or base needed to react.
*For % Error in a titre value, the uncertainty should be of TWO measurements because you measure the INITIAL and FINAL burette readings.
Uncertainty of ONE measurement = ± (half of smallest division of the apparatus)
% error of a digital apparatus. The uncertainty (for ONE measurement) in this case would be the precision of the apparatus.
eg. 3 d.p. electronic balance → Uncertainty = ±0.001g (express in 3DP when recording in table)
:)
Source of Error for NaOH(s)
NaOH(s) absorbs H2O and CO2 from the atmosphere and dissolves, making it less alkaline.
*This can be applied to other solids as well.
[ALWAYS LINK TO TITRE VOLUME BASED ON CONTEXT]
