[Hendawg]

Hi there

I am hoping to evaluate the UV absorbing properties of a variety of sunblocks.

I hoped to use the following compounds and equipment

• 120 cm3 Ethanedioic acid solution (8g per litre)
• 3 cm3 Potassium hexacyanoferrate III solution (30g per litre)
• 120 cm3 Iron III nitrate solution (12 g per litre)
• 12 Petri dishes
• Source of UV light (preferably a UV lamp)
• 3 different sun screen samples
• Bottle wrapped in aluminium foil for the combined reagent
• 1 Colorimeter

I originally intended to use the following method:

Preparation
1.   Make up the three solutions.
For each test, you need about 20 cm3 to cover the bottom of the petri dish, so for 9 samples (3 for each sunscreen) and 3 controls you will need
120 cm3 of ethanedioic acid solution
3 cm3 of potassium hexacyanoferrate III
120 cm3 of Iron III nitrate solution
2.   Add 3 cm3 of the potassium hexacyanoferrate III to the ethanedioic acid solution and transfer to the light-proof bottle.
3.   Set up your samples. Put 5 g of sunblock on each lid of the petri dishes – remember to keep three free of it as a control
4.   Add 120 cm3 of iron III nitrate solution to the reagent bottle and swirl to mix.

The Experiment
1.   Pour about 20 cm3 of the combined reagent into each petri dish, replace the lids and then expose to the light source. Measure and note down the reading of the solution from the colorimeter
2.   Within a minute or two, depending on the light, you will see a dark blue colour appearing. The more UV light the dish is exposed to, the darker blue the solution.
3.   After a set amount of time, use the colorimeter to measure the change in colour that has occurred.
Repeat experiment at least three times.

Explanation:

Interaction of light with the ethanedioate ions leads to their oxidation and releases carbon dioxide and an electron (equation 4a) which then reduces Fe(III) to Fe (II).

C4 H4 O6 2- + hν = 2CO2 + 2C(OH2) + 2e-
Fe 3+  +  e   =  Fe 2+
The Fe(II) formed combines with CN - present in the solution to form the complex [Fe(CN)6]4-  which, in turn, gives the insoluble blue Prussian blue, Fe(III)4[Fe(CN)6]3.

Fe 2+  +  6 CN -  =  [Fe (CN6)]4-
[Fe (CN6)]4- +  4 Fe 3+  = Fe(III)4[Fe (CN6)]3


Unfortunately my school has informed me that I am not permitted to use potassium hexacyanoferate for health and safety reasons.

Can you please recommend an alternative to hexacyanoferate or an entirely different combined reagent that is photosensitive to UV light and will decompose in a linear fashion that I will be able to measure.

Thanks
Henry

[wildfyr]

Can you get access to a UV-vis spectrophotometer and possibly a spincoater? A direct measurement is possible with a UV-Vis, as opposed to the indirect method you have proposed counting on 100% conversion of UV degradation to prussian blue. Do you see what I am getting at?

Also, ethandioic acid is known as oxalic acid to most chemists.

[Hendawg]

Hi wildfyr

Thanks for the suggestions. I will try and find out if my school has access to a UV-vis spectrophotometer.

I'm concerned that if I do this however, the investigation will become more physics-oriented rather than one focused on chemistry. I am doing this for an IB chemistry internal assessment and it is important that I demonstrate understanding of concepts relating to chemistry and that my findings aren't obvious to anyone who has any background knowledge of the subject.

At this point I might flag this route and pursue another option. I'll let you know how I go.


Thanks
Henry

[DrCMS]

What are the schools reason for not allowing the use of potassium ferricyanide?  It is non-hazardous for both transport and supply.

[Borek]

Exactly my thought, DrCMS was just a few seconds faster: pretty sad situation when you are asked to do a chemistry based experiment, but you are not allowed to use quite safe and basic reagents

[DrCMS]

A flaw in this set up I did not spot in my first read was the use of the glass petri dish lid; glass itself is pretty good at blocking UV.

[billnotgatez]

plastic petri dish?

[Babcock_Hall]

It depends somewhat on the plastic.  Some plastic spectrophotometric cuvettes are only good for the visible region because they absorb in the UV region, but others let in wavelengths to roughly 280 nm.

[Hendawg]

Yes, unfortunately in New Zealand we have incredibly stringent health and safety regulations, especially in high schools. I was told that there is a risk that the oxalic acid could result in the Potassium ferricyanide decomposing into hydrogen cyanide. My understanding is that this requires conditions far more acidic than that of dilute oxalic acid however, so this seems rather pedantic. This happened recently apparently as the technician told me they only recently disposed of all their ferricyanides.

If I do go ahead with this experiment I will ensure that the petri dish lid I use blocks a minimum amount of UV light. Are you able to suggest an alternative to a petri dish lid that will likely block less UV light?

Thanks
Henry

[wildfyr]

Glass starts blocking around 320 nm or so, so its not a great choice. Most consumer polymers start to absorb around there too. Quartz is the gold standard, it starts absorbing around 220 nm. However, it is much more expensive.

Let us know about the UV-Vis, it opens a vastly larger set of experiments. Some of instruments are quite cheap and easy to use.

If you are totally stuck with colorimetry, you could use a digital camera to take pictures (carefully) and analyze the color by computer using software like ImageJ (its free, and free use!). Or you could dip a swatch of paper in the solution and do colorimetry on that after it dries.

[Hendawg]

Ok, thanks! I'll keep you posted on how things go.