Download Preparing a sample for titration

Ice Maker
Uses
- Instant making ice
- Three different ice sizes
- 12 pieces of ice in 10 minutes
Operation
- fill water in reservoir. Make sure the water is not over the ice collecting tray.
- connect to power
- select the desired ice cube size
- Press start and the ice maker begins to work
- if indicator’ DEFICIENT’ with noise sound ‘DI’ take out basket and add water
- if indicator’ ICE FULL’ red with noise ‘DIDI’TAKE OUT ICE WITH A SCOOP
Press STOP
Remove drain plug to drain remaining water, screw the drain plug back in place after water has been
drained
Caring
- clean the inner liner, ice collecting tray, water box, ice shovel and evaporator frequently. When
collecting, unplug the unit and remove the ice cubes. Use diluted water and vinegar to clean the inside
and outside surface. Do not spray the ice maker with chemicals or diluted agents such as acids,
gasoline or oil
-keep ventilation openings in the appliance enclosure or in the built in structure, clear of obstruction.
-if the machine is reused for a long time and the water pump cannot pump sufficiently due to air
inside, deficient water indicator turns on. Press the start button again it starts to work properly
-if the compressor just stops, wait for 3mins then restart
- Always change with fresh water before starting ice production at initial instillation or after long shut
off periods
Automatic Autoclave Sterilizer
Uses
-
For the sterilization of equipment
For the sterilization of microbial media
Operation
-
Open door and take out heater cover
Turn the water valve to ’FILLING’ to fill water in the chamber. Then turn it to ‘CLOSE’ to
stop filling
Put heater cover back to chamber. Then arrange sterilization items into the chamber.
Select ’STERILIZATION TEMP’
Press the ’POWER’ switch
Select ‘STERILIZATION TIME’
Close the door. Press the START BUTTON
Start HEATING by heater. The heat indicator should be lightened. Cold air will be exhausting
by the steam trap. Pressure rises to a preset value
Complete sterilization Buzzer sound for 40seconds.
Turn water valve to DRAINING
Take out your items only when the pressure gauge reads zero. Turn the valve to close then
open the door
Maintenance
Daily
Wipe inside chamber. Door and gasket with a damp lint cloth
Check the water level and top up with distilled water only.
Drain off the water inside the chamber every time after running a cycle
Weekly
Drain off the remaining water of water reservoir and refill with distilled water
Remove the cap of water reservoir and find the safety valve. Put the ringlet of the safety valve to
check its working status.
Monthly
Clean the chamber and piping system with ’CHAM-MATE’
Use clean water to scrub the water reservoir thoroughly
Annually
Contact distributor for maintenance
Check the silicon door gasket, steam trap safety valve and heater
Check function of printed circuit board
Check temperature during sterilisation
Spectrophotometer
It measures transmittance that is the amount of light that passes through a solution at a given
wave length
The transmittance is usually converted to absorbance.
According to Beer –Lambert law
Absorbance =-log transmittance
Absorbance is directly depended to the concentration of the solution and
A=abc
Where A=absorbance
a=molar absorptivity
b=path length of a sample holder
c=concentration of the solution
Calibration
Done to verify working status of the spectrophotometer
This is usually done by careful inspection of the standard curve of a chemical species that is
known to obey the Beer Lambert law. Linearity of the standard curve is a good indication of
instrument linearity and low stray light level
A solution of potassium dichromate i usually used prepared in 0,01N sulphuric acid
Maintenance
Turn the machine off and disconnect power cord before performing any maintenance
procedure
Several of the maintenance procedures involve components located in the Maintenance Lamp
compartment
Allow the bulbs to cool before replacing them. They can be either tungsten-halogen lamps or
deuterium lamps
Printer maintenance; the built in printer requires thermal paper. Before installing a new roll of
paper clean the printer head
Centrifuge
It is used for separating mixtures with differences in particle size. Those with a bigger
particle will settle at the bottom of the mixture.
Operation
Connect correctly to power. It is very important to balance the centrifuge when putting
samples into the bucket.
First close and lock the centrifuge lid
Select time by first pressing the set button and then +or- button the press set
Select speed then +or- then press set
To open the centrifuge make sure it has stopped. This is noted by reflection of 0.0 on the
speed screen. One can then unlock the centrifuge rid, open and remove buckets and tubes. If
the centrifuge is still in motion the lid will not unlock.
Maintenance
Clean all buckets thoroughly at regular intervals. Very care full remove all glass splinters
which may be in the buckets before returning them in position. The motor bearings are
grease, packed before leaving the factory and require no maintenance.
The centrifuge body needs to be cleaned periodically with silicon or wax polish.
Incubator
Uses; for the growth of microbial cultures using various microbial media.
Operations
Connect cord to a200to250AC mains
Turn the electrical button mains
Turn the of-off one
Temperature is controlled using the far right knob
Turn the knob to the required temperature of which the temperature will reflect after 5 to
10mins
For a temperature of 100degrees the reflection takes longer
A thermometer can be put in one of the shelves to control the temperature
Maintenance
Switch power off
Clean the inside regularly
Remove shelves and swab using absolute alcohol to kill most bacteria and yeast
Wash outside regularly. ALL THE PETRI DISHES SHOULD BE BURNT after every
microbiology practical session for they are an environmental hazard.
Magnetic Stirrer
Uses
It has the functions of heating, stable temperature and digital temperature display and timing.
It is also an ideal instrument for all kinds of analytical instruments to stirring solutions.
Measurement of pH
- pH is a measure of the acidity or basicity of a solution
- Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater
than 7 are basic or alkaline.
- In a solution pH approximates but is not equal to p[H], the negative logarithm (base
10) of the molar concentration of dissolved hydronium ions (H3O+); a low pH
indicates a high concentration of hydronium ions, while a high pH indicates a low
concentration.
Universal indicator components
Indicator
Low pH color Transition pH range High pH color
Thymol blue (first transition) red
1.2 – 2.8
yellow
Methyl red
red
4.4 – 6.2
yellow
Bromothymol blue
yellow
6.0 – 7.6
blue
Thymol blue (second transition) yellow
8.0 – 9.6
blue
Phenolphthalein
colorless
8.3 – 10.0
fuchsia
pH meter
A pH meter is an electronic instrument used to measure the pH (acidity or alkalinity) of a
liquid (though special probes are sometimes used to measure the pH of semi-solid
substances). A typical pH meter consists of a special measuring probe (a glass electrode)
connected to an electronic meter that measures and displays the pH reading.
How a pH meter works
When one metal is brought in contact with another, a voltage difference occurs due to their
differences in electron mobility. When a metal is brought in contact with a solution of salts or
acids, a similar electric potential is caused, which has led to the invention of batteries.
Similarly, an electric potential develops when one liquid is brought in contact with another
one, but a membrane is needed to keep such liquids apart.
A pH meter measures essentially the electro-chemical potential between a known liquid
inside the glass electrode (membrane) and an unknown liquid outside. Because the thin glass
bulb allows mainly the agile and small hydrogen ions to interact with the glass, the glass
electrode measures the electro-chemical potential of hydrogen ions or the potential of
hydrogen. To complete the electrical circuit, also a reference electrode is needed. Note that
the instrument does not measure a current but only an electrical voltage, yet a small leakage
of ions from the reference electrode is needed, forming a conducting bridge to the glass
electrode. A pH meter must thus not be used in moving liquids of low conductivity (thus
measuring inside small containers is preferable).
The pH meter measures the
electrical potential (follow the
drawing clock-wise from the
meter) between the mercuric
chloride of the reference
electrode and its potassium
chloride liquid, the unknown
liquid, the solution inside the
glass electrode, and the
potential between that solution
and the silver electrode. But
only the potential between the
unknown liquid and the
solution inside the glass
electrode change from sample
to sample. So all other potentials can be calibrated out of the equation.
The calomel reference electrode consists of a glass tube with a potassium chloride (KCl)
electrolyte which is in intimate contact with a mercuric chloride element at the end of a KCL
element. It is a fragile construction, joined by a liquid junction tip made of porous ceramic or
similar material. This kind of electrode is not easily 'poisoned' by heavy metals and sodium.
The glass electrode consists of a sturdy glass tube with a thin glass bulb welded to it. Inside is
a known solution of potassium chloride (KCl) buffered at a pH of 7.0. A silver electrode with
a silver chloride tip makes contact with the inside solution. To minimise electronic
interference, the probe is shielded by a foil shield, often found inside the glass electrode.
Most modern pH meters also have a thermistor temperature probe which allows for automatic
temperature correction, since pH varies somewhat with temperature.
Calibration and use
For very precise work the pH meter should be calibrated before each measurement. For
normal use calibration should be performed at the beginning of each day. The reason for this
is that the glass electrode does not give a reproducible e.m.f. over longer periods of time.
Calibration should be performed with at least two standard buffer solutions that span the
range of pH values to be measured. For general purposes buffers at pH 4 and pH 10 are
acceptable. The pH meter has one control (calibrate) to set the meter reading equal to the
value of the first standard buffer and a second control (slope) which is used to adjust the
meter reading to the value of the second buffer. A third control allows the temperature to be
set. Standard buffer sachets, which can be obtained from a variety of suppliers, usually state
how the buffer value changes with temperature.
The calibration process correlates the voltage produced by the probe (approximately 0.06
volts per pH unit) with the pH scale. After each single measurement, the probe is rinsed with
distilled water or deionized water to remove any traces of the solution being measured,
blotted with a clean tissue to absorb any remaining water which could dilute the sample and
thus alter the reading, and then quickly immersed in another solution.
When not in use, the glass probe tip must be kept wet at all times to avoid the pH sensing
membrane dehydration and the subsequent dysfunction of the electrode.
A glass electrode alone (i.e., without combined reference electrode) is typically stored
immersed in an acidic solution of around pH 3.0. In an emergency, acidified tap water can be
used, but distilled or deionised water must never be used for longer-term probe storage as the
relatively ionless water "sucks" ions out of the probe membrane through diffusion, which
degrades it.
Combined electrodes (glass membrane + reference electrode) are better stored immersed in
the bridge electrolyte (often KCl 3 M) to avoid the diffusion of the electrolyte (KCl) out of
the liquid junction.
Occasionally (about once a month), the probe may be cleaned using pH-electrode cleaning
solution; generally a 0.1 M solution of hydrochloric acid (HCl) is used, having a pH of about
one.
In case of strong degradation of the glass membrane performance due to membrane
poisoning, diluted hydrofluoric acid (HF < 2 %) can be used to quickly etch (< 1 minute) a
thin damaged film of glass. Alternatively a dilute solution of ammonium fluoride (NH4F) can
be used. To avoid unexpected problems, the best practice is however to always refer to the
electrode manufacturer recommendations or to a classical textbook of analytical chemistry.
pH meters range from simple and inexpensive pen-like devices to complex and expensive
laboratory instruments with computer interfaces and several inputs for indicator (ionsensitive, redox), reference electrodes, and temperature sensors such as thermoresistors or
thermocouples. Cheaper models sometimes require that temperature measurements be entered
to adjust for the slight variation in pH caused by temperature. Specialty meters and probes are
available for use in special applications, harsh environments, etc. Pocket pH meters are
readily available today for a few tens of dollars that automatically compensate for
temperature (ATC, Automatic Temperature Compensation).
Titration
Titration is a common laboratory method of quantitative chemical analysis that is used to
determine the unknown concentration of a known reactant. Because volume measurements
play a key role in titration, it is also known as volumetric analysis. A reagent, called the
titrant or titrator, of a known concentration (a standard solution) and volume is used to react
with a solution of the analyte or titrand, whose concentration is not known. Using a
calibrated burette or chemistry pipetting syringe to add the titrant, it is possible to determine
the exact amount that has been consumed when the endpoint is reached. The endpoint is the
point at which the titration is complete, as determined by an indicator. This is ideally the
same volume as the equivalence point—the volume of added titrant at which the number of
moles of titrant is equal to the number of moles of analyte, or some multiple thereof (as in
polyprotic acids). In the classic strong acid-strong base titration, the endpoint of a titration is
the point at which the pH of the reactant is just about equal to 7, and often when the solution
takes on a persisting solid color as in the pink of phenolphthalein indicator. There are
however many different types of titrations. Many methods can be used to indicate the
endpoint of a reaction; titrations often use visual indicators (the reactant mixture changes
color). In simple acid-base titrations a pH indicator may be used, such as phenolphthalein,
which becomes pink when a certain pH (about 8.2) is reached or exceeded. Another example
is methyl orange, which is red in acids and yellow in alkali solutions.
Not every titration requires an indicator. In some cases, either the reactants or the products
are strongly colored and can serve as the "indicator". For example, a redox titration using
potassium permanganate (pink/purple) as the titrant does not require an indicator. When the
titrant is reduced, it turns colorless. After the equivalence point, there is excess titrant present.
The equivalence point is identified from the first faint persisting pink color (due to an excess
of permanganate) in the solution being titrated.
Preparing a sample for titration
In a titration, both titrant and analyte are required to be in a liquid (solution) form. If the
sample is not a liquid or solution, the samples must be dissolved. If the analyte is very
concentrated in the sample, it might be useful to dilute the sample.
A measured amount of the sample can be given in the flask and then be dissolved or diluted.
The mathematical result of the titration can be calculated directly with the measured amount.
Sometimes the sample is dissolved or diluted beforehand, and a measured amount of the
solution is used for titration. In this case the dissolving or diluting must be done accurately
with a known coefficient because the mathematical result of the titration must be multiplied
with this factor.
Many titrations require buffering to maintain a certain pH for the reaction. Therefore, buffer
solutions are added to the reactant solution in the flask to maintain the pH of the solution.
Some titrations require "masking" of a certain ion. This can be necessary when two reactants
in the sample would react with the titrant and only one of them must be analysed, or when the
reaction would be disturbed or inhibited by this ion. In this case another solution is added to
the sample, which "masks" the unwanted ion (for instance by a weak binding with it or even
forming a solid insoluble substance with it).
Some redox reactions may require heating the solution with the sample and titration while the
solution is still hot, in order to increase the reaction rate. For instance, the oxidation of certain
oxalate solutions requires heating the solution to approximately 60 degrees in order to
maintain a reasonable rate of reaction.
Indicator
Color on Acidic
Side
Range of Color
Change
Color on Basic
Side
Methyl Violet
Yellow
0.0 - 1.6
Violet
Bromophenol Blue
Yellow
3.0 - 4.6
Blue
Methyl Orange
Red
3.1 - 4.4
Yellow
Methyl Red
Red
4.4 - 6.2
Yellow
Litmus
Red
5.0 - 8.0
Blue
Bromothymol
Blue
Yellow
6.0 - 7.6
Blue
Phenolphthalein
Colorless
8.3 - 10.0
Pink
Alizarin Yellow
Yellow
10.1 - 12.0
Red
Chromatography
It is the collective term for a set of laboratory techniques for the separation of mixtures. It
involves passing a mixture dissolved in a "mobile phase" through a stationary phase, which
separates the analyte to be measured from other molecules in the mixture based on
differential partitioning between the mobile and stationary phases. Subtle differences in a
compound's partition coefficient result in differential retention on the stationary phase and
thus changing the separation.
Chromatography may be preparative or analytical. The purpose of preparative
chromatography is to separate the components of a mixture for further use (and is thus a form
of purification). Analytical chromatography is done normally with smaller amounts of
material and is for measuring the relative proportions of analytes in a mixture. The two are
not mutually exclusive.