Hı: my name ıs Brıan
Wrıght
Lets take a moment so you can
get to know me a lıttle.
I LOVE TEACHING CHEMISTRY
However ıt was not my fırst
career...
Acids
HCl, Hydrochloric acid
H2SO4 ,Sulfuric acid
HNO3, nitric acid
All are soluble in water. When placed in
water they disassociate to become
H+ Cl-
2H+ SO42-
H+ NO3-
H+ , Hydronium, Proton
When acid dissolve in water they release a
positive charged hydrogen ion.
A bare proton H+ cannot exist in solution because of its strong tendency to attach
itself to atoms or molecules with electrons. However, the term 'proton' is used
loosely to refer to positively charged or cationic hydrogen, denoted H+.
To avoid the convenient fiction of the naked "solvated proton" in solution, acidic
aqueous solutions are sometimes considered to contain the hydronium ion (H3O+),
which is organized into clusters to form H9O4+.[38] Other oxonium ions are found
when water is in solution with other solvents.
HCl(aq) +H2O(l) H3O+(aq)+ Cl-(aq)
These are the most common models for
hydronium.
Keep it Simple Students (KiSS)
You can certainly think of the hydrogen
ion from acids as hydronium. However
for many aspects of chemistry it is far
easier to think of it only as a proton.
Thus
HCl (aq) H+ Cl-
When the hydrogen and the chlorine in this
example disassociate the chlorine keep the
electron and leave only the hydrogen proton
behind.
Brønsted-Lowery definition of
acid
Definitions
Acid proton donor [H+] [OH-]
Base proton acceptor [H+] [OH-]
There are more complicated definitions
however this will work well for this now.
Acids simplified
So in the most simple form when you add
acid to water you end up with a lot of this
H+ H+ H+ H+ H+ H+ H+
H+ H+ H+ H+ H+ H+ H+
H+ H+ H+ H+ H + H+ H+
H+ H+ H+ H+ H + H+
In a aqueous solution.
Bases simplified
When you add base to water you get a lot of
this
OH- OH- OH- OH- OH-
OH- OH- OH- OH- OH- OH-
OH -OH-OH- OH- OH- OH- OH-
OH-
OH- OH- OH- OH- OH-
So what?
Indicators
One way to determine if a solution is either
an acid or base is to use an indicator.
Acid base indicators can also be used to
provide a rough estimate regarding the
concentration of the acid or basic solution.
Indicators are weak acids or weak
bases.
The generalized formula for a Acid base
indicator is Hin when protanated and In-
when it is depotonated.
HIn H+ In-
Just like a normal acid
HCl H+ + Cl-
Various acid base indicators
We will use phenolphthalein
Limitations to Indicators
Indicators can not be effectively used to
measure instantaneous pH.
If there is a dynamic chemical reaction, like
a titration, indicators are not useful for
providing pH data as a function of time.
pH meters
A pH meter can measure the concentration
of H+ quickly and accurately.
This makes them Ideal for measuring H+
concentration while conducting a titration.
HOW DOES IT WORK
A pH probe is a sensitive galvanic cell.
It has two half cells each using silver and silver
chloride.
Ag+ + 1e− ↔ Ag0(s)
Ag+ + Cl− ↔ AgCl(s)
or an overall reaction can be written:
Ag0(s) + Cl− ↔ AgCl(s) + e-
1. a sensing part of electrode, a bulb made from a
specific glass
2. sometimes the electrode contains a small
amount of AgCl precipitate inside the glass
electrode
3. internal solution, 0.1M HCl for pH electrodes
4. internal electrode, usually silver chloride
electrode
5. body of electrode, made from non-conductor
6. reference electrode, usually the same type as 4
7. junction with studied solution, usually made
from ceramics or capillary with asbestos or
quartz fiber.
Calibrate pH probe
Buffer 4
Rinse
Buffer 7
Rinse
If probe needs to be adjusted please
follow instructions.
Laboratory setup
How to stir
(be very careful the probe is very fragile)
White paper underneath
(if lab counter is dark)
May be easiest to set up two stands
One for the buret
One for the probe.
How to Pipette
Use you index finger
Twist
Let gravity work
Touch once to the glass
Your class may prefer to use two burets
instead of pipetting the acid.
This is not last years Lab
Last year when you did a titration it was only
necessary to find the end point. In the this
lab you will be monitoring the pH level.
Then the pH level goes up .15 you then stop
the flow of base and record the buret volume.
It is critical to this lab that you are skilled at
reading a buret.
How Read a Buret
This is the most important skill to the
successful completion of this lab.
Reading you buret accurately will provide
you with precise and accurate data.
Significant Figures and the
Buret
The buret has 1.0 mL graduations
These are further divided into 0.1 mL
graduations
It is then possible to estimate to the 0.01
mL
The uncertainty is +/- 0.01 mL
Reading a Buret
Remember you will be reading to the
bottom of the meniscus.
This sound pretty easy however there are
3 visible layers to the meniscus.
Always read to the bottom layer.
Keep in mind that the thickness of the line
on the buret is typically about 0.02mL
One other thing to remember.
One any precision analog device the
thickness of the graduations must be
considered when taking measurements
Keep in mind that the thickness of the line
on the buret is typically about 0.02mL
Lets Try a Few readings.
Remember you will be reading to the
bottom of the meniscus.
This sound pretty easy however there are
3 visible layers to the meniscus.
Always read to the bottom layer.
Keep in mind that the thickness of the line
on the buret is typically about 0.02mL
The reading is
1.28 mL
The reading is
6.89 mL
The reading is
19.48 mL
The reading is
46.36mL
Lets Make it Easier
Place something behind the buret.
You can use your finger
or even better a card that is ½ black and ½
white
The reading is
1.95 mL
The reading
is
A little hard
to read
because
the
graduations
are over to
the side.
15.11 mL
Lab Group Assignments
Each task must be complete by the same
person for the whole trial. Do enough trials
that each person get a chance to do each
task.
Data collection, Qualitative and quantitative.
Pipette acid or deliver acid with a buret
Stir and run the buret
Read the buret
Read the pH data.
Clean up.