November 24, 2013

On the Nature of Water: viii) Creating Solutions

   Dissolving. Water is as water does. Water is wet – it likes water. Materials dissolve in water up to the limit of their solubility. Ionic compounds are called acids. bases and salts. Acids and bases are generally completely soluble in water. Some salts dissolve in water readily, other salts have a very limited solubility. The Ksp is the solubility product constant – which is defined mathematically as a means of organizing solution effects.

    The solution is made up of a solute that is dissolved in a solvent. A known amount of solute is added to a fixed volume of water (or other solvent) to provide a concentration value in grams per liter. Depending on the identity of the chemical species involved – the grams can be converted to moles and the basis set configured to a one to one molecular basis. Moles are grams per formula weight. Let's prepare a liter of one Molar solution of sodium hydroxide as an example. 
    First things we do is collect our equipment that we will need in the lab. We will need several types of glass containers – a large beaker for mixing, a volumetric flask for holding exactly one liter – one thousand milliliters, a final container. We might ask ourselves if we really need a whole liter of caustic solution and consider making less solution. We will need a glass stirring rod, a small funnel, a dropper and must have rubber gloves and safety glasses. A box of baking soda will serve as spill protection and running cold water should be available on demand. The reagent water for the solution is a gallon of bottled distilled water. We will need a balance to weigh the sodium hydroxide – NaOH. The form that we choose is called red devil lye and is an off-the-shelf drain cleaner at the local farm cooperative. Lye will cause caustic burns – handle with care.

    The formula weight of NaOH can be looked up quickly using the periodic table. Sodium weighs 23, oxygen weighs 16 and the hydrogen weighs one – our three atoms together weigh 40 atomic mass units. Thus one mole of sodium hydroxide contains 40 grams and when added to a liter of water makes a 1M solution. Sodium hydroxide is a flaked powder – a mask might be appropriate if you have NaOH in your breathing space. The solution we are preparing here is a strong base – it is similar to strong acids, but works at the opposite end of the pH scale. Please handle with caution and store in a safe location away from children and pets (I know, I was a child once!)

    Turn on your electronic balance or isolate your beam scale. Glove and mask up and use a scupula or a tablespoon to handle the NaOH. Place the beaker (400 ml or larger) onto the balance and get an accurate weight in grams. This is your tare value – the weight of the empty container. Add 40 grams to this value – or set the beam to tare plus 40 and weigh the sodium hydroxide directly into the beaker. NaOH ends to be hygroscopic and will pick up moisture from the air, so work with focus when handling. Cap the container and set aside – add distilled water carefully to the white powder in he beaker and stir with the glass rod to dissolve. 
    The amount of water used to dissolve the solid here should be about 200 ml – the solid will go into solution readily – be cautious about any splashing. After the material is dissolved – pour the solution through the funnel into the one liter volumetric flash. This container is designed to make very exact solutions and should not be used as a storage vessel. The glass is carefully calibrated to hold a certain volume at a given temperature – the variance is not germane to the scale hat we are working with here. This is a 'to contain' device – volumetric glassware also comes in 'to deliver' forms, like burets and pipettes. Chemists like their glass toys – which tend to be fragile, take good care.

    Take some of the distilled water and rinse the beaker that held the sodium hydroxide and add this rinse to the volumetric flask. For good quantitative work, we might wish to use more rinses – once the rinsing is done – distilled water can be added to the vol flask up to the line on the neck. Use the dropper to add water as you get close – the accuracy and precision of future tests may depend on your precise methods of making a proper solution. 
    When mixing up solutions of acids, it is generally good protocol to add the solute to the solvent. Water tends to spit when added to concentrated acids – the more you think you know about chemistry – the less you end to use your safety gear – this is a mistake. Have confidence in your ability and always use your safety gear – it is there to protect you if you need it – just don't plan to need it.

    Once the volumetric flask is full – put in the stopper and mix by inversion. The entire solution should be uniformly dissolved, then transferred to the storage container, again with the assistance of the funnel. Realize that the 1M NaOH solution is caustic – any spill should be neutralized with acid or sodium bicarbonate. The solution should be handled with care and neutralized with acid before discarding (unless you wish to unplug your drain – which is why you generally purchase this stuff.)

    All ionic solutions of acids, bases and salts can be prepared in this fashion. If you are stocking a qualitative lab with test solutions, you will need small amounts of a variety of chemicals on a one molar basis. To make the solution, you need to be able to calculate the number of grams per mole of the chemical involved and have the appropriate glass on hand to make the dilution. Materials that are not soluble in water will not make solutions. Here are some ways of altering the solubility, using heat or other appropriate reagents that will promote the desired chemistry.

    For instance – the element iodine is not terribly soluble in water. However, tincture of iodine is prepared by adding potassium iodide to the water – creating the soluble molecular form KI3 . This material – available as Lugol's solution is very useful for purifying drinking water and disinfecting small abrasions. The KI3 dissociates to K+ and I3- ions – which now can dissolve the non-polar iodine in the polar solvent water.  Doc will have 2% Lugol's Iodine available for sale after the new year.

    Ah, the little tricks of the trade. You don't find too many older chemists these days, because the toxicity of so many chemicals was revealed after the fact of common use – the regular folk hardly notice chemistry in action since cars made of plastic and fiberglass don't rust. The chemistry of solutions is a colorful field – the measurement of amounts is known as quantitative analysis. This field relies on breaking substances down to their component parts and telling us what is there.  

Namaste' ...  doc

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