Making up solutions

Table of contents

1 Diluting Concentrated Acids 1.1 Hydrochloric acid 1.1.1 36% 1.1.2 32% 1.1.3 Safety Notes 1.2 Nitric Acid 1.3 Sulphuric acid 1.4 Ethanoic acid 2 Concentrations 3 Help! It won't dissolve 4 Reagents used for testing foodstuffs 4.5 Benedict's Qualitative Reagent 4.6 Benedict's Quantitative Reagent 5 Iodine Solution 6 Buffer Solutions

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 84mL 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 97mL 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.

0.1M (mole/litre) solutions and above should be labelled IRRITANT.

1M solutions and above should be labelled CORROSIVE.

Sulphuric acid

Concentrated sulphuric 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.

In a fume cupboard, add 54mL of concentrated sulphuric 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 29mL 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

• How to make a molar solution
• How to make a % solution

Help! It won't dissolve

Some substances do not dissolve very readily in water and require special methods.

• Sodium alginate

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.

• Starch

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.

• IronIII salts

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

Biuret reagent

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 sulphate solution

For the second (Quantitative) recipe, in about 600mL of distilled water, dissolve in order 3g of copper sulphate. 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 ofanhydrous Sodium Carbonate in about 600ml of distilled water. Then 17.3g copper sulphate5H₂O 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 sulphate.6H₂O .

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,

Citric Acid-Phosphate buffers

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