Kodak ECN-0002/1 manual Introduction, Precision and Bias, Repeatability, Reproducibility

Titrimetric Determination of Hypo Index, Thiosulfate, and Sulfite in EASTMAN Color Films, Process ECN-2 Fixer

ECN-0002/1

Process	ECN-2	ECP-2D	VNF-1/LC	RVNP
Formulas	F-34a/F-34aR	—	—	—

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 I2 consumed by 1.0 mL of sample.

2 S

O

3

= + I

2

→ 2I-

+ S O

=

(reaction 1)

2

4

6

HSO =

+ I + H2O →

SO

=

+ 2I- + 3H+

(reaction 2)

3

2

4

IO -

+ 5I- + 6H+ →

3I

2

+ 3H

2

O

(reaction 3)

3

Na2SO3 + HCHO + H2O →

CH3(OH) SO3Na + NaOH

(reaction 4)

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 (S2O3=).

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

Processing KODAK Motion Picture Films, Module 3, Analytical Procedures • H24.03 1