Stock & Working Solutions

- It is very important before you start laboratory work to be familiar with the universal measurement units and unit conversions. - In Molecular Techniques, usually small units are used such as gram (g), milligram (mg), microgram (µg), nanogram (ng), micron (u), micrometre (um), nanometre (nm), and angstrom (A).

Prefixes used in common ‘SI Units’ (The International System of Units) Multiple

Prefix

Symbol

10-3 (1/103)

milli

m

10-6 (1/106)

micro

µ

10-9 (1/109)

Nano

n

10-12 (1/1012 )

Pico

p

10-15 (1/1015 )

Femto

f

103

kilo

k

106

Mega

M

109

Giga

G

Measurement Amount of substance

Mass

Volume (space occupied by something, e.g: solution)

Length

Basic unit name mole

Kilogram

3-dimensional, measured in 'cube' of a length unit, e.g: Cubic metre - [it is large for practical purposes] – The 'SI-derived unit' for volume is litre (l).

metre

symbol mol

kg

Most common units - millimole (mmol) = 10-3 mol - micromole (µmol) = 10-6 mol - nanomole (nmol) = 10-9 mol - gram (g): = 10-3 kg - milligram (mg): = 10-3 g - microgram (µg): = 10-6 g - nanogram (ng): = 10-9 g

l

- millilitre (ml) = 10-3 l - microlitre (µl) = 10-6 l

m

- millimetre (mm) = 10-3 m - micrometre (µm) = 10-6 m - nanometre (nm) = 10-9 m - angstrom (Ǻ) = 10-1 nm

What these symbols mean?

mol m-3 OR mol l-1

This means mole per cubic metre OR mole per litre which represent the concentration of a solution.

Also, What these symbols mean?

Kg m-3 OR

g l-1

This mean kilogram per cubic metre OR gram per litre which also represent the concentration of a solution.

**So, solutions can be prepared with respect to their molar concentrations or mass concentrations and both represent: Amount of substance per unit Volume of solution.

Concentration = Amount / Volume

Making Solutions 1- Molar solution

(M): is the solution which contain the ‘molecular weight’

(MW) = 'molecular mass' of a chemical per 1 litre of solution. e.g. 'MW' of NaCl = 58.44g So, 1M of NaCl contains 58.44g NaCl in 1l of water.

NB: To convert the amount of a chemical from mole to gram Multiply the number of moles by the 'MW' of the chemical.

2- Making a solution of known concentration from solid material - Decide the concentration, and the volume of solution you need. - Find out the molecular mass of the chemical (MW) - Suppose you need 250ml of 0.1mol l-1 of NaCl

[i.e. 0.1M].

* start by expressing all volumes in the same units, either millilitres or litres so, 250ml = 0.25 litre. * apply the equation Concentration = Amount / Volume to get the mass of the chemical, so, 0.1 = amount (mol) / 0.25 So, amount = 0.025 moles - Convert from mole to gram by multiplying by the relative molecular mass of the NaCl [58.44g] so, 0.025 x 58.44 = 1.461g - Finally, add 1.461g NaCl to 250ml distilled water.

3- Making Weight in Volume (w/v) solution e.g: To make 1 litre of 0.55 % NaCl - '1% solution' is the solution which contains 1 gram of a substance in 100 ml of solvent 'water'. So, 0.55 % solution of NaCl contains 0.55g of NaCl in 100ml water. And to make 1 litre i.e.1000ml it is 10x of 100ml volume, so, we multiply 0.55 x 10 = 5.5 gram NaCl in 1000 ml water.

Golden Rule for diluting Stock solutions to reach a lower concentration working solution C1 V1 = C2 V2 C1= initial concentration & V1= initial volume / C2= required concentration & V2= required volume

**NB: units must be the same on both sides of the equation.

- e.g: if you want to prepare 0.1% solution of SDS 'sodium dodecyl sulphate' from 20% stock solution. - Decide the volume you need from the required concentration say 100ml and then apply the equation to determine how much you need to take from the initial concentration 'stock solution' to be diluted to the required concentration 'working solution'. So, 20 % x ? ml = 0.1% x 100 ml ? = 0.1 x 100 / 20 =0.5 ml that is to say: take 0.5 ml from the 20% 'stock solution' and then top up to 100ml with water to get 100ml of 0.1% SDS.

Do you like to save your time? Do you like to conserve your bench space and containers? Then, You have to use 'Stock Solutions' - Stock solutions are concentrated solutions which can be used to prepare a less concentrated solutions by different dilutions, OR enter in the composition of a more complex solution. -The final solution we use is refereed to as 'working solutions'. - At the end, it is your decision to work with 'Stock Solutions', but actually, it is worth doing.

-To prepare a stock solution, we multiply the concentrations of different components of the solution by a factor e.g. 5 or 10 and have it all in the solution, then, it is called 5x or 10x stock solution [x is the times symbol], it can also be called a solution of 10x strength or a 10x concentrate or 10 times concentrated solution. - To prepare a working solution [usually 1x] from a [5x] stock solution, we do 5 times dilution of the stock solution i.e. have 1 litre of the stock solution in final of 5 litres by dilution with water. - Stock solution can be used to prepare a more complex solution by mixing with other stock solutions with dilution of each to reach 1x concentration in the final volume of the working solution.

- It is sometimes difficult to accurately weigh some chemicals which are Hydroscopic such as Magnesium chloride as it absorbs moisture from the atmosphere while stored on the shelf. So, it is better to prepare a Stock solution from a know amount e.g. 500g even without weighing. - To prepare a 'Molar' solution we dissolve the 'molecular weight' of the chemical in 1 litre of water. - The molecular weight 'MW' or called 'formula weight' or 'molecular mass' of a chemical is written on its container, which is the sum of the atomic masses of the component elements. [if the chemical is hydrated, i.e. in water, water atoms masses must be included when calculating the mass required.

- e.g: To prepare a 0.5M stock solution [i.e: 500mM] of MgCl2, MW: 95.211g/mol We dissolve 47.6g of MgCl2 in 900ml water and then top up water to 1000ml. - And to reach to 5mM concentration we have to do 100 times dilution from 500mM stock solution.

References: - Books: - practical skills in biology (2003).