Intro Lab Methods
Chapter 7
Laboratory Measurements
These notes will only look at those measurements which will be made in Intro Lab
Methods. A full discussion of other measurements is available in the text Practical
Laboratory Skills.
This chapter looks at the following physical properties:
• pH
• refractive index
• density
• conductivity
pH
Measurement of pH is a widely applied technique for monitoring agricultural,
industrial and biomedical processes. pH is a measure of the acidity of a sample, a
low pH (ie <7) indicates the sample is acidic, a high pH (>7) indicates the sample is
basic.
pH may be measured using:
1. indicator papers: paper impregnated with coloured dyes, whose colour is
sensitive to the pH of the solution to which they are exposed
2. indicator solutions: solutions which are made up from the same coloured dyes
which are used for indicator papers. They are added directly to the test material
where the coloured form is used to assign a pH value.
3. pH meter: an electronic instrument which gives a direct readout of pH. It uses a
glass probe which generates an electrical signal in proportion to the hydrogen ion
concentration of the solution in which it is immersed. The pH meter must be
calibrated prior to use to ensure the validity of the result.
Refractive index
The refractive index of a substance is defined as the ratio of light velocity in a
vacuum to its velocity in the particular substance.
For routine laboratory applications, velocities are not measured. Instead the bending
caused by the velocity change between the vacuum and the material is measured.
Because the substance always slows down or retards the passage of the light beam,
the light beam is bent or refracted and the amount of the refraction or refractive index
is measured.
In routine laboratory practice, the major factors which affect RI values are:
• the material
• the temperature
• the wavelength of light used
• the purity of the sample
The standard instrument for measuring RI is the refractometer.
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Conductivity measurements
Solutions which contain ions will conduct electricity, just like a metal does.
Conductivity is the ease with which a current can be carried by a solution.
Conductivity is measured using a special probe attached to a digital or analogue
meter. The meter must be calibrated in order for the value obtained to have valid
significance
Measurement of density
The density of any gas, liquid or solid is its mass per unit volume.
mass
Density
=
volume
Mass is measured with a balance
Volume is measured by direct reading of a volume scale on the container or from
displacement measurements by immersion in a suitable liquid. The object is
immersed in the liquid and the apparent rise in level equals the volume of the object.
Problems arise when:
• the material is soluble
• the void spaces absorb fluid
• the material dissolves, swells, expands or shrinks
Liquid density measurements
Two basic methods are used
1. density bottles (pycnometers): highly precise volumetric vessels which are filled
with the liquid in question and weighed.
2. hydrometers: specially weighted floats which are immersed in the liquid.
Refractive index can be correlated with density. The producers and users of sugarbased materials make extensive use of density as measured by hydrometry and
refractometry to analyse for sugar content.
Practical work 7.1
pH measurements
The practical tasks provide experience with various detection methods for the
determination of pH. Your teacher will demonstrate the use of each method. Whilst
working with each consider the accuracy and efficiency of the method.
Practical work 7.2
Calibration of pH meter
Practical work 7.7
Measurement of density
The practical is designed to give the student skills in determining the density of a
variety of different types of material, (irregular solids, granular solids, liquids etc)
using a variety of techniques (hydrometry, pycnometer etc).
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Density of an irregular solid
1. obtain three objects and record their identity
2. measure and record the mass of the objects
3. partially fill an appropriate measuring cylinder with water and record the initial
volume
4. Carefully immerse the object in the measuring cylinder
5. record the new volume
6. complete the worksheet
Bulk density of various granular solids
1. obtain three different granular solids
2. record the mass of an appropriately sized dry measuring cylinder
3. add a predetermined volume of sample to the measuring cylinder
4. reweigh the measuring cylinder, record the mass
5. gently tap the measuring cylinder to pack down the granular solid
6. record the compacted volume
7. add a known quantity of water to the measuring cylinder
8. record the new volume in the measuring cylinder
9. calculate the true volume of the granular solid
10. complete the worksheet
Density of liquids
1. weigh and record the mass of a clean, dry 25mL measuring cylinder
2. add 25 mL of the liquid sample to the measuring cylinder
3. reweigh the measuring cylinder and record the mass
4. complete the worksheet
5. follow the teacher’s instructions regarding disposal of the organic solvents
Density of liquids by hydrometry
1. follow the teachers instructions regarding use of the hydrometer
2. carefully float the appropriate hydrometer in a measuring cylinder of the allocated
liquid
3. read the value from the bottom of the meniscus
Determination of sugar content by hydrometry and refractometry
1. carefully measure the hydrometer reading of each of the sugar solutions
provided, ensuring that the hydrometer is dried before placement in a new
solution
2. carefully measure the refractive index of each of the solutions following
instructions given by the teacher
3. determine the concentration of sugar in each of the unknowns by plotting
i) Hydrometer reading vs sugar content
ii) Refractive index vs sugar content
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Student Name:
Practical:
pH measurements
Practical Number: 7.1
Date Performed:
Text book References
Date Submitted:
Procedure:
Indicator solutions, test papers and the pH meter are used to measure pH. Different
requirements for accuracy and speed will dictate which method is appropriate.
Complete the table below by determining the pH of the samples provided, using the
methods as shown by your teacher
Results:
Sample
Litmus
Red
Litmus
Blue
BTB
Phenolphthalein
Universal
indicator
Other?
Dip
stick
pH
meter
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Questions:
Which method do you consider the most reliable?
Which household chemical was the most acidic?
Which household chemical was the most alkaline?
What information does litmus paper give?
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Student Name:
Practical:
To determine melting point.
Practical Number: 7.3
Date Performed:
Date Submitted:
Text book References see text book P. 152, 153
Results:
Sample
First sign of melting
Completely liquid
Literature value
Sample 1:
benzoic acid
Sample 2:
2 – naphthol
Mixture of samples
1 & 2:
Sample 3:
Possibilities are:
Unknown Code
Mix of Sample 3 +
first suspected
compound
conclusion
Mix of Sample 3 +
second suspected
compound
conclusion
Unknown code …
was found to be
Sample 4:
Possibilities are:
Unknown Code
Mix of Sample 4 +
first suspected
compound
conclusion
Mix of Sample 4 +
second suspected
compound
conclusion
Unknown code …
was found to be
Questions:
Answer the questions at the end of Practical exercise 7.3 in the textbook.
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Student Name:
Practical:
To measure the refractive index of liquids.
Practical Number: 7.5
Date Performed:
Date Submitted:
Text book References see text book P. 155
Results:
A. Refractive index of a variety of solvents:
Substance
RI reading
Temperature
Corr. factor
Corr. RI
Conclusion
Water
Trichloromethane
Unknown
Unknown
Unknown
B. Refractive index of sugar solutions using sucrose calibration standards:
RI reading
Temp.
Corr. factor
Corr. RI
5%
10%
20%
Unknown 1
Unknown 2
Unknown 3
C. Refractive index of wine solutions using ethanol calibration standards
RI reading
Temp.
Corr. factor
Corr. RI
5%
10%
20%
Unknown 1
Unknown 2
Unknown 3
Questions:
Answer questions at the end of Practical exercise 7.5 in the text book.
You will need to prepare calibration graphs for each set of sugar and wine solutions and obtain the
answers for the amount of sugar and ethanol in each of the unknowns.
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Student Name:
Practical:
To measure the conductivity of various liquids.
Practical Number: 7.6
Date Performed:
Date Submitted:
Text book References see text book P. 156
Results:
Conductivity of aqueous materials
Solute
Concentration Instrument
reading
Units
Water
KCl
KCl
KCl
NaCl
NaCl
NaCl
NaCl
NaCl
HNO3
NaNO3
KNO3
Ca(NO3 ) 2
Mg (NO3 ) 2
Zn (NO3 ) 2
Questions
What trends exist for your NaCl solutions?
Comment on which cations and anions are better conductors.
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Student Name:
Practical:
Density
Practical Number: 7.7
Date Performed:
Text book References
Procedure:
Results:
1. Density of an irregular solid
Sample
mass (g)
Initial
Identity
Volume
or code
(mL)
Date Submitted:
Final
Volume
(mL)
True
Volume
(mL)
Density
(g/mL)
Literature
Density
You need to reference the source of your literature value data here.
2. Bulk density of various granular solids
Sample
I.D.
Empty
cylinder
mass (g)
Sample +
Cylinder
Mass (g)
Mass
sample
(g)
Bulk
volume
(mL)
Settled
volume
(mL)
True
volume
(mL)
Bulk
density
Settled
density
True
density
Summary.
Bulk
Settled
True density
Sand densities are
Styrene bead densities are
Saw dust densities are
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3. Density of liquids &
4. Density of liquids by hydrometry
Sample
I.D.
Empty
cylinder
mass (g)
Sample +
cylinder
mass (g)
Sample
mass (g)
Volume
sample
(mL)
Density
(g/mL)
Hydrometer
reading
Lit. Density
5. Sugar content by hydrometry
Sample identity
Hydrometer reading
Refractive index
5% sugar
10% sugar
20 % sugar
30% sugar
unk 1
unk2
You must plot a calibration graph (manually or by excel) for your RI and hydrometer
results and use it to calculate the sugar content of each of your unknowns.
Concentration unk 1
=
=
_________________ % (by hydrometer)
_________________ % (by RI)
Concentration unk 2
=
=
_________________ % (by hydrometer)
_________________ % (by RI)
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