SODIUM HYDROXIDE STANDARD SOLU

1.
Yes, NaOH is hygroscopic.

2.
many salts (including table salt), sugar, calcium chloride, sulfuric acid etc.
more special is CaCl2, NaOH and KOH, in which they absorb so much water that they dissolve in it (deliquescence) ---- on standing, solid turns into solution.
the moisture-absorber like 吸濕大笨象 utilizes this property; it contains solid CaCl2 in the container.

3.
notice that NaOH CAN be made into standard solutions.
It can give Secondary standard, but NOT a Primary one.

First some solid NaOH is dissolved in water (mass is not very important, but should be approximately the mass needed for target concentration).

For example, to make 1000ml of ~1M NaOH solution, you need to weigh ~1mole = ~40g of NaOH; around 40g is okay, no need to be exact.

Then, titrate the NaOH solution with a standard acid (either primary or secondary is okay), to determine the exact concentration of the NaOH solution. Then, the NaOH solution is said to be standardized, making it a secondary standard.

Primary standard: prepared by Direct dissolution of Exact mass of pure solid solute (like oxalic acid dihydrate, ammonium iron(II) sulphate-12-water), or Direct dilution of Exact mass/volume of pure liquid solute (like diluting heavy water D2O by light water H2O).

Secondary standard: prepared by standardizing a solution with other standards (primary or secondary) (like determining concentration of an unknown NaOH solution with a standard HCl solution).
OR: by diluting another standard solution (primary or secondary) (like diluting a 1.000M sol into a 0.500M solution).

There's no primary standard for NaOH; it's secondary standard can be prepared.


4.
CO2 is an acidic gas, which dissolves in water to give acidic solution. As a base, NaOH reacts with CO2 to give more stable salt (sodium carbonate).

1) SODIUM HYDROXIDE 既STANDARD SOLUTION 唔可以溶舊SOLID 落DISTILLED WATER 去預備 係咪因為佢hygroscopic??
Ans : You are right, NaOH is hygroscopic, it readily absorbs water in air.

2) 仲有D咩係hygroscopic?
Ans : There are a lot of substances that are hygroscopic, like sugar, solid acids, solid alkali, salts and etc.
Specific examples : NaCl(s), KOH(s), MgCl2(s), K2CO3(s), ammonium ferric citrate.....

3) 如果想預備standard solution of sodium hydroxide 要點做
Ans :
Method 1 : Dilute a standard solution using pipette and volumetric flask.
Method 2 : Dissolve suitable amount of primary standard in deionized water in volumetric flask.

4) NaOH(s) 除左會absorb water in air外 點解仲會 reacts with CO2 in air?
Ans :
I don't know why it reacts, but I know how it reacts.
CO2 + 2NaOH ---> Na2CO3 + H2O (when NaOH is excess)
CO2 + NaOH ---> NaHCO3 (when CO2 is excess)



黑白灰_唯物主義者_魑魅魍魎


2011-08-24 19:45:41 補充：
In Method 1, the standard solution means NaOH solution with known molarity. And as you said, this method is to dilute a NaOH solution with known molarity

In Method 2, dissolve primary standard in deionized water means dissolve pure solid NaOH in deionized water.

2011-08-24 19:51:57 補充：
I want to ask why do you say that NaOH standard solution cannot be prepared by dissolving its solid in distilled water?
As I know, you can heat the solid till it is anhydrous,
you can keep it anhydrous till you dissolve it in distilled water.

2011-08-24 22:31:24 補充：
For more, you can refer to another question you asked.