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CHEMISTRY : Quantitative Analysis

Percentage of Aluminium in a sample

PROJECT TITLE: Percentage of Aluminium in a solid sample
RELEASE DATE: 20th - 21st September 1997
LAST UPDATE: 14th December 1997
VERSION HISTORY: 1.0 - Procedure for solid samples containing Aluminium
1.1 - Procedure for additional text for salts/soluble samples with Aluminium


This procedure is used to calculate the concentration of Aluminium in a sample, such as an alloy or solution. Since the procedure is a volumetric Analysis and involves several preparation steps, the minimum percentage of Aluminium which can be determined is taken 5%. The procedure is slightly altered depending the sample is in a soluable form (salt or unsoluble form (metal or alloys)

First, the Aluminium component is extracted from the sample, and turned into a pure aluminium salt solution. Its concentration depends on the quantity of Al in the sample. The concentration of the Al solution is found out and the mass or % of Alumium in the sample is then calculated.

This is an easy procedure which can be performed in a small home lab, using basic chemicals and glassware, perhaps the exeption being that of an analytical balance. The procedure, hence is not very sensitive and accurate, but it is excellent to find percentage of Aluminium in alloy metals for example. It is estimated this procedure has about +/- 4% accuracy error.


The whole test includes the following 6 parts until the final result is obtained:

1) Confirmation of Al in the sample:

This is done by a set of qualitative reactions, and the pattern of results is matched with that of Aluminium results

2) Weighing and preperation of the Sample in the form of an adequate solution:

The sample is turned into a solution for further Analysis. If its already a solution or soluble salt, there is no need of doing anything, but in case of solids or metals, the sample is usually dissolved in acids.

3) Extraction of Al from the sample in a pure solution form:

This is done by precipitating other metals leaving Al in solution. Filtration and washing then follows. The choice of reagents to ppt other metals must be studied.

4) Conversion the Al salt to the carbonate with xs Sodium Carbonate:

The aluminium salt to be measured is added an xs Sodium Carbonate solution of known concentration.

5) Finding concentration of the unreacted Sodium Carbonate in xs:

The unreacted xs Sodium carbonate is then titred against Sulphuric Acid and its concentration is calculated.

6) Calculations to find Molarity of Al salt and % of Aluminium in sample:

The difference of the concentration between the initial sodium carbonate solution and that after the precipitation is the concentration of sodium carbonate used to precipitate the aluminium salt, and is used to determine the concentration of Aluminium salt.


1) Confirmation of Al in the sample:

First, the presence of Al must be confirmed in the sample, and its concentration will be roughly estimated. Some of the sample is dissolved either in water in case of a soluble sample, or in acids in case of metals or undissolved powder. A set of reagents are added to the solution for precipitation reactions. From the pattern of results obtained, Al and any other metal(s) present in the sample will be discovered. This set of qualitative tests serves to know:

  1. Wether if Al is present
  2. What other metal elements are present and hence how to extract Al from them.
  3. A rough estimation of its quantity in the sample, by the precipitate ammount resulted from specific reagents which reacts with Al but not with any other metals present in the sample.

Once Al is confirmed in the sample, the procedure can be continued otherwise it is safely to report that there is less from 5% of Al in the sample. The exact procedure how these tests are performed is described in another document tited Metal cation Qualitative identification tests.

2) Sample Preparation:

The sample which is tested for the ammount of Aluminium can be of various types. It can be in a metallic form such as in an alloy, in a soluble Alumiium salt , or in an unsoluble solid (eg Aluminium oxide or Aluminium carbonate). This part of the procedure is designed to convert the Aluminium in the sample in a solution, that is a salt of Aluminium of a known anion, for example Aluminium Sulphate or Aluminium chloride. Examine the sample and treat it using one of the following preparations according to the type of sample.

a) Metals & alloys (most common)
  1. a1) Weigh about 0.8 to 1.2 grams of the sample metal.

  2. a2) Place metal in fairly concentrated Hydrochloric Acid. The concentration of the acid is not important, but the best concentration and volume to use is 10mls of water and 15mls of Concentrated acid. Heat to speed up the reaction. Leave enough time to make sure that most of the Aluminium content have been dissolved in the acid. Nitric Acid is not suggested since this dissolves many more metals than Hydrochloric Acid.

  3. a3) Filter off any solid unreactable particles, making sure that they are left in the solution because they are inert to the acid, not because the reaction is not yet complete or acid is depleted. For this reason add some concentrated HCl to the undissolved particles. If they dissolve, add this solution to the previously filtered solution, else it can be discarded.

Extraction of Al can now be performed following step 3

b) Unsoluble solids.
  1. b1) The sample is heated to free any moisture, and about 1-2g are weighed.

  2. b2) They are dissolved in fairly concentrated Hydrochloric acid. The concentration is not important, but it must be mild to leave lot of the powder undissolved, or severly concentrated and so the suggested volume for 1g sample is 15mls of Acid diluted with 10mls water is ideal.
Extraction of Al can now be performed following step 3

This method is used if the sample is a mixture of soluble and insoluble solids, but then the procedure is continued from c2 to make the whole solution being of a single known anion, eg chloride.

c) Salts or soluble samples.
  1. c1) The solid sample is heated to free any water of crystalization or moisture, and about 1 - 2g of salt is weighed and dissolved in say 50ml of hot water

Extraction of Al can now be performed following step 3

d) Mixture of a completely unkown content (Soluble & unsoluble mixture)
If one does not know if the sample constitutes of soluble or insoluble consituents or a mixture of both then use procedure B above (unsoluble solids) using 20ml HCl and 30ml water mixture. The soluble parts of the sample will dissolve in water while the unsoluble Al parts will dissolve in the acid. Unreacted solid residue remaining can be discarded Extraction of Al can now be performed following step 3


Aluminium should be now present as a solution, and specificaly as Aluminium Chloride for sample preparation a, b above. This however can be mixed with other metals in solution such as metals with a soluble chloride like zinc, iron, magnesium, and also Sodium and Potassium salts. The scope of the extraction is to precipitate aluminium as a solid leaving the others in solution, and then filtering off the solid aluminium compound.

From the qualitative tests performed earlier, the metal elements present in the sample should be known. For example if an alloy metal of Aluminium, the metals Lead and Tin are also present, the solution will contain a mixture of the three salts. The lead salt if present since it will not usually form a soluble chloride, can be removed by adding xs sulphate, and the Tin salt can be removed by adding xs Ammonium Molybdate or Sodium Thiosulphate which both precipitates the unwanted metals, leaving aluminium salt in solution.

This part requires some study of how to precipitate the unwanted metals leaving aluminium salt in solution, considering that the xs solutions added will not interfere with the rest of the procedure, which basically means that this will not react with Sodium Carbonate. It is important that the reagents are added in xs to precipitate all the unwanted metals.

After filtering the precipitate of the unwanted metal some drops of the reagents used to ppt the unwanted metals are dropped. If precipitation results by any of the reagents, this procedure is repeated until no precipitation results when adding the reagents to the clear filtrate, and hence the solution is pure Aluminium salt.

The remaining filtrate solution will contains:
  1. Any xs acid which did not reacted when forming the metal solution
  2. Some xs solution used to precipitate the unwanted metals (in our example sodium sulphate, sodium molybdate)
  3. The aluminium salt. :-)

Extraction of the Aluminium from solution mixture
(1) All the dissolved salt is converted to a carbonate by adding Sodium Carbonate in xs. With a maximum weight of 2g of sample, 4 grams of Carbonate are more than enough

The sodium carbonate will first neutralize any xs acid and precipitate the Aluminium as Aluminium carbonate. Sodium Hydroxide cannot be used, since the xs hydroxide can form the soluble Aluminium complex - the Aluminate salt.

(2) The gelatinous white unsoluble carbonate is filtered and washed from the soluble reagents, several times with hot water.

It is important that all aluminium is converted to the solid carbonate, and to check that no aluminium salt is still present some further sodium carbonate is added to the filtered solution, and if no precipitate is formed, then no Aluminium is present. On the other hand if a faint white precipitate appears, filtration is repeated, until no precipitate is formed on adding sodium carbonate to the filtered solution.

(3) The filter paper and carbonate are transferred in a beaker and about 20 mls of water are added. The beaker is heated.

(4) Diluted Hydrochloric acid is added drop wise to the hot mixtures until all the white precipitate is dissolved. An acid indicator such as Litmus powder can be used so that no xs acid is added.

(5) To neutralize the xs acid, dilute sodium carbonate is added to the clear solution, while mixing continously, until traces of white aluminium carbonate remains undissolved even on boiling.

Care should be taken so that neither ecxess acid nor excess carbonate are present in solution.

(6) The solution is filtered from the pieces of remaining filter paper leaving a solution of Aluminium Chloride and sodium chloride (which will not interfere with the procedure )>
Quantitative titration
The next part is to perform a quantitative titration to know how much Aluminium we have in solution. A known strong concentration (in terms of moles) of Sodium Carbonate is reacted with the Aluminium salt. The concentration of Sodium carbonate left (not reacted) is determined. The difference between the two concentrations of Sodium carbonate reflects the amount of Aluminium present. For instance if little Aluminium is present, few carbonate is used for precipitation, and the carbonate concentration after precipitation is relatively high. On the other hand if the Aliminium in solution is abundant, plenty of Carbonate is used from solution to precipitate the metal cations and therefore the carbonate concentration after the precipitation reaction is low.
Preperation of the Aluminium salt solution
The Aluminium solution is transferred in a 100ml volumetric flask and diluted with water till the mark. If there is more from 100ml solution, this is either evaporated till it reaches little less from 100ml, or else transferred in a larger volumetric flask. It's important that all the salt is transferred.

Preperation of the Sodium Carbonate solution
The sodium carbonate solution to be prepared is used to precipitate ALL the aluminium salt to its carbonate. For our 0.8-1.2g of sample, 100ml of 0.4M Sodium Carbonate are required while for 1-2g sample 100ml of 0.8M Sodium Carbonate are required.

To prepare 0.4m Sodium Carbonate anhydrous sodium carbonate is used and to make sure that there is no water present in the hygroscopic compound, some of this compond is preheated to evaporate any water before weighing.

1M Na2_CO3 = 106g in 1000ml
1M Na2_CO3 = 0.6g in 100ml
0.4M Na2_CO3 = 10.6 x 0.4 = 4.24 g in 100ml
Thus 4.24g are weighed, transferred in a 100ml volumetric flask and diluted till the mark.

The Molarity of this Carbonate solution is noted [M-CO3]

Preparation of the Sulphuric Acid solution
The concentration of Sulphuric acid to be prepared is approximately of the same or slightly more from the concentration of the Sodium Carbonate. 0.5M Sulphuric acid are prepared as follows:
1M H2SO4 = 98g in 1000mL
0.5M = 49g in 1000ml (98g/2)
0.5M = 9.8g in 200ml (49g/5)

Specific gravity of 98% Sulphuric Acid is 1.96g/ml :
1.96g of 98% Sulphuric Acid measure 1mL
9.8g are made by 9.8g/1.96g = 5ml Sulphuric Acid.

The Molarity of the Acid is noted [MOL-Acid]

N.B. To be more precise, check specific gravity of the Concentrated Sulphuric Acid used by weighing 25ml of acid. Specific gravity = Mass/Volume (or mass obtained / 25ml)
5ml of 98% Sulphuric Acid are transfered in a 200ml volumetric flask and diluted to the mark to produce 0.5M solution H2SO4

4) The Reactions : Convert Aluminium Salt to Aluminium Carbonate

First, the precipitation between the the Aluminium salt and the 0.4M sodium carbonate in xs. It is important that the carbonate is in xs, as afterall that xs carbonate is going to be measured. 25mls or 40mls of the salt are reacted with 50mls Carbonate.

If the solution of the Aluminium salt is taught to be more concentrated, 15ml, 10ml or 5ml are taken instead.

The volume of the Aluminium solution used [ VOL-AL ], and volume of the carbonate solution added [VOL-CO3] are noted.

A gelatinous white ppt of Aluminium Carbonate is formed. The reaction is warmed and let to stand for few minutes to be complete

The mixture is filtered, in a 50 or 100 ml Volumetric flask. The flask should have a capacity more from the mixture volume due the washing is also collected in the flask. The residue is washed several times with hot water, so that all soluble carbonate is transferred in the volumetric flask.

The volume of the volumetric flask is noted. [V1]

To to test that the carbonate was really in xs, and so there is no Aluminium left, a few drops of caronate solution is added to the filtered solution . If a precipitate is formed, then the carbonate used was not in xs, so the procedure has to be started using a more dilute aluminium solution (say 10ml) or a more concentrated carbonate solution (say 100ml 0.8M). If no ppt forms, the procedure can be continued.

5) The Titration of unreacted Sodium Carbonate Solution

The 0.5M sulphuric acid is transferred in the burrette, and 25cm of the xs Sodium Carbonate solution just filtered is transferred in two or three small flasks, depending the volume gathered.

The volume of carbonate used for the titration is noted. [V2]

In each flask, 4 drops of Bromocresol Purple indicator is dropped.

The Sodium carbonate in the flasks is titred with the 0.5 Sulphuric Acid, and the two or three titre results (depending on how many flasks were used) were averaged.

The titrated volume resulted is noted [Vol-Titre]

6) Calculations

The concentration in moles of the Aluminium Salt is worked by the formula:

M.Al = ( M-CO3 x VOL-CO3) - [ V1/V2 ( M-Acid x VOL-Titre) ]

Ratio factor x VOL-Al


M.Al = Molarity of Aluminium sample salt

M-CO3 = Molarity of Sodium carbonate solution (start solution)

VOL-CO3 = Volume of Sodium Carbonate reacted with aluminium salt

VOL-Al = Volume of Aluminium salt reacted with Sodium Carbonate

M-Acid = Molarity of Sulphuric Acid

VOL-Titre = Volume of titre

V1 = Volume of xs Sodium Carbonate gathered in volumetric flask

V2 = Volume of xs Sodium Carbonate used for Titration

RF = Ratio factor depending Aluminium salt used (Since Hcl was used 1.5 should be used)
    -> Aluminium Sulphate = 3
    -> Aluminium Chloride = 1.5
    -> Aluminium Nitrate = 1.5

The ratio factor is determined from the equilibrium factors in the reaction between the salts and the carbonate.

Sulphate: Al(SO4)3 + 3 Na2CO3 --> Al2(CO3)3 + 3 Na2SO4 : 3/1 = 3.0

Chloride: 2 AlCl3 + 3 Na2CO3 --> Al2(CO3)3 + 6 NaCl : 3/2 = 1.5

Nitrate: 2 Al(NO3)3 + 3 Na2CO3 --> Al2(CO3)3 + 6 Na NO3 : 3/2 = 1.5

The mass of aluminium salt is calculated by the formula:

Weight Al = VOL-Al x M-Al x RMM of Al salt


VOL-Al = Volume of aluminium salt initially used.

M-Al = Moalarity of Aluminium salat calculated previously

RMM = Relative Molecular mass of Aluminium salt used

Aluminium Chloride = 133.5
Sulphate = 342
Nitrate = 213

To find immediately the percentage, the mass of Al is calculated from the mass of salt, and the percentage is found by simple proportion. Otherwise it can be found by the following formula instead of that above:

% Al = VOL-Al x M-Al x 27 x Al Factor

10 x Mass

Al factor    is the number of Aluminium atoms found in its compound salt.
For example for Al(NO3)3 and Al(Cl)3 is 1, but for AL2(SO4) is 2.

Mass     is the mass of the dry sample weighed

10    is the ratio between 100% / 1000

% of Al in pure AL_Cl3 = 100 x 27 / 133.5 = 20.22% Al
% of Al in pure Al2_(SO4)3 = 100 x 54 / 342 = 15.79% Al
% of Al in pure Al_(NO3)3 = 100 x 27 / 213 = 12.68% Al


  1. Determine the percentage of Al in an alloy
  2. Determine percentage of Al in a solid sample (soluble or unsoluble)
  3. Determine the concentration of an aluminium salt solution
  4. If a pure aluminium salt is used, it's anion (Cl, Br, SO4, NO3, etc) can be found out by comparing the percentages obtained with the actual percentages of Al present in the various salts.


I tested this procedure several times and got good results, not more from 5% of error. The errors arising in this procedure are the non-complete transfer of solutions from one container to another. This can be eliminated by washing well the container. The solutions must be prepared very carefully, and avoid spilling and loss of solution during mixing, boiling or pouring.

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