Glossary of Chloridometry

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What are the 4 types of analysis used in chloridometry?
1. Manual titration
2. Autoanalysis
3. Ion selective electrodes
4. Coulometry
what is manual titration?
adding a definite amt of a known substance to another substance being assayed until an end point is reached.
what is the reference method for chloride measurement?
What type of sample is measured?
Schales and Schales method.

-Protein free patient sample filtrate.
What reaction takes place?
2 Cl- + Hg++ -> HgCl2
-Excess mercury combines with the indicator and forms a color.
what are the disadvantages of the schales and schales method?
-Very sensitive endpoint to pH
-Halogens can cause false Pos.
-Pigment in serum masks the color change.
-Variability btwn techs.
-Time consuming
How does autoanalysis work for chloride measmnt?
-with an autoanalyzer!!
-Requires a dialyzer, room temp.
-Colorimetric endpoint.
what are the disadvantages of the autoanalyzer for cl-?
Only linear between 80-120 mM so it is not applicable for sweat or urine chloride msmt.
What is coulometry?
a titration in which the titrant is ELECTROCHEMICALLY generated.
how does coulemetric titration work in a nutshell?
titrant reacts with equivalent amounts of sample to produce product.
what is faraday's law?
96487 coulombs generate one equivalent weight of element.
How is the concentration of unknown found?
X unknown X standard
---------- = --------------
T unknown T standard
what are the major components of a coulometer?
1. Generator electrode
2. Auxiliary electrode
3. Conducting medium
4. Stirrer
what takes place at the generator electrode?
oxidation - this is the anode!

Ag wire -> Ag+ and e-
what takes place at the auxiliary electrode?
reduction - this is the cathode. Nitric acid provides the proton.

2H+ and 2e- -> H2
What reaction takes place after generation of the titrant?
Ag+ and Cl- make AgCl precipitate.
How is the end point detected in coulometric titration?
After all Cl- is precipitated, excess Ag+ increases the current in the conducting medium. Amperiometric electrodes detect that change in I.
What stops the titration process?
a pre-determined value of Ag+ excess; records the time at which it stops.
what is a chloridometer?
an instrument for determining Chloride by generating silver titrant coulometrically.
what 3 things need to be considered in chloridometry?
-Titration rate
what is the blank?
the time required for the indicator current to trip the relay circuit by rising 10 uA above the initial current after equiv point reached.
why do you need to consider a blank?
b/c otherwise the analyte measurement will be too high
how do you use the blank to correct msmt?
subtract the blank time from both the standard and unknown T
one mo time, why run a blank?
to find the time needed to generate a 10 mA current - that's the current that trips the switch to stop msmt.
why must titration RATE be considered?
because different analytes react at different rates. A POTENTIOMETER is used for adjustment.
What photodecomposition occurs in chloridometry?
the solid, AgCl, decomposes in light, so the chloridometer CAN'T BE IN DIRECT SUNLIGHT.
what are the 2 advantages of chloridometry?
1. Accuracy - within 0.5%, whereas the other methods are 10% (manual) or 12-15% (colorim)
2. Precision - within 0.3%!
What the 3 disadvantages of chloridometry?
1. Interference from other halides
2. Cyanate and Thiocyanate and Sulfhydryl ions come from the rubber stoppers, interfere w/ msmt.
3. Light sensitive
how do you correct for those disadvantages?
-Only use glass/plastic bottles and lids for reagents.
-Keep the chloridometer out of sunlight.
What needs to be done to maintain and care for the chloridometer?
-Replinish silver wire and polish it daily.
-Keep electrodes wet to prevent oxidation.
-Condition the electrodes before running the unknown

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