OVERVIEW

A 85% (w/w) Phosphoric acid can be purchased from several commercial suppliers. It is a clear colorless liquid and can be diluted to prepare solutions of known concentrations. Here we describe a procedure to prepare a 1 M Phosphoric acid solution using commercially supplied 85% stock solution.

REQUIREMENTS

Reagents and solutions
85% (w/w) Phosphoric acid solution
Distilled water

Equipment and disposables
Measuring cylinder/volumetric flask
Beaker (1 Liter)
Magnetic stirrer

OBJECTIVE
Preparation of 1 Liter of 1M Phosphoric acid using concentrated stock solution (85%, w/w)

PROCEDURE

Use personal protective equipment (lab coat, gloves, goggles etc) for your safety and follow the guidelines of your institute.

PART I: Calculate how much 85% (w/w) concentrated stock Solution is required to prepare 1 L of 1M Phosphoric acid solution

Step 1: Calculate the molarity of 85% (w/w) concentrated Phosphoric acid solution
The procedure to calculate molarity of 85% (w/w) concentrated Phosphoric acid solution is described in detail in the post “Molarity of 85% (w/w) Phosphoric acid (H3PO4)”. From this post, you will know that the molarity of 85% (w/w) concentrated Phosphoric acid solution is 14.615 M.

Step 2: Calculate how much concentrated stock solution is required to prepare 1L of 1M Phosphoric acid solution using dilution method.

Dilution method is described in detail in the post “Preparing Working Solution from Stock Solution by Dilution”.

Use the following formula to calculate the volume of concentrated solution required for the preparation of 1 liter of 1M solution of hydrochloric acid solution

Formula:   C_f x V_f = C_S x V_S

C_f (concentration of diluted (final) solution) = 1M
V_f (volume of diluted (final) solution) = 1 liter
C_S (concentration of stock solution) = 14.615 M
V_S (volume of stock solution) = ? (unknown)

Place all values in the formula:

1 x 1 = 14.615 x V_S

V_S = 1 x 1/ 14.615 = 0.0684 liter (68.4 ml)

So you need 0.0684 liters of 85% (w/w) concentrated Phosphoric acid solution to prepare a 1 liter of 1M Phosphoric acid solution.

Step 3: Calculate the volume of solvent to be taken.

Final volume of diluted solution = 1Liter
Volume of concentrated solution = 0.0684 Liter
Volume of solvent (water) required = 1 – 0.0684 = 0.9316 Liter

So you need to mix 0.0684 liter (68.4 ml) of concentrated solution to 0.9316 liter (931.6 ml) water to prepare 1 liter of 1 M Phosphoric acid solution.

PART II: Solution preparation

Step 4: Transfer 931.6 ml water in a beaker/volumetric flask/measuring cylinder. Add 68.4 ml of 85% Phosphoric acid stock solution and mix it to prepare a homogeneous solution.
Note: In case you have not calculated the amount of water to be added, just take ≈ 500 ml water in a volumetric flask/measuring cylinder (capacity 1L) and add 68.4 ml of  85% Phosphoric acid stock solution. Now adjust the final volume to 1 liter with water. Mix it.

STORAGE
Solution can be stored at room temperature.