Calculating Quantities From Master Formula

Calculating Quantities From

 Master Formula

In extemporaneous dispensing a product is prepared and usually includes several ingredients. A List of the ingredients is provided on the prescription or is obtained from a recognised reference source where the quantities of each ingredient are indicated. It may be that this 'formula' is for the quantity requested, but more often the quantities provided by the master formula have to be scaled up or down depending on the quantity of the product you wish to prepare.

Recognised reference source in Sri Lanka

• The British pharmacopeia (BP)
• British Pharmaceutical Cordex (BPC)
• United States Pharmacopoeia (USP)
• European Pharmacopoiea ( Pharmacopoeia Eropeana, Ph Eur)
• International pharmacopoeia (Ph.l)
• Japanese pharmacopoeia (JP)

Fromularies

• The British National Formulary(BNF)
• Sri Lanka Hospital Formulary (SLHF)
• Ceylon Hospital Formulary (CHF)

Reducing and Enlarging Formulae

    In the course of pharmaceutical compounding and in professional practise activities, it is often necessary to reduce or enlarge a master formula in order to prepare the desired amount of product. The standard formula or the master formula contains the quantitative amount of each ingredient needed to prepare a specified quantity of product.

Different types of Formulae

      The original formulae may be available in different forms, but they have the same ratio.

01. Precentage Formula

  

          In these formulae indicate a particular concentration of an ingredient expressed as a percentage. There are certain convention which apply when dealing with ingredients indicated as a percentage in a formula.

• A solid in a formula where the final quantity is stated as a weight is calculated as weight in weight (w/w).  
• A solid in a formula where the final quantity is stated as a volume is calculated as weight in volume (w/v).
• A Liquid in a formula where the final quantity is stated as a volume is calculated as volume in volume (v/v).
• A liquid in a formula where the final quantity is stated as a weight is calculated as volume in weight (v/w).

1% w/w  = 1g in 100g

1% w/v   = 1g in 100 ml

1% v/v    = 1 ml in 100 ml

1% v/w   = 1 ml in 100g 

eg 01.  

  

          Order - Prepare 120 ml of the following preparation.

eg 02.

   

       Order - Prepare 250 g  of the following  Hydrophylic Ointment (O/W Type emulsion base)

eg 03.

     Order - Prepare 250g of the following ointment 

Source - Pharmacy Practicals by chula edirisinghe.