Appendix A to Part 425 - Potassium Ferricyanide Titration Method
40:32.0.1.1.1.9.2.8.1 : Appendix A
Appendix A to Part 425 - Potassium Ferricyanide Titration Method
Source
The potassium ferricyanide titration method is based on method
SLM 4/2 described in “Official Method of Analysis,” Society of
Leather Trades' Chemists, Fourth Revised Edition, Redbourn, Herts.,
England, 1965.
Outline of Method
The buffered sulfide solution is titrated with standard
potassium ferricyanide solution in the presence of a ferrous
dimethylglyoxime ammonia complex. The sulfide is oxidized to
sulfur. Sulfite interferes and must be precipitated with barium
chloride. Thiosulfate is not titrated under the conditions of the
determination (Charlot, “Ann. chim, anal,”, 1945, 27, 153; Booth;
“J. Soc. Leather Trades' Chemists,” 1956, 40, 238).
Apparatus
Burrette, 10 ml.
Reagents
1. Preparation of 0.02N potassium ferricyanide; Weigh to the
nearest tenth of a gram 6.6 g. of analytical reagent grade
potassium ferricyanide and dissolve in 1 liter distilled water.
Store in an amber bottle in the dark. Prepare fresh each week.
2. Standardization of ferricyanide solution: Transfer 50 ml. of
solution to a 250 ml. Erlenmeyer flask. Add several crystals of
potassium iodide (about 1 g.), mix gently to dissolve, add 1 ml. of
6N hydrochloric acid, stopper the flask, and swirl gently. Let
stand for two minutes, add 10 ml. of a 30 percent zinc sulfate
solution, and titrate the mixture containing the gelatinous
precipitate with standardized sodium thiosulfate or phenylarsine
oxide titrant in the range of 0.025-0.050N Add 1 ml. of starch
indicator solution after the color has faded to a pale yellow, and
continue the titration to the disappearance of the blue color.
Calculate the normality of the ferricyanide solution using the
equation:
3. Preparation of 6M ammonium chloride buffer, pH 9.3: Dissolve
200 g. ammonium chloride in approximately 500 ml. distilled water,
add 200 ml. 14M reagent grade ammonium hydroxide and make up to 1
liter with distilled water. The buffer should be prepared in a
hood. Store in a tightly stoppered container.
4. Preparation of 0.05M barium chloride solution: Dissolve 12-13
g. barium chloride dihydrate in 1 liter of distilled water.
5. Preparation of ferrous dimethylglyoxime indicator solution:
Mix 10 ml. 0.6 percent ferrous sulfate, 50 ml. 1 percent
dimethylglyoxime in ethanol, and 0.5 ml. concentrated sulfuric
acid.
6. Preparation of stock sulfide standard, 1000 ppm: Dissolve 2.4
g. reagent grade sodium sulfide in 1 liter of distilled water.
Store in a tightly stoppered container. Diluted working standards
must be prepared fresh daily and their concentrations determined by
EPA test procedure 376.1 (see 40 CFR 136.3, Table IB, parameter 66
(49 FR 43234, October 26, 1984, with correction notice at 50 FR
690, January 4, 1985)) immediately prior to use.
7. Preparation of 10N NaOH: Dissolve 400 g. of analytical
reagent grade NaOH in 1 liter distilled water.
Sample Preservation and Storage
Samples are to be field filtered (gravity or pressure) with
coarse filter paper (Whatman 4 or equivalent) immediately after
collection. Filtered samples must be preserved by adjustment to
pH>12 with 10N NaOH. Sample containers must be covered tightly
and stored at 4 °C until analysis. Samples must be analyzed within
48 hours of collection. If these procedures cannot be achieved, it
is the laboratory's responsibility to institute quality control
procedures that will provide documentation of sample integrity.
Procedure
1. Transfer 100 ml. of sample to be analyzed, or a suitable
portion containing not more than 15 mg. sulfide supplemented to 100
ml. with distilled water, to a 250 ml. Erlenmeyer flask.
2. Adjust the sample to pH 8.5-9.5 with 6N HC1.
3. Add 20 ml. of 6M ammonium chloride buffer (pH 9.3), 1 ml. of
ferrous dimethylglyoxime indicator, and 25 ml. of 0.05M barium
chloride. Mix gently, stopper, and let stand for 10 minutes.
4. After 10 minutes titrate with standardized potassium
ferricyanide to disappearance of pink color. The endpoint is
reached when there is no reappearance of the pink color after 30
seconds.
Calculation and Reporting of Results.
where A =
volume in ml. of potassium ferricyanide solution used, and B =
normality of potassium ferricyanide solution.
2. Report results to two significant figures.
Quality Control
1. Each laboratory that uses this method is required to operate
a formal quality control program. The minimum requirements of this
program consist of an initial demonstration of laboratory
capability and the analysis of replicate and spiked samples as a
continuing check on performance. The laboratory is required to
maintain performance records to define the quality of data that is
generated. Ongoing performance checks must be compared with
established performance criteria to determine if the results of
analyses are within precision and accuracy limits expected of the
method.
2. Before performing any analyses, the analyst must demonstrate
the ability to generate acceptable precision and accuracy with this
method by performing the following operations.
(a) Perform four replicate analyses of a 20 mg./l. sulfide
standard prepared in distilled water (see paragraph 6 under
“Reagents” above).
(b)(1) Calculate clean water precision and accuracy in
accordance with standard statistical procedures. Clean water
acceptance limits are presented in paragraph 2(b)(2) below. These
criteria must be met or exceeded before sample analyses can be
initiated. A clean water standard must be analyzed with each sample
set and the established criteria met for the analysis to be
considered under control.
(2) Clean water precision and accuracy acceptance limits: For
distilled water samples containing from 5 mg./l. to 50 mg./l.
sulfide, the mean concentration from four replicate analyses must
be within the range of 50 to 110 percent of the true value.
3. The Method Detection Limits (MDL) should be determined
periodically by each participating laboratory in accordance with
the procedures specified in “Methods for Chemical Analysis of
Municipal and Industrial Wastewater,” EPA-660/4-82-057, July 1982,
EMSL, Cincinnati, OH 45268. For the convenience of the user, these
procedures are contained in appendix C to part 425.
4. A minimum of one spiked and one duplicate sample must be
performed for each analytical event, or five percent spikes and
five percent duplicates when the number of samples per event
exceeds twenty. Spike levels are to be at the MDL (see paragraph 3
above for MDL samples) and at x where x is the concentration found
if in excess of the MDL. Spike recovery must be 40 to 120 percent
for the analysis of a particular matrix type to be considered
valid. If a sample or matrix type provides performance outside
these acceptance limits, the analyses must be repeated using the
modified Monier-Williams procedures described in appendix B to this
part.
5. Report results in mg./liter. When duplicate and spiked
samples are analyzed, report all data with the sample results.
[53 FR 9183, Mar. 21, 1988]