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29 Cards in this Set
- Front
- Back
What are the 4 types of analysis used in chloridometry?
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1. Manual titration
2. Autoanalysis 3. Ion selective electrodes 4. Coulometry |
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what is manual titration?
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adding a definite amt of a known substance to another substance being assayed until an end point is reached.
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what is the reference method for chloride measurement?
What type of sample is measured? |
Schales and Schales method.
-Protein free patient sample filtrate. |
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What reaction takes place?
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2 Cl- + Hg++ -> HgCl2
-Excess mercury combines with the indicator and forms a color. |
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what are the disadvantages of the schales and schales method?
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-Very sensitive endpoint to pH
-Halogens can cause false Pos. -Pigment in serum masks the color change. -Variability btwn techs. -Time consuming |
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How does autoanalysis work for chloride measmnt?
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-with an autoanalyzer!!
-Requires a dialyzer, room temp. -Colorimetric endpoint. |
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what are the disadvantages of the autoanalyzer for cl-?
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Only linear between 80-120 mM so it is not applicable for sweat or urine chloride msmt.
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What is coulometry?
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a titration in which the titrant is ELECTROCHEMICALLY generated.
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how does coulemetric titration work in a nutshell?
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titrant reacts with equivalent amounts of sample to produce product.
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what is faraday's law?
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96487 coulombs generate one equivalent weight of element.
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How is the concentration of unknown found?
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X unknown X standard
---------- = -------------- T unknown T standard |
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what are the major components of a coulometer?
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1. Generator electrode
2. Auxiliary electrode 3. Conducting medium 4. Stirrer |
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what takes place at the generator electrode?
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oxidation - this is the anode!
Ag wire -> Ag+ and e- |
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what takes place at the auxiliary electrode?
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reduction - this is the cathode. Nitric acid provides the proton.
2H+ and 2e- -> H2 |
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What reaction takes place after generation of the titrant?
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Ag+ and Cl- make AgCl precipitate.
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How is the end point detected in coulometric titration?
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After all Cl- is precipitated, excess Ag+ increases the current in the conducting medium. Amperiometric electrodes detect that change in I.
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What stops the titration process?
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a pre-determined value of Ag+ excess; records the time at which it stops.
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what is a chloridometer?
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an instrument for determining Chloride by generating silver titrant coulometrically.
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what 3 things need to be considered in chloridometry?
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-Blank
-Titration rate -Photodecomposition |
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what is the blank?
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the time required for the indicator current to trip the relay circuit by rising 10 uA above the initial current after equiv point reached.
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why do you need to consider a blank?
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b/c otherwise the analyte measurement will be too high
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how do you use the blank to correct msmt?
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subtract the blank time from both the standard and unknown T
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one mo time, why run a blank?
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to find the time needed to generate a 10 mA current - that's the current that trips the switch to stop msmt.
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why must titration RATE be considered?
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because different analytes react at different rates. A POTENTIOMETER is used for adjustment.
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What photodecomposition occurs in chloridometry?
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the solid, AgCl, decomposes in light, so the chloridometer CAN'T BE IN DIRECT SUNLIGHT.
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what are the 2 advantages of chloridometry?
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1. Accuracy - within 0.5%, whereas the other methods are 10% (manual) or 12-15% (colorim)
2. Precision - within 0.3%! |
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What the 3 disadvantages of chloridometry?
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1. Interference from other halides
2. Cyanate and Thiocyanate and Sulfhydryl ions come from the rubber stoppers, interfere w/ msmt. 3. Light sensitive |
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how do you correct for those disadvantages?
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-Only use glass/plastic bottles and lids for reagents.
-Keep the chloridometer out of sunlight. |
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What needs to be done to maintain and care for the chloridometer?
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-Replinish silver wire and polish it daily.
-Keep electrodes wet to prevent oxidation. -Condition the electrodes before running the unknown |