PHOSPHATE BUFFER PRACTICAL Flashcards
WHAT IS A BUFFER?
solution that prevents a drastic pH chage
-consists of a conjugate pair -> acid and conjugate base or base and conjugate acid
-weak acid - salt = negative ion (conjugate base)
-weak base - salt = positive (conjugate acid)
-2 different salts of the same polyprotic acid
-an amphoteric electrolyte (protein)
WHAT ARE THE MOST IMPORATANT BUFFERS IN THE HUMAN BODY?
-carbonate, phosphate, hemoglobin
WHAT ARE BUFFERS USED FOR IN THE LAB?
buffers are used to prepare solutions of known stable pH - they are used as standard solutions to calibrate pH meter or help maintain pH of some reaction mixtures
WHAT DO PROTONS REACT WITH?
conjugate base
WHAT DO HYDROXIDE IONS REACT WITH?
conjugate acid
WHAT HAPPENS WHEN AN ACID IS ADDED TO THE SOLUTION?
most of the extra protons are removed / neutralized by the base
WHAT HAPPENS WHEN A BASE IS ADDED TO THE SOLUTION?
most of the extra hydroxide ions are removed by a reaction with undissociated acid
WHAT IS THE BEST RATIO FOR A BUFFER?
ratio 1:1 of the conjugate pair is the most efficient for handling the addition of either a base or an acid
-> If the buffer contains much
more conjugate acid than conjugate base, it is less efficient in handling in acid. Alternatively, a buffer with much more of the conjugate base than of conjugate acid cannot efficiently counteract the addition of a base.
For example, the H2CO3 / HCO3
- buffer in blood has a 1:20 ratio of acid to its conjugate base. Blood is then more effective in counteracting acids, which are the products of metabolism
ACIDIC BUFFER CALCULATION
pH = pKa + log (csalt/cacid)
BASE BUFFER CALCULATION
pOH = pKb + log (csalt/ cbase)
pH = 14 - pOH
WHAT IS pKa?
negative common logarithm of the dissociation constant of the acid (pKa = - log Ka)
WHAT IS Ka?
an equilibrium constant describing a dissociation of the acid: HA <–> H+ + A-
(i.e. Ka is andissociation constant of the acid)
WHAT IS pKb
negative common logarithm of the dissociation constant of the hydroxide (pKb = - log Kb)
WHAT IS Kb?
an equilibrium constant describing a dissociation of the hydroxide: BOH B+ + OH-
(i.e. Kb is an dissociation constant of the hydroxide)
WHAT IS THE MOST EFFICIENT BUFFER?
pH = pK (the ratio of concentrations is 1/1 because log(1/1) = 0), sufficient efficiency is found if a buffer pH = pK ± 1 (it is related to the ratio of 1/10 or 10/1)
ACIDIC BUFFER WITH MOLAR CONCENTRATION
pH = pKa + log (c´salt x Vsalt / c´acid x Vacid)
BASIC BUFFER WITH MOLAR CONCENTRATION
pOH = pKb + log (c´salt x Vsalt / c´base x Vbase)
pH = 14 - pOH
WHAT IS THE CAPACITY OF A BUFFER?
the substance amount of acid or base it must be added into the buffer to change its pH by 1 pH unit (substance amount of acid or base divided by related pH change).
-buffer has its highest capacity at equal concentrations of weak acid and conjugate base, when p H = p K a
-to increase capacity -> increase total concentration
DEFINE POTENTIOMETRY
electrochemical analytical method that is based on the measurement of voltage of an electrochemical cell when no current flows
WHAT IS A WORKING / INDICATING ELECTRODE?
sensitive for a compound of interest – its potential is changed if an activity (concentration) of the compound is changed
WHAT IS A REFERENCE ELECTRODE?
one whose potential is stable
- The electrode potential of those electrodes is constant, it is not influenced by
ion of interest.
EG: metal wire covered by layer of an insoluble salt that has common cation with the wire; it is
dipped into a solution that has the same anion with the insoluble salt (e.g. argentchloride electrode:
Ag/AgCl/solution of saturated KCl; calomel electrode: Hg/Hg2Cl2/solution of saturated KCl)
WHAT IS THE ELECTRODE POTENTIAL?
“expression of energy” of processes proceeding on the electrode. It is defined as the voltage of the electrochemical cell that consists of this electrode as a cathode and a standard hydrogen electrode (SHE) as an anode
WHAT IS THE ELECTRON POTENTIAL OF THE SHE ELECTRODE?
0.00 volts under all temperatures
WHAT DOES THE NERNST PETERSON EQUATION REPRESENT?
relationship between the electrode potential and the activity of any ion of interest
WHAT DOES POTENTIOMETRIC ANALYSIS CONTAIN?
-voltmeter
-reference electrode
-salt bridge
-analyte
-indicator electrode
-magnetic mixer
NERNST PETERSON EQUATION
IF REDOX PAIR IN STANDARD THE STATE THE EQUATION IS SIMPLIFIED
DEFINE ANODE
the electrode at which oxidation occurs; anions move toward the anode because it is
positively charged
DEFINE ANODE
the electrode at which oxidation occurs; anions move toward the anode because it is
positively charged
DEFINE CATHODE
the electrode at which reduction occurs; cations move toward the cathode because it is
negatively charged
DEFINE CURRENT
the flow of charge
-> current of 1 ampere = 1 coulomb per sec
DEFINE POTENTIAL
it is an expression of energy; the gradient (slope) of a potential causes charge to move;
the movement of one coulomb of charge from a potential of V to a potential of V-1 volts requires one joule of energy
WHAT IS A SALT BRIDGE?
a device that allows charge to flow as ions between two separate containers of
electrolyte that are also connected by metallic conductors
WHAT IS FARADAYS CONSTANT?
the number of coulombs in a mole of electrons which is 96 485 coulombs per mole
WHAT IS POTENTIOMETRY USED FOR IN VOLUMETRIC ANALYSIS?
for indication of a point of equivalence during neutralization or redox reactions
WHAT ARE MEMBRANE / ION SELECTIVE ELECTRODES?
potentiometric electrodes that develop a potential in the presence of one ion (or class of ions) but not in the presence of a similar concentration of other ions
EG: electrodes specific to H+, Na+, K+, Cl-
WHAT IS THE GLASS ELECTRODE?
glass membrane sensitive to H+ and it is commonly used for determination of solution pH
WHICH ELECTRODE IS DIFFERENT TO OTHER TYPES BASED OF THE PRINCIPLE OF ELECTRODES?
ion selective electrodes
WHAT IS A COMBINATION ELECTRODE?
glass electrode and reference electrode
IONIC PRODUCT OF WATER
EQUILIBRIUM CONSTANT OF WATER
DEFINE EQUILIBRIUM CONSTANT
a value that links the amounts of reactants and products in a reversible reaction at equilibrium
WEAK ACIDS AND BASES
DISSOCIATION OF TRIHYDROGEN PHOSPHORIC ACID
EXAMPLES OF REFERENCE ELECTRODES
argentchloride
calomel
- metal wire and layer of insoluble salt in common with the cation of the wire
WHAT IS USED TO CALIBRATE THE PH MACHINE?
Two buffers of known solution are used to set relationship between known value of ph and a measured value of ph
