CHEMISTRY : Quantitative Analysis

Acid vs Metal Reaction Meter

INTRODUCTION:

This is a procedure to estimate at which state an Acid + Metal reaction has arrived, and hence knowing wether the reaction is still in the initial phase or nearly complete. These Acid + Metal reactions involve those between a metal and a strong (mineral) acid to form Hydrogen gas and the salt of the metal. A such reaction usual takes days or weeks to be all complete, hence when (nearly) all the acid has reacted with the metal.

ACID (aq) + METAL (s) ---> SALT (aq) + HYDROGEN (g)

However one cannot tell easily when the reaction, is complete, or how much acid has reacted, - since some acid has still not reacted.

xx Acid + yy Metal --> Salt + Hydrogen + x Acid + y Metal (still unreacted)

This quantitative procedure, as mentioned, is used to estimate when the reaction is or will be nearly complete, without leaving lot of xs unreacted products. This is useful so that when extracting the salt by evaporation, it will not be contamitated with unreacted acid.

PRINCIPLE:

The principle involves the determination of the concentration of the acid still not reacted by a simple acid/alkali titration over a gradual period of time. When the acid concentration left in the mixture does not change between different readings taken after some time, then it is safe to say that the acid+metal reaction is complete. It is very unlikely that you find no acid left because traces will always remain unreacted.

PROCEDURE:

A: Preperation of a constant concentration of the acid+metal mixture

1) The mixture of Acid + metal + formed salt is transfered from the preperation flask to a 50ml measuring cylinder, and added water till 50ml solution is obtained. If there is more from 50ml, heat up, so that water is evaporated and the volume of the mixture becomes a little less from 50ml. Otherwise use a 100ml measuring cylinder. Always work with a cool solution, since volumetric measurments are at 20°C.

2) Once the mixture is made to 50ml and cooled take 1 ml, using a measuring pipetteand transfer it in a 75 or 100 ml flask, and added about 25 - 40 ml of water. Amount of water is not important in any titration. It will not effect the moles of H+ ions of acid to be calculated

3) 4-5 drops of BromoCresol Purple (or any suitable indicator), are added and the flask is shaken gently.

B: Preperation of the titrant

4) Prepare standard solutions of 1.0M and 0.1M NaOH (or Sodium Carbonate).

  1M NaOH = 4g in 100ml
0.1M NaOH= 0.4g in 100mL ( RMM NaOH = 40g)

5) Dissolve well and transfer into a clean washed burette

C: Titration and calculations.

6) Titre the acid solution with the 1.0M Sodium Hydroxide solution. The colour change for the Bromocresol Purple indicator is usually from bright yellow to fairly dark purple. Note that the metal salt may produce a colourful precipitation reaction with the alkali so the colour change may be masked by anothe colour of the metal base produced. If the solution in the flask is heated, it will enhance the reaction between the solid Hydroxide formed and any xs acid, back to the salt, hence a clear solution is maintained. Theoretically, the alkali should first attack the acid then if in xs it precipitates the metal base.

If the colour change was immediate (NaOH very strong) then use the weaker 0.1M solution. A titre of less then 2 or 3ml should be considered as an immediate change and the weaker solution should be used. Note down the volume used to neutrelize the xs acid in the mixture.

7) Transfer back the acid+salt solution from the measuring cylinder to the preperation flask + remaining metal, to let the reaction continue.

8) After the reaction is let to procede for enough time. (eg 1 or 2 days, the above procedure is repeated again, and a new titre value, which must be always less from the previous value is obtained.

9) The reaction is let to continue until the titre value of the NaOH becomes more or less between 3 consecutive readings. Plotting a graph will result a logarithmic (half life) shape, and theoretically, the values will never reach 0 (hence no acid is left) but should be very close. Also, some weak acid always remains in the solution, and it would take many many time for all of it to react. This is called latent acid, and it is that little amount of acid remaining which is not strong enough to further attack the metal into salt and Hydrogen.

Typical NaOH titre readings (hence amount of acid present after the indicated time for reaction to take palce:)

1. initially: 8.4 ml
2. after 1 day: 40ml
3. after 3 days: 35ml
4. after 4 days: 32ml
5. after 7 days: 27ml
6. after 9 days: 26ml
7. after 10 days: 25ml
8. after 15 days: 23ml
9. after 18 days: 22ml
10. after 20 days: 22ml

CONCLUSIONS

Well, it is a good procedure, using little of the sample (1ml each time), reliable and easy but has a severe drawback, it takes lot of time for a reaction to be nearly complete. In the above example it took me 20 days. I have deviced a new fast method if one needs to extract the produced metal salt free from acid as much as possible, in a much shorter time. This can be read in the document QNA02.

It is important that the conditions and preferences used in the initial reading are maintained constant for all the preceding readings, such as choice of indicator and concentrations of solutions used.

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