November 2015

# Sharing chemistry with the community: The solubility and alkalinity of ammonia

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Kenneth Lyle, Lecturing Fellow, Duke University, Durham NC

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Penney Sconzo presented this nifty demonstration at the ChemEd 2015 Generations Symposium in Kennesaw State University, Kennesaw GA. This is an easy, colorful and safe microscale version of the Ammonia Fountain.

Ammonia gas is very soluble in water. The relatively high solubility is attributed to the hydrogen bonding that takes place between the ammonia and water molecules. The dissolving of ammonia in water forms a basic solution. A small amount of the dissolved ammonia reacts with water to form ammonium hydroxide, which dissociates into ammonium and hydroxide ions. All of these processes are reversible. For both reactions, heating the system favors the reverse direction.

NH3(g) ⇔ NH3 (aq)

NH3(aq) + H2O(l) ⇔ NH4OH(aq) ⇔ NH41+(aq) + OH1-(aq)

The Ammonia Fountain, as described in many demonstration texts and online, is typically performed on a large scale and requires a great deal of setup. Repeating the demo for different indicators is often not feasible. As well, it often includes materials (multiple flasks, luminol and anhydrous ammonia) that can potentially distract viewers from the desired learning outcomes. These issues along with safety concerns limit the Ammonium Fountain to a front-of-the-class, teacher-presented demonstration.

Fig. 1. Pipet apparatus: The smaller stem pipet bulb contains the concentrated ammonia solution. The larger stem pipet is gently placed on top to collect the ammonia gas — it should not be jammed together but have enough room for the air to be able to be displaced.

This version of the demonstration is performed on a small scale and would allow, if desired, for students/the audience to take an active role in the presentation by placing the ammonia-filled jumbo pipet into a solution with an acid/base indicator. It is simple to set up and perform, as well as being free of distractions. Participants enjoy SEEING, HEARING and FEELING the reaction!

Fig 2. The ammonia gas-filled pipet is placed in water with universal indictor — the color green indicates a pH around 7. When the solution is drawn into the pipet, the color is purple indicating pH around 10.
Funding for the Duke Chemistry Outreach program provided by the Powell Family Trust, the Duke-Durham Neighborhood Partnership and Bio-gen Idec – Research Triangle Park. The Duke Department of Service Learning funded Ken to attend the ChemEd 2015 conference and participate in the Generations Symposium.

Clip off the stem of a jumbo pipet, leaving about 1 cm of the stem. Simply insert the jumbo pipet over the thin-stem of another pipet, which is partially filled with concentrated ammonia solution. The pipet apparatus (right) is placed in a hot water bath to increase the rate of the production of ammonia gas. The top larger pipet bulb collects the ammonia gas. The top pipet is removed once the gas has been collected.

The ammonia-filled pipet is inserted into a clear cup with water (or a slightly acid) solution with an added acid/base indicator. The bulb is gently squeezed to prepare room for a small amount of solution to be drawn in. When the acidic solution is drawn up into the pipet, it appears immediately as the color of the indicator in a basic solution, as shown in Fig. 2.

While the participants perform this procedure, another pipet is placed over the ammonia source filling with ammonia gas and preventing the ammonia from going into the open air. Because the procedure is fast and simple — with little concentrated ammonia solution consumed — the participant can repeat this demo multiple times with different acid/base indicators.

Fig 3. Penney Sconzo (green goggles) at ChemEd 2015 with a variety of acid/base indicators. The pipet apparatus is shown sitting on the hot plate. Penney had participants come up and perform the demonstration.

Fig. 4. Above are the four acid/base indicators tested (clockwise from top left: phenolphthalein, bromothymol blue, thymolphthalein and phenol red). When the slightly acidic solution is pulled into the ammonia gas-filled pipet, the color of the solution in the pipet indicates that it is now a base.

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