Parr 4744, 4745, 4746, 4747, 4748 OPERATING INSTRUCTIONS
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OPERATING INSTRUCTIONS
Parr Acid Digestion Bombs
No. 249M
Scope
These instructions are to acquaint the user with the procedures to be followed and the precautions to be taken
when using any ParrAcid Digestion Bomb, except those
designed specifically for microwave heating.(Microwave
bombs are covered by separate instructions, No. 243M.)
Parr PTFE-lined digestion bombs provide a convenient
means for dissolving analytical samples rapidly in strong
acids or alkalis,it is important that the user understand
the capabilities and limitations of the equipment and
will be well aware of the safety precautions to be observed in its operation. Pages 1 through 6 of these instructions apply equally to all Parr Acid Digestion
Bombs with metal bodies and removable PTFE liners.
The user should read these basic instructions carefully
before starting to use any of these bombs. Then turn to
pages 8 through 11 for the special instructions which
apply to each individual model. It must be understood
that the excellent mechanism for sample digestion and
dissolution provided by these bombs can be hazardous if
a bomb is misused. Therefore careful reading and full
compliance with these instructions must be observed in
all applications.
Contents
Scope ......................................................................... 1
Contents.................................................................... 1
Assumption of Risk .................................................. 2
The Nature of PTFE................................................. 2
Potential Hazards .................................................... 2
Sample Selection......................................................3
Loading Limits ......................................................... 3
Acid Selection........................................................... 4
Pressure and Temperature Limits.......................... 4
Heating and Cooling the Bomb ............................... 5
Pre-Treating PTFE Parts ........................................ 5
Liner Maintenance................................................... 5
Liner Lifetimes......................................................... 5
Bomb Maintenance .................................................. 6
References................................................................. 6
Vapor Pressure Tables............................................. 7
Operating Procedure
4745 General Purpose Bomb.................................8
4744 and 4749 General Purpose Bombs...............9
4746 and 4747 High Strength Bombs................. 10
4748 Large Capacity Bomb................................. 11
Assumption of Risk
Potential Hazards
The Parr Instrument Company offers its PTFE-lined
Acid Digestion Bombs to skilled analytical chemists as
an attractive means for digesting and dissolving analytical samples for analysis. Parr designed and manufactures these bombs to be as effective as possible when
used within the limitations prescribed for each individual model. But, since the pressures generated within
these bombs are solely dependent upon the nature of the
materials being treated, the filling level and the amount
of heat applied to promote the reaction, Parr will not be
responsible for personal injuries or damage to the bomb,
to the oven or to other equipment resulting from the use
of these bombs. As with all laboratory operations, the
user must assume responsibility for and institute safety
procedures to protect all personnel from any hazards associated with this equipment. Rigid controls must be established to guarantee that the operator does not
overcharge or overheat the bomb.
The Nature of PTFE
PTFE offers such unique inertness and high temperature usefulness that it is an obvious choice as the material of construction for lining these acid digestion bombs.
PTFE does, however,have two characteristics which
make it somewhat less than perfect for this application,
and the user who understands these deficiencies will be
able to minimize the effect upon his work.
First, PTFE has a tendency to creep or flow under pressure or load. This tendency is present even at room temperature and it is accentuated at higher temperatures.
At operating temperatures below 150ºC the creep effect
will become more pronounced, making it more difficult to
maintain tight seals and resulting in deformation and
shorter life for the PTFE components. The extent of the
creep effect will be roughly proportional to the maximum
operating temperature.
Secondly, PTFE is a porous material. Although the materials and designs used in Parr Acid Digestion Bombs
minimize the effects of this porosity,users of these
bombs can expect to see evidence of vapor migration
across the cover seal and through the wall of the liner itself. Parr is able to minimize these problems by machining these parts from virgin PTFE which has been
molded at an optimum pressure selected to reduce any
porosity to an absolute minimum. The thick walls and
long path seals used in Parr bomb liners also help to
overcome these undesirable properties. Experiments
have shown that the amount of solute lost in this manner during a normal digestion is negligible, but vapor
migration will occur and frequently it will be sufficient
to produce noticeable discoloration on the inner metal
wall of the bomb body and the screw cap.
While many thousands of these bombs have been used
safely and routinely for treating a great variety of samples with different digestion media under a broad range
of operating conditions,it is possible to create conditions
under which these bombs will explode. The bulk of the
reported incidents of this type have been caused by failure of the operator to recognize one or more of the
following potential hazards.
Excessive temperature. When a bomb is overheated,
two factors come into play: (1) the vapor pressure of the
materials in the bomb increases exponentially with temperature and (2) the strength of the materials from
which the bomb is made falls off (again exponentially)
if the bomb is heated above its maximum temperature
limit. Dangerous overheating can be produced by ovens
with defective temperature controls, by water baths boiling dry, or by operator inattention or carelessness.
Excessive pressure. Excessive pressure can be produced not only by overheating, as mentioned above, but
also from uncontrolled gaseous reactions and from high
vapor pressure or explosive materials, or from overloading the bomb as mentioned below.
Excessive loading. When organic materials are treated
in these bombs, they may liberate gases as well as heat.
Since the PTFE liner is an excellent thermal insulator,
this internal heat will be translated into higher internal
temperatures and pressures. The loading limits prescribed for these bombs are purposely conservative to
ensure that the energy released from the sample will not
over-stress the bomb.The user must also remember that
when a water-based solution is heated to 250ºC it expands to fill a space approximately 25 percent larger
than its volume at room temperature. If there is insufficient vapor space in the bomb to accommodate this expansion, the tremendous hydrostatic pressure which
will be generated will destroy the bomb.
Explosive materials. The nitro compounds produced
when nitric acid reacts with certain organic materials
may have explosive properties capable of destroying the
bomb, even when present in quantities well within the
normal recommended charging limits.Consider, for example, what might happen if nitroglycerin were produced by reactions in the bomb. For this reason, fats,
fatty acids, glycerin and similar materials must not be
treated with nitric acid in these bombs, and cellulosic
materials must not be treated with mixed nitric and
sulfuric acids. Similarly, because of its unpredictable
nature, perchloric acid must not be used in these
bombs.
2
Sample Selection
Inorganic materials. Most inorganic digestions proceed smoothly without unusual hazards, using not more
than 1.0 gram of sample in a 23 mL bomb, 2.0 grams in a
45 mL bomb and 5.0 grams in the larger,125 mL size. As
in all reactions, the bomb must never be completely filled
as there must always be vapor space above the surface of
the charge.To besure that there is adequate free space,
the total volume of the charge must never exceed twothirds (66%) of the capacity of the cup when working
with inorganic materials. By observing these limits and
taking precautions to prevent overheating, there should
be no unusual hazards in treating inorganic samples
with mineral acids.
Ores, rock samples, glass and other inorganic materials
can be dissolved in Parr acid digestion bombs using
strong mineral acids: HF, HCl, H2SO4, HNO3, Aqua Regia and others.Digestion times for these materials can
vary anywhere from 2 hours to several days. Ordinary
glass materials (SiO2) will mandate the use of HF,sometimes in combination with HCl, or aqua regia. Temperatures in the range of 100 to 150ºC are routinely used.
Alumina is routinely digested using 10% sulfuric acid.
Temperatures used for these samples are typically in excess of 200ºC.It is advantageous, from the standpoint of
minimizing the digestion time, to reduce the sample to
granular or powder form prior to digestion. The increase
in surface area of the sample has a significant impact on
the reaction with the digestion aid.
The following
used for glass and other silicate samples using the Parr
23 mL Acid Digestion Bomb. For the dissolution of glass,
sand and mineral silicate samples, weigh 0.4 g of the
powdered sample into the PTFE liner.Moisten the sample with water and cautiously and 4 mL of 40 to 50% HF
to the liner. Cover the liner and allow it to stand until
the initial reaction has taken place, then seal the bomb.
Place the bomb assembly into a preheated lab oven for 2
hours at 130-150ºC.Remove the bomb from the oven, and
after cooling the room temperature, the bomb may be
opened.
Elements that form insoluble fluorides, such as Al, Ba,
Ca, and Mg can be dealt with effectively by adding 1
gram of boric acid, after cooling, and re-heating again for
1 hour. A blank, used throughout the dissolution and
analysis procedure,should contain the same amount of
HF and boric acid. For example, it is important to have
the boric acid blank subtracted from the sample spectrum in ICPES analysis to account for the boron interferences with other elemental lines.
Organic materials. Many organic materials can be
treated satisfactorily in these digestion bombs but careful attention must be given to the nature of the sample
and to possible explosive reactions with the digestion me-
general digestion procedure can be
dia. In all cases the size of the sample and the amount of
oxidant used must be carefully controlled. For nitric
acid digestions (Carius decompositions) of organic compounds, the dry weight of organic matter must not exceed 0.1 gram in a 23 mL bomb, 0.2 gram in a 45 mL
bomb and 0.5 gram in the 125 mL size. The sample does
not have to be dried before it is placed in the bomb, but
the amount used must not exceed the above limits when
converted to a dry weight basis. The amount of concentrated nitric acid (sp. g 1.42) added to the above charges
must fall within the amounts shown in the table of loading limits. Notice that both minimum and maximum
amounts of acid are specified. If the sample contains less
than the specified maximum amount of dry organic matter, the amount of nitric acid must be reduced proportionately.
As stated above, fats, fatty acids,glycerin and similar materials which form explosive compounds in an intermediate stage must not be treated with nitric acid in these
bombs. Digestions involving other organic materials
must also be handled cautiously since it is impossible to
list all of the potentially dangerous combinations which
might arise. For best protection, the user and his supervisor should study each reaction carefully before proceeding to use the Parr digestion bomb or any other pressure
vessel, asking such questions as: Is the reaction exothermic? What intermediate and final products might be produced and what will be their behavior?
When working with new or unfamiliar materials it is always advisable to run preliminary experiments using
small samples and observing the behavior of the reactants carefully.This initial screening is best conducted in
the heavier 4746 bomb which has a safety rupture disc.
Organic samples are typically treated with concentrated
nitric acid. Nitric - sulfuric mixtures are not recommended for digesting organic samples due to the possibility of forming potentially unstable reaction products.
Digestion with perchloric acid can be dangerous,for the
same reason, and must not be used. Typical heating
Loading Limits
Minimum and
Bomb
No.
4744
4745
4746
4747
4748
4749
Size
mL
45
23
23
23
125
23
Maximum
Inorganic
Sample
2.0 gram
1.0
1.0
1.0
5.0
1.0
Maximum
Organic
Sample
0.2 gram
0.1
0.1
0.1
0.5
0.1
Maximum Nitric
Acid to be used
with an
Organic Sample
5.0-6.0 mL
2.5-3.0
2.5-3.0
12-15
2.5-3.0
3
times range from 1 to 8 hours. Temperatures in the 150
to 200ºC range are generally quite effective.
The following
ganic samples using the Parr 23 mL Acid Digestion
Bomb. Weigh no more than 0.1 g of sample (dry weight
basis) into the PTFE digestion vessel. Add 2.5 mL hot
concentrated nitric acid and let stand open for 15 minutes. Add 0.2 mL of hydrofluoric acid to dissolve siliceous
materials contained in the sample. (In case that the sample does not contain siliceous material, this step can be
omitted.) Close the vessel and place into a preheated
oven (100-150ºC) for 1 to 4 hours,as required until the
solution is clear. If the solution is colloidal, heat until
clear. Remove the vessel from the oven and let cool to
room temperature (2 hours) before opening.Sample
sizes and amount of reagents used are proportional to
the free volume of the PTFE liner.
general digestion procedure for or-
Acid Selection
With the vigorous action that can be achieved in these
bombs at elevated temperatures and pressures, most
organic samples can be decomposed in nitric acid alone
without using mixed nitric and sulfuric acids as in oxidations at atmospheric pressure. Users should always try
nitric acid alone and resist the temptation to add sulfuric acid to the charge. Sulfuric acid may not be needed at
the higher temperature and it may tend to dehydrate the
system and promote the formation of unstable nitro
compounds.
Do not use perchloric acid. We repeat again the
warning that perchloric acid should not be used in these
bombs because of its unpredictable behavior when
heated in a closed vessel. Also, other reactions which are
highly exothermic or which might be expected to release
large volumes of gas should be avoided.
Pressure and Temperature Limits
Maximum working pressures for each acid digestion
bomb are shown in the individual listings on pages 8
through 11. Extreme care must be exercised to ensure
that pressures do not exceed these prescribed limits.The
user must understand that in acid systems the solubility
of gases such as NO
the temperature rises, having the effect of adding noncondensable gas to the vapor phase in a closed vessel. As
a result, the amount of acid present, the acid concentration and the free head space above the liquid will all
have a bearing on the pressure developed in a closed
bomb. Free head space must therefore be provided in
all procedures,and the volume and concentration of
the acid must be held to a minimum.
Maximum operating temperatures for each acid digestion bomb are shown in the individual listings. But these
limits must be used cautiously because, with certain
acids, pressures higher than the allowable limit will be
generated if the bomb is heated to the listed maximum
temperature. For example,Table I on page 6 shows the
heating only 3 mL of fuming nitric acid to 256ºC in a 23
mL bomb will generate a pressure of 1565 psi which is
well above the 1200 psi limit for the 4745 general purpose bomb. Table II shows that heating 10 mL of concentrated hydrochloric acid to 255ºC in a 23 mL bomb will
generate 2150 psi, which is well above the allowable
limit for the 4744, 4745 and 4749 bombs. But if the acid
is diluted, Table III shows that much lower pressures are
developed at temperatures to 250ºC.Diluting or reducing
the amount of acid in the charge will generally allow the
use of higher temperatures to develop a given pressure.
But in all operations the user must use good judgment in
selecting the operating temperature, and he must control
the heating medium carefully to be sure that both temperature and pressure are held within prescribed limits.
, HCl and SO3will be reduced as
2
The 4744 and 4749
Bombs can be held
4749 Bomb
with A284AC
Tumbling Ring
4
firmly in an A285AC
Holding Fixture while
tightening the cap with a
264AC2 Hook Spanner
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