Note: Descriptions are shown in the official language in which they were submitted.
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Detail Description
The present invention pertains to an indicator system
which visually displays the time-temperature integral to which a
product has been exposed.
The desirability of detecting whether or not a frozen
product has been allowed to thaw has long been recognized and
numerous tell-tale devices are described in the literature.
One class of these relies upon material which is frozen but which
melts at some preselected temperature so as to irreversibly
activate an indicator, either chemically or physically. Typically
of these devices are those described in the following U.S.
patents:
Nos. 1,917l048Nos. 2,753,270Nos. 2,955,942
2,216,127 2,762,711 3,047,405
2,277,278 2,788,282 3,055,759
2,340,337 2,823,131 3,065,083
2,553,369 2,850,3g3 3,194,669
2,617,734 2,852,394 3,362,834
2,662,018 2,951,405 3,437,010
All of the above devices merely signal "thaw" with no
attempt to measure the period during which the product is thawed
or the temperature which the product attains while thawed.
A second class of known indicators utilizes diffusion
or capillary action of a liquid on a wick or similar permeable
member. These devices while often cumbersome, provide some
degree of gradation and are typified by the devices of the
following U.S. Patents:
Nos. 2,560,537Nos. 3,243,303
2,716,065 3,414,415
302,951,764 3,479,877
3,118,774
The majority of the prior art devices however are
directed primarily at the phenomenon of thawing and the attendant
damage which occurs. It is now recognized that various natural
and synthetic materials deteriorate with the passage of time
even when taking the precaution of storing under adequate
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refrigeration. This is true even with such additional or
alternative precautions as packaging in an inert atmosphere,
sterilization or adding spoilage retardants. Thus, for example,
foods, films, pharmaceuticals, biological preparations and the
like, can demonstrate decomposition with the passage of time,
even when sterilized or maintained at sufficiently low temperatures
to preclude microbiological degradation. Such decomposition
occurs for various reasons, including strictly chemical reactions,
such as oxidation, and enzymatic processes. Frozen foods and
ice cream show deterioration even when held in a frozen state.
A system which would monitor such decomposition or deterioration
would be extremely valuable.
The deterioration kinetics involved in such processes
however, are exceedingly complex. For example, while it is
clear that deterioration is a function of temperature, the rate
of this deterioration of such products can also vary with
temperature. One rate of deterioration will exist at a first
temperature while a different rate obtains at a second temperature.
The total amount of deterioration will depend upon the time at
which the product is held at each temperature; i.e. the integral
of time and temperature.
The quotient of (a) the rate of change at one temperature
of an article's property whose deterioration is being monitored
to (b) the rate of change at a lower temperature is often expressed
for ten degree increments and represented by the symbol "Qlo"
for the Celsius scale and "ql0" for the Fahrenheit scale. This
quotient is substantially constant within limited temperature
ranges.
The practical effect of the foregoing can be seen for
example from two comparable samples of frozen food which are
processed and packaged at the same time. If in the course
of distribution or storage one package is allowed to rise in
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temperature by 10 or 20C, even without thawing, its life will
be reduced as compared with the other package which was maintained
at a lower temperature for its entire storage life since the
rate of decomposition of the contents of the first package is
accelerated during the storage at the higher temperature. A
consumer about to purchase these packages, both of which are
now stored at normal freezer temperature, has no way of
ascertaining this difference in temperature histories.
Systems have been suggested for monitoring the
temperature history of a product. Thus U.S. Patent No. 2,671,028
utilizes an enzyme such as pepsin in indicator systems while
U.S. Patent No. 3,751,382 discloses an enzymatic indicator in
which urease decomposes urea with the reaction products causing
a change in the pH of the system. The activity of the enzyme,
and thus rate of decomposition, is dependent on temperature so
that the change in pH resulting from this decomposition can be
monitored by conventional acid-base indicators. This type of
system, which appears to be directed at the spe~ific problem of
microbiological putrefaction rather than the broader problem
of monitoring temperature histories, suffers from the inherent
limitation of any enzymatic reaction. Thus while enzyme activity
is a function of temperature, it is also sensitive to the very
passage of time being measured, enzymatic activity generally
decreasing with time. Enzyme activity is also sensitive to pH
change and such change is the operative factor in, for example,
the system of U.S. Patent No. 3,751,382. A more sophisticated
system is described in U.S. Patent No. 3,768,976 in which time
temperature integration is achieved by monitoring permeation of
oxygen from the atmosphere through a film, utilizing a redox
dye to provide a visual read out. This device is however
dependent upon the presence of atmospheric oxygen and somewhat
cumbersome in configuration and dimensions.
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A further problem is that the change in rate of quality
loss per unit of temperature change differs for di~ferent products.
Thus the change in the rate of deterioration per unit of
temperature change for certain fruits and berries is vastly
different fron the change in rate for lean meats. The values
for dairy products are different from both. For example, within
the range of 0 to -20C, raw fatty meat and pre-cooked fatty
meat have Qlo ~ s of about 3, whereas raw lean meat and pre-cooked
lean meat have Qlo's between 5 and 6. Vegetables generally have
a Qlo of between 7 and 8, whereas fruits and berries have a Qlo
of approximately 13. Consequently, a system which is dependent
on a single enzymatic reaction or the permeability of a given
film will be suitable as an indicator only for those materials
having a similar slope for their relationship of change of rate
of decomposition to temperature. Although U.S. Patent No.
3,751,382 describes a method for modifying the time at which the
indicator's color change occurs, the activation energy of the
enzyme system is modified only slightly and the ratio of change
in reaction rate per temperature unit remains substantially the
same. The same is true of the device described in U.S. Patent
No. 3,768,976 which is dependent solely on gas permeability.
The present invention pertains to an indicator system
which overcomes the above problems yet is extremely simple and
reliable in structure and operation. Moreover, the device is
extremely well suited for remote sensing: i.e., monitoring
the time-temperature integrals at the interior of a package,
while providing an immediate read-out of that integral on the
exterior of the package.
The present system is not limited in application to
monitoring long storage periods at low temperatures. The same
considerations apply to short periods and to high temperature.
The present system can also be used to insure, for example, that
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products have been adequately heat sterilized. The indicator
is thus admirably suited to insure that canned goods which are
autoclaved have been subjected to the appropriate time-temperature
integral required to obtain a necessary degree of microorganism
kill. In this case, the indicator provides visual information
as to whether the necessary parameters of temperature and time
have been reached or exceeded. Similarly, the present indicator
can be used to insure that surgical instruments have been subjected
to appropriate sterilization conditions, that pharmaceuticals
have not been stored for periods in excess of that which is
permissible, that dairy products have been properly pasteurized,
and the like. Various other applications in which it is
desirable to know the temperature history of a product are
immediately apparent.
The present invention will be described in conjunction
with the appended drawings in which:
FIGURE 1 is a plan view of a temperature-time-integrating
indicator device constructed in accordance with the principles
of the present invention, portions of the upper wall and the
ampule positioning strip being broken away for purposes of
clarity in depicting constructional details.
FIGURE 2 is a longitudinal vertical sectional view as
taken along the line II-II in FIGURE 1.
FIGURE 3 is a transverse sectional view on enlarged
scale as taken along the line III-III in FIGURE 2.
FIGURE 4 is a fragmentary plan view of the device
showing an additional manner in which the sealing together
of the envelope walls can be carried out.
FIGURE 5 is a side view of the ampule in which the
gas generating material is confined, the ampule being enclosed
in a resilient sleeve.
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DESCRIPTION OF TH~ PR~F`ERRE~ EMBODIMENT
ThiS inventio~ relates to a device for monitoring
the quality of a product. More specifically, it relates to
a monitoring device which is capable of giving a visual display
of the integral of time and temperature to which a product has
been exposed.
With continuing reference to FIGURES 1-3, there is
depicted a temperature time indicator which includes an envelope
10 comprised of elongated, generally co-extensive upper and lower
walls 12 and 14 of gas impermeable material. The walls 12 and 14
while depicted as single ply components of transparent material
could be plural ply and be laminated to include a metal foil
layer as well as being in part opaque. The important
consideration is that said walls be gas impermeable. Walls 12
and 14 are joined together to form the envelope structure by
sealing them together in a continuous course extending about the
periphery of each, e.g., by heat-sealing, the material of the walls
of course being compatible to that purpose, and such peripheral
seal being shown generally at 16 in FIGURE 2. The device also
embodies a wick 18, the wick being disposed longitudinally of the
envelope 1~, in a longitudinal portion thereof which constitutes
an indicating section 26, and being treated with an indicator
composition.
The device also includes an ampule 22 disposed in
another longitudinal portion of the envelope constituting a gas
generation section 26 in which is confined a gas generating
material, the ampule being disposed intermediate the upper and
lower walls 12 and 14 and being fixedly positioned therebetween
as by connection of an overlaying gas permeable sheet 24 with one
of said walls, the wick 18 having one tip end as at 19 in gas
generation section 26 and its other tip end 21 remote from said
gas generation section.
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In acc~rdance with the present invention, there is
provided a gas barrier 40 at each longitudinal side of the wick
18, the gas barrier extending between walls 12 and 14 and in the
instance where walls 12 and 14 are amenable to heat sealing
being provided by effecting a heat sealed joinder of the walls
in the pattern depicted best in FIGURE 1. The heat seal is
positioned immediately adjacent the said wick longitudinal side
margins. "Immediately adjacent" as used herein is intended to
mean effecting the heat seal as close to the wick as practical
manufacturing will permit without causing adherance of any melted
wall material to the wick material. Thus any spacing 51 as
may exist between the sides of the wick at the barrier is of
insignificant consequence with respect to the possibility of gas
transport occurring along said space without making a contact
with the wick 18 at or very close to tip end 19. In this manner
the possibility of random gas molecules transport through said
space and into first contact with the wick at location remote
from tip end l9 is inhibited.
The important requirement in the construction of the
device is that the longitudinal gas barrier extend immediately
adjacent the wick side margins substantially along the full
length of the wick. If desired, however, the sealed joinder of
the envelope walls can be extended laterally outwardly from the
wick sides in the pattern 55 depicted in FIGURE 4.
Further in accordance with the present invention, the
gas generating component is confined within ampule 22, and the
ampule 22 is fixedly secured to the inner surface of one of
the envelope upper and lower walls, in the depicted embodiment
the ampule 22 being fixedly positioned by securing the same to
the inner surface of lower wall 14 with the gas permeable sheet
24, the latter being heat sealed to the lower wall in the
generally oval course seal pattern 57 depicted in FIGURE 1. The
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ampule 22 in which the gas generatin~ material is confined
desirably is an elongated component, closed at its ends and made
of a frangible material, glass being preferred. Thus, when it is
desired to activate the devicel the user need only apply a bending
- force to the envelope in the reyion of the position of the ampule
and gelierally applied intermediate the ends of the ampule to
fracture the same and permit the gas to escape in the first
section 28 of the envelope from whence it can flow onto the wick
located in the second section 26. To provide that when ampule 22
is ruptured, resulting jagged particles of the same will not
pierce or damage any of the envelope structure, the ampule can
be enclosed in a resilient sleeve 60 as shown in FIGURE 5, the
resilient sleeve for example being a braided fiberglass member.
It will be obvious to those skilled in the art that
the gas generating material need not necessarily be sealed in an
ampule. The only necessary requirement is that it be contained
and isolated from the wick prior to activation. Furt'nermore,
the ampule or other means for isolating the gas generating
material can be completely enclosed in a pouch of the gas
permeable sheet, 24. In that event the pouch must have a gas
tight seal about its periphery. The pouch iLself need not be
heat sealed to the walls of the gas barrier.
Upon rupture of the ampule 22 and after an initial
induction period during which the partial pressure of the gas
rises in chamber formed by the gas permeable sheet, 24, the gas
permeates across film 24 to the wick, 18. The gas is then
absorbed into wick 18. The rate of gas generation by the gas
generating material is a function of temperature and the amount
of gas which thus passes through the permeal film, 24, is in turn
a function of temperature. If wick 18 is constructed with a
substantially-constant cross-section, the distance which the gas
advances along wick means 18 will thus be a direct function of
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the time-temperature integral to which the device has been
subjected.
Deposited on wick 18 is an indicator composition which
produces a color change in the presence of the gas generated
by gas generating material. This indicator composition can vary
widely but is selected so as to be responsive to the particular
gas generated by gas generating material. Since this indicator
composition produces a color change in the presence of the gas,
an advancing front will be observed on wick means 18 in the
indicating section, 26. The length of advancement corresponds
to the time-temperature integral to which the device has been
exposed and can be read through the incorporation of a graduated
scale and appropriate indicia associated with the wick means.
The indicator composition may be a pH sensitive dye.
Alternately, it may be a composition which complexes with the gas
generated to produce a color change.
Illustrative non-limiting examples of pH sensitive dyes
useful as indicator compositions in the practice of this invention
are phenolphthalein, xylenol blue, nile blue A, m-cresol purple,
bromocresol green, o-cresol red, cyanidine chloride, bromocresol
purple, alizarin, thymol blue, bromophenol red, methyl red, acid
fuchsin, brilliant yellow, logwood extract, bromthymol blue,
phenol red, phenolphthalexon, etc.
Various compounds such as copper or cobalt halides
which can form complexes (e.g. with ammonia) which exhibit a
color change upon complexing may be used as the indicator.
An additional compound preferably included in the wick
is a quantifier material whose function is to fix the time
interval over which the time-temperature indicator is operative.
Although the temperature and hence the Qlo sensitivity of
the time-temperature indicator is determined by the temperature
coefficients of both the vapor pressure of the gas generated and
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the permeability of the rate controlling film, 24, (RCF); the
timing response of the indicator, on the other hand, is
determined by the amount of quantifier impregnated on the wick,
as well as the thickness and effective area of the RCF.
Variations in the quantity of quantifier are best
accomplished by controlling its concentration in an impregnating
solution. For example, where the quantifier material is tartaric
acid, a solution is prepared of 0.2N tartaric acid in ethanol and
glycerol, the glycerol comprising 20% in volume of the solution,
and 0.2~ of % phenol red based on the total solution. The wick
is immersed in the solution and the excess material squeezed out
by passing the saturated wick through a roll nip and allowing
the wick to air dry.
Where the RCF is polypropylene of an area of about
525 mm and the gas generating material is (NH4)2CO3, the
indicator based on a wick prepared in the above manner, (NH4)CO3
has a time scale at 0F of about 600 days for a 1/4 x 4-inch
wick of 6 mil Whatman #114 filter paper. This time scale may be
shortened by reducing the concentration of quantifier material
in the impregnating solution.
The quantifier material can be any non-volatile material
which reacts to neutralize the gas generated. Hence, the
quantifier materials of choice are acidic or basic compounds
which can be solvated for deposition on the wick. Illustrative
examples of such quantifier are tartaric acid, potassium acid
phosphate, cinnamic acid, quinine, guanidine, sodium hydroxide,
sodium carbonate, etc. Certain quantifiers are preferably used
with particular pH sensitive dyes as shown in the table below:
Gas Generated Quantifier pH Sensitive Dyes
NH Tartaric acid Phenol red
"3 Potassium acid phosphate Cresol red
" Cinnamic acid Ethyl red
Acetic acid Sodium hydroxide Methyl red
" Quinine Methyl orange
" Sodium carbonate Cresol red
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In lieu of a mechanical barrier, such as an ampule,
the gas generating material may be isolated ~rom wick 18 prior
to use by encapsulation, the details of which being well known
to the art need not be elaborated here. Upon fracturing the
protective coating around the individual particles of the
encapsulated material/ which fracturing can be done mechanically
or in the course of subjecting the particles to low temperatures,
gas generation begins. The gas passes through permeable film 24
and then to the wick means 18.
An alternative to the longitudinal seals described
above is a seal transverse and perpendicular to the wick 18,
at or near the end of the wick l9, near the gas generating section
28. This transverse seal divides the device into its two
sections 26 and 28. The function of the transverse seals or the
heretofore described longitudinal seals is to prevent access to
the wick, 18, of the gas generated except by capillary wicking
action along the wick, 18, beginning at the end, l9, which
protrudes into the gas generating section, 28. Absent, these
seals gas would be free to diffuse toward the far end of the
wick 21, thereby giving erroneous readings.
The gas generation section, 26, can utilize a variety
of physical or chemical processes. In its simplest embodiment,
the gas generation may involve simple sublimation or vaporization
and thus one may utilize any substance which has a high vapor
pressure, as for example, water (or ice); iodine, aliphatic and
aromatic alcohols such as thymol; hydrogen peroxide; lower
alkanoic and aromatic acids, such as acetic acid; acid anhydrides
such as maleic anhydride; acid halides; ketones, aldehydes and
the like. Alternatively the gas generating material can be a
salt which decomposes with the generation of a gas, as for example
ammonium carbonate, sodium bicarbonate, ammonium acetate,
ammonium oxalate, ammonium formate and the like.
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In those instances in which the rate of gas gen-
eration corresponds to the rates being monitored, it i8 unneces-
sary to include the barrier film, and gas generating section of
the envelope, 28, can have a single chamber. Even in such em-
bodiments, however, it is often aesirable to interpose a highly
permeable physical barrier which separates the gas generating
material from the wick. The permeability of such barriers should
be substantially independent of temperature since the rate
determining step i8 the generation of gas. Typical of these are
6uch materials as microporous polypropylene (Celgard ) and
microporous acrylic polyvinyl chloride on woven nylon cloth
(Acropor ). When no film is employed, or the film is highly
permeable, the rate of sublimation is in part dependent on the
available surface area of the gas generating material. In such
instances, it is often desîrable to impregnate the material on
a carrier so that a uniform surface is provided.
Alternatively, the film, 24, can divide the gas
generating section, 28, into a first and second chamber, as
shown in Figure II. The film may have a more limited gas
permeability and one which is temperature dependent. Typical of
these temperature dependent rate controlling films (RCF) are
polyethylene, polypropylene, nylon, cellulose films and the like.
It can be shown mathematically that the contribution of the gas
generation and the contribution of gas transport to the Qlo of
the system are cumulative so that by judicious selection of the
two sy~tems it is possible to achieve an overall effect in which
the change in rate of gas availability at the wick with changes
in temperature parallels the Qlo of the product being minotored.
Moreover, when a film of limited permeability is utilized, the
effect of surface area of the gas generating material is elimin-
ated since gas transport across the film is the rate controlling
step.
The gas generation process and optionally also the
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permeability throuyh the film are thus selected so that the
change in rate of gas availability at the wick per unit change
in temperature approximates the Qlo of the product being monitored.
The activation energy values of the operative components are
useful in this selection since the relationship between Qlo and
the activation energy is as follows:
(Equation 1) Q1o = elEa/Tl T2 R
where Ea = the activation energy
Tl = a first temperature in degrees (absolute)
T2 = a second temperature ten degrees lower than Tl and
R = the gas constant
Within, for example, the range of -10 to -20C, an
important region for frozen foods, the following values are
obtained:
Ea Qlo qlO Ea Qlo qlO
Kcal/mole Kcal/mole
0.1 1.001.00 20.0 4.54 2.31
5.0 1.461.23 22.0 5.28 2.52
8.0 1.831.40 25.0 6.63 2.86
2010.0 2.131.52 27.0 7.71 3.11
12.0 2.481.66 30.0 9.61 3.52
15.0 3.111.88 33.0 12.0 4.00
34.0 13.0 4.16
It is thus possible to select gas generating materials
and films in which the rates of gas generation and permeability
parallel the decomposition rates of various materials, even in
the course of temperature fluctuation over a period of time.
The wick means can be selected from a wide variety
of known materials. These may be simple cellulosic products
such as paper or fiber, various synthetic polymeric materials,
such as polypropylene, polyesters, or polyamides, glass fiber
paper, alumina, silica gel and the like. The nature of the
wick means is relatively unimportant, provided it possesses a
sufficient affinity for the gas and indicator composition and
is substantially inert to both.
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The indicating composition which is deposited on
the wick means and which results in a color change in the presence
of gas can be a single component or a mixture of components
operating together. The particular indicating composition must
be selected for the particular ~as generated. When, for example,
the gas generated is ammonia, the indicator composition can
simply include an aqueous medium and a pH sensitive dye, such as
methyl red or thymol blue, and an acidic substance of low
volatility such as trichloroacetic, benzoic, oxalic or the like
acid. Prior to absorption of any ammonia, the dye will display
its first color which color will change as ammonia is absorbed.
Analogous systems are employed with acidic gases.
The indicating composition can alternatively use a
redox system to produce the requisite color change. For example,
the wick may be impregnated with a potassium permanganate solution.
In such an instance, the gas or vapor generated is one which is
susceptible to oxidation, as for example, thymol or another
oxidizable alcohol. As the thymol is absorbed on the wick and
advances along its length, it is oxidized by the permanganate
which in turn loses its characteristic red color.
It is also possible to utilize an indicator composition
which, while not responding to the gas directly, converts it to
a material which can be monitored. Thus, for example, in the
case of maleic anhydride, the wick may be impregnated with an
aqueous base of with an alcohol serving as a solvolysis agent.
As the anhydride is absorbed in the wick, it is hydrolyzed by
the water or alcohol with the generation of maleic acid. This
acid can then be monitored by incorporation in the composition
of a pH sensitive dye.
The indicator composition can also complex the gas,
as with potassium iodine and starch for iodine gas.
The following examples will serve to typify other
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systems and configurations but should not be construed as a
limitation on the scope of the present invention, the invention
being defined only by the appended claims.
Example l
A time-temperature integrating indicator is prepared
in a configuration similar to that shown in Figures l and 2.
The upper wall is a laminate of 2 mil polyethylene and l mil
trifluorochloropolyethylene while the bottom wall is l mil
aluminum foil laminated to l mil polyethylene. The gas permeable
film is 2 mil polyethylene having an available area of l sq. inch.
The gas generating material is ammonium carbonate. The wick is
Whatman No. l filter paper having a width of 0.5 inch. The
indicator composition is .05 molar aqueous trichloroacetic acid,
20~ by volume glycerol and 0.1% methyl red.
Upon activation and equilibration, the ammonia generated
by the ammonium carbonate migrated through the polyethylene film
and produces a color change in the wick. At -18C, the front
advances at a rate of .017 mm/hr. If the sensor is held at -1C,
the front advances at a rate of 0.15 mm/hr. The change in the
0 rate with 10C increments corresponds to a Qlo of 3.7.
Example 2
An indicator is prepared as above utilizing, however,
iodine as the gas generating material. The indicator composition
consists of lO~ potassium iodine and 0.1~ starch. At -1C, the
front advances at 0.033 mm/hr. while at 22C, the front advances
at 0.15 mm/hr., corresponding to a Qlo of from 2.5 to 3Ø
Example 3
An indicator is prepared in a configuration similar to
that shown in Figures 3 and 4, omitting however, the gas
permeable film, 24. Paraformaldehyde is employed as the gas
generating material. The indicator composition consists of
l.l molar hydroxylamine hydrochloride, 0.8 molar sodium acetate
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and 0.1~ bromphenol blue and thymol blue. At -18C, the front
advances at a rate of 0.065 mm/hr. while at 10C, the front
advances at 0.12 mm/hr., corresponding to a Qlo of 1.5.
Example 4
An indicator is prepared in a configuration similar to
that shown in Figures 3 and 4, omitting however, the gas
permeable filmr 24. Thymol is utilized as the gas generating
material. The wick is glass fiber paper which is impregnated
with 0.01 molar potassium permanganate. A brownish yellow front
advances along the initially red strip at a rate of 0.06 mm/hr.
at 21C and 0.0002 mm/hr. at -1C, corresponding to a Qlo of
about S.
Example 5
An indicator is prepared as in Example 3. Maleic
anhydride is employed as the gas generating material to give a
Qlo of approximately 4. The indicator composition comprises O.lM
octadecanol, which hydrolyzes the anhydride, and a wide range pH
indicator such as lacmoid.
Example 6
An indicator is prepared as in Example 1, utilizing
glacial acetic acid as the gas generating material. This is
sealed below a 2 mil film of polyethylene as the gas permeable
film, 24. The indicator composition comprises 0.1 molar sodium
hydroxide, together with 0.1% thymol blue. The initially blue
strip demonstrates a sharp yellow front advancing at a rate of
0.02 mm/hr. at -18C and 0.25 mm/hr. at 4.5C, corresponding
to a Qlo of 3.1.
