Newborn infants are screened for cystic fibrosis by measuring the chloride in th
ID: 889333 • Letter: N
Question
Newborn infants are screened for cystic fibrosis by measuring the chloride in their sweat. Pilocarpine, a sweat inducer, is applied to a small spot on the skin. The sweat is than collected and analyzed using an ion selective electrode. If the chloride is above 70 mMol/liter, there is a strong suspicion of cystic fibrosis. The schematic of the one piece electrode is below:
Ag/AgCl reference// Cl- selective cell
Note the selective membrane is the cathode!
The potential when the electrode is placed in a 0.065 Mol/liter solution is 0.2235 volt. The potential of the electrode in the infant's sweat is 0.2329 volt. What is the chloride ion concentration?
Explanation / Answer
Since it is cathode,
reaction occuring is:
1/2 Cl2 + e- --->Cl-
E= Eo - 0.059 * log [Cl-]
case 1:
Potantial of 0.065 Mol/liter solution is 0.2235 volt
0.2235= Eo - 0.059 * log [0.065] .....eqn 1
case 2:
Potantial of 0.2329 volt in infant. Let concentration be M
0.2329 = Eo - 0.059 * log [M] ....eqn 2
subtract eqn 1 from eqn 2
0.2329 - 0.2235 = Eo - 0.059 * log [M] - { Eo - 0.059 * log [0.065] }
0.0094 = 0.059 * log [0.065] - 0.059 * log [M]
0.0094 = (-0.07004) - 0.059 * log [M]
0.059 * log [M] = -0.07944
M=0.045 M
Answer:0.045 M