106.3.2.21 TM-21, Determination of Cobalt in Tungsten Carbide Inserts

This method determines the percent of Cobalt in Tungsten Carbide Inserts used in snow plow blades through the use of Inductively Coupled Plasma Optical Emmission Spectrophotometer (ICP-OES).

106.3.2.21.1 Equipment and Reagents

1. Inductively Coupled Plasma Optical Emmission Spectrophotometer (ICP-OES) capable of internal standard correction (Varian Vista-MPX ICP-OES equipped with a Charged Coupled Device (CCD) Simultaneous detector) or comparable instrument.

2. Digestion Block with capability to hold 70 ml polyethylene digestion vessels.

3. Trace Metal Grade Hydrofluoric Acid (HF), 48 percent.

4. Trace Metal Grade Nitric Acid (HNO₃), 1.42 specific gravity.

5. 1000 mg/l (ppm) Cobalt Stock Solution – this solution can be purchased from a number of commercial sources.

6. 1000 mg/l (ppm) Yitrium Stock Solution - this solution can be purchased from a number of commercial sources.

7. Adjustible pipet capable of accurate dispensing of 100-1000 μl.

8. Balance with an accuracy of 0.1 mg.

9. 100 ml PFA Volumetric Flask, Class A.

10. Teflon Watch Glass.

106.3.2.21.2 Preparation of Standards

Blank solution: Prepare a blank by adding 2-5 ml of 1:24 HNO₃ into a 100 ml PFA volumetric flask. Pipet 0.400 ml (400 μl) of 1000 mg/l Yitrium and bring to the 100 ml graduated volume mark with 1:24 HNO₃. Place the supplied cap on the PFA volumetric flask and mix well. This solution will contain 4.00 mg/l Yitrium.

5.00% Cobalt standard solution: Add 2-5 ml of 1:24 HNO₃ into a 100 ml PFA volumetric flask. Pipet 0.400 ml (400 μl) of the 1000 mg/L Yitrium Stock Solution and 0.125 ml (125 μl) of the 1000 mg/l Cobalt Stock Solution, bring to 100 ml graduated volume mark with 1:24 HNO₃. Place the supplied cap on the PFA volumetric flask and mix well. This solution contains 1.25 mg/l Cobalt and 4.00 mg/l Yitrium.

10.00% Cobalt standard solution: Add 2-5 ml of 1:24 HNO₃ into a 100 ml PFA volumetric flask. Pipet 0.400 ml (400 μl) of the 1000 mg/l Yitrium Stock Solution and 0.250 ml (250 μl) of the 1000 mg/l Cobalt Stock Solution and bring to 100 ml graduated volume mark with 1:24 HNO₃. Place the supplied cap on the PFA volumetric flask and mix well. This solution contains 2.50 mg/l Cobalt and 4.00 mg/l Yitrium.

20.00% Cobalt standard solution: Add 2-5 ml of 1:24 HNO₃ into a 100 ml PFA volumetric flask. Pipet 0.400 ml (400 μl) of the 1000 mg/l Yitrium Stock Solution and 0.500 ml (500 μl) of the 1000 mg/l Cobalt Stock Solution and bring to 100 ml graduated volume mark with 1:24 HNO₃. Place the supplied cap on the PFA volumetric flask and mix well. This solution contains 5.00 mg/l Cobalt and 4.00 mg/l Yitrium.

NOTE: The concentration of Yitirum MUST be exactly 4.00 mg/l in ALL of the standards and samples if Yitrium is used as an internal standard.

106.3.2.21.3 Procedure

Using a Plattner’s Motar and Postle or suitable device, crush or grind an insert until approximately 2.0-5.0 grams of the particles will pass a No. 20 sieve. Weigh, to the nearest 0.1 mg., 0.250 grams of the material passing the No. 20 sieve, and place in a 70 ml polyethylene digestion vessel.

NOTE: This digestion procedure uses concentrated Hydrofluoric Acid. Use ONLY plastic or teflon labware. HF will aggresively attack glass labware. Use EXTREME CAUTION when measuring and handling any HF solutions.

Add 10 ml (using plastic or teflon labware) of HF and 2 or 3 drops of HNO₃. Cover the vessel immediately with a teflon watch glass and heat in the digestion block at 95°C. Add 2 or 3 drops of HNO₃ each time the reaction subsides, keeping the digestion vessel covered as much as possible. When solution of the sample is complete, remove the teflon watch glass, rinse with 1:24 HNO₃, and cool to room temperature.

Quantitatively transfer solution into a 100 ml PFA Volumetric Flask and bring to volume with deionized water. Thoroughly mix the sample in the volumetric flask and pour a suitible aliquot back into a digestion vessel. (Filter through a Whatman 41 filter paper if necessary). Pipet exactly a 1.00 ml aliquot of the sample solution and 0.400 ml (400 μl) of the 1000 mg/l Yitrium into a 100 ml PFA Volumetric Flask and bring to the 100 ml graduated volume mark with 1:24 HNO₃. Place the supplied cap on the PFA volumetric flask and mix well.

Calibrate the instrument using the blank solution, the 5.00%, 10.00%, 20.00% Cobalt standard solutions. The preferred analytical line for Cobalt is located at 236.379 nm or an appropriate sensitive wavelength. It is recommended that several wavelengths be used to eliminate any possible spectral interferences.

106.3.2.21.4 Quality Control

A Method Blank, Laboratory Control Sample, Duplicate Sample and Matrix Spike Sample must be digested and analyzed in the same manner as the unknown samples. They are prepared as follows:

Method Blank: Use a separate, clean 70 ml digestion vessel without any sample added to it. Add all reagents and follow all steps outlined in 106.7.21.3, Procedure.

Laboratory Control Sample (LCS): Using an avaliable National Institue of Standards and Technology NIST Standard Reference Material SRM of Cemented Carbide Powder, follow the procedure outlined in 106.7.21.3, Procedure. Since this sample is in powder form, the crushing, grinding and sieving step may be omitted from 106.7.21.3. The percent recovery is calculated using the following formula:

$LCS\ \%\ Recovery=100\times {\frac {LCS\ result}{Certified\ Value}}$

Duplicate Sample: Using a separate aliquot of any of your unknown samples, follow the prodecure outlined in 106.7.21.3, Procedure. The result is compared to the result for the original sample. The relative percent difference is calculated using the following formula:

$RPD=100\times {\frac {|Duplicate\ Result-Original\ Result|}{\left({\frac {Duplicate\ Result+Original\ Result}{2}}\right)}}$

Matrix Spike Sample: Using still another aliquot of same sample used for the Duplicate Sample, follow the procedure outlined in 106.7.21.3, Procedure. However, add exactly 0.125 grams of the NIST SRM Cemented Carbide Powder to the 0.125 to 0.250 grams of unknown sample keeping the total result within the calibration curve. When this sample is taken through the exact procedure in 106.7.21.3, Procedure, the result is a cobalt addition of half the SRM certified value to the percent of original sample used. The percent recovery is calculated using the following formula:

$Spike\ \%\ Recovery=100\times {\frac {Spike\ Result-\left(Sample\ Result\times {\frac {Weight\ of\ Unknown\ Used}{Original\ Weight\ Used}}\right)}{Theoretical\ Spike\ Amount}}$

106.3.2.21.5 Calculation and Report

Since the calibration curve for the instrumentation is calculated using the 5.00%, 10.00% and 20.00% levels as the standards for the curve, no additional calculations are required. Report the Cobalt in the sample to the nearest 0.1% as follows:

% Cobalt (Co)