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Gravimetric analysis apparatus

Apparatus used in gravimetric analysis should be appropriate and suitable  for the required purpose. They should also be thoroughly cleaned with distilled water. Glasswares and crucibles used should not have scratches on their surfaces.

(a) Beakers

The beakers used are usually 400 –500cm³  . They are used for ;

1. Making solutions
2. Carrying out  precipitation
3. For heating solutions  
4. For filtering solutions

Beakers are made of Pyrex  or corning glass  because  the kind of glass  are hat tolerant ( they tolerate frequent heating  and cooling )

Sprouted beakers  are preferred  because they allow  pouring ,  and steam to go out even when   the beaker  is covered  with a cover glass or cork glass. Sprouted beakers  also provide  space at which  glass  rods  protrudes (come out) from the beaker . Ensure that  the beaker  is clean  and dry  before use

Beakers should be heated  on an asbestos  filter covered  with a wire gauze . Never  heat  them on a naked flame

Before placing the beaker in the beaker  for heating .its outer surface  should be dried  by wiping the moisture from its surface  so as to prevent uneven expansion  of the beaker and hence  cracking

When boiling in a beaker, a stirring rod  should  not be left in the beaker  because the rod   may bump up  and down  during  the boiling and may  break  the bottom of the beaker

The content of the beaker must be covered with a clock to avoid contamination

The inner surface of the beaker should be smooth and not scratched since the scratched surface may adsorb substances  from the solution  and therefore leading to wrong results .

b.Cover or clock glass

Cover glass are used for covering beakers containing  solution for gravimetric analysis  . Cover glass should be larger diameter than the beaker.

Cover glass  should be  placed on the beaker  on the beaker  with convex (bulge) downwards

Ensure that the  beaker  is  covered with  a cover glass  when heating  so as  to leave space  for exit of steam

When cover glass is removed  from  the beaker , it should  be washed with spray of water  from the  wash bottle .

1. Glass rod

This is a rod with a rounded tip  for stirring  gravimetric solutions and for removing precipitates  from the container  during filtration

The length of  the glass rod  should be of  suitable  size and length of the vessel  (beaker) . if the glassrodis  fitted with  a short  piece of rubber tubing  over one  end , then the tubing is called  policeman

 The policemen  is used  to remove  detached  particles  of precipitates  sticking on the  sides of the  beaker  which cannot be removed  by stream  of water from  the wash bottle

When the glass rod is being used , the policeman  should not be used  for stirring  and it should  remain in solution  because the rubber  is attacked  by chemicals  and hot water

The policeman end should not be used  for directing   the flow of liquid during  pouring

The rod should not be  placed  on the bench  as it can pick  dust and other  impurities  but should be instead placed on  an empty beaker  to rest on  a  sprout  rim.

When the rod is taken  out of  the solution , it should be washed  with spray  from the wash bottle

1. Wash bottle

This bottle is designed in such a way  as to   deliver a fine jet or stream of distilled water  for transfer  or washing of the precipitate. Wash bottles are either  made of glass or plastic ,but plastic wash bottles  are the most commonly used  because they are cheap, easy to clean and not fragile  and resistant to chemical attack .they are also flexible  i.e. only a  slight application of pressure  on the sides  of the bottle  gives  a jet of water

1. Funnels

Funnels are used for directing liquids  from one container to another with a narrow neck .they are made of Pyrex  or plastic  .the commonly used ones are  those bearing an angle of 60^o  to the stem  and have a diameter of 9cm .the stem diameter is 4cm

Funnels should be cleaned and washed  with distilled water

1. Weighing bottle

This is  a small  bottle  with a glass stopper .its weight is known .its used for weighing in gravimetric analysis

Weighing glasses that are fitted  with glass stoppers outside are better than those fitted  with stoppers inside because

the ones  with stoppers inside  have a danger  of picking up small  particles  from inside during weighing   hence the weight may not be accurate

Crucibles

Crucibles are small containers with loosely fitted lid for heating  substances in gravimetric analysis

Crucibles are made up of fine quality porcelain that is glazed outside and inside. Their capacity is usually between 25-30ml

Crucibles made of fussed silica  are better than porcelain  though they are more expensive .silica   crucibles are highly resistant  to heat  and therefore  good for heating, They do not react with acids  at high temperatures  except hydrochloric acid  and phosphoric acid

Tarring

Tarring is a process followed when cleaning crucibles and their lids. Crucibles should be washed ,cleaned and dried  and finally

Ignited or heated before using them using the following systematic

Procedure.

• Place the crucibles in concentrated  potassium dichromate  to remove stains.
• Wash the crucibles  in distilled water ,add some dilute nitric acid  to the crucibles then heat the crucible .
• Pour away the HNO₃, wash it in distilled water ,if the stain still persist  then rub  the crucible  with moist sand.
• Wash it with tap water, heat it with non-luminous flame  for 15 min
• Incase the stain still persist, leave it alone  because these means  that its now part of the crucible and therefore its harmless.
• Clean the lid using a similar process. Place the crucible and lid  in the desiccator  using a pair of tongs.
• Always handle crucibles using pairs of tongs  and not fingers  because  the moisture and dust  from your fingers  can make  the crucible contaminated
• Never put a wet crucible on a flame  because it might break

Crucible tongs

 These are scissors like apparatus used  for handling  crucibles  and other hot substances .They are  made up of nickel or steel

Before using it ensure that its  arms  move  freely  and its tips move  freely. When using crucible tongs  , the tips should be  cleaned ,if its not clean ,rub  with sand  paper and tap  water  then heat with non- luminous flame  for  a while and let it cool.  When not using it , place  it on the table  with the tips facing  upwards  so that  they don’t pick dust  from the table

When lifting the crucibles, the tips of the tongs should not touch the content of the crucibles to avoid  loss of content

• Clay-pipe triangle

This is used for supporting the crucibles on a retort stand  while heating it on  a Bunsen burner  or while placing it in the desiccator for drying . It is  formed  by passing  three pieces of iron wire through small length of heat resistant clay tube . the wires are then twisted  at the end to form  a triangle

• Desiccator

This is a large vessel  with a tightly fitting  cover in which  the atmosphere  is kept  free from moisture  or water vapor  by use of a drying agent called a desiccant which is placed in the lower chamber  or compartment .it  is usually used for holding crucibles  so as to protect them  and their contents  from contact with moisture.

The ground glass rims of the desiccators  are coated  with Vaseline  or grease to avoid  air getting in . When using a desiccator;

• Hold firmly when opening  on the table with your  left hand . hold the knob of the cover with  your right hand  and pull it aside  by sliding it sideways.
• Do not keep the cover on the table  but hold it in your  left hand upside down  in order to avoid dirt  getting  in contact with the grease.
• If the cover have to be placed on the table , the grease surface should face upward  with the knob resting on the table.
• Place the hot crucible in the desiccator using a pair of tongs. If the over have to be replaced , slide it on the rim  until the rims coincides . Also remove the crucible from the desiccators using  a pair of tongs
• Do not pace a red-hot crucible in a desiccators  but allow it to  at least cool.
• Use both hands when carrying a desiccator
• Filtering media

These includes all categories of those devices used in filtration eg  filter papers  and filter crucibles . Different filter papers are used  for different  types of precipitates depending on

1. The nature of the precipitate
2. The treatment given to the precipitate  to convert it to the final product for weighing

There are two types  of filtering media’s

• filter paper
• filter crucibles
  1. Filter paper

In quantitative work , accuracy is very important, hence therefore special type of filtering  media’s are used i.e. ashless filter papers and harden filter papers is important

Ashless filter papers are produced by removing  the mineral components  of the paper  by treating them with HCL and HF acid . Such papers are called  acid washed  filter papers . They have general diameters  of 7 and 12.5cm

 Filter papers come in  various grades  and  the type of filter papers one will choose   will be determined by  the fineness  of the precipitate  to be filtered eg FeOH  precipitate  have large particles  and therefore  requires  a dense paper with  fine pores . The denser the  precipitate ,  the slower the filtration .

 Because of the hygroscopic nature of the filter papers , the precipitated collected cannot  be weighed  in a filter paper  after washing and drying  and therefore igniting the content  in a crucible  is necessary   and the residue  is weighed . the weight of the ash formed is negligible

1. Filter crucibles  

They are used for collection of precipitates to be weighed  after heating in them . the precipitate is  filtered , washed  and weighed  in the same vessel  they are porous and the precipitated is sucked by applying sunction pressure .

There are  generally three types ;

1. Gooch crucibles

They are made of glazed porcelain with a perforated bottom upon which there is a  glass mat  or glass fiber or asbestos

1. Sintered glass crucibles

These are made of only sintered glass

• Porous porcelain  filtered glass– They are made of porous porcelain .

NB Filtering crucibles can be heated in an oven

1. Buchner funnel

Applications Of Gravimetric Analysis :

(a) In the analysis of standards which are to be used for the testing and/or calibration of instrumental techniques;

 (b)in the analyses requiring high accuracy, although the time-consuming nature of gravimetry limits this application to small numbers of determination