A free publication from Allan Heywood Enamels - all links will open in a separate browser window |
|
|
|
When making up solutions from solids, or diluting liquids using some instructions, there is not often any information upon the best ways of doing the job, and I propose here to suggest safe, simple and easy methods of going about it. There is no necessity for ordinary folk to try working to any great accuracy; accurate solutions are best left to qualified people, even if it costs a little more. First, however, wear eye protection. It is a good idea to have on hand a simple weighing scale for the solids, and measures for the liquids; small measures up to say, 100 mls (millilitres) and bigger ones up to a litre. (1000 mls) You will also need a stirrer made of some material resistant to corrosion - plastic is quite OK. You can often get very cheap battery powered electric hand held stirrers, and these are very useful; but ensure that the business end is clean and won't corrode. A couple of plastic funnels are also useful; a 6 inch and a 3 inch funnel will help when pouring. Be careful to use a thoroughly clean container of suitable size in which to keep the solution, having a good stopper or cap. A container to be used for solids must have a wide mouth and be completely dry with a watertight cap. Let's consider the dilution of a liquid with water. Just imagine say, 20 mls of some liquid has to be diluted with water to make one litre. Make sure the measuring jug to hold the diluted liquid holds at least a litre, and is very clean ; it doesn't have to be dry. Half fill the jug with water, and now pour the 20 mls of strong liquid into the water slowly and with constant stirring, but avoid foaming. If the solution has become hot, allow it to cool a little, then add more water to reach the 1000 mls mark and stir well. Using the funnel, transfer the diluted solution into it's final container, and stopper it. Immediately label the bottle correctly and very clearly in large block letters. Quite often it is necessary to add the word “POISON” and it is a good idea to add the date. It is imperative that the bottle be labelled. Will you know for certain 6 months later what the bottle contains? In any laboratory the strict rule is that the contents of any unlabelled bottle or container must be discarded. No guessing allowed! Mop up spills at once. Never use bottles which have been sold containing food or drink; and don't keep them near food. It is too easy to pick up the wrong bottle. Now suppose we have to make up a solution containing (say) three powders or crystals. You do really need weighing scales for this, unless the recipe tells you how many spoons full to use and whether heaped or flat. If any of the constituents contain lumps, then they should be crushed, otherwise the substance may be hard to dissolve. Have all the substances weighed out or otherwise measured out on small pieces of clean paper which have previously been folded down the middle and creased. Most solids dissolve more easily in warm water, but don't have it very hot. Pour about half the required amount of water into the measuring jug then add each of the solids slowly and carefully with continuous stirring. The order is usually unimportant. When all has dissolved, make up to the final volume with cold water. In the case of caustic soda (sodium hydroxide, Drano, lye) the solution will get very hot - constant stirring is essential, and it will be necessary to allow the liquid to cool. Anhydrous (very dry) substances are likely to clump and so take longer to dissolve; constant stirring and slow addition will help avoid this. Bottle and label when cool. Be careful not to contaminate bottles of solids with other chemicals; carefully clean the spatula or spoon between each solid. Dispose of the papers used to weigh out each chemical as soon as used. Making up solutions is neither difficult nor inherently dangerous; just proceed slowly, think a little ahead, and read all the instructions and precautions on the bottles. Ordinary tap water will be OK for making up solutions unless the instructions say otherwise. Distilled or deionised water is not usually necessary, unless you need to make up solutions like silver nitrate; any chlorine in the water will give a cloudiness of silver chloride. Keep everything away and out of reach of children. If you want to know what a substance smells like, never take a direct sniff at it. Hold the open container about a foot from your face and waft any vapour emanating from it towards you with the other hand. |
|
There is no standard 'use by' time for many thousands of chemicals, and it is really not practicable to offer much in the way of advice. However, most inorganic chemicals are stable provided the container has an air tight lid. The shelf life of things like sodium chloride, ferrous sulphate, alum, sodium carbonate and bicarbonate, sodium bisulphite and so on, is long, but many will absorb moisture, which helps destroy metal caps, and corrodable things like corks. Nitric acid (conc & dilute); Keep tightly stoppered in glass or polythene and away from light. It is extremely corrosive. In good conditions it will last years. Sulphuric acid (conc and dilute); The concentrated acid is avid for moisture, and bottles must be kept airtight; it will corrode cork and rubber - it is extremely corrosive. In good conditions it will also last years. Methylene Iodide (Iodomethane) Benzyl benzoate Bromoform Monobromonapthalene Ether* (Diethyl ether) Amyl acetate All the above are volatile liquids, are flammable so must be well stoppered, will all attack rubber and cork and all will soon deteriorate in the presence of light, but are otherwise fairly stable; by that I mean there should be little change in several years. All are poisonous and none of the vapours should be breathed. *In the case of ether (diethyl ether); in the presence of a little air and and light it will react with air to form, in time, ether peroxides which are explosive. To guard against this occurring, about 10 mls of strong ferrous sulphate solution in water should always be present at the bottom of the bottle. As the boiling point of ether is only 35C, it is extremely volatile and its vapour is heavier than air and will therefore pool on the floor, awaiting the slightest spark to cause a bad explosion.. Chemical solutions used in film development (black & white) are all poisonous, and care should be taken not to get any on the hands or body, as they are likely to cause skin irritation and dermatitis. Wear a face shield and protective gloves. Since these chemicals are used in dim lighting, it is well to wear a protective apron too. Almost all will rapidly deteriorate, and won't last more than a week, except sodium or ammonium thiosulphate ('Hypo') which may be kept in solution for months. Cheers for now, John Burgess Mapua, Nelson NZ |
| USE BROWSER "BACK" BUTTON TO RETURN TO PREVIOUS PAGE |