Reliable Measurements 6. Container Transpiration By Paul Gaines, Ph.D. • Edited by Brian Brolin Overview[ Back ]
A problem that has gone relatively unnoticed when achieving reliable measurements is that of transpiration. Transpiration
is defined as the passage of vapor from within a container to the
outside. This loss of vapor can occur through the container walls or
between the cap and threads, resulting in an increase in concentration.
Transpiration becomes a problem when samples are stored for a long
period of time. It is a greater problem for standard solutions, since
they typically remain in use or storage for periods of up to one year.
Inorganic
Ventures has been studying the transpiration loss of containers for
several years. The data presented in this section will be for LDPE.
This is one of the best container materials because it's the cleanest
of the plastics and it's inexpensive. It has been gaining considerable
popularity for use in the handling of both samples and aqueous
standards.
Initial Transpiration Study:
Figure
6.1 shows the rate of transpiration for 125 mL and 500 mL LDPE bottles
with conventional PP caps. This study, which was conducted over a five
year period, showed a significant transpiration rate difference between
the two bottles. Initially, it was thought that transpiration was
occurring through the container walls. However, no correlation could be
found that would explain the difference of the slopes. Surprisingly, it
was discovered that there was a direct correlation between the
transpiration rate and the circumference of the opening.
Figure 6.1: Transpiration - 125 and 500 mL LDPE Bottles
The
transpiration rate for the study shown in Figure 6.1 was found to be
directly proportional to the ratio of the circumference of the bottle
opening to the surface area of the solution. No correlation of the
transpiration rate to the bottle surface area could be found. These
findings suggest that transpiration is occurring around the cap/bottle
opening and not through the container walls. This discovery was
unexpected. Therefore, additional transpiration studies were conducted
in an attempt to better define the transpiration mechanism.
Additional Transpiration Studies:
The
next study was conducted for LDPE bottles of 30, 125, 250, and 500 mL
capacity. The purpose was to determine if the correlation of the
transpiration rate to the bottle opening held up for a variety of
bottle sizes. In addition, other variables were added as illustrated in
Figures 6.2 and 6.3.
Figure 6.2: Transpiration - 125 ml Bottles
No tape, Never opened
Tape sealed, Never opened
No tape, Opened periodically (30 times)
Tape sealed, Opened periodically (30 times)
No tape, Half-full, Opened periodically (30 times)
The
results for the 250 ml and 500 ml bottles are congruent with the data
shown above, with the exception that the transpiration rates are
proportionately lower. So the question then becomes, What factor is proportionately effecting the rate of transpiration?
Figure
6.4 shows a comparison of the transpiration rates for all of the
bottles capacities as compared to each bottle size surface area. No
correlation could be found, indicating that the mechanism for
transpiration is not through the bottle walls.
Figure 6.4: Transpiration Rates vs Bottle Surface Area
Figure
6.5 shows a comparison of the transpiration rates for all of the
bottles as compared to the ratio of the bottle opening circumference to
the solution surface area -- i.e. Transpiration Rate vs.
[π * dc] � [π * (Rb)2] where:
π = 3.14
dc = cap diameter in centimeters
Rb = radius of bottle (main body)
Figure 6.5: Transpiration Rates vs Bottle Opening Geometry
This
study revealed a correlation that fits to within 6 % relative,
indicating that transpiration around the cap is the correct mechanism
and not through the bottle as previously assumed.
As
additional support for this argument, Figure 6.6 shows a dramatic
illustration of the fact that there is a near perfect correlation of
transpiration rate to bottle opening geometry and no correlation to the
bottle surface area.
Figure 6.6: Bottle Opening vs Area Geometries
Summary of Findings:
Through this study, the following conclusions were drawn:
Transpiration occurs around the bottle opening and not through the bottle as previously thought.
Taping the bottle has no influence upon the transpiration rate.
Bottles that are half full and opened periodically have the highest transpiration rate.
Opening the bottle periodically greatly increases the transpiration rate.
Smaller bottles transpire faster.
30 ml bottles can not have a 1 year expiration date if certified to an uncertainty of ≤ ±0.6 % relative.
Studies
involving different bottle construction materials and 'bottle vapor
space' have also been conducted and will be published at a later
date.
1. Transpiration Controlled Technology
(TCT) - a patented technology that virtually eliminates transpiration
loss, greatly increasing a standard's shelf life. Developed by
Inorganic Ventures for the optimal storage of standards after packaging.
