CH 455 exam 1
what indicator will you be using for the purposes of this experiment- EDTA
EBT for Ca and Mg... Murexide for Ca
why do we use EBT as an indicator? what does the end point signify?
EBT is selective for Mg and endpoint tell total hardness
why do we use buffers for the titrations- EDTA
EDTA titrations have clearer endpoints (due to larger formation constants) at higher pH values.
describe the detector used in this experiment (GC)
Flame ionization detector—burns organic sample—make ions—current measured
describe how you will construct your calibration curve (HPLC)
From dilutions from a supplied standard stock solution
what is the primary reason to use HPLC instead of GC
HPLC can analyze non- volatile materials
what is joule heating (CE)
Heating that occurs within the solution bc of the motion created by charge carriers
describe the components of the GC instrument
Helium carrier gas, injector, column inside oven, detector
what two types of injection methods will you use today? describe how each of them works (CE)
Hydrodynamic (pressure sucking)// and electrokinetic (charge dif)
does EDTA bind equally to Ca and Mg? which has the higher complex ion formation constant
No- Kf around 10 for Ca... Kf for mg around 10^9. EDTA has a higher complex ion formation constant with Ca
what is the standard potential- CV
Potential we use to compare everything else to
what are the major components of an HPLC system and what is the function of each
Pump - forces mobile phase through at a specific flow rate Injector - injects sample into flow stream of mobile phase Column - separates the sample components of interest Detector - detects individual molecules that elute from the column Computer - determines time of elution
define spike recovery and explain why we use it in this lab (HPLC)
Spike recovery is a known quantity of analyte added to a sample to test whether the response to the sample is the same as that expected from a calibration curve
based on the structures of capsaicin and diyhydrocapsaicin, which will elute first and why (HPLC)
There is a soluble bond in the dihydrocapsaicin so it will come off first
describe how the detector used in this experiment works (HPLC)
UV visible detector- detects absorption of light. (aromatic ring can be detectible w UV vis)
conjugate base
a base that forms when an acid loses a proton... formate
what does a CV for a reversible reaction look like
a duck
chelating ligand
a ligand having more than one atom with a lone pair that can be used to bond to a metal ion.
what should potassium cyanide never come in contact with
anything near u
why do we use potentiometric titration for this experiment instead of just using an indicator
bc we are doing a precise measurement of pH—so the ability to take a first and second derivative to see inflection point is better
explain how determining the fist and second derivative will aid you in finding the experimental endpoints
better job at finding the peak- better inflection point
masking agent- EDTA
chemical that can be added to the sample solution to prevent the interfering ions from binding with EDTA, allowing it to complex the analytes.
how will you know when you have reached the titration endpoint- EDTA
color change
what is the function of counter electrode
controls for potential
define elctroosmotic flow (CE)
due to plug flow. any molecule will follow the flow of ions/ analytes
purpose of EDTA lab
figure out Ca and Mg ions in water
Define electrophoretic flow
flow occurs from attraction from opposite charges
what are the three ionization steps of phosphoric acid
h3po4 -> h2po4-, hpo4-2, po4-3
what is a polyprotic acid
has more than 1 proton
which gas is used as the carrier gas (GC)
helium
why is EDTA chosen for this titration
it is good at complexing (high K value)
how might the mobile phase be modified to provide greater separation of the two peaks (HPLC)
make the particle sizes smaller
how many endpoints will you observe in the experiment (potentiometric titration)
only 2 because the third dissociation is so strong that hydroxide is not able to remove proton
what does a pH meter measure
pH
define weak acid
partially ionized in aqueous solution
how does a pH meter work
pourus glass and known concentration of acid sitting inside the inner container of the pH meter and unknown amount outside—depending on the difference—u look at electro chemical potential difference of inside to outside.
what are the electrodes in a three- electrode cell?
reference, working, counter
why do we use murexide as the indicator for the second set of titrations
specificity for calcium
what are the characteristics of a good reference electrode?
stability and not easily polarizable
how do we vary the applied potential in a chronoamperometry experiment
stop function charge
solubility product
the constant for the equilibrium expression representing the dissolving of an ionic solid in water
when the reaction begins, how does the current change
the current increases
what is reverse phase HPLC
the stationary phase is non polar and the liquid phase (solvent) is polar.
describe what is happening chemically at each of the endpoints
the stronger acid protons fall off and at the second endpoint, the second proton of the triprotic acid falls off.
why dont we measure the water hardness of distilled water
there arent any metal ions in there
how will you know when you reached the first and second endpoints
there will be a steep climb in pH
complexometric titration
titration based on complex formation with metal ion
what is the purpose of the supporting electrolyte and what are you using in this experiment? (CV)
to minimize migration
where should u dispose of used filters and SPE cartridges (HPLC)
trash
why is all glassware acid washed- EDTA
we are looking for metal ions that easily attach to glass—acid wash to remove all lewis acid
what dont we calibrate the pH meters?
we are only interested in the differences in pH
is EDTA a strong acid or weak acid
weak hexodente ligand- hexoprotic acid (it can bind 6 times, forming a coordination complex)
when would you use a masking agent? -EDTA
when there can be an interfering ion
what is the function of working electrode
where potential change happens
what is the function of reference electrode
working against comparison
what process is responsible for the shape of the chronoamperometry curve after the potential step
Diffusion process as u watch a return back down because u see a slowing down in current generated as current
why do multidente ligands form more stable metal complexes than similar monodente ligands
it is much more thermodynamically stable (greater increase in entropy from binding 6 molecules as opposed to binding one)
what color will your solution be at the first endpoint? the second endpoint?
no color because we are not adding an indicator
what is normal phase HPLC
polar stationary phase and nonpolar liquid phase (solvent)
when a substance is being reduced, what type of current is produced
positve
conjugate acid with example
product that is different from a base by one proton... formic acid
what are the three acid dissociation constants and/or pka values for phosphoric acid
2,7,12
how many endpoints would expect to observe in the titration of a tripotic acid with a strong base
3 points
how many deprotonations need to occur in order for EDTA to effectively bind to a metal?
5 or 6
how many pka values does EDTA have
6
which metal ion must be present in the water for a color change to be produced and constant-EDTA
?
how do we vary the applied potential in a CV experiment
???
how are we eliminated electromigration and convection in this experiment
Adding potassium chloride salt/// convection by not stirring
why cant the hot sauce be injected directly into the instrument (HPLC)
Because the amount of particles traveling through must be minimized to prevent clogging. Also remove dissolved gases
what property of the compounds in this experiment determines their elution order (GC)
Boiling point
how does your internal standard make your analysis more accurate (GC)
By using an internal standard we can make sure that the prep work that precedes it is done in exactly same way
define pH
-log[H+]
how do you convert from the standard potential to the potential vs. the Ag/ AgCl electrode
.197 (add this to silver silver chloride electrode)
what is your internal standard (GC)
1-butanol
what metal ions are you going to test for in your experiment- EDTA
Ca and Mg ions
what metal ions generally contribute to water hardness-EDTA
Ca and Mg ions
what causes water hardness in nature?
Ca ion and Mg ion that form with hydroxides
what are the health hazards of capsaicins (HPLC)
Can burn mucous membranes and cause heartburn
how can you determine the water hardness due to calcium and magnesium individually
Can determine individual hardness of calcium by using murexide/NaCl compound which ONLY binds calcium, and EBT binds both Mg and Ca. We can subtract the hardness of Ca (murexide) from the total hardness (EBT)
what are some advantages for using a GC
Cheap and easy/ provides high resolution
what are some of the parameters that can be adjusted to achieve better separation (GC)
Choice of temp on the column—this is why we can vary the temperature.
Define gas chromatography
Chromatography w gas as mobile phase
define strong acid
Completely ionized in aqueous solution
by which three methods can an ion in solution reach an electrode surface
Convection, migration, diffusion
explain electrophoretic mobility (CE)
Every different chemical molecule has size and charge—and ability to move thru buffered soln .. small things high charge are more mobile
why are we increasing the temperature during a sample run (GC)
Increase temp bc higher BP would never come off... if we did really high then everything would come off... this is why some people use a temperature gradient.
formation constant
K value associated with ligand complexing with lewis acid
explain the internal standard method (GC)
Known amount of 1-butanol and know exactly its concentration—u can measure its response... quick way to calibrate
what type of molecules/ chemicals/ compounds are best suited for separation by CE
Mc that are not volatile/ dissolve in water/ dif in polarity and shape.. pretty much any kind can be separated
what is the purpose of CV experiment
Measure different coefficients for ferri to ferro... follow formation of interesting products by irreversible CV reaction thr-> tyr
which metal ion must be present in the water for a color change to be produced and observed- EDTA
Mg ion
what must u do to your sample before injecting it into the HPLC
Must run thru solid phase extraction
explain what a response factor is and how it is used in to calculate the concentrations of the compounds in this experiment (GC)
Response factor is a absorptivity factor In beers law—the multiplier for the relationship between concentration and the standard and the signal detected by flame ionization detector.
what can you tell me about the properties of each phase in reverse phase HPLC? relate this to stationary phase and mobile phase composition
Reverse phase has a nonpolar stationary and polar solvent Eluent strength is increased by making the mobile phase more like the stationary phase
how would the CV shapes differ for reversible and irreversible reactions
Reversible look like duck// irreversible look like dead duck
how are the components separated while passing through the column (GC)
Separated bc at any given temp- their likelihood of condensing down onto the stationary phase is dependent upon the type of molecule that it is... by varying temp u can get things off column
water hardness
The amount of dissolved calcium and magnesium in the water
describe how you will determine the concentrations of the mixture of two acids by a single titration
The first endpoint represents = amount NaOH required to remove all protons from HCl and one proton from H3PO4. The second end point= amount NaOH required to remove one more proton from H2PO4-. o Subtract amount NaOH at first endpoint from second endpoint -> concentration of H2PO4- could be determined. oSubtract moles H2PO4- from the moles of total acid calculated from the concentration of NaOH at the first endpoint.
describe how to use the solid phase extraction (SPE) (HPLC)
The sample is passed through the cartridge containing a hydrophobic stationary phase. Many of the components in the sample matrix, including the capsaicinoid analytes will be retained. This will then be washed out by eluting methanol and 1% acetic acid solution.
what reference electrode is being used... what is its reduction potential? CV
The silver- silver chloride electrode... .13 V difference
