Processing kodak motion picture films, module 3 analytical procedures

Titrimetric Determination of Hypo Index, Sulfite, Bisulfite, and Thiosulfate Content of Fixers
ECR-1308J
Process ECN-2 ECP-2D VNF-1/LC RVNP
Formulas — — F-36 F-38
PRINCIPLE
The Hypo Index (HI) of a fixer is defined as the millilitres of
standardized 0.1 N iodine consumed by the sulfite, bisulfite,
and thiosulfate in a specified volume of the fixer. H.I. is
determined (Part I) by adding the sample to an excess of
iodine (formed by acidifying potassium iodate and adding
potassium iodide) and titrating the remaining iodine with
standard sodium thiosulfate. The difference between the mL
of standardized 0.1 N potassium iodate added originally and
the mL of standardized 0.1 N sodium thiosulfate used in the
titration is, therefore, the Hypo Index of the sample.
Ferrocyanide interferes in the determination of both Hypo
Index and sulfite in fixers. Zinc sulfate is added to remove
the ferrocyanide.
The theoretical consumption of iodine by sulfite, bisulfite,
and thiosulfate is a close approximation of the actual
consumption (see Table 1) The “theoretical” Hypo Index is
calculated from a consideration of the equivalent weights of
ammonium thiosulfate, sodium sulfite, sodium bisulfite, and
sodium thiosulfate pentahydrate. Photographic-grade
ammonium thiosulfate is supplied as a waster solution
having an assay of 57.0 to 61.0 percent and containing a
maximum of 0.7 percent sulfite (ANSI PH4.252-1987 or
revision). Photographic-grade sodium sulfite is usually less
than 100 percent pure with a minimum assay of 97.0 percent
(ANSI PH4.275-1987 or revision). Photographic-grade
sodium bisulfite is a mixture of sodium sulfite, sodium
metabisulfite and water. Its purity, as determined by an
iodine titration and expressed as sodium bisulfite, is
approximately 107.5 percent with a minimum assay of
105 percent. If it were pure sodium metabisulfite, it would
assay 110 percent (ANSI PH4.276-1987) as sodium
bisulfite. Photographic-grade sodium thiosulfate has an
assay of 99.0 to 101.0 percent as sodium thiosulfate
pentahydrate (ANSI PH4.250-1987 or revision).
In order to determine only the thiosulfate content (Part II),
a separate sample is adjusted to pH 8.5. Formaldehyde
solution, 37.5 percent, is then added to form a complex with
the sulfite. At this pH the complex forms rapidly. The
solution is then made acid to prevent the complex from
reacting with iodine, which is subsequently used to titrate the
thiosulfate.
The sulfite content is calculated (Part III) by subtracting
the volume of iodine consumed by the thiosulfate from the
volume of iodine consumed by the sum of sulfite, bisulfite,
and thiosulfate (the Hypo Index).
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.
All pipets and volumetric glassware should be “Class A”
as defined by NIST (National Institute of Standards and
Technology, formerly National Bureau of Standards).
REAGENTS
Use reagents that are ACS Reagent Grade unless specified
otherwise.
0.1 N Potassium Iodate, KIO 3 ,
(standardized to four decimal places)
2.0 N Acetic Acid, CH 3 COOH
50 g/L Zinc Sulfate. ZnSO 4
0.60 M Potassium Iodide, KI
0.1 N Sodium Thiosulfate, Na 2 S 2 O 3
(standardized to four decimal places)
Starch Indicator
37.5% Formaldehyde Solution, CH 2O
Phenolphthalein Indicator
1.0 N Sodium Hydroxide, NaOH
0.1 N Iodine, I 2
(standardized to four decimal places)
PROCEDURE
Hypo Index
(Total Bisulfite, Sulfite, and Thiosulfate)
Sample Treatment
1. Pipet (wipe the pipet before leveling) 50.0 mL of
standardized 0.1 N potassium iodate into a 250-mL
conical flask.
2. Add 10 mL of 2.0 N acetic acid from a tip-up pipet.
3. Add 5 mL of 50 g/L zinc sulfate from a tip-up pipet.
4. Stir the solution with a magnetic stirrer and add 25 mL
of 0.60 M potassium iodide from a tip-up pipet.
5. Immediately pipet (wipe) 3.00 mL of fixer into the
250-mL flask while the solution is stirring.
Titration
1. Titrate with standardized 0.1 N sodium thiosulfate to a
light yellow color.
2. Add 5 mL of starch indicator from a tip-up pipet and
continue the titration until the blue color disappears.
Calculation:
Hypo Index (3 mL sample) = (50.00 – mL Na 2 S 2 O 3 )
Note: Always specify the sample size when reporting
results.
Processing KODAK Motion Picture Films, Module 3, Analytical Procedures H24.03 1