Teamwork at Sunderland Analytical Chemistry Competition

The 3 highest achieving chemists from our school were selected to take part in an analytical chemistry day at Sunderland University, and I was asked to write up the day's event for the school newspaper. Although we did not win, I believe we did well as we seemed to all get results that matched up with the motives and the culprit in the end.

Marrow murderer at large!

A year 12 team of Anu Krishna, Fraser Gaines and Will Bowles representing the school were invited for a day of a analytical Chemistry at the labs of Sunderland University on Tuesday. They were each given a booklet containing a story of a man called Dave who wanted to grow a prizewinning marrow in his garden; however he had made some enemies in the village and his marrow ha
d subsequently been destroyed by a mystery marrow murderer. 

Three suspects were narrowed down: firstly, Jason, a boy who had his football confiscated by Dave for kicking it into his garden who was believed to have held a grudge and be seen with a large bag of salt near the time of the crime. Next was Ms.Dale, a fertiliser retailer in the village who threatened to sell Dave fertiliser canister that's contents had been replaced with water. Finally was Mrs. Pembroke who had been recently spotted with copious amounts of silver nitrate, a toxic chemical, which she had no conceivable need for and also happened to hold a grudge against Dave and his marrow contest. 

Flame Photometer

Each of these suspects were to be investigated using analytical chemistry methods by each of the three members of the team, with Anu sorting the Silver nitrate scandal, Fraser testing for Phospherous in the Fertiliser and Will working on the salt scheme. 

Will made samples of different concentrations of salt water to be compared with both water obtained from the marrow plant's surrounding soil and soil from elsewhere in the garden. Using flame spectroscopy (where a sample  of a chemical is sent through a methane flame and the energy absorbed is an indication of the proportion of a certain element in the sample) Will found there to be less than 0.01 ppm (part per million) of Na⁺ in the water, and there was no difference between samples taken directly from the marrow soil and elsewhere in the garden, thus exonerating Jason from having committed such a terrible crime.

The Silver Thiocyanate ion titration

Anu was next, using a Ferrous thiocyanate indicator to detect a colour change from milky to grapefruit pink with the thiocyanate ion ([SCN]⁻) when combined with silver nitrate ([Ag]⁺[NO₃]⁻) and 6 molar nitric acid (HNO₃) to form a Silver thiocyanate precipitate when all of the iron had been used in conjunction with the thiocyanate

([Fe]³⁺3[SCN]⁻ with excess [Ag]⁺ means [Ag]⁺[SCN]⁻ silver thiocyanate precipitate is produced.

 By titrating this solution Anu was able to work out there was no significant difference in the amount of AgNO₃ in the soil around the marrow plant, and it was much less than the weakest concentration solution she had prepared in the lab, meaning that Mrs. Pembroke had also not poisoned the prized marrow.

It all came down to Fraser Gaines' phosphate fertiliser activity, as he used the analytical test for phosphate ions; ammonium molybdate-(2[NH₄]⁺[MoO₄]⁻²)

If phosphate was present the solution turned a dark marine blue, and the concentration of phosphate was proportional to the depth of the blue colour. 

Ammonium Molybdate

Fraser mixed varying concentrations of phosphate to use as comparisons, and then tested the sample from the marrow patch and the soil from elsewhere in the garden, finding a distinct difference between the two; the one from the rest of the garden was a dark blue, with a high concentration of phosphate [PO₃]⁻ ions, where the Dave had previously used fertiliser before Ms. Dale had started to be suspected of selling watered down fertiliser, the other from the marrow patch being a light transparent sky blue, indicating a paucity of phosphate ions, which, after using analytical photospectroscopy techniques ( which measure the absorption of light of a substance), Fraser could identify as having the same concentration as the weakest solution he had prepared, showing that Ms. Dale had indeed watered down the fertiliser to sabotage the growth of the prized marrow.

The 'Smoking Gun'

 Having taken longer to complete the analytical side of the practical work, the team was left to write a conclusion with fo
llow up questions, which were answered in a shortage of time and unfortunately did not secure them first place. 

The experience was both interesting and difficult, while improving their understanding of chemistry through analytical practical skills which will no doubt be good experience for university and later life for all three members of the Year 12 team.