Titration of Acid using a PH meter Flashcards
what is a pH meter
an instrument used to measure the pH of a solution
what provides a direct method of obtaining a titration curve
pH meter
what is a titration curve
a graph of measured pH values verses the volume (mm) of the titrant added
how is the volume of one reagent needed to EXACTLY react with the other reagent measured
using a burette
what is a titration
a technique where a solution of known concentration is used to determine the concentration of an unknown solution
what is the analyte
the substance that is being analyzed (what was in the flask)
what is the titrant
the substance that is being used to analyze the analyte (what is run through the burette)
what was the titrant of this experiment
NaOH
what was the analyte of this experiment
the unknown acid
what is the equivalence point
the point where an equal amount (number of moles) of base has been added to the amount of acid present
where can the equivalence point be found on a titration curve
there is a relatively LARGE change in pH with a relatively SMALL change in volume
what is a monoprotic acid
acids that only have 1 acidic H to give up to the NaOH
what is the generalized formula for monoprotic acids
HA
how many equivalence points does a monoprotic acid have
1
how many equivalence points does a polyprotic acid have
more than 1
what is a polyprotic acid
acids that have more than 1 acidic H to give up to the NaOH
what is the generic formula for a diprotic acid
H2A
what is a diprotic acid
an acid that has 2 acidic Hs to give up to the NaOH
how many equivalence points does a diprotic acid
2
what is the generic formula for a triprotic acid
H3A
what is a triprotic acid
an acid that has 3 acidic Hs to give up to the NaOH
how many equivalence points does a triprotic acid have
3
what determines how precise the equivalence point has been determined
based on how steep the curve in the region is
- those with a steeper curve have been more precisely established
what is an endpoint
the point during a titration where the indicator changes colour
how can the equivalence point of our titration curve be determined
a) use the steepest tangent line to the smooth curve where the pH changes rapidly
b) the equivalence point will be the midpoint between the two lines intersecting the VOLUME axis
what is a disadvantage of the titration curve method
the time and effort required to make measurements and to construct the graph
what is the equation for determining pH of a solution
what is the equilibrium equation for a monoprotic acid in water AND how can the Ka value be found
what does Ka stand for
ionization constant
how can you determine the PKa of an acid from the titration curve
the Pka is equal to the Ph and can be read right off the graph for that acid
how much of the unknown solid sample was obtained to start this experiment
1.25 to 1.75 grams
how was the unknown acidic analyte solution prepared for this lab
1) first between 1.25 and 1.75 grams of the unknow solid sample was weighed using the technique weighting by difference (using weighing paper)
2) the weighed sample was transferred to a 250 mL beaker
3) then the weight sample was dissolved in about 50 mL of RO water
what was done with the supplied 1 M NaOH solution for the experiment
about 100 mL of the 1 M NaOH solution was transferred to a clean, dry 150 mL beaker
what is a hazard of working with NaOH
it is a strong base and can cause burns if left on the skin
how were we suppose to clean the burette prior to doing the experiment
- first clean the burette 2 or 3 times with RO water
- filling up a chunk of it with RO water and swirling the burette to ensure the entire surface of the glass is covered before opening the stop cock and letting the RO water drain down - once it has been drained with RO water, run NaOH through the beaker once or twice to coat the sides in the base before filling it up the the zero mark to begin the titration
if the NaOH does not exactly line up with zero when you start what must be done
just subtract that initial starting value from all the NaOH readings to ensure you calculate the NaOH that was actually added to the acidic solution
what must be noted when the pH is meter is set up along with the rest of the apparatus
it is clamped so that the electrode is immersed in the acidic analyte solution and off to one side of the beaker so that the burette will drip the NaOH solution into the solution but NOT on the pH meter electrode
how should the drops of the NaOH change once there is a rapid change in pH of the solution
drops should be reduced to about 0.5 mL or less compared to the initial 0.1 mL
when should you conclude adding the NaOH to the acid solution
once the equivalence point(s) have been reached OR if the pH is about 11 or 11.5
how can the titrated solution be disposed of
down the sink with LOTS of tap water (after being diluted)
what is done with the magnetic stir rod after the titration
MUST NOT go down the sink and give back to the TA
what can be done with excess NaOH
diluted to 1 M and poured down the sink with lots of water
generic equation of NaOH reacting with HA
NaOH (aq) + HA(aq) —–> H2O(l) + ANa(aq)
generic equation of NaOH reacting with H2A
2NaOH (aq) + H2A(aq) —–> H2O(l) + ANa2(aq)
generic equation of NaOH reacting with H3A
3NaOH (aq) + H3A(aq) —–> 3H2O(l) + ANa3(aq)
how to determine Ka from Pka
Ka = 10^-pka
