Processing kodak motion picture films, module 3 analytical procedures

Potentiometric Determination of Silver in Process ECP-2 Fixing Baths
ECP-0024-01
INTRODUCTION
The sample containing silver is titrated potentiometrically
with standardized sodium sulfide using a silver billet/double
junction electrode pair. The sample is made alkaline to
prevent the decomposition of sodium thiosulfate, which
occurs in acid solutions. Ethylenedinitrilotetraacetic acid
(EDTA) is added to minimize interference of other metal
ions. The EDTA reagent does not prevent interference from
zinc ions. Gelatin is added to prevent the coagulation of the
silver sulfide that is formed. This prevents the coagulated
silver sulfide from occluding the silver ions.
Changes in the volume of sample and of sodium
hydroxide/EDTA reagent affect the silver results. If a small
amount of sample is used, the sample volume must be
adjusted to about 300 mL with 1.0 M sodium thiosulfate.
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
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” fixing tank 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 1.0745 g/L Ag.
3. A “seasoned” fixing tank analyzed potentiometrically
as received, at 0.1547 g/L Ag.
4. The same “seasoned” solution as in number 3, above,
reanalyzed after making an analytically weighed,
standard addition of 0.0537 g/L Ag.
Sample
Mean
(g/L Ag)
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.
Instead, a recovery was calculated for the component in a
fresh sample.
Statistically the recovery of 101.68 percent was
significantly different from 100 percent at the 95 percent
confidence level, but judged to be not practically significant.
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.
Statistically the recovery of 103.63 percent* was not
significantly different from 100 percent at the 95 percent
confidence level.
* Note: Recovery was calculated by accounting for a
1 percent error from dilution of 1 liter of seasoned fixer by a
10.00 mL aliquot of silver nitrate.
Processing KODAK Motion Picture Films, Module 3, Analytical Procedures H24.03 1
Ag
N
Repeatability
Standard
Deviation, 1s r
(g/L Ag)
95 Percent
Confidence
Estimate
(g/L Ag)
“Fresh”
“Fresh” plus
0.0080 3 0.00346 ± 0.0149
Standard
Addition
“Seasoned”,
1.1006 3 0.00294 ± 0.0127
As
Received
“Seasoned”
0.1547 3 0.00167 ± 0.0072
plus
Standard
Addition
0.2088 3 0.00184 ± 0.0079
Example:
(0.2088) - (0.1547 x 0.99)
0.0537
x 100 = 103.63%