Qualitative Investigation Of Water Samples Biology Essay

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When a chemist conducts a qualitative analysis, the primary goal is to identify the substances that are present in a mixture. It is important to separate substances from each other in a mixture, so that a confirming test can be as definitive as possible. After a thorough qualitative analysis investigation, it would be nice to say, for example, that potassium is present instead of saying merely that an alkali metal is present. Conducting a qualitative analysis, or "qual scheme" as it is sometimes nicknamed, is a methodical process that requires great care. A qual scheme can be used to identify a wide variety of substances; in this experiment, you will focus on ions. Chemical analysis can be either qualitative or quantitative in nature. In qualitative analysis we want to know which elements or characteristic chemical species are present. In quantitative analysis we are interested in the relative amounts of the components present. We will be concerned with qualitative analysis in this laboratory. The classical qualitative analysis scheme has been around for well over 100 years, but it continues to be an important part of any chemist's training. It offers an effective means for presenting descriptive inorganic chemistry in the laboratory and it illustrates not only descriptive chemistry, but also important chemical principles, especially those involving ionic equilibrium. We will use three different type of water sample such as standard solution which will contain all the ions to familiarize with the test , tap water and lake water. We need to check what are the ions presence in tap water and lake water.

Procedure :

When solid reagents are called for, a few crystals were added from the tip of a spatula. A beaker of boiling water was used for heating test tubes where directed. Cm3 portions of water were used in small test tubes for all tests except Ca2+ and Mg2+.

Part 1 Standard. Solutions containing all the ions were used to familiarize you with the tests. A blank sample of demineralized water was run alongside it.

Part 2 Tap water. 200 - 250 cm3 of tap water was evaporated to about one- third of the original volume. Ions present was analyzed and indicated in notebook. If test is very faint, you may indicate a trace.

Part 3 Unknown. A sample of solution was obtained in a small, clean beaker. The results were analyzed and recorded.

Ca2+ . 1 mole dm-3 NH4OH was added to about 4cm3 of water sample until blue to litmus. If dark precipitate forms, filter it off, a little NH4Cl and (NH4)2C2O4 was added then. It was placed in water bath and warm for a minute. A white precipitate indicates calcium in the form of calcium oxalate.

Mg2+ . The calcium oxalate was filtered off from the above sample and a little NH4OH and NH4Cl were added to the filtrate and then a few grains of Na2HPO4 was added. White crystalline precipitate, ammonium magnesium phosphate formed upon shaking indicates magnesium.

Fe3+ . A few drops of 1 mole dm-3 HCl and a little potassium hexacyanoferrate (II) (K4Fe(CN)6) was added to a fresh sample of water. A dark blue precipitate(Prussian blue) shows presence of Fe2+.

Cl- . 0.2 mole dm-3 of AgNO3 solutions was added.a cloudy white turbidity was looked for to develop which will turn purplish in sunlight. (A yellow precipitate here could indicate the presence of PO43- . This will dissolve in a drop of conc. H2SO4 , the AgCl will not )

PO43- . A litmus paper was used as an indicator to check if is either strongly basic or acidic. Neutralize with 1 mole dm-3 if acidic , or 1 mole dm-3 HNO3 if basic, then about 1 -2 cm3 of 0.25 mole dm-3 (NH4)2MoO4 solution was added warm in water bath and look for the formation of bright yellow precipitate.

NH4+ . 3 cm3 sample of water was placed into a boiling tube. 10 drops 20% of sodium hydroxide solution and an anti- bumping chips were added then boiled.

NO3- . emission of ammonia indicated an ammonium salt. 5mg Devarda's alloy was added after boiled off all the ammonia and then was boiled again. Emission of ammonia at this stage indicates nitrate.

Test for ammonia :


A drop of conc. HCl carried by the pipette to the mouth of the tube.

A piece of filter paper soaked in mercury(I) nitrate solution.

Results :

Table 1 : observation of various ions in the test

Type of water


Standard solution

Tap water

Lake water


Litmus paper turn red to blue.

Light yellow to orange precipitate.

White precipitate formed.

Litmus paper turn red to blue.

White crystals formed after boiled.

White precipitate formed.

Litmus paper turn red to blue.


White crystalline precipitate formed.

White crystalline precipitate formed

No precipitate formed


Light yellow to dark blue solution.

Dark blue precipitate formed

Light yellow solution.

No precipitate


Light yellow to brown solution.

Dark blue precipitate formed

Yellow solution.

Solution change to pale yellow


Solution turns from light yellow to cloudy .

Under sunlight, grayish and purplish precipitate formed.

Solution turns colorless to cloudy .

Under sunlight dark purplish precipitate formed.

Solution change to grey under sunlight.


Solution change from light yellow to orange.

Bright orange precipitate formed.

Litmus paper turns blue to red

No change in litmus paper.

Yellow solution after added (NH4)2MoO4.

Bright yellow after warmed.


No precipitate.

Colorless solution.


Yellow precipitate formed

Wet litmus turn red to blue

No change in litmus paper.

Litmus paper turns red to blue.

Bubble released.


Effervescence occurs.

Wet litmus paper turns red to blue.

No changes to litmus paper.

Litmus paper turns red to blue.

Effervescence occurs.

Discussion :

Chemists have developed many tests for the identification of ions found in water. Several such tests will be used in this lab to determine if certain ions are present in an unknown sample, tap water sample and natural water sample. These ions are detected by the formation of a new colour or a precipitate (ppt) when special reagents are added to the water such as NH4Cl , (NH4)2C2O4 , potassium hexacynoferrate(II) and (NH4)2MoO4. For standard solution , this solution contains all the ions which we will test to familiarize with the test and by this we can observe and compare accurately. For calcium ion test both tap water and lake water the litmus paper change from red to blue colour indicates alkaline solution. Only lake water contain calcium ions because of the formation of white precipitate after react with NH4Cl and (NH4)2C2O4. precipitating agent for Ca2 + is the oxalate anion, C2O42-. A series of reactions, starting with eq. 1, occurs. The solution starts off being acidic with undissociated oxalic acid, H2C2O4. In this form, the calcium ion will not be precipitated. However, upon heating, the urea, (NH2)2CO, slowly decomposes to form -OH, which in turn, neutralizes the oxalic acid to liberate the oxalate anion. Precipitation occurs slowly allowing for slow crystal growth, hence larger and purer crystals develop.

(NH2)2CO + 3 H2O 2 NH4+ + CO2 + 2 -OH eq. 1

2 -OH + H2C2O4 C2O42- + 2 H2O eq. 2

Ca2+ + C2O2- CaC2O4.H2O (precipitate) eq. 3

The formation of white crystalline precipitate , ammonium magnesium phosphate indicate the presence of magnesium in tap water and for the lake water there is no formation of precipitate which means no magnesium ions in lake water.

Potassium ferrocyanide also know as potassium hexacynoferrate II is the inorganic compound with formula K4[Fe(CN)6]•3H2O. It is the potassium salt of the coordination complex [Fe(CN)6]4-. This salt forms lemon-yellow monoclinic crystals. The most famous reaction involves treatment with ferric salts to give Prussian blue. With the approximate composition KFe2(CN)6, this insoluble but deeply coloured material is the blue of blueprinting. Prussian blue, the deep blue pigment in blue printing, is generated by the reaction of K3[Fe(CN)6] with ferrous (Fe2+) ions. The reaction with hydrochloric acid is as follows:

6 HCl + K4[Fe(CN)6] → 6 HCN + FeCl2 + 4 KCl

Potassium hexacynoferrate II is used to test the presence of Fe3+ and Fe 2+ in water sample. There is no blue precipitate for both tap and lake water thus there is no Fe3+ and Fe 2+ in both the water sample. For chloride ion test, the tap water observed in our experiment was taken from the laboratory. The appearance of tap water was transparent and may contain minerals, disinfectant, and contaminants. Chloride may also be present in tap water. When each of the water samples were added with AgNO3 ,the AgNO3 ions dissociates:

AgNO3(aq)→ Ag+(aq)+ NO3- (aq)

Then, the Ag+ions reacted in the presence of Cl-ions:

Ag+(aq) + Cl- (aq) → AgCl(s)

AgCl is insoluble in water and it precipitated to become solid. The colour of AgCl solid is white . When it was put into tap water, the water became turbid and also turns purplish under sunlight. This shows that the tap water contains chloride ions. Whereas for lake water there is small changes in solution which turn grey under sunlight. Thus, only very small amount of chlorine can be found in lake water.

For the phosphate test, only the tap water contain phosphate. The solution turns bright yellow after added by (NH4)2MoO4.

PO4 + I2(NH)2MoO4 + 24H2O -> (NH4)3PO4.12MoO3 + 2INH4 + 12H2O 

the tap water phosphate (PO4) can react with Ammonium Molybdate (I2(NH)2MoO4), which can exist in tap water to form Ammonium Phosphomolybdate ((NH4)3PO4.12MoO3). Under laboratory testing this Phosphomolybdate is obtained as a yellow precipitate. It is more commonly called as the Ammonium Molybdate Phosphomolybdic Acid or the Ammonium Phospho-molybdate. For the ammonium ions only the lake water contain ammonium when it react with sodium hydroxide. The litmus paper turn red to blue which indicate the the gas is alkaline. For the nitrate test, there is no nitrate ions in tap water whereas in lake water there is a nitrate. Devarda's alloy (Cu/Al/Zn) is a reducing agent. When reacted with nitrate in sodium hydroxide solution, ammonia is liberated. The ammonia formed may be detected by its characteristic odor, and by moist red litmus - very few gases other than ammonia evolved from wet chemistry are alkaline.

3 NO−3 + 8 Al + 5 OH− + 18 H2O â†’ 3 NH3 + 8 [Al(OH)4]−

Aluminium is the reductant in this reaction.

Precaution steps :

Potassium ferricyanide has very low toxicity, its main hazard being that it is a mild irritant to the eyes and skin. Hydrochloric acid is corrosive and can cause irritation to the skin. Exposure to very high concentrations of gaseous ammonia can result in lung damage and death. It is very toxic.