Limit Test for Iron and Limit Test for Chloride

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limit test for iron

Limit Test for Iron

Theory :

  • The limit test for iron is formed on the reaction between iron (ferrous) and thioglycollic acid in a specific medium buffered with ammonium citrate to produce purple colour, which is afterwards compared with the standard colour produced with a known amount of iron (0.04 mg of Fe).
  • Ferrous thioglycollate is a complex chemical compound that cause production of the purple colour, Later thioglycollic acid converts the entire ferric ions to ferrous ions (Fe3+ into Fe2+).
  • The reactions involved in limit test for iron may be described as follows:



Fe2++2HS.CH2.COOHFerrous Thioglycollate+2H+



Materials Required

Nessler cylinder:1 pair, Ferric ammonium sulphate 1.726 grams, sulphuric acid (0.1 N) 10 ml, iron free citric acid (20% w/v) 2 ml, Thioglycollic acid 0.1 ml, Ammonium solution which must be free from iron 20 ml.

Standard Solution of Iron:

Weigh exactly 0.1726 grams of ferric ammonium sulphate which is also known as iron alum with formula NH4Fe(SO4)2 and dissolve in 10 ml of 0.1 N (Normal) sulphuric acid and sufficient quantity of water to produce 1 Litre solution. Each ml of this solution contains 0.02 mg of Ferrous (Fe).

Standard Colour:

  • Dilute 2 ml of standard iron solution with 40 ml Distilled water in a Nessler cylinder.
  • Add 2 ml of a 20% w/v solution of iron/ferrous free citric acid and 0.1 ml of thioglycollic acid, mix them well and make alkaline in nature with iron-free ammonia solution, dilute to 50 ml with distilled water and allow it to stand for 5 minutes.

Procedure Of Limit test for Iron

Dissolve the specified quantity of the substance being examined in 40 ml distilled water, and transfer to a Nessler cylinder.

Add to it 2 ml iron-free citric acid solution and 0.1 ml thioglycollic acid, mix well and make alkaline with iron-free ammonia solution, dilute to 50 ml with distilled water and allow standing for 5 minutes.

Any colour obtained is not Darker or intense than the standard colour.

Limit Test for Iron Practical Values

Chemical TestResults
1Calcium LactateDissolve 0.5 grams in distilled water then, add 20%w/v (2 ml) solution of iron free citric acid,Add 0.1 ml thioglycollic acid and mix it,make it basic with iron-free ammonia solution, Dilute to 50 ml with distilled water and keep for 5 minutes.standard colour is more intense than any other colour obtained,
2Calcium CarbonateDissolve 0.2 gram in 5 ml distilled water and 0.5 ml of HCl, Boil and dilute it to 40 ml with H2O.Complies with limit test for iron
3Tri-basic Calcium PhosphateDissolve 0.l gram in a mixture of 5 ml distilled water, 0.5 ml
HCl and 1 gram of citric acid. Dilute the solution to
40 ml with water.
Complies with limit test for iron
4Heavy kaolinTriturate 2 grams in a mortar with a 10 ml distil water and add
0.5 gram sodium salicylate.
Mixture colour is slight reddish.
5Heavy Magnesium OxideDissolve 40 mg in 5 ml distil water and 0.5 ml HCl-Do-
6Heavy magnesium CarbonateDissolve 40 mg in 5 ml distil water and add 0.5 ml HCl.Complies the test
7Magnesium SulphateDissolve 2 gram in 20 ml distil water.Complies the test
8Salicylic Acid(I) Boil 12 grams with 14 ml of dilute ammonia solution.
and 35 ml distil water. Cool and adjust the pH to 5 to 6 by the drop wise addition of dilute, NH4OH/
dilute. H2SO4.
(II) Boil 2 g with 1 ml stand. iron soln., 2 ml of
dil. NH4OH and 45 ml DW, adjust the pH to
5.0 to 6.0 and dilute to 50 ml with DW
Pink Colour in (I) should not be intense than (II).
9Potassium Chloride20 ml in 20%w/v solution in carbon-dioxide free distil water and carry test as given in 2 above.Complies the test
10Sodium bicarbonate2.5 grams in 20 ml distil water and 4 ml Hydrochloric and dilute to 40 ml with distil water.Complies the test
11Sodium Phosphate20 ml of 10% w/v sol. in distil water and copy test no 2.Complies the test
12Starchobtained residue is dissolved in the test for sulphated ash in 4ml Hydrochloric acid by heating softly, dilute with distil water to 50 ml and mix.25 ml complies Iron limit test
13Zinc Sulphateadd 2.5 gram in suffient CO@ free Distil water to get 50 ml. Dilute 2 ml of this solution of iron free citric acid and add0.5 ml of thioglycolic acid, make it alkaline with ammonia solution and dilute it to 50 ml by distil water and allow to stand for 5 minutes.Complies the test
14Zinc oxideDissolve 0.1 grams in mixture of 5 ml distil water and 0.5 ml HCl. and dilute to 40 ml with water.Complies the test.


Acid radical impurities contains an important but inescapable source of impurities in numerous pharmaceutical industrial chemicals.

Although, the two most commonly found acid radical impurities are chloride (Cl) and sulphate (SO4) that obviously arise from the unavoidable use of raw tap water in numerous manufacturing operations.

As these two acid radical impurities are found in abundance due to contamination, the Pharmacopoeia categorically stipulates limit tests for them which after due minor modifications are applicable to a number of pharmaceutical substances

In limit test for acid radial impurities, two commonly found impurities, there are numerous and which is present in pharmaceutical substances, most commonly seen are oxalate, nitrate, arsenate, carbonate, cyanide, phosphate and silicate.

All acid radical impurities is given briefly as under:

  • Limit test for Iron
  • Limit test for Chloride
  • Limit test for Lead
  • Limit test for Sulphate

Limit Test for Chloride

The limit test for chlorides is based or formed on its precipitation with silver nitrate (AgNO3).

In the presence of dilute nitric acid (HNO3), and comparing the opalescence (milky whitish appearance) produced due to the formation of silver chloride (AgCl) with a standard opalescence obtained with a known quantity of chloride (Cl) ions.

The equation may be expressed as: NaCl + AgNO3 → AgCl + NaNO3

Materials Required:

Nessler cylinder 1 pair, dilute nitric acid (10% w/w of HNO3) 10ml, silver nitrate solution (5.0% w/v in distil water) 1ml.

Standard Opalescence:

  • Place 1 ml of a 0.05845% w/v solution of Na-CI in 10 ml of dilute HNO3 in a Nessler cylinder.
  • Dilute to 50 ml with distilled water and add 1 ml of AgNO3 solution. Stir immediately with a glass rod and permit facing for five minutes.

Procedure of Limit test for Chloride

Dissolve the specified quantity for the substance in distilled water, or prepare a solution as directed in the text and transfer to a Nessler cylinder.

Add 10 ml of dilute nitric acid, except when it is used in the preparation of the solution, dilute to 50 ml with distil water, and add 1 ml of AgNO3 solution.

Shake well and save it aside for five minutes.

The opalescence produced is not greater than the standard opalescence, when viewed transversely.

Limit Test For Chloride

1Aluminium Hydroxide GelDissolve 0.5 gram in 5ml Dilute Nitric acid (HNO3) boil it, cool it, dilute to 100 ml with distil water and filter 2.5 ml used for the test.Complies with limit test for chlorides
2Benzocainedissolve 0.2 grams in 5ml alcohol, which is before acidified with a few drops of dilute HNO3 and add few drops of Silver nitrate Solution.No Turbidity is seen immediately
3Bephenium Hydroxy-napthoneDissolve 0.2 grams with 100 ml Distil water, cool in ice and filter to 20 ml of filtrate, add 10 ml dilute.nitric acid shake well and filter.Complies the Test
4Calcium Amino salicylateDissolve 0.99 grams in 10 ml distil water and add 3 ml of
acetic acid, filter, it wash the residue with 5 successive quantities each of 2 ml distil water, mix the filtrate
and washings and dilute 50 ml with distil water. Use
10 ml for the test.
5Calcium GluconateDissolve 1 grams in distil water, add 10 ml of dilute HNO3,
dilute to 50 ml with distil water and add 1 ml of 0.1 M
silver nitrate. Stir immediately with a glass rod and
allow to stand for 5 minutes protected from light.
When viewed transversely
against a black background any
opalescence produced is not
more intense then that obtained
by treating a mixture of 10 ml
of chloride standard solution
(25 ppm Cl) and 5 ml of distil water
in the same manner.
6ChloramphenicolShake 50 mg with 10 ml distil water and filter to filtrate add a few drops of AgNO3 solutionNo opalescence is produced
7Dextrose20 ml of a 10% w/v solution in DW is treated as
stated in 5 above.
Complies the limit test for chloride 125 ppm.
8EphedrineDissolve 0.1 g in 1 ml DW and 1 ml dilute HNO3
and add 0.l ml AgNO3 solution.
No turbidity is produced
9FurosemideShake l.0 g with 40 ml DW for 5 minutes and
filter. Test with filtrate as stated in 5 above.
Complies with limit test for chloride 25 ppm.
10Glycerin20 ml of a 50.0% w/v solution in CO2

free DW
is treated as described in 5 above.
Complies with limit test for chloride 25 ppm.
11IsocarboxazidBoil 0.5 g with 5 ml H2
O2 soln. (30%) and 10 ml
NaOH soln. (20% w/v in DW) for 2 minutes. Cool,
neutralize to litmus with HNO3 and add sufficient
DW to produce 40 ml. Test with the resulting
Complies with limit test for chloride 25 ppm.
12Heavy Magnesium OxideDissolve 0.4 g in DW by addition of 2 ml HNO3.-do-
13PhenybutazoneBoil l.0 g with 30 ml DW for 5 minutes, cool and
filter. To 10 ml of the filtrate add 1 ml dil. HNO3
and 1 ml AgNO3 solution
No opalescence is produced
14Promethazine Theoclate.Shake 2.0 g with 20 ml DW for 2 minutes and
filter. Use 10 ml of filtrate for the test.
Complies with limit test for chloride

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