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Red Litmus paper turned blue after heating

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Red Litmus paper turned blue after heating

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Singapore is known as the “Ornamental Fish Capital of the World”, being the top exporter of ornamental fish worldwide. The term ‘Ornamental Fish’ is used to describe coloured fish that are bred and kept in captivity for the purpose of decoration. One of the most sought after ornamental fish in Asia is the Koi that symbolises good luck and fortune. To expand this ornamental fish business in Singapore which has land and water scarcity, your fish company has decided to bid for this land and convert a pond near an animal farm in Singapore for breeding of koi. As a water analyst, you have been called in to analyse the suitability of the water for the breeding of the Koi fish.

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Due to the presence of NH3, Cl2, which are unsafe for koi, thus the water is not suitable for raising koi.

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NaOH added: Blue and white ppt present - white ppt soluble in excess-presence of blue ppt indicates presence of CU2+ and absence of Fe2+, Fe3+

NH3 added: Dark blue and white ppt present

NaOH + heating: turns red litmus paper blue, pungent smell, thus NH3 is present in water

AgNO3 added: White ppt present-indicates presence of CL2+

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After doing background research, we found that koi fish thrive in water that does not have ammonia, and with nitrites and nitrates of levels less than 0.2 ppm and less than 50 ppm respectively. The ideal pH for koi fish would be between 7.0 to 7.5. There should be no copper and iron ions present in the water.

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We decided to carry out qualitative analysis to determine if the pond water is suitable for the breeding of koi fish. Based on our research, we decided to test for the presence of ammonia (NH4+), nitrites (NO2-), nitrates (NO3-), copper (CU2+), sulfate (SO42-) and Iron(Fe2+ and Fe3+)

To test for ammonia, we added NaOH to a small sample of pondwater and heated the it. If ammonia is present in the sample, a pungent gas which turns red litmus paper blue, will be observed

Testing for nitrites and nitrates included firstly adding NaOH and Al foil to a small sample of pondwater. We then heated the sample. If effervesence and brown fumes are observed, the results will be inconclusive due to the possible CO32- presence. To differentiate between the two ions, we added acid.

The test for Cu2+ consisted of adding a few drops of NaOH to a sample of pond water. A blue precipitate will indicate the presence these copper ions.

The test for SO42- consists of adding dilute hydrochloric acid and a few drops of barium chloride solution to a sample of pond water containing sulfate ions. As the barium sulfate formed in the reaction is insoluble in water, it will be seen as a white precipitate.

The test for Fe2+ and Fe3+ consisted of adding a dew drops of NaOH to a sample of pond water. A dirty dark green precipitate indicates the presence of Fe2+ ions, and a brown precipitate indicates the presence of Fe3+ ions.