Absorption Test Chemistry
The formation of peroxychromic acid by the reaction of hydrogen peroxide with potassium dichromate provides a convenient reaction for the purpose of demonstrating and teaching the use of absorption stopped-flow on both the SX20 and PiStar spectometers. Using sequential stepping of the monochromator and a photomutlitpler detector, kinetic measurements can be collected at multiple wavelengths under automatic control and, from these data, time resolved spectra can be produced. In addition, this chemistry is also suitable for demonstrating and teaching the use of the photodiode array accesssory when this is present as part of the overall instrument package.
Sample Preparation
| Solution 1:- | 10 -3 M potassium dichromate with 10 -1 M potassium nitrate |
| Solute 1: | Potassium dichromate |
| Source: | Sigma P-5271 |
| MW: | 294 |
| Conc: | 10 -3 M |
| Solute 2: | Potassium nitrate |
| Source: | Sigma P-6162 |
| MW: | 101 |
| Conc: | 10 -1 M |
Preparation of working solution:-
Dissolve 147mg Potassium dichromate + 5.05g Potassium nitrate in 500mL of water
| Solution 2:- | Acidified 10 -1 M hydrogen peroxide |
| Solute 1: | Hydrogen peroxide (30% w/w) |
| Source: | Sigma H-1009 |
| MW: | 34 |
| Conc: | 10 -1 M |
| Solute 2: | Nitric acid (1M) |
| Source: | Aldrich 31,916-3 |
| MW: | 63 |
| Conc: | 10 -1 M |
Preparation of working solution:-
Fill a 500mL volumetric flask with about 250mL of distilled water. Add 5.7mL hydrogen peroxide + 50mL of 1M nitric acid to flask and then make up to 500mL with distilled water.
Risk Assessment
Dichromate should be treated as a suspected carcinogen, protective disposable gloves should be worn (particularly when preparing the stock solution). Wash hands after handling.
Hydrogen peroxide (30% w/w) is a strong oxidising agent. Do not get on skin, eyes or clothing. Avoid any spillage. Wear suitable protection when preparing the working solution. Refer to safety data sheets in case of accidents.
The concentration of dichromate and hydrogen peroxide in the working solutions is considered low enough to be of minimal hazard.
However, observe usual hygeine precautions and wash hands after use.
| Measurement Conditions | |
| Syringe Ratio: | 1:1 (both syringes 2.5mL) |
| Dichromate conc. in cell: | 5x10 -4 M |
| Wavelength range: | 300 - 600nm in 5 or 10 nm steps 350nm - absorbance decrease 580nm - absorbance increase 465nm - isosbestic |
| Slit settings: | 0.5mm |
| Cell optical path: | 2mm |
| Acquisition period: | 0.1s |
| Absorbance range: | 0 to 0.3 AU |
| Reagent volume: | 50 to 75µl of each reagent (2 to 2.5 turns of the autostop adjustment) |