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IS0
I N TERN AT I O NA L ORGAN IZATl O N
FOR STAND AR D I ZATl O N
IS0 RECOMMENDATION
R 419
SPECIFICATION FOR PHOTOGRAPHIC GRADE
SODIUM THIOSULPHATE, CRYSTALLINE
1st EDITION
March 1965
COPYRIGHT RESERVED
The copyright of IS0 Recommendations and IS0 Standards
belongs to IS0 Member Bodies. Reproduction of these
documents, in any country, may be authorized therefore only
by the national standards organization of that country, being
a member of ISO.
For each individual country the only valid standard is the national standard of that country.
Printed in Switzerland
Also issued in French and Russian. Copies to be obtained through the national standards organizations.
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BRIEF HISTORY
The IS0 Recommendation R 419, SpeciJication for Photographic Grade Sodium
Thiosulphate, Crystalline, was drawn up by Technical Committee ISO/TC 42, Photography,
the Secretariat of which is held by the American Standards Association, Inc. (ASA).
Work on this question by the Technical Committee began in 1956 and led, in 1958,
to the adoption of a Draft IS0 Recommendation.
In August 1961, this Draft IS0 Recommendation (No. 394) was circulated to all the
IS0 Member Bodies for enquiry. It was approved, subject to a few modifications of an
editorial nature, by the following Member Bodies :
Sweden
Belgium Italy
Switzerland
Brazil Japan
Canada Netherlands United Kingdom
Chile New Zealand U.S.A.
Germany Romania U.S.S.R.
No Member Body opposed the approval of the Draft.
The Draft IS0 Recommendation was then submitted by correspondence to the IS0
Council, which decided, in March 1965, to accept it as an IS0 RECOMMENDATION.
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IS0 / R 419 - 1965 (E)
IS0 Recommendation R 419 March 1965
SPECIFICATION FOR PHOTOGRAPHIC GRADE
SODIUM THIOSULPHATE, CRYSTALLINE
1. SCOPE
This IS0 Recommendation is one of a series to establish criteria of purity of chemicals suitable
for processing photographic materials. A " Photographic grade " chemical is one which meets
purity requirements as described. This specification states the limiting concentrations and the
test methods for certain inert or photographically harmful impurities that may be present.
2. PHYSICAL APPEARANCE
Crystalline sodium thiosulphate (crystal hypo) (Na,S,O,. 5H,û) is in the form of colourless
crystals.
3. SUMMARY OF REQUIREMENTS
Assay (as Na,S,03.5H,0) : 99.0 per cent minimum.
Alkalinity (as NaOH) : 0.016 per cent maximum.
i
Acidity (as H,SO,) : 0.005 per cent maximum.
Sulphide (as Na,S) : 0.001 per cent maximum.
Insoluble matter, calcium, magnesium and ammonium hydroxide precipitate : 0.2 per cent maximum
residue.
Heavy metals (as Pb) : 0.001 per cent maximum.
Iron (Fe) : 0.005 per cent maximum.
Appearance of solution : to pass test.
4. ASSAY (as Na,S,O,. 5H,O)
(99.0 per cent minimum)
4.1 Reagents
4.1.1 Standard iodine solution. Weigh accurately about 12.75 g of freshly resublimed iodine *
into a tared weighing flask. Add 36 g of potassium iodine and 100 ml of distilled water.
After solution is complete, dilute to exactly 1 litre at 20 "C in a volumetric flask. From
the mass of the iodine, calculate the normality.
4.1.2 Starch indicator. Stir 5 g of soluble starch with 100 ml of 1 per cent salicylic acid solu-
tion. Add 300 to 400 ml of boiling distilled water and boil until the starch dissolves.
Dilute to 1 litre.
4 -2 Procedure
Take about 1 g of the sample, weigh accurately and dissolve in 50 ml of distilled water.
Titrate with standard iodine solution, using starch indicator.
1 ml 0.1 N I = 0.0248 g Na,S,03.5H,0
5. ALKALINITY (as NaOH)
(0.016 per cent maximum)
Dissolve 5 i 0.1 g of the sample in 100 ml of freshly boiled distilled water. Add 3 drops of
phenolphthalein indicator. If a pink colour is produced, it should be discharged by the addition
of not more than 0.2 ml of 0.1 N hydrochloric acid. If no pink colour is produced, proceed as
in section 6.
* Reagents used in making the tests should be recognized reagent grade chemicals normally used for careful analytical
work. In all the directions, the acids and ammonium hydroxide referred to should be of full strength, unless dilution is
specified. Dilution is specif
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