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Файл:English in Analytical Chemistry communicating about Methods & Techniques. Учебное пособие. Книга для студента
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3.
CHEMICAL
6 Match the words from column A with the words from column B to make up
some frequently used collocations in analytical chemistry.
A B
to meet analyte
target furnace
suitable impurities
precipitating matter
suspended process
high procedure
pure product
muffle reagent
final reagent
gravimetric requirement
to remove solids
particulate standard
certified substance
primary temperature
overall weight
11
7 Now answer the questions to the text.
1. What is the purpose of gravimetry?
2. How do we determine the concentration of an analyte?
3. How do we obtain the solid form?
4. What is the requirement for the precipitating agent?
5. Where do they apply gravimetry according to the text?
6. How is an analyte removed from the solution?
7. How do they collect and ignite the precipitate?
8. What is the principal requirement in gravimetry concerning the mass of the
weighed substance?
9. What is a gravimetric factor?
Further Reading and Speaking
8 Read the text. Then close the text and fill in the correct prepositions in the
phrases below.
Gravimetric Analysis
Gravimetric analysis is one of the most accurate and precise methods of
macroquantitative analysis. In a gravimetric analysis, a measurement of mass or
change in mass provides quantitative information about the amount of analyte in a
sample. The most common form of gravimetry uses a precipitation reaction to
generate a product whose mass is proportional to the analyte. In many cases, the
precipitate includes the analyte; however, an indirect analysis in which the analyte
causes the precipitation of another compound is possible. Precipitation gravimetric
procedures must be carefully controlled to produce precipitates that are easily
filterable, free from impurities, and of known stoichiometry.
In volatilization gravimetry, thermal or chemical energy is used to decompose the
sample containing the analyte. The mass of residue remaining after decomposition, the
mass of volatile products collected with a suitable trap, or a change in mass due to the
loss of volatile material are all gravimetric measurements.
When the analyte is already present in a particulate form that is easily
separated from its matrix, then a particulate gravimetric analysis may be feasible.
Examples include the determination of dissolved solids and the determination of fat in
foods.
Gravimetric analysis is capable of exceedingly precise analysis. Gravimetry does
not require a series of standards for calculation of an unknown since calculations are
based only on atomic or molecular weights. Only a precise analytical balance is
needed for measurements. Gravimetric analysis, due to its high degree of accuracy, can
also be used to calibrate other instruments in lieu of reference standards.
(based on D. Harvey – Analytical Chemistry 2.0)
1) change ________mass 7) to be separated ________the matrix
2) to be needed
_______measurements
3) to be proportional _______ 9) to be based ________ atomic weights
4) to be free _________impurities 10) a series of standards ____ calculation
8) to be capable _______precise analysis
12
5) to be _______ known stoichiometry 11) ____ lieu of reference standards
6) due _____ the loss 12) method_______ analysis
9 Make sentences using the completed phrases from Task 8. Ask your costudents to listen carefully and to translate these sentences into Russian.
10 Match the words with their Russian equivalents.
1. similarities a) различия
2. moisture b) измерение массы
3. purity c) сходства
4. differences d) предварительно взвешенный фильтр
5. definitive analysis e) гравиметрия
6. gravimetric analysis f) удерживать
7. volatile g) гравиметрическое осаждение
8. suspension h) взвесь
9. preweighed filter i) чистота
10. reaction completion j) определяющий анализ
11. volatilization k) влага
12. suspended solid l) завершение реакции
13. direct / indirect method m) гравиметрическое выделение
14. to retain n) взвешенные частицы
15. particular gravimetry o) летучий – о веществе
16. gravimetric precipitation p) гравиметрическая отгонка
17. mass measurement q) прямой и непрямой метод
11 Read the text “Overview of Gravimetric Methods”, explain the words in bold
and complete the gaps 1-6 with the words provided in a) and b).
Overview of Gravimetric Methods
Before we consider specific gravimetric methods, let us take a moment to
develop a broad survey of gravimetry. Later, as you read through the descriptions of
specific gravimetric methods, this survey will help you focus on their similarities instead
of their differences. You will find that it is easier to understand a new analytical method
when you can see its relationship to other similar methods.
13
a) direct analysis & indirect analysis
Suppose you are to determine the total suspended solids in the water released
by a sewage-treatment facility. Suspended solids are just that — solid matter that has
yet to settle out of its solution matrix. The analysis is easy. After collecting a sample,
you pass it through a preweighed filter that retains the suspended solids, and dry the
filter and solids to remove any residual moisture. The mass of suspended solids is the
difference between the filter’s final mass and its original mass. We call this a
1._____________________ because the analyte—the suspended solids in this
example — is the species that is weighed.
What if our analyte is an aqueous ion, such as Pb
2+
? Because the analyte is not
a solid, we cannot isolate it by filtration. We can still measure the analyte’s mass directly
if we first convert it into a solid form. If we suspend a pair of Pt electrodes in the sample
and apply a sufficiently positive potential between them for a long enough time, we can
force the reaction to completion: Pb
2+(
aq) + 4H2O (l) = PbO2(s) + H2(g) + 2H3O+(aq).
Oxidizing Pb2+ deposits PbO2 on the Pt anode. If we weigh the anode before and after
applying the potential, the change in its mass gives the mass of PbO2 and, from the
reaction’s stoichiometry, the amount of Pb2+ in the sample. This is a direct analysis
because PbO2 contains the analyte.
Sometimes it is easier to remove the analyte and let a change in mass serve as
the analytical signal. Suppose you need to determine a food’s moisture content. One
approach is to heat a sample of the food to a temperature that vaporizes the water,
capturing it in a preweighed absorbent trap. The change in the absorbent’s mass
provides a direct determination of the amount of water in the sample. An easier
approach is to weigh the sample of food before and after heating, using the change in
its mass as an indication of the amount of water originally present. We call this an
2.____________________ because we determine the analyte using a signal that is
proportional to its disappearance. The indirect determination of a sample’s moisture
content is done by difference. The sample’s initial mass includes the water, but its final
mass does not. We can also determine an analyte indirectly without its ever being
weighed.
b) particulate gravimetry, electrogravimetry, volatilization gravimetry,
precipitation gravimetry
There are four different ways in which the measurement of mass may serve as an
analytical signal. When the signal is the mass of a precipitate, we call the method
3._________________. The indirect determination of PO
3-
by precipitating Hg2Cl2 is an
3
example, as is the direct determination of Cl- by precipitating AgCl.
In _4.____________________, we deposit the analyte as a solid film an
electrode in an electrochemical cell. The deposition as PbO2 at a Pt anode is one
example of 4.____________________. The reduction of Cu2+ to Cu at a Pt cathode is
another example of 4.____________________.
When we use thermal or chemical energy to remove a volatile species, we call
the method 5.__________________. In determining the moisture content of bread, for
example, we use thermal energy to vaporize the water in the sample. To determine the
amount of carbon in an organic compound, we use the chemical energy of combustion
to convert it to CO2.
Finally, in 6.___________________ we determine the analyte by separating it
from the sample's matrix using a filtration or an extraction. The determination of total
suspended solids is one example of 6.___________________.
14
c) definitive technique
2
Except for particulate gravimetry, which is the most trivial form of gravimetry, you
probably will not use gravimetry after you complete this course. Why, then, is familiarity
with gravimetry still important? The answer is that gravimetry is one of only a small
number of definitive techniques whose measurements require only base SI units,
such as mass or the mole, and defined constants, such as Avogadro’s number and the
mass of
12
C. Ultimately, we must be able to trace the result of an analysis to a definitive
technique, such as gravimetry, that we can relate to fundamental physical properties.
Although most analysts never use gravimetry to validate their results, they often
verifying an analytical method by analyzing a standard reference material whose
composition is traceable to a definitive technique.
(based on D. Harvey – Analytical Chemistry 2.0)
12 Fill in the correct preposition, and then make sentences using the completed
phrases.
1) to read ________ the descriptions
2) to focus_______ similarities
3) instead ________ differences
4) released _____ a sewage-treatment facility
5) to settle ______of solution matrix
6) pass _____ filter
7) determination of Cl~ ____ precipitating AgCl
8) the reduction of Cu
+
_____ Cu
9) to determine the analyte____separating
10) separate_______ matrix
13 Match the lines in the table provided below and write the full definitions for all
the terms provided in the table.
For your definitions, use the structure:
A is B that / which / where C, where A is a term, B is a class or category, and C is
a description.
Remember: that normally goes with essential information, which introduces
nonessential one, and where is linked to location or space.
Term (А) Class (B) Attribute (C)
Direct analysis (1) type of method
a) uses vaporization
Indirect analysis b) uses the presence of
analyte
Precipitation gravimetry (2) way to analyze
c) uses the absence of
analyte
Electrogravimetry d) uses solely SI system
units
Volatilization gravimetry e) uses conversion from
15
(3) advantage of method liquid into solid
Particulate gravimetry f) uses electrochemical
process
Definitive technique g) uses filtration or
extraction techniques
14 Give your own examples of precipitation gravimetry, electrogravimetry,
volatilization gravimetry, and particulate gravimetry. Use the verbs below to
convey the information in English.
Determine, settle out, collect, pass (through), retain, dry, remove, isolate, measure,
convert, suspend, apply, force (to completion), weigh, deposit, contain, serve, heat,
vaporize, capture, provide, include, precipitate, add, produce.
15 Read the following excerpt from a lab report on determining the percentage
(by mass) of sulfate in an unknown sulfate salt by gravimetric analysis. Think of
Russian equivalents of the vocabulary in red.
Objectives
To learn the techniques associated with gravimetric analysis.
To use stoichiometry to calculate the percentage by mass of sulfate in an
unknown sulfate salt.
Background
Gravimetric analysis is a quantitative method for accurate determination of the
amount of a substance by selective precipitation of the substance from an aqueous
solution. The precipitate is separated from the remaining aqueous solution by filtration
and is then weighed. Assuming that the chemical formula for the precipitate is known
and that the precipitation reaction is stoichiometric (goes to completion), the mass of the
substance in the original sample can be determined.
In this experiment, you will determine the percentage (by mass) of sulfate in an
unknown sulfate salt by gravimetric analysis. First you will dissolve a measured mass
of the unknown salt in water. Next you will add an excess of aqueous barium chloride to
the aqueous solution of the unknown salt. This will result in the precipitation of the
sulfate as barium sulfate.
BaCl2 (aq) + M2SO4 (aq) — BaSO4 (s) + 2 MCl (aq) (assuming +1 cation)
BaCl2 (aq) + MSO4 (aq) — BaSO4 (s) + MCl2 (aq) (assuming +2 cation)
The barium sulfate precipitate is collected by filtration, dried, and weighed. The
number of moles of sulfate can be determined from the mass of the barium sulfate.
Since barium chloride is added in excess and since the precipitation reaction is
assumed to go to completion, the number of moles of sulfate recovered in the
precipitate can be assumed to be equal to the number of moles of sulfate in the original
sample allowing for the calculation of the percentage by mass of sulfate in the original
sample.
In order to obtain the optimal results, the BaSO4 crystals should be as large as
possible. This facilitates filtration and washing of the crystals, and the decreased
surface area minimizes the amount of impurities adsorbed onto the crystals. Generally
16
the largest crystals are obtained when the rate of precipitation is as low as possible. The
rate of precipitation is minimized by slow addition of the BaCl
solution to the aqueous
2
mixture containing the unknown salt while continuously stirring the mixture. The
precipitation rate can be further decreased by a slight increase in the solubility of
BaSO4 (remember that a substance that is said to be insoluble is in fact slightly soluble).
The increase in solubility is reached by lowering the pH with 6M HCl and by increasing
the temperature. The resulting decrease in the yield of BaSO4 is insignificant.
Procedure
Materials and Equipment
Crucible and cover
Ashless filter paper (110 mm diameter)
0.1 M BaCl2 solution
6 M HCl solution
Safety
Be careful while handling 6 M HCl. If any HCl comes in contact with your skin or
eyes, you should immediately wash with water for several minutes. You should be
wearing your goggles at all times. Make sure to allow ample time for the crucible to cool
after it has been heated.
(based on Santa Monica College lab reports)
16 Study the second part of the same lab report and put the steps 1-12 into the
correct order.
1. Add 50 mL of distilled water to the sample in the beaker. Next add 20
drops of 6 M HCl to the beaker. Stir the contents of the beaker until the sample has
entirely dissolved. Leave the stirring rod in the beaker.
2. After 20 minutes has passed slowly pour the mixture containing the
BaSO4 precipitate down your stirring rod into the funnel. Be careful that the level of
liquid in the funnel is never more than three-fourths of the way to the top of the filter
paper. When the transfer is complete use your wash bottle (filled with distilled water) to
rinse the residual precipitate from the beaker and the stirring rod into the funnel.
3. After all the liquid has drained from the funnel very carefully press the top
edges of the filter paper together, and fold the filter paper into a compact package that
will fit into the crucible. In order to avoid tearing the filter paper it is important that you do
not use too much force. Place the folded filter paper into the crucible.
4. Allow the crucible to cool. When the crucible has cooled to room
temperature, record the mass of the crucible, the cover and its contents to the nearest
0.001g on the analytical balance.
5. Discard the BaSO4 in the proper waste container.
6. Heat the solution containing the sample in the 250 mL beaker until it is
nearly (but not quite) boiling. Turn the burner off and slowly pour small portions of the
BaCl2 solution into the 250 mL beaker containing the sample. This step should take at
least 3 minutes otherwise the BaCl2 is being added to rapidly. Stir the contents of the
beaker as you add the BaCl2 solution. You should observe the formation of the white
BaSO4 precipitate. Rinse any precipitate that remains on the stirring rod into the solution
with a small amount of distilled water and allow the precipitate to settle in the beaker for
about 20 minutes.
17
7. Measure 25 mL of 0.1 M BaCl
solution in a graduated cylinder. The
2
graduated cylinder should be clean and rinsed with distilled water but does not need to
be dry.
8. Measure and record the mass of a clean, dry 250 mL beaker using the
analytical balance to the nearest 0.001 g. Add between 0.30 g and 0.35 g of your
unknown sample to the 250 mL beaker and record the mass of the beaker plus sample
to the nearest 0.001 g.
9. Obtain a piece of ashless filter paper and fold it into quarters. Open the
folded paper into a cone, place it into a funnel and wet the filter paper with distilled
water so that it adheres to the funnel. Place a 500 mL Erlenmeyer flask under the funnel
to collect the filtrate.
10. Once all the filter paper has turned black heat the crucible vigorously with
the cover off in the hottest part of the Bunsen burner flame so that the bottom of the
crucible is red hot. The charred filter paper (carbon) will gradually combust and be
converted into CO2 gas. When the filter paper is entirely combusted only the white
BaSO4 should remain in the crucible. The crucible should be heated vigorously until
there is no charred filter paper remaining.
11. Set up your ring stand in the fume hood and support the crucible in a clay
triangle that is attached to your ring stand. Gently heat the crucible without the cover to
remove the water. After several minutes when you are sure the paper is dry, heat the
crucible more vigorously so that the filter paper begins to char (turning from white, to
brown, to black) but not so vigorously that the filter paper bursts into flame. If the filter
paper bursts into flame you should smother it with your crucible cover and lessen the
amount of heat. Continue to heat moderately until all of the filter paper has turned black.
12. While the precipitate settles prepare your crucible by heating it (with the
cover on) in the hottest part of the Bunsen burner flame for about 2 minutes. After the
crucible has cooled to room temperature record the mass of the crucible and the cover
to the nearest 0.001 g using the analytical balance.
(based on Santa Monica College lab reports)
17 Read an instructive example of a typical gravimetric procedure and answer the
questions below.
Determination of Mg
+
in Water and Wastewater
2
Description of Method. Magnesium is precipitated as MgNH4PO4 • 6H2O using
(NH4)2HPO4 as the precipitant. The precipitate's solubility in neutral solutions (0.0065
g/100 mL in pure water at 10 °C) is relatively high, but it is much less soluble in the
presence of dilute ammonia (0.0003 g/100 mL in 0.6 M NH3). The precipitant is not very
selective, so a preliminary separation of Mg
+
from potential interferents is necessary.
2
Calcium, which is the most significant interferent, is usually removed by its prior
precipitation as the oxalate. The presence of excess ammonium salts from the
precipitant or the addition of too much ammonia can lead to the formation of
Mg(NH4)4(PO4)2, which is subsequently isolated as Mg(PO3)2 after drying. The
precipitate is isolated by filtration using a rinse solution of dilute ammonia. After filtering,
the precipitate is converted to Mg2P2O7 and weighed.
Procedure. Transfer a sample containing no more than 60 mg of Mg
+
into a
2
600-mL beaker. Add 2-3 drops of methyl red indicator, and, if necessary, adjust the
volume to 150 mL. Acidify the solution with 6 M HCI, and add 10 mL of 30% w/v
(NH4)2HPO4. After cooling, add concentrated NH3 dropwise, and while constantly
stirring, until the methyl red indicator turns yellow (pH > 6.3). After stirring for 5 min, add
5 mL of concentrated NH
, and continue stirring for an additional 10 min. Allow the
3
resulting solution and precipitate to stand overnight. Isolate the precipitate by filtration,
18
rinsing with 5% v/v NH
. Dissolve the precipitate in 50 mL of 10% v/v HCI, and
3
precipitate a second time following the same procedure. After filtering, carefully remove
the filter paper by charring. Heat the precipitate at 500 °C until the residue is white, and
then bring the precipitate to constant weight at 1100 °C.
(based on D. Harvey – Analytical Chemistry 2.0)
Questions
1. Why does the procedure call for a sample containing no more than 60 mg of
Mg2+?
2. Why is the solution acidified with HCI before the precipitant is added?
3. Why is the acid-base indicator methyl red added to the solution?
4. Explain why the formation of Mg(PO3)2 in place of Mg2P2O7 increases the
mass of precipitate.
5. What additional steps in the procedure, beyond those discussed in questions 2
and 3, are taken to improve the precipitate's purity?
6. Why is the precipitate rinsed with a solution of 5% v/v NH3?
Practice Makes Perfect
18 Complete the statements below.
1. In gravimetry, they determine the concentration of an analyte by...
2. The precipitating agent should be...
3. Gravimetry is a principal method for...
4. The precipitate is collected by... Then it is rinsed to...
5. After rinsing the precipitate...
6. To obtain a pure form, it might be necessary to.. It is normally done with...
7. The mass of the weighed substance must...
8. The gravimetric factor includes...
19 Put the steps of iron gravimetric determination into the logical order. Add the
purposes of the steps if they are not provided. Translate them into Russian in
writing.
Gravimetric Determination of Iron
Ignite sample to convert iron hydroxide to iron oxide.
Add ammonium hydroxide to precipitate iron
hydroxide.
Add dilute HCl.
Clean and dry crucibles of constant weight.
Accurately weigh test portions of sample.
Filter and collect all solid.
Calculate iron in each test portion.
Add hydrogen peroxide to oxidize iron.
19
Wash precipitates with hot water and pour through filter paper.
Cool crucible in desiccator and then weigh.
Dry precipitate and filter paper. Transfer them to crucible.
Heat solution to remove peroxide.
20 Match the steps of iron gravimetric determination with the purposes they
pursue.
Steps’ Purposes
A. Constant weight is the repetitive drying and weighing of a crucible until the
weight does not change. This process is necessary for the clean crucibles and when
they contain the precipitate so that the difference in weights is due only to the
precipitate.
B. The hydrogen peroxide is added to oxidize all iron to Fe3+ because
Fe(OH)3(s) has a much lower K_sp than Fe(OH)2(s).
C. The filter aid is added because iron hydroxide tends to form small colloidal
particles that clog the filter paper.
D. The filter aid and filter paper are both removed by combustion.
E. The thorough washing is necessary to remove soluble interferences.
F. Handbooks provide detailed procedures for gravimetric analysis, including
precipitating agents and common interferences. Note that the weighed form is often
different from the initial precipitate; for example, calcium oxalate and Al and Fe
hydroxides are converted to their oxide. The conversion provided by this step creates a
more stable form that is free of waters of hydration and other impurities.
(based on B.M. Tissue – The Basics of Analytical Chemistry)
21 Translate the statements into English.
1. Добавьте разбавленную соляную кислоту к каждой части тестируемого
раствора.
2. Прокалите образец для перевода гидроксида железа в оксид железа.
3. Гравиметрия – это количественный метод определения концентрации
анализируемого вещества путём взвешивания его чистой формы.
4. Основные шаги в гравиметрии – осаждение, фильтрование, прокаливание и
высушивание.
5. Гравиметрический фактор – это отношение молекулярной массы аналита к
молярной массе его чистой формы.
Discussion
22 Prepare a visual retelling of Gravimetric Determination of Iron. Visual retelling
means that your speech is accompanied by visual images. You may use Power
Point slides, you may use pictures, and you may draw while speaking.
20
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