Case Study: HexChecks® Hexavalent Chromium Test Swabs Sensitivity, Specificity, and Temperature
Sensitivity
HexChecks reliably detect levels of hexavalent chromium below 0.1 micrograms. Low levels of hexavalent chromium show as light pink. Higher levels of hexavalent chromium will show as darkening pink to purple color development.
Temperature Range
HexChecks were tested on Mil-Spec chromate primed panels (MIL-PRF-23377 Type 1 Class C, PPG CA 7233) in a temperature range of -54C to 70C (-65F to 158F). Panels were cooled via immersion in dry ice. Panels were heated via a Lindberg BlueM convection oven. Temperatures were verified with a Fluke infrared thermometer.
A hex check was rubbed on the chromate primed surface for 20 seconds at each temperature. Results are recoded below.
The liquid in hex checks has a freezing point of approximately -7C (19F). Above its freezing point, the hex check will change color at a slower rate at cooler temperatures but will still indicate the presence of hexavalent chromium. At high temperatures, the reaction proceeds at a rapid rate with flash evaporation of the liquid the limitation at extremely high temperatures.
| Temperature °C (aprox. °F) |
Result |
|---|---|
| -15 (5) | Frozen, no color |
| -10 (15) | liquid, no color |
| -5 (25) | liquid, light purple |
| 0 (32) | purple |
| 10 (50) | purple |
| 20 (70) | purple |
| 30 (85) | purple |
| 40 (105) | purple |
| 50 (120) | purple |
| 60 (140) | purple |
| 70 (160) | light purple, flash evaporation of liquid |
| 70+ (160+) | flash evaporation |
Specificity
HexChecks are highly specific to hexavalent chromium. To demonstrate specificity and understand any interferences, HexChecks were tested against a selection of metals and metal salts that may be encountered at worksites.
Atomic Absorption Spectroscopy (AAS) standards and pure metals were obtained from established suppliers such as Alfa Aesar and Millipore Sigma. 1 milliliter of each material was placed into a Teflon dish. A hex check was rubbed into each solution. Samples sat undisturbed for 5 minutes to allow any color to develop.
Note: Upon drying, the wick of used swabs will turn pink. This is normal and is does not indicate the presence of Cr(VI). If color does not develop within 2-3min, the result is negative.
| Material | Concentration | Color Change |
|---|---|---|
| Aluminum [as Aluminum(III) Chloride] | 1mg/mL | None |
| Aluminum [metallic] | bar stock | None |
| Arsenic [as Arsenic (III) Nitrate] | 1mg/mL | None |
| Cadmium [as Cadmium(II) Nitrate] | 10mg/mL | None |
| Chromium [metallic] | bar stock | None |
| Chromium, Hexavalent [as Ammonium Dichromate(VI)] | 1mg/mL | Purple |
| Chromium, Hexavalent [as Strontium Chromate(VI)] | 1mg/mL | Purple |
| Chromium, Trivalent [as Chromium(III) Oxide] | 10mg/mL | None |
| Cobalt [as Cobalt(II) Nitrate] | 10mg/mL | None |
| Copper [as Copper(II) Sulfate] | 10mg/mL | None |
| Copper [metallic] | bar stock | None |
| Iron [as Iron(III) Nitrate] | 10mg/mL | None |
| Iron [metallic] | bar stock | None |
| Lead [as Lead(II) Nitrate] | 1mg/mL | None |
| Magnesium [as Magnesium(II) Nitrate] | 10mg/mL | None |
| Nickel [as Nickel(II) Nitrate] | 1mg/mL | None |
| Selenium [as Selenium(IV) Oxide] | 1mg/mL | None |
| Silver [as Silver(I) Nitrate] | 10mg/mL | None |
| Silver [metallic] | bar stock | None |
| Sulfur [elemental, crystaline] | 0.995 | None |
| Thallium [as Thallium(I) Nitrate] | 1mg/mL | None |
| Tin [as Tin(II) Chloride] | 10mg/mL | None |
| Iron [as Iron(III) Nitrate] | 10mg/mL | None |
| Titanium [as Titanium (IV) Chloride] | 1mg/mL | None |
| Titanium [metallic] | bar stock | None |
| Zinc [as Zinc(II) Nitrate] | 10mg/mL | None |
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