Processing kodak motion picture films, module 3 analytical procedures

Potentiometric Determination of Ferricyanide in Process ECP-2 Ferricyanide Bleach
ECP-00021/1
Process ECN-2 ECP-2D VNF-1/LC RVNP
Formulas —
SR-27
SR-27R
INTRODUCTION
Excess iodide ions and a zinc reagent are added to the bleach
sample. The ferricyanide reacts with the iodide to produce an
equivalent amount of iodine. The iodine is titrated with
standard sodium thiosulfate, using either an automatic
titrator to record a potentiometric end point, or it is titrated
manually using starch indicator to detect the end point
visually. The potentiometric titration is recommended over
the visual end point titration. However, for those unable to
use an automatic titrator, the visual titrimetric technique is
included. Judging end points with a visual color change can
differ from person to person. The potentiometric method
overcomes this problem because the end point is detected
potentiometrically and displayed graphically by the titrator.
For the potentiometric measurement, a Metrohm
Potentiograph, Model E536 or equivalent should be used.
The potentiometric titration requires a platinum indicator
electrode and a double-junction reference electrode.
The reaction of ferricyanide and iodide is quantitative as
long as zinc ions are present in excess. Any ferrocyanide
present in the bleach, as well as the ferrocyanide produced by
the reduction of ferricyanide, is precipitated as zinc
ferrocyanide. See reactions 1-3, below.
Persulfate ions and some other oxidizing agents will also
oxidize iodide. Thus, if present, they will be measured as
ferricyanide by this method.
2[Fe(CN) 6 ] 3- + 2I - → 2[Fe(CN) 6 ] 4- + I 2
2[Fe(CN) 6] 4- + 2K + + 3Zn 2+ → K 2Zn 3[Fe(CN) 6] 2
— —
(reaction 1)
(reaction 2)
2 S 2O 3 2- + I 2 → (S 4O 6) 2- + 2I - (reaction 3)
Use of this method requires handling potentially
hazardous chemicals. Consult the Material Safety Data Sheet
for each chemical before use. MSDS's are available from
your chemical supplier.
PRECISION AND BIAS
Note: Separate statistics presented for potentiometric and
visual titration methods.
Potentiometric Titrations
Repeatability Standard Deviation (1sr) ) & 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 will
include the mean value 95 percent of the time.
To obtain the repeatability data, a single skilled analyst
performed five (5) replicates on each of the following
solutions during methods development (this procedure was
performed by both potentiometric and visual end point
detection):
1. A “fresh” ferricyanide bleach prepared with all
components at their respective “working tank” aim
concentrations.
2. A “seasoned” ferricyanide bleach analyzed
potentiometrically as received at 32.636 g/L.
3. The same “seasoned” solution as in number 2, above,
reanalyzed after making an analytically weighed,
standard addition of 9.751 g/L K3Fe(CN) 6 .
Sample
“Fresh” at
“Aim”
(30.004 g/L
K3Fe(CN)6)
“Seasoned”,
As Received
“Seasoned”
with
Standard
Addition
K 3 Fe(CN) 6 (Potentiometrically)
Mean
(g/L
K 3Fe(CN) 6)
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 is not determined
for “seasoned” samples, since the component concentration
level was not determined independently of the test method.
A statistically significant high bias of (+0.139 g/L) for
K3Fe(CN)6 was found for a Process ECP-2 “fresh” tank
developer sample. However, the bias was judged not to be
practically significant.
Recovery
Recovery is used instead of bias 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
99.0 percent for Process ECP-2 was not statistically different
from 100 percent.
Processing KODAK Motion Picture Films, Module 3, Analytical Procedures H24.03 1
N
Repeatability
Standard
Deviation, 1S r
(g/L K 3 Fe(CN) 6 )
95 Percent
Confidence
Estimate
(g/L
K 3Fe(CN) 6)
30.143 6 0.058 ± 0.15
32.636 5 0.015 ± 0.04
42.290 5 0.117 ± 0.30