The Student Room Group
Competition - post a photo you've taken of nature >>
Competition - post a photo you've taken of nature >>
Competition - post a photo you've taken of nature >>

Calibrating a pH glass electrode and determining pH level.

Hi, i'm stuck on part b) of the question below:

Q: In order to perform pH determinations with a glass electrode, the cell potential was measured for threestandard solutions with the following pH values at 25 Celsius: 2.04, 7.05, and 9.20. The cell voltage readout(in mV) for each of the above solutions was 238.0, -37.5 and, -164.5, respectively. Calculate: (a) thesensitivity of the pH sensor; (b) the pH of an unknown sample yielding a cell voltage of 20.5 mV; (c) thepH deviation from the actual value if the sample temperature is 35 Celsius.

My ans:

So to get the sensitivity (s) I just use the nernst equation i.e.

s=2.303RTnF s = \frac{2.303 RT}{nF}
Where: R is the gas constant 8.3144J.K−1.mol−1 8.3144 J.K^{-1}.mol^{-1}
T is temperature in Kelvins
n is the charge of a Hydrogen ion = 1
F is Faraday's constant 96485C.mol−1 96485 C.mol^{-1}

⇒2.303×8.3144×29896485=0.0591\Rightarrow \frac{2.303 \times 8.3144 \times 298}{96485} = 0.0591 V/pH unit

But now I'm stuck on part b). My method is either to rearrange the nernst equation like so:
E=E0+2.303RTnF.pH E = E^0 + \frac{2.303 RT}{nF}. pH

⇒pH=E−E02.303RT/nF\Rightarrow pH = \frac{E - E^0}{2.303 RT/ nF}

where E0=238mV−37.5mV−164.5mV=36mVE^0 = 238mV - 37.5mV - 164.5mV = 36mV

=0.0205v+0.036v0.0591v/pHunit = \frac{0.0205v + 0.036v}{0.0591 v/pH unit}

But this gives me a pH value of 0.96 which doesn't seem right.The other method I am using is to work out the value given the pH scale and sensitivity. i.e. If we know that the cell voltage of the unknown pH is 0.0205v, then if we find the difference between this and the the voltage for pH 7.05 (0.0375v) gives 0.058v. Then divide by the sensitivity of 0.0591v/pH unit to give a pH difference of 0.981. Then then take this value away from the reference pH value of 7.05 to give a pH of 6.068.

Any thoughts on my methods, are they correct or am I way off?
(edited 8 years ago)
Reply 1
Avatar for biosensing
biosensing
OP
Sorry I've edited the OP.

Quick Reply

Related discussions

Latest

Trending

Trending

Articles for you