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Processing KODAK Motion Picture Films, Module 3, Analytical Procedures • H24.03 1
Titrimetric Determination of Hypo Index, Thiosulfate, and Sulfite in EASTMAN Color Films,
Process ECN-2 Fixer
ECN-0002/1
INTRODUCTION
This method describes the titrimetric determination of hypo
index (total reductants), thiosulfate, and sulfite in
EASTMAN Color Films, Process ECN-2, fixers. It is
recommended that these determinations be carried out by a
potentiometric titrimetric approach, using an auto-titrator.
However, for those unable to use instrumentation, the
manual titrimetric technique, using the visual starch
indicator, is included.
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 Hypo Index (HI) or total reductants of a fixer is
defined as the millilitres of 0.1 N iodine consumed by the
thiosulfate and sulfite combined (reaction 1 & 2), in a
specified volume of fixer. The fixer is added to an excess of
iodine (liberated from the reaction of potassium iodate and
potassium iodide under acidic conditions - reaction 3). The
unreacted iodine is titrated either potentiometrically or
visually with standardized sodium thiosulfate from the
appropriate capacity burette. The difference between the
blank titration and the sample titration represents the
milliequivalents of iodine consumed by the sample.
Dividing the milliequivalents of iodine by 0.1 meq/mL
yields the HI of the sample. Hypo index is reported in the
terms of HI(1), mL which is the millilitres of 0.1000 N I
2
consumed by 1.0 mL of sample.
The thiosulfate is determined potentiometrically by
adding 6 percent formaldehyde to a second sample aliquot in
reagent water. Under these conditions, the sulfite in the
sample forms a formaldehyde bisulfite complex (reaction 4).
This sample is then added to an excess of acidified iodine.
The unreacted iodine is titrated either potentiometrically
with standardized sodium thiosulfate from a 50-mL capacity
burette. The difference between the blank titration and the
sample titration represents the milliequivalents of iodine
consumed by the thiosulfate in the sample. The thiosulfate is
expressed as g/L thiosulfate ion (S
2
O
3
=
).
Process ECN-2 ECP-2D VNF-1/LC RVNP
Formulas F-34a/F-34aR — — —
2 S
2
O
3
=
+ I
2
→ 2I
-
+ S
4
O
6
=
(reaction 1)
HSO
3
=
+ I
2
+ H2O → SO
4
=
+ 2I
-
+ 3H
+
(reaction 2)
IO
3
-
+ 5I
-
+ 6H
+
→ 3I
2
+ 3H
2
O (reaction 3)
Na
2
SO
3
+ HCHO + H2O → CH
3
(OH) SO
3
Na + NaOH (reaction 4)
The thiosulfate is determined by the visual titration by
adjusting the pH of a sample aliquot to 8.5. At this pH, the
sulfite rapidly forms the stable sulfite - formaldehyde
adduct. Upon acidification, which prevents the adduct from
reacting with iodine, the thiosulfate from the sample is
titrated with standardized iodine reagent to a starch end
point.
The sulfite content is calculated by subtracting the
milliequivalents of iodine consumed by the thiosulfate from
the milliequivalents of iodine consumed by the thiosulfate
and sulfite. The sulfite is reported as sodium sulfite.
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
Repeatability
To obtain the repeatability data, a single skilled analyst
performed five (5) replicates on each of the following
solutions (this procedure was done by both potentiometric
and visual end point detection):
a. A “fresh” EASTMAN Color Films, Process ECN-2,
Fixer prepared with all components at their respective
aim concentrations in a working tank.
b. A “seasoned” EASTMAN Color Films, Process ECN-2,
Fixer analyzed as received at 125.67 g/L thiosulfate ion
and 28.92 g/L sodium sulfite.
c. The same “seasoned” solution as in number b, above,
reanalyzed after making standard additions of
37.850 g/L thiosulfate ion and 8.415 g/L sodium sulfite.
Reproducibility
Three EASTMAN Color Films, Process ECN-2, Fixer
samples were analyzed by four analysts, each using different
titration stations, on two different days. Each analyst
analyzed each sample by both the potentiometric and the
visual end point technique. Duplicate analyses were
performed on each sample, on each of the two days. These
samples were:
a. a “fresh” tank solution prepared at 109.212 g/L
thiosulfate ion and 21.335 g/L sodium sulfite.
b. an EASTMAN Color Films, Process ECN-2 “seasoned”
tank fixer sample analyzed, as received, in the same
manner as the “fresh” fixer.
c. the same (as in number b, above) EASTMAN Color
Films, Process ECN-2 “seasoned” tank fixer sample
reanalyzed in the same manner as the “fresh” fixer, after
standard additions of thiosulfate and sulfite were made.
The “seasoned” sample of EASTMAN Color Films,
Process ECN-2 fixer, analyzed to be 115.17 g/L
thiosulfate ion and 15.69 g/L sodium sulfite. Standard