Cardinal Environmental Laboratories, LLC
Standard Operating Procedure
Procedure No.: 1090 Revision Date: 17/07
Subject: (Method 3500 CRD) Hexavalent Chromium in Solids:
Approved By: Title: Date Revised:
Hexavalent Chromuim in Solids
Method 3500 CRD
Description
Method 3060A is an alkaline digestion procedure to solubilize both water-insoluble and water soluble Cr+6 compounds in solid waste samples. After preparation, samples are analyzed utilizing UV-Vis colorimetry.
Precautions
General requirements (see procedure 0010)
Corrosives (see procedure 0020)
Sample Preservation
Samples should be collected in glass or plastic containers and stored at 4°C until analysis. Samples must be analyzed within 30 days from collection. The digestate may be stored for up to 7 days from extraction date.
Interferences
Oxidizing/reducing agents will bias results. For samples containing Cr+3 concentrations grater than 4 x the detection limit, Cr+6 results will be biased high due to oxidation. This oxidation can be suppressed by the addition of Mg+2 in a phosphate buffer. Elevated iron concentrations will cause the formation of a yellow color in the sample after the addition of reagents. If iron is suspected, confirmation is usually made by ICP screening of the sample.
Apparatus
- 250 ml beakers
- 100 ml graduated cylinder
- 100ml Volumetric flasks, class A
- Vacuum filtration apparatus
- 0.45 um filter membranes; preferably cellulose or polycarbonate membranes.
- Hot plate; capable of maintaining digestion solution at 90-95°C
- Volumetric pipets, class A
- Calibrated pH meter
- Calibrated balance
- NIST traceable thermometer, capable of measuring up to 100°C
- Watch glasses for 250 ml beakers
- Whatman #4 filter paper
Reagents
- Concentrated Sulfuric Acid (H2SO4): Analytical reagent grade.
- Sodium Carbonate (Na2CO3), anhydrous, analytical reagent grade
- Magnesium Chloride (MgCl2) anhydrous, analytical reagent grade (400 mg MgCl2 = 100mg Mg+2)
- Potassium Phosphate dibasic (K2HPO4), analytical reagent grade.
- Potassium Phosphate monobasic (KH2PO4), analytical reagent grade.
- Phosphate Buffer: 0.5 M K2HPO4 / 0.5 M KH2PO4 buffer at pH 7. Dissolve 87.09g K2HPO4 and 68.04g KH2PO4 in 700mls ultra pure water. Dilute to 1-L
- Lead Chromate (PbCrO4) analytical reagent grade, used for insoluble matrix spike.
- Digestion solution: Dissolve 40.0± 0.1g NaOH and 60.0 ± 0.1g Na2CO3 in ultra pure. Dilute to 2-L. The pH of this solution must be >11.5, if not discard and remake.
- Magnesium Chloride solution: 80g/L. Dissolve 80g MgCl2 in 800ml ultra pure water. Dilute to 1L.
- Potassium Chromate stock standard (1000 mg/L) commercially purchased. Stable for 1 year.
- Matrix spike standard: (100 mg/L) pipet 10 ml of 1000mg/L stock standard into a
100 ml volumetric flask and dilute to volume with ultrapure water. Expires after 1 month
- QC standard (1000 mg/L) commercially purchased. Stable for 1 year.
- Reagent grade ultrapure water.
- Acetone: ACS grade or better.
- Diphenylcarbazide indicator: Dissolve 250 mg diphenylcarbazide in 50 ml acetone. Store in brown bottle. Should be made daily.
Procedure
1.0 Monitor a temperature blank consisting of 50 ml digestion solution in a 250ml beaker. Adjust the temperature setting to maintain a digestion solution temperature of 90 - 95°C, measured with an NIST traceable thermometer.
1.1 Weigh approximately 2.5g ± 0.10 g of the sample into a clean, labeled 250 ml beaker. Mix the sample thoroughly before removing the aliquot.
1.2 Add the spike material to the matrix spike and matrix spike duplicate at this time.
1.2.1 The soluble matrix spike is spiked with 1.0 ml of the 100 mg/L spiking solution or twice the sample concentration, whichever is greater.
1.2.2 The insoluble matrix spike is prepared by adding 10-20 mg of PbCrO4 to a separate sample aliquot.
1.3 Add 50 ± 1 ml of digestion solution to each sample using a graduated cylinder. Add approximately 5 ml of Magnesium Chloride solution and 0.5 ml of phosphate buffer. Cover all samples with watch glasses.
1.4 Stir samples, unheated, for at least 5 minutes.
1.5 Heat the samples to 90 - 95°C. Maintain this temperature for at least 60 minutes, with frequent agitation of samples.
1.6 Cool to room temperature while continuing to agitate.
1.7 Add 2 - 3 drops Phenolphthalein to the sample. Slowly add conc H2SO4 to the
beaker dropwise until the solution just becomes clear.
CAUTION: This will react vigorously. CO2 will be generated. Add acid slowly to prevent sample from foaming over.
1.8 Quantitatively filter through Whatman #4 filter paper. If samples are still cloudy, filter through 0.45 um filter. Make sure to rinse the beaker a minimum of 3 times with ultra pure, add the rinsates to the filter apparatus. Filter through a 0.45 um membrane filter. Rinse the filter and filter flask with ultrapure. Transfer to a clean 100ml volumetric flask.
1.9 Sample Analysis
1.9.1 Transfer 50 mls of sample or an aliquot diluted to 50 mls to a clean beaker.
1.9.2 Using pH paper, adjust blanks, standards and samples to pH 2 to 5 using 6N or concentrated H2SO4.
1.9.3 To each sample add 2 ml of diphenylcarbazide indicator. Mix.
1.9.4 After exactly 10 minutes, the samples are ready to be read on the spectrometer.
1.9.4.1 The spectrometer should be allowed to warm up for at least one 1/2 hour.
1.9.4.2 Adjust wavelenght to 540 nm.
1.9.5 Place reagent blank in 1 cm cell and insert into spectrometer.
1.9.5.1 Zero the spectrometer to this blank by keying the second function key followed by the zero key.
1.9.6 Remove reagent blank and insert digested blank, standards and samples.
1.9.6.1 A pink color indicates the presence of Cr+6. If a sample is above the detection limit, run a spike to confirm positive Cr+6.
1.9.6.2 If the sample reads high on the spectrometer, but has no visible pink color, you must read a sample blank on the spectrometer. Subtract sample blank absorbance from the sample absorbance for the corrected sample absorbance.
1.9.6.3 If sample turns a yellow or rust color after the addition 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.
1.10 Prepare a second aliquot for dry weight analysis by SOP 2040.
2.0 Standard curve
2.1 Pipette the following volumes of the 100mg/L standard solution into
100 ml volumetric flasks. Analyze 50 mls.
mls of working standard conc. mg/L
0 0
0.1 0.1
0.5 0.5
1.0 1.0
2.0 2.0
5.0 5.0
2.2 Proceed with samples analysis as in steps 1.9 thru 1.9.6.3.
Calculations
Concentration mg/Kg dry wt = (A - F) x LC x D x E
B x C
Where: A = Sample absorbance
B = Initial moist sample weight (g)
C = % solids / 100
D = Dilution factor (if necessary)
E = Final digest volume (ml)
F = Sample blank absorbance
LC = Linear coefficient
Quality Control
· Each analytical batch must be accompanied by a digested method blank, which is taken through all analytical steps.
· Each analytical batch must be accompanied by a standard consisting of 0.1 ml of 1000mg/L standard added to 50 ml of digestion solution.
· Each analytical batch must be accompanied by a QC standard consisting of 1 ml of the aqueous spiking solution AND a QC standard consisting of 10 - 20 mg of the solid matrix spike (PbCrO4) added to 50 mls of digestion solution. Recovery must be between 80% and 120% or the entire batch must be reanalyzed.
· Every 5th sample shall be analyzed as a duplicate spike - spiked with 1 ml of the 100 mg/L aqueous spiking solution. Every 10th sample shall be analyzed as a duplicate spike - spiked with 10 - 20 mg of the solid spike material. RPD must be £ 20% and spike recovery must be 75 - 125%. Both soluble and insoluble matrix spikes must be analyzed with each batch. If spike recoveries are outside of limits, a post-digestion spike must be analyzed.
· A post-digestion matrix spike must be analyzed with every batch. Spike recovery should be 85 - 115%.
· All weights, volumes, comments, etc. for each batch must be entered in notebook form for data review.
· Any sample with absorbance >10% above the curve must be diluted and reanalyzed.
Failure to meet quality control acceptable limits requires corrective action. (See attachment 0150)
References:
- SW846 Method 3060A, pp. 3060A-1 to
3060A-15, Revision 1, December 1996
- Standard methods for the examination of water and wastewater, 18th Edition, 1992. Method 3500CrD, pages 3-59 thru 3-60.
- SW846, Method 7196A, pp 7196A - 1 thru 7196A -6 Revision 1, July 1992.
Procedure 1090
Revised 17/07