acid and base 2 (urgent)

1.
Actually, when A is titrated against B, either one of A and B can be put in theconical flask, and the other one is put in the burette. However, many questions in titration arewritten as "25.00 cm³ of A is titration against B", many people havethe WRONG IDEA that A must be put in the conical flask.

Usually, the acid and the alkali can be exchanged in performing the titration.However, difference results may be obtained in some titrations if the acid andthe alkali are exchanged.

For example, when sodium carbonate solution is put into conical flask and istitrated against hydrochloric acid, the reaction is two-stage and there are twoend points which can be shown by using phenolphthalein indicator and methylorange indicator respectively.
Na2CO­3(aq) + HCl(aq) →NaHCO3(aq) + H2O(l)
NaHCO3(aq) + HCl(aq) → NaCl(aq) + H2O+ CO2(g)

When hydrochloric acid is put into conical flask and is titrated against sodiumcarbonate solution, the reaction is only one-stage because hydrochloric acid isin excess throughout the titration before the end-point. Thus, methyl orange isa suitable indicator, but phenolphthalein is NOT.
NaHCO3(aq) + HCl(aq) → NaCl(aq) + H2O+ CO2(g)


2.
Ammonia chloride reacts with NaOH to give ammonia, but potassium chloride doesnot react with NaOH. The ammonia formed is very soluble in water, and thusalmost all the ammonia formed is dissolved in water. It is necessary to heatthe solution to make the ammonia gas escape from the reaction mixture and thus theammonia gas can be detected.


3.
A precipitate of BaSO4 is formed when barium nitrate solution anddilute sulphuric acid are mixed, because BaSO4 is insoluble in wateror dilute acids. The equation is:
Ba(NO3)2(aq) + H2SO4(aq) → BaSO4(s) + 2HNO3(aq)
Ba²⁺(aq)+ SO4²⁻(aq) → BaSO4(s)

2011-07-09 00:38:15 補充：
No matter whether the strong acid or the weak alkali is put into a conical flask in a titration between them, methyl orange can be used as an indicator. If the strong acid is put in the conical flask, the end point colour change is from red to orange.

2011-07-09 00:38:36 補充：
If the weak alkali is put in the conical flask, the end point colour change is from orange to red. In both cases, the same result would be obtained. There is NO effect on the indicator when the strong acid and the weak alkali are exchanged.

2011-07-09 00:38:54 補充：
Similarly, there is also NO effect on the indicator when the strong alkali and the weak acid are exchanged.

2011-07-09 00:58:09 補充：
Nowadays, there is almost no problem of NaOH attacking glass because stopcocks of burette are made with PTFE instead of glass, and thus there is no risk that the stopcock and the burette together.

2011-07-09 00:58:24 補充：
Besides, the concentration of NaOH used in titration is dilute, and thus the ability of NaOH to attack glass is very weak.

2011-07-09 00:58:49 補充：
In titration between NaOH and an acid using phenolphthalein as indicator, it is recommended to put NaOH in the burette and put the acid in the conical flask. If so, the colour change of indicator at the end point is from colourless to pale pink.

2011-07-09 00:59:29 補充：
If NaOH is put in the conical flask and the acid in the burette, the end point colour change is from pink to pale pink, and it is impossible to recognize the end point. Therefore, slightly excess acid is added to change the solution to colourless for clear obervation. This will cause an error.

2011-07-09 01:06:07 補充：
However, when a burette is filled with NaOH solution, it is recommended to used a dilute acid to rinse the burette after experiment in order to remove the NaOH solution completely.

2011-07-09 01:07:00 補充：
Although strong alkali (NaOH, KOH) can react with CO2 to form a carbonate, there is no risk to block moving parts of the burette. This is because both Na¬2CO3 and K2CO3 are soluble in water. Besides, Ca(OH)2 reacts with CO2 to form insoluble CaCO3, but we usually do not used Ca(OH)2 in titration.

2011-07-09 01:07:36 補充：
請看「意見」。

2011-07-09 20:14:07 補充：
The higher the temperature, the lower of the solubility of a gas.

2011-07-13 03:50:51 補充：
As NaOH used to fill the burette is very dilute and the time for titration is not too long, dilute alkali solution practically has no reaction with the glass of the burette. However, never allow the dilute alkali solution to be dried, as concentrated alkali would attack glass.

2011-07-13 03:51:23 補充：
Therefore, we usually use dilute acid (e.g. dilute hydrochloric acid) to rinse the burette after cleaned with water in order to remove the remained alkali in the burette, as I have mentioned before.

do you worry about residual strong alkalis after titration that can attack the glass, blocking the burette by forming silicate or lowering accuracy of the readings?