Cardinal Environmental Laboratories, LLC Standard Operating Procedure
Procedure No.: 1030 Revision Date: 17/07
Subject: (Method 3500 CRD) Hexavalent Chromium:
Approved By: Title: Date Revised:
Hexavalent Chromium Method 3500 CRD
Procedure 1030 Revised 17/07 Description
Chromium in a hexavalent state (Cr+6) can be determined colorimetrically by reaction with diphenylcarbazide. The method is sensitive to 10 ppb.
Precautions
General Requirements (see procedure 0010) Corrosives (See procedure 0020)
Interferences
The primary interference faced under typical circumstances will be from iron. Some industrial effluents and waste materials contain elevated levels of iron. Elevated iron concentrations will cause the formation of a yellow color in the sample after the addition of the reagents. If iron is suspected, confirmation is usually made by ICP screening of the sample. The initial corrective action will be to dilute the sample and reanalyze.
Sample Preservation
Samples to be analyzed for hexavalent chromium may be collected in either glass or plastic and must be analyzed as soon as possible, but no later than 24 hrs. from the time of sample collection.
Apparatus
- Spectrometer - 5cm cell for spectrometer - 5cm cell holder for spectrometer - Filtration apparatus - 0.45um filter paper - 250ml plastic beakers - 100ml graduated cylinders - Volumetric pipettes, various - pH paper (1-14 range) - 100ml volumetric flasks
Reagents
- Sulfuric Acid (H2SO4) concentrated: ACS grade or better.
- Sulfuric Acid (H2SO4) 6N: Carefully add 167ml of concentrated H2SO4 to 900ml of ultra pure
Procedure 1030 Revised 17/07 water. Dilute to l liter.
- Acetone: ACS grade or better.
- Diphenylcarbazide indicator: Dissolve 250mg diphenylcarbazide in 50ml acetone. Store in a brown bottle. Discard when solution becomes discolored.
- Standard Chromium solution (1000ppm): commercially available, expires after 1 year.
- Stock Chromium solution (10ppm): Using a volumetric pipette, transfer 1ml of 1000ppm standard to a 100ml volumetric flask. Cap and invert to mix. Make monthly.
- Working Chromium solution (1ppm): Using a volumetric pipette, transfer 10ml of 10ppm standard to a 100ml volumetric flask. Cap and invert to mix. Make daily.
Procedure
1.0 Sample Preparation
1.1 Samples should be unpreserved.
1.2 Filter sample through 0.45m filter paper. You should filter at least enough for sample (90ml) and for sample blank (90ml).
1.3 Using pH paper to check, adjust blanks, standards, samples, and
sample blanks to pH 2 0.5 using 6N or concentrated H2SO4. A minimum amount of acid should be used.
1.4 To each sample, add 2ml of diphenylcarbazide indicator. Mix.
1.4.1 Do not add the indicator to the sample blank.
1.5 After exactly 10 minutes the samples are ready to be read on the spectrometer.
1.5.1 The spectrometer should be allowed to warm up for at least 1/2 hour.
1.5.2 Adjust wavelength to 540 nm.
1.5.3 Remove the l cm cell holder and replace it with the 5 cm cell holder.
Procedure 1030 Revised 17/07 1.6 Place reagent blank in 5cm cell and insert into the spectrometer.
1.6.1 Zero the spectrometer to this blank by keying the second function key followed by the zero key.
1.7 Remove reagent blank and insert standards, samples and/or sample blanks.
1.7.1 A pink color indicates the presence of Cr+6. If a sample is above detection limit, run a spike and spike duplicate to confirm a positive Cr+6.
1.7.2 If the sample reads high on the spectrometer but has no visible pink color, or if sample becomes turbid with the addition of acid- you must read the sample blank on the spectrometer. Subtract sample blank absorbance from the sample absorbance for the corrected sample absorbance.
1.7.3 If the sample turns a yellow or rust color after the addition of of indicator, this could indicate Fe interference. Check with supervisor to see if metals area could screen for high Fe. Less sample may have to be used for analysis.
2.0 Standard Curve
2.1 Pipette the following volumes of the working standard solution into 90 mls of ultra pure water:
mls of working standard ug in 90ml sample
0 0 1.0 1.0 2.0 2.0 4.0 4.0 5.0 5.0 8.0 8.0
Procedure 1030 Revised 17/07 10.0 10.0
2.2 Proceed with sample analysis step 1.3 - 1.7.3.
2.2.2 Absorbance data is input into excel file CR6STD5M.XLS to generate a linear coeffiient. A printed copy of the current LC is found in the active LC file.
Calculation
( A - B ) X LC ppm Cr+6 = ------X Dilution Factor sample volume in mls
where: A = sample absorbance B = sample blank absorbance LC = linear coefficant
Quality Control
* Each analytical batch analyzed for hexavalent chromium must be accompanied by a reagent blank consisting of 90ml ultra pure water taken through all steps.
* Each batch analyzed must also be accompanied by two standards and two QC standards with concentrations of 2ml of 1 ppm std 90ml ultrapure and 5ml of1ppm std 90ml chromium.
* All weights, volumes, comments etc. for each batch must be entered in notebook form for data review.
Procedure 1030 Revised 17/07 * Every 5th sample of a similar matrix shall be analyzed in duplicate and % RPD calculated. Every 10th sample shall be analyzed as a duplicate spike and % recoveries and % RPD calculated. (See attachment 0110 for calculations).
* Required recoveries for duplicate and spike duplicates shall be determined using historical data. The current acceptable limits for these parameters are listed in attachment 0120.
* Any sample with an absorbance > 10% above the curve must be diluted and re-run.
* Failure to meet quality control acceptable limits requires corrective action. (See attachment 0150).
If correlation coefficient is <0.995 reprep standards and rerun calibration curve. If standards are outside acceptable limits, re-prep and re-analyze standards. If still outside acceptable limits, prepare a new standard curve and reanalyze standards and samples.
References
Standard Methods for Examination of Water and Wastewater, 18th edition, 1992. Method 3500 Cr D. pages 3 - 59 thru 3 - 60.
SW-846, Third Edition, Method 7196A, Revision 1, July 1992. pp 7196A-1 thru 7196A-6.
Procedure 1030 Revised 17/07