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Chemical formula: NH4Fe(SO4)2.12H2O
Synonyms: ammonium ferric sulphate dodecahydrate, ferric alum, ferric ammonium alum, ferric ammonium sulphate, iron alum, FAS, FAA.
Amount: 100 g
Appearance and odour: mixture of small and larger crystals, off white, pale violet to light brown in colour, without appreciable odour
Packaging: professional wide necked, white HDPE bottle with blue conical seal cap and tamper evidence ring. Labelled. Bottle and cap are UN approved and reusable.
Description and uses: a double salt of ferric (III) sulphate and ammonium sulphate used as a water treatment agent (flocculant) it is a primary source of soluble iron (III) salt. Well soluble in cold water and highly soluble in hot water it is an ideal substance for crystal growing. It forms large regular octahedral crystals with flattened corners and can be grown into very large crystals with some patience. Mixed crystals with chromium alum or aluminium alum can also be grown.
As a highly stable iron (III) compound with a high gravitational lever and a medium oxidiser it is often used as a titrant solution (0.1; 0.05 or 0.01 N) in redox titrations. For such analytical purposes it is recommendable (but not strictly speaking necessary) to dry the material overnight on filter paper in a dry, cool place. For most purposes ferric ammonium alum titrant solutions require standardising.
Ferric ammonium sulphate solutions are slightly thermochromic: hot solutions are considerably darker than cold ones, due to hydrolysis of the Fe (III) ion. It is recommended to suppress hydrolysis in titrant solutions by dissolving the compound in 0.1 M strong acid (sulphuric or hydrochloric).
For some purposes you may want to extra purify the product. Ferric ammonium alum is very easy to recrystallise. Take 2 parts (weight) of product and add 1 part (weight or volume) of distilled, deionised or very soft water and gently heat till the crystals have dissolved completely. There is no need to boil. Transfer the liquid to a sturdy but plastic and slightly flexible container (PP is ideal for this). Dip a glass rod or spatula in the liquid and allow it to cool and dry, then scrape some of the crystals thus formed into the liquid: these will act as seed crystals. Allow to cool to room temperature, then put on ice bath or in the fridge overnight. Decant off the supernatant liquid and set it aside. By squeezing the container gently the crystals will separate easily from the container walls (the alum tends to stick somewhat to glass and can be hard to safely remove from a glass vessel). Allow them to dry in a cool dark place on a porous plate, filter paper or good quality kitchen towel.