急求美国epa7196a方法

CD-ROM 7196A - 1 Revision 1
July 1992
METHOD 7196A
CHROMIUM, HEXAVALENT (COLORIMETRIC)
1.0 SCOPE AND APPLICATION
1.1 Method 7196 is used to determine the concentration of dissolved
hexavalent chromium [Cr(VI)] in EP/TCLP characteristic extracts and ground
waters. This method may also be applicable to certain domestic and industrial
wastes, provided that no interfering substances are present (see Paragraph 3.1
below).
1.2 Method 7196 may be used to analyze samples containing from 0.5 to
50 mg of Cr(VI) per liter.
2.0 SUMMARY OF METHOD
2.1 Dissolved hexavalent chromium, in the absence of interfering amounts
of substances such as molybdenum, vanadium, and mercury, may be determined
colorimetrically by reaction with diphenylcarbazide in acid solution. A redviolet
color of unknown composition is produced. The reaction is very sensitive,
the absorbancy index per gram atom of chromium being about 40,000 at 540 nm.
Addition of an excess of diphenylcarbazide yields the red-violet product, and its
absorbance is measured photometrically at 540 nm.
3.0 INTERFERENCES
3.1 The chromium reaction with diphenylcarbazide is usually free from
interferences. However, certain substances may interfere if the chromium
concentration is relatively low. Hexavalent molybdenum and mercury salts also
react to form color with the reagent; however, the red-violet intensities
produced are much lower than those for chromium at the specified pH.
Concentrations of up to 200 mg/L of molybdenum and mercury can be tolerated.
Vanadium interferes strongly, but concentrations up to 10 times that of chromium
will not cause trouble.
3.2 Iron in concentrations greater than 1 mg/L may produce a yellow
color, but the ferric iron color is not strong and difficulty is not normally
encountered if the absorbance is measured photometrically at the appropriate
wavelength.
4.0 APPARATUS AND MATERIALS
4.1 Colorimetric equipment: One of the following is required: Either
a spectrophotometer, for use at 540 nm, providing a light path of 1 cm or longer,
or a filter photometer, providing a light path of 1 cm or longer and equipped
with a greenish-yellow filter having maximum transmittance near
540 nm.
CD-ROM 7196A - 2 Revision 1
July 1992
5.0 REAGENTS
5.1 Reagent water: Reagent water should be monitored for
impurities.
5.2 Potassium dichromate stock solution: Dissolve 141.4 mg of dried
potassium dichromate, K Cr 2 2O7 (analytical reagent grade), in reagent water and
dilute to 1 liter (1 mL = 50 ug Cr).
5.3 Potassium dichromate standard solution: Dilute 10.00 mL potassium
dichromate stock solution to 100 mL (1 mL = 5 ug Cr).
5.4 Sulfuric acid, 10% (v/v): Dilute 10 mL of distilled reagent grade
or spectrograde quality sulfuric acid, H2SO4, to 100 mL with reagent water.
5.5 Diphenylcarbazide solution: Dissolve 250 mg 1,5-diphenylcarbazide
in 50 mL acetone. Store in a brown bottle. Discard when the solution becomes
discolored.
5.6 Acetone (analytical reagent grade): Avoid or redistill material that
comes in containers with metal or metal-lined caps.
6.0 SAMPLE COLLECTION, PRESERVATION, AND HANDLING
6.1 All samples must have been collected using a sampling plan that
addresses the considerations discussed in Chapter Nine of this manual.
6.2 Since the stability of Cr(VI) in extracts is not completely
understood at this time, the analysis should be carried out as soon as possible.
6.3 To retard the chemical activity of hexavalent chromium, the samples
and extracts should be stored at 4oC until analyzed. The maximum holding time
prior to analysis of the samples or extracts is 24 hr. The 24 hr holding time
begins after extraction.
7.0 PROCEDURE
7.1 Color development and measurement: Transfer 95 mL of the extract to
be tested to a 100-mL volumetric flask. Add 2.0 mL diphenylcarbazide solution
and mix. Add H2SO4 solution to give a pH of 2 + 0.5, dilute to 100 mL with
reagent water, and let stand 5 to 10 min for full color development. Transfer
an appropriate portion of the solution to a 1-cm absorption cell and measure its
absorbance at 540 nm. Use reagent water as a reference. Correct the absorbance
reading of the sample by subtracting the absorbance of a blank carried through
the method (see Note below). An aliquot of the sample containing all reagents
except diphenylcarbazide should be prepared and used to correct the sample for
turbidity (i.e., a turbidity blank). From the corrected absorbance, determine
the mg/L of chromium present by reference to the calibration curve.
NOTE: If the solution is turbid after dilution to 100 mL in Step 7.1,
above, take an absorbance reading before adding the carbazide
CD-ROM 7196A - 3 Revision 1
July 1992
reagent and correct the absorbance reading of the final colored
solution by subtracting the absorbance measured previously.
7.2 Preparation of calibration curve:
7.2.1 To compensate for possible slight losses of chromium during
digestion or other operations of the analysis, treat the chromium
standards by the same procedure as the sample. Accordingly, pipet a
chromium standard solution in measured volumes into 250-mL beakers or
conical flasks to generate standard concentrations ranging from 0.5 to
5 mg/L Cr(VI) when diluted to the appropriate volume.
7.2.2 Develop the color of the standards as for the samples.
Transfer a suitable portion of each colored solution to a 1-cm absorption
cell and measure the absorbance at 540 nm. As reference, use reagent
water. Correct the absorbance readings of the standards by subtracting
the absorbance of a reagent blank carried through the method. Construct
a calibration curve by plotting corrected absorbance values against mg/L
of Cr(VI).
7.3 Verification:
7.3.1 For every sample matrix analyzed, verification is required to
ensure that neither a reducing condition nor chemical interference is
affecting color development. This must be accomplished by analyzing a
second 10-mL aliquot of the pH-adjusted filtrate that has been spiked with
Cr(VI). The amount of spike added should double the concentration found
in the original aliquot. Under no circumstances should the increase be
less than 30 μg Cr(VI)/liter. To verify the absence of an interference,
the spike recovery must be between 85% and 115%.
7.3.2 If addition of the spike extends the concentration beyond the
calibration curve, the analysis solution should be diluted with blank
solution and the calculated results adjusted accordingly.
7.3.3 If the result of verification indicates a suppressive
interference, the sample should be diluted and reanalyzed.
7.3.4 If the interference persists after sample dilution, an
alternative method (Method 7195, Coprecipitation, or Method 7197,
Chelation/Extraction) should be used.
7.4 Acidic extracts that yield recoveries of less than 85% should be
retested to determine if the low spike recovery is due to the presence of
residual reducing agent. This determination shall be performed by first making
an aliquot of the extract alkaline (pH 8.0-8.5) using 1 N sodium hydroxide and
then respiking and analyzing. If a spike recovery of 85-115% is obtained in the
alkaline