Gravimetric Analysis

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Chapter 3

• Gravimetric Analysis

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Introduction

• Working the data
• Three basic ways to do this
• Wet chemical methods
• Physical methods
• Instrumental methods
• Gravimetric analysis
• First of the two wet chemical methods of working
  the data
• The measurement of weight is the primary
  measurement/only measurement
• The sample
• Its components
• Its reaction products
• Weight measurements
• Calculation of results
• Often only measurements used in this calculation

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Introduction

• Figure 3.1 (pg. 38) Gravimetric analysis strategy
• Obtain the Sample
• Prepare the Sample
• Carry out the gravimetric analysis method
• Obtain weight or volume data on the prepared
  sample
• An initial sample weight or volume is always
  needed to calculate results
• Prepare a solution of a substance with which the
  analyte will react and/or carry out a physical or
  chemical operation on the sample
• A solution may be needed to react with the
  analyte in the sample
• Work the data
• Requires the weight data to be used in percent-or
  similar-calculation
• Stoichiometry or statistics may be involved
• Calculate and report the results

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Details of Gravimetric Methods

• 3 steps
• The weight or volume of the prepared sample is
  obtained
• The analyte is either physically separated from
  the sample matrix or chemically altered and
  separated from the matrix
• The weight of the separated analyte or its
  derivative is obtained
• Obtained data used to calculate the desired
  results

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Details of Gravimetric Methods

• Physical Separation Methods an Calculations
• Separations vary according to the matrix and form
  of the analyte relative to matrix
• Evaporation
• Filtered or sieved
• Calculation as of the whole

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Experimental Considerations

• Weighing Bottles
• Small glass bottle-ground glass top and stopper
• Containing, drying, weighing, dispensing finely
  divided solids for all types of analysis
• Gravimetric analysis
• Used for containing samples in loss-on-drying
  experiments
• Other times when sample must be dried before
  proceeding, then weighed and dispensed later
• Drying
• Takes place in drying oven
• Can be set to 100OC or higher
• Usually placed in the oven-lid ajar in small
  beaker
• Beaker covered with a watch glass

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Experimental Considerations

• Weighing by Difference
• Act of determining weight of a sample by
  obtaining two weighing bottle weights
• One before and one after dispensing the sample
• Dispensing the sample
• Subtracting the two weights
• No utensil or weighing paper contacts the sample
• Sample shaken into receiving vessel directly

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Experimental Considerations

• Isolation and Weighing Precipitates