Processing kodak motion picture films, module 3 analytical procedures

Determination of Sodium Metabisulfite in Process ECP-2 Accelerator Tank
ECP-0025/01
INTRODUCTION
The concentration of sodium metabisulfite (Na 2 S 2 O 5 ) in an
ECP-2 bleach accelerator sample is determined by a manual
or potentiometric titration using an iodine (I 3 -) titrant. In the
method an excess of iodine is added to the sample. Sodium
metabisulfite and Kodak persulfate bleach accelerator,
PBA-1, present in the sample react with a portion of the
iodine. The iodine that did not react with sodium
metabisulfite and PBA-1 is then titrated with standardized
sodium thiosulfate, using manual (starch indicator) or
potentiometric endpoint detection.
In order to determine the sodium metabisulfite
concentration, the contribution of PBA-1 in the titration
must be accounted for. This is done by subtracting a factor in
the calculation of sodium metabisulfite concentration. The
factor is calculated based on the PBA-1 concentration as
determined by method ECP-0027-01 and it's iodimetric
equivalent weight.
The measured level of sodium metabisulfite in the sample
may be lower than the amount added during mixing. This is
due to either a reaction of PBA-1 and sodium metabisulfite
that can result in a lower equilibrium concentration of
sodium metabisulfite, or aerial oxidation of sodium
metabisulfite.
Use of this method requires handling potentially
hazardous chemicals. Material Safety Data Sheets (MSDS)
should be consulted for each chemical before use. These can
be obtained from each chemical supplier.
PRECISION AND BIAS
Note: Separate statistics presented for potentiometric and
visual titration methods.
I. Potentiometric Titrations
A. Repeatability Standard Deviation, 1sr and
95 Percent Confidence Estimate (not including Bias)
Repeatability standard deviation is an estimate of the
variability one trained analyst should be able to obtain under
favorable conditions (analyzing a sample, with one
instrument, within one day).
The 95 percent confidence estimate (calculated using the
repeatability standard deviation) around a single test result
will include the mean value 95 percent of the time.
To obtain the repeatability data, a single skilled analyst
performed three (3) replicates on each of the following
solutions during methods development.
1. A “fresh” Accelerator tank solution prepared with all
components at their respective “working tank” aim
concentrations.
2. The same “fresh” solution as in number 1, above,
reanalyzed after making an analytically weighed,
standard addition of 0.8080 g/L Na2S2O5 (assay
99.04 percent).
3. A “seasoned” Accelerator tank solution analyzed as
received, at 2.2536 g/L Na2S2O5 .
4. The same “seasoned” solution as in number 3, above,
reanalyzed after making an analytically weighed,
standard addition of 0.7036 g/L Na2S2O5 (assay
99.04 percent).
Sample
Na 2 S 2 O 5
Mean
(g/L Na 2 S 2 O 5 ) N
Repeatability
Standard
Deviation, 1s r
(g/L Na 2 S 2 O 5 )
95 Percent
Confidence
Estimate
(g/L Na 2 S 2 O 5 )
“Fresh”
“Fresh” plus
2.39 3 0.038 ± 0.16
Standard
Addition
“Seasoned”,
3.24 3 0.066 ± 0.29
As
Received
“Seasoned”
2.18 2 0.008 ± 0.11
plus
Standard
Addition
2.83 3 0.000 ± 0.00
Bias
Bias is a statistically significant deviation of the mean from
the known mix level at a 95 percent confidence level. It is
determined for fresh samples only. Bias was not determined
for this sample because the component concentration level
was not determined independently of the test method. An
equilibrium is established between sodium metabisulfite and
PBA-1 that effectively lowers the concentration of sodium
metabisulfite in freshly mixed solutions. Therefore bias is
estimated for fresh solutions by spiking a known amount of
sodium metabisulfite to a fresh mix that has been allowed to
sit overnight, and the recovery is then calculated.
The recovery of 106.28 percent was not statistically
significantly different from 100 percent at the 95 percent
confidence level.
Recovery
Recovery is used for seasoned samples, since the component
concentration level was not determined independently of the
test method. It is defined as the calculated mean for the
seasoned sample with a standard addition of the component
minus the mean for the seasoned sample, divided by the
actual amount of the standard addition. It is expressed as a
percentage.
The recovery of 93.23 percent was statistically
significantly different from 100 percent at the 95 percent
confidence level, but was judged not to be practically
significant.
Processing KODAK Motion Picture Films, Module 3, Analytical Procedures H24.03 1