Making up solutions
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Diluting Concentrated Acids
Dilution of concentrated acid should always be done in a fume cupboard.
It is important only to pour acid into water, not the other way around, especially with concentrated acids. Acids may quickly absorb water, creating a lot of heat in the process. When acid is poured into water, the heat can quickly become evenly distributed in the water. If water is poured into acid, the water may quickly boil, spraying acid everywhere.
Hydrochloric acid
Hydrochloric acid can be purchased as either 36% or 32% solution so carefully check the bottle first.
To make a 1 molar solution (1M):
36%
Add 83.5mL of 36% hydrochloric acid to about 600mL of distilled water in a 1 litre measuring cylinder in a fume cupboard. Make up to 1L, mix well and pour into a labelled bottle.
32%
As above, except use 96mL of hydrochloric acid.
Safety Notes
Concentrated hydrochloric acid is highly corrosive and very irritating to the lungs, wear a face shield and use a fume cupboard.
- 1M (mole/litre) solutions and above should be labelled IRRITANT.
- 5M solutions and above should be labelled CORROSIVE.
Nitric Acid
Assuming the concentrated nitric acid is 70%w/v, then to make a 1M solution:
Add 62mL of concentrated nitric acid to about 700mL of water then dilute to 1L.
Add slowly, stirring constantly with a polypropylene or glass stirring rod. If the solution gets too hot, stop and let it cool down.
- 0.1M (mole/litre) solutions and above should be labelled IRRITANT.
- 1M solutions and above should be labelled CORROSIVE.
Sulfuric acid
Concentrated sulfuric acid is highly CORROSIVE and a dehydrating agent. It causes severe burns. It should only be handled under close supervision by an experienced person. Wear gloves and protect the eyes with safety goggles or even better, a face shield.
Add 54mL of concentrated sulfuric acid to about 700mL of iced water and dilute to 1 litre. This is a 1M solution.
- 1M (mole/litre) solutions and above should be labelled IRRITANT.
- 4M solutions and above should be labelled CORROSIVE.
Ethanoic acid
Known also as acetic acid. The concentrated acid is called Glacial acetic acid as it freezes at 17°C. Glacial acetic acid is CORROSIVE and the vapour is an extreme IRRITANT. Wear gloves and protect the eyes with safety goggles or even better a face shield. Work in a fume cupboard.
To make a 1M (mole/litre) solution, add 57mL of the concentrate to about 600mL of distilled water and dilute to 1 litre.
- 1M (mole/litre) solutions and above should be labelled IRRITANT
- 4M solutions and above should also be labelled CORROSIVE
Concentrations
Help! It won't dissolve
Some substances do not dissolve very readily in water and require special methods.
This dissolves very slowly in water. The best thing to do is to start making it up the day before you need it. Mix the powder into agitated water, then put into the fridge overnight. When you come back in the morning it will have dissolved.
In general, starch is insoluble in water, but some types of starch e.g. corn starch will dissolve in water provided they are made into a paste first with cold water then dissolved in boiling water.
Most iron (III) salts do not form stable aqueous solutions. In order to get them to dissolve you need to add a small amount of acid. Add hydrochloric acid to the chloride etc...
Reagents used for testing foodstuffs
This is used to test for the presence of protein. There are two recipes the first consists of two reagents Biuret A and Biuret B.
- A is 0.1M sodium hydroxide or potassium hydroxide the bottle should be labelled CORROSIVE.
- B is 0.01M copper (II) sulfate solution
For the second (Quantitative) recipe, in about 600mL of distilled water, dissolve in order 3g of copper (II) sulfate · 5H2O, 5g of potassium iodide, 9g of potassium sodium tartrate.4H2O, and 8g of sodium hydroxide. Make the dissolved solids to 1 litre. Label the solution IRRITANT
Benedict's reagent is used to test for reducing sugars. It has two recipes. Benedict's qualitative and Benedict's quantitative.
Benedict's Qualitative Reagent
The reagent is made up by first dissolving 173g of sodium citrate and 100g of anhydrous sodium carbonate in about 600ml of distilled water. Then 17.3g copper (II) sulfate · 5H2O is dissolved in about 100ml of distilled water. The two solutions are then mixed together and when cool are made up to 1L with distilled water.
Benedict's Quantitative Reagent
In about 600ml of hot water dissolve
- 200g of sodium citrate
- 75g sodium carbonate
- 125g potassium thiocynate
In about 100ml of water dissolve
- 18g of copper (II) sulfate · 5H2O
When the solutions have cooled, mix them together stirring constantly. Add
- 5ml of 5% potassium ferrocyanide then make up to 1L.
Iodine Solution
- Iodine solution is used to test for starch.
Recipes vary but 1g of iodine plus 1g of potassium iodide in 100mL water is suitable. Dissolve the iodide then the iodine.
Buffer Solutions
Buffer solutions help to keep the pH of a sample constant. Make up 0.1M citric Acid and 0.2M phosphate solutions then mix as follows,
| pH | 0.2M Na2HPO4 /ml | 0.1M Citric Acid /ml |
|---|---|---|
| 3.0 | 20.55 | 79.45 |
| 4.0 | 38.55 | 61.45 |
| 5.0 | 51.50 | 48.50 |
| 6.0 | 63.15 | 36.85 |
| 7.0 | 82.35 | 17.65 |
| 8.0 | 97.25 | 2.75 |