Download science background - CMA

Titration with step motor
burette
Science Background
CHEMISTRY
Acids
and bases
What does it mean for a solution to be acidic or basic?
An acid is a substance that donates hydrogen ions. Because of this, when an acid is
dissolved in water, the balance between hydrogen ions and hydroxide ions is shifted. Now
there are more hydrogen ions than hydroxide ions in the solution. This kind of solution is
acidic.
A base is a substance that accepts hydrogen ions. When a base is dissolved in water, the
balance between hydrogen ions and hydroxide ions shifts the opposite way. Because the
base "soaks up" hydrogen ions, the result is a solution with more hydroxide ions than
hydrogen ions. This kind of solution is basic (alkaline).
The pH scale
The pH scale was first used by the Danish biochemist Sören Sörensen in 1909 to create a
more efficient notation for the enormous range of the H 3O+ concentrations confronted
within his experiments. He defined pH as the negative logarithm (with base 10) of the
hydronium ion concentration and in that way reduced an inconvenient number to a simple
number. Henceforth we will write log instead of log10
𝑝𝐻 = −log⁡([𝐻3 𝑂+ ])
[H3O+] is expressed in powers of 10:
[𝐻3 𝑂+ ] = 10−𝑝𝐻
In neutral solution: [H3O+] = [OH-] = 10-7mol/L
In acidic solution:
[H3O+]
In basic solution:
[H3O+] < [OH-]
>
[OH-]
pH = pOH =7.
(pure water at 25°C)
pH < 7.
pH>7.
Conductivity
The Conductivity sensor measures the ability to conduct electricity in water solutions.
When salts and other inorganic chemicals dissolve in water, they break apart into
electrically charged ions. Ions increase the water’s ability to conduct electrical current.
Common ions in water that conduct electrical current include sodium, chloride, calcium,
and magnesium. Organic compounds, such as sugars, oils, and alcohols, do not form ions.
The principle by which the sensor measures conductivity is simple - two graphite plates
(cells) are placed in the sample, a potential is applied across the plates and the current is
measured. The Conductivity sensor actually measures the conductance of the solution
(the inverse of the resistivity R), which is determined from the voltage and current values
according to Ohm's law (G = 1/R = I/V).
Titration with step motor burette – Science background
1
The specific cell constant (K) of the conductivity electrode is used to determine the
conductivity (C). The conductivity is the cell conductance multiplied by the cell constant,
C=G*K. The electrode separation distance divided by the electrode area determines the
cell constant. The supplied electrode has a nominal cell constant K of 1.0 cm-1.
The SI unit of conductance is Siemens (S). Since S is a very large unit, conductance of
aqueous samples is commonly measured in S, and conductivity in S/cm. Some typical
conductivity ranges of hydrous solutions are:
Sample
Conductivity (μS/cm)
Pure water
0.055
Distilled water
0.5
Deionized water
0.1 – 10
Rain water
20 - 100
Drinking water
50 - 200
Tap water
100 - 1500
River water
250 - 800
Brackish water
1000 - 8000
KCI 0.01 M
1410
MgSO4
5810
KCI 0.1 M
12900
Ocean water
53000
H2S04
82600
KCI 1.0 M
112000
Titration
Titration is an analytical method in which a standard solution with the known concentration
is used to determine the concentration of another solution.
During titration the standard solution (also known as titrant) is slowly added to the solution
of unknown concentration by means of a burette. The endpoint of the reaction can be
observed by the colour change when using an indicator (for example phenolphthalein) or
detected by pH measurement. At the end point an amount of standard solution has been
added that just completely reacts with the solution titrated.
2
CMA Learning and Teaching Resources
The moles of standard solution can be calculated by multiplying the volume of standard
solution used by its molarity.
nstandard solution = Vstandard solution * cstandard solution
The moles in the titrated solution of unknown concentration are then found using the
coefficient in the chemical equation. Then, dividing the moles of the titrated solution by the
volume of that solution gives the concentration of the titrated solution.
ctitrated solution = ntitrated solution / Vtitrated solution.
Step motor burette
The step motor burette (titrator) is an actuator with which a liquid can be added to a
solution automatically and relatively accurately. That way, a certain volume of a liquid can
be added at constant speed, allowing for the creation of a graph in which you can plot pH
versus volume added. This might also be achieved by using a drop counter and photo
gate, but the step motor burette is much more convenient.
More information about the function of the step motor burette, connecting it to an interface
or tips about possible experiments can be found in the actuator’s manual.
Conductivity titration using step motor burette – Science background
3