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AIMS
1.To learn to determine the solution concentration with the primary standard solution.
2.To determine the nitrogen content in ammonium salt with formaldehyde.
Common ammonium salts,such as ammonium sulfate,ammonium chloride,ammonium nitrate,are strong acid and weak alkali salts.
Although
is acidic,it can not be titrated directly because of
cK
a
＜ 10
-8
,where c is the concentration,
K
a
is the acid dissociation constant,
K
w
is the ion product of water,
K
b
is the base dissociation constant.
There are three more or less standard methods in general use for analyzing ammonium nitrate,i.e.,the Kjeldahl method,the nitrometer method,and the impurity method.The first two are very accurate,but take from
to 1
hrs for duplicate analyses,which is too long for most control work.The third method,while commonly used,is unsatisfactory,in that any errors of omission or commission are combined here.
The method in our experiment,making use of formaldehyde,is much more rapid and quite accurate.The method depends upon the ease with which formaldehyde reacts with ammonia or ammonium salts to form hexamethylenetetramine:
In the circumstance, cK a ［(CH 2 ) 6 N 4 H ＋ ］＞ 10 -8 ,meet the requirements of direct titration.
The method is very simple and in general is as follows:A neutral 20 percent solution of formaldehyde is added to the ammonium nitrate solution,and titrated with standard NaOH,using phenolphthalein as indicator,since the latter is not sensitive to hexamethylenetetramine.The amount of NaOH used gives the amount of ammonium nitrate present.
(CH 2 ) 6 N 4 H ＋ ＋3H ＋ ＋4OH- ＝(CH 2 ) 6 N 4 ＋4H 2 O
Mole relations:
n
(N)＝
n
(
)＝
n
(H
＋
)＝(
cV
)
NaOH
● Analytical balance,100 mL and 250 mL volumetric flasks,50 mL burettes,250mL conical beakers,25 mL beaker,10 mL graduated cylinder,droppers,glass rods,rubber bulbs
● Sodium hydroxide (NaOH),potassium hydrogen phthalate (KHP),ammonium sulphate((NH 4 ) 2 SO 4 ),40％formaldehyde solution,10 g·L -1 phenolphthalein(PP)solution,distilled and deionized water
Formaldehyde often contains trace formic acid,which is caused by the oxidation by air.Formic acid should be removed,or otherwise it will bring positive error to the total reaction.Take the supernatant of the 40％ formaldehyde in beaker,dilute it twice with water.Add 1—2 drops of phenolphthalein solution as indicator,and then titrate with 0.1 mol·L -1 NaOH standard solution until the solution is light pink,finally drop with unneutralized formaldehyde to just colorless.
About 1.0 g sodium hydroxide is weighed (record the exact weight on your paper,and the weighing accuracy should be to±0.0001 g),and dissolvedin water in a small beaker(25 mL).After all dissolved,transfer to 250 mL volumetric flasks and add water to the calibration line.
Standardization(3 times)
Accurately weigh 0.4—0.5 g potassium hydrogen phthalate (the weighing accuracy should be to ±0.000 1 g),dissolve in a small beaker(25 mL)and then transfer to a conical beaker.The beaker needs to be rinsed three times with distilled water,and all the solution after each rinse should be transferred to the conical beaker.The total amount of water is 50 mL,including dissolution and rinsing.
Add 1—2 drops of phenolphthalein solution to the conical beaker as indicator,and record the initial burette reading,and then titrate with 0.1 mol·L -1 NaOH solution slowly while swirl the conical beaker.When the color turns to light pink,and does not fade for 30 seconds,the end point is reached.Record the final burette reading.The burette accuracy should be 0.01 mL.Repeated three times.
The reaction for potassium hydrogen phthalate with sodium hydroxide:
Accurately weigh 0.6—0.8 g (NH 4 ) 2 SO 4 in beaker (the weighing accuracy should be to±0.000 1 g),dissolve it with distilled water,then move it quantitatively to 100 mL volumetric flask,and finally dilute it with distilled water to the mark and shake it evenly.
Transfer 25.00 mL of sample solution into a conical beaker with pipette.Add 4 mL neutral formaldehyde solution,shake well and let stand for 1 minute.1 drop of phenolphthalein indicator is added to the solution,and then titrated with the calculated NaOH solution until the solution is light pink for 30 seconds.Record the initial and final burette reading with accuracy of 0.01 mL.The endpoint should be determined for three times in parallel.
1.If there is little amount of formicacidin the formaldehyde sample,what effects does it have?How to eliminate its influence?
2.Except potassium hydrogen phthalate(KHP)primary standard,what else can be used to standardize the concentration of NaOH solution?
3.In the titration of NaOH solution with potassium hydrogen phthalate(KHP),why is weighting the mass of KHP for 0.4～0.5 g?