Pharmaceutical Analysis: Second Year - Pharmacy

Pharmaceutical Analysis

Second Year - Pharmacy

Water Analysis

Dissolved gases in water

1. Oxygen 2. Chloride 3. Hydrogen sulfide 4. Carbon dioxide

(1) Dissolved Oxygen •

It measures the amount of dissolved oxygen present in water due to its aeration.

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A good water should contain about 11 ppm of dissolved oxygen.

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Bacteria is readily oxidized and destroyed by dissolved oxygen, therefore it is a good sign for water.

Determination a) Winkler method: •

Water sample is treated with MnSO4, NaOH and KI and set aside in a dark place for about 15 minutes. MnSO4+ NaOH  Mn(OH)2 + 2Na2SO4

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In the presence of dissolved oxygen, the white precipitate of Mn(OH)2 is oxidized to brown Mn(OH)3. 2Mn(OH)2 + O + H2O  2Mn(OH)3

•

Upon acidification, manganese hydroxide dissolves and reacts with KI to liberate free iodine (I )

Notes:

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The experiment is carried out in a closed bottle of known volume and care should be taken to exclude air bubbles.

•

Blank experiment should be carried out.

Interferences: •

Reducing agent, such as ferrous, sulfide and sulfite reduce iodine to iodide leading in low results. Solution: Dilute KMnO4 solution  faint pink color. Excess KMnO4 is removed by sodium oxalate solution.

•

Oxidising agent such as Ferric, nitrite react with iodide and liberate free iodine, giving high results. The difficulty caused by the presence of nitrite in the sample can be avoided by addition of sodium azide. NaN3 + H+  NH3 + Na+ NH3 + NO2- + H+  N2O + H2O + N2

2- Rideal-Stewart modification :

•

It is used to remove the undesirable effect of reducing substances .

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It depends on removal of organic compounds by addition of KMnO4 , then the excess KMnO4 is removed by adding K-oxalate,

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After the removal of organic compounds, the procedure is completed as in Winkler method .

(2) Chlorine •

Chlorine is added to public water supplies, swimming pool and sewage treatment plant effluents to destroy bacteria.

•

1 mg/L of free chlorine generally is sufficient to control bacteria without causing noxious odor or taste to water.

•

Chlorine can be present in water either as free chlorine or combined available chlorine. • •

•

Free chlorine is present as hypochlorous acid and or hypochlorite ion. Combined chlorine exists as monochloramine, dichloramine, nitrogen tetrachloride and other chloro derivatives. Both forms can exist in water and can be determined together

Determination: a) Iodometric method: • Water sample is acidified and KI solution is added then the liberated iodine is titrated with sodium thiosulfate using starch as indicator. •

Many oxidizing substance can interfere with the determination.

•

To overcome this problem, the method is carried out in neutral medium. In this case chlorine liberate iodine by displacement not by oxidation

b) O-toludine method: •

O-toludine solution in dilute hydrochloric acid is added to the water sample in Nessler tube.

•

The mixture is kept in dark place for about 5 minutes at pH below 4.

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The color produced is varied from pale yellow to orange color according to the amount of chlorine present in the sample.

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Since standard chlorine solution is difficult to be prepared the comparison is carried out using “artificial standard color” prepared by using CuSO4 and K2Cr2O7 solutions in different proportion.

•

Colored glass discs may be also used.

Ortho-toludine reaction with chlorine

O -toludine

H

+

Pale yellow to orange color

(3) Hydrogen sulfide •

Sulfide is poisonous by-product of the anaerobic decomposition of organic matter. It is commonly found in sewage and industrial waste waters.

•

The toxicity of H2S is equivalent to that of HCN, but its odor is detectable before toxic level is reached.

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Effects on human: 1. Hydrogen sulfide inhibits aerobic respiration, 2. inhibits muscle contractions, including breathing, and 3. promotes excess breakdown of glucose.

Detection and determination: •

The test is based on the ability of hydrogen sulfide to convert p-amino-dimethyl aniline into methylene blue under suitable condition.

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The intensity of the formed blue color is directly proportional to the amount of H2S present in the water sample.

•

Interferences: Strong reducing agents such as sulfites, thiosulfate and hydrosulfite interfere by reducing the intensity of the blue color or prevent its development.

Reaction of p-aminodimethyl aniline with H2S

p-aminodimethyl aniline

HCl /FeCl3

Methyline blue

(4) Carbon dioxide •

Carbon dioxide present in all surface water in amount generally less than 10 ppm.

•

Many ground water have high level of CO2.

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Although CO2 is not harmful for human, high concentration of dissolved CO2 in water have a corrosive effect and have been known to kill fish.

Determination: •

The analysis for CO2 is similar to that for acidity determination.

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Water sample is titrated with standard sodium hydroxide solution using ph. ph as indicator.

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This will give both mineral acidity and acidity due to dissolved CO2.

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Mineral acidity can be determined alone by using M.O. as indicator.

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Care must be taken during the analysis to minimize the loss of CO2 from water sample as a result of aeration, during sample collection and swirling.

METALS IN WATER. •

Iron

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Copper

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Zinc

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Lead

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Manganese

(1) Iron 0.3 mg/l (0.3 ppm) •

Natural water contain variable but minor amounts of iron despite its universal distribution and abundance.

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Iron in natural water may exist in ferrous or ferric form.

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Ferrogenous sand is the source of iron in natural water.

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High concentrations can indicate runoff from mining operations or industrial effluent.

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Concentrations of iron above 1 mg/L will impart a foul taste to the water.

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The level of iron in drinking water must not exceed 0.3 mg/L (0.3 ppm).

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Water containing higher level should be rejected.

a) Phenanthroline method •

Iron is brought into solution by boiling with acid.

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Any ferric ions present is reduced to ferrous by addition of hydroxylamine. 4 Fe3+ + 2H2N-OH  4 Fe2+ + H2O + N2O

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Finally the solution is treated with 1,10-phenanthroline solution.

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A red color is produced due to the formation of complex between three molecules of phenantholine and one ion of ferrous.

Phenanthroline reaction

N + Fe2-

3 N

N Fe N 3

1,10-phenanthroline

Red colored complex

b) Thioglycolic acid method: •

Thioglycolic acid (mercapto acetic acid) reacts with ferric ions to form a red purple color which can be measured spectrophotometrically at 535 nm.

•

This reaction of a great importance because it is relatively free from interferences.

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Precipitation of Al3+ and Cr3+ ions are prevented by the addition of ammonium citrate. Fe3+ + 2HS-CH2-COOH + 3OH-  Fe(OH)(SCH2COO)2 + 2H2O

c) Other colorimetric methods: •

Iron in the ferric state can be determined colorimetric methods as a red soluble complex by using thiocyanate in acid medium or as a blue precipitate by using ferrocyanide.

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For determination of total iron, ferrous should be first oxidized to ferric state by the addition of very dilute KMnO4 solution which is added to the acidified sample till faint pink color.

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Excess permanganate is removed by repeated filtration of the treated sample through a filter paper which act as reducing agent.

(2) Zinc 1 mg/l (1 ppm) •

The average acceptable level of zinc in potable water is 1 ppm.

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The average of zinc concentration in most water supplies is about 1 ppm, but in some area it may be as high as 50 ppm.

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The deterioration of galvanic iron and leaching of brasses can add substantial amount of zinc to water.

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Zinc may also be found in spring and stream waters in mining districts.

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Industrial effluents may also contribute large amount of zinc.

Effect of Zinc: •

Zinc is essential to human metabolism.

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High concentration of zinc have irritant effect on stomach but this effect is temporary.

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Concentration above 5 ppm show no harmful physiological effect but can cause a bitter taste and/ or an opalescence in alkaline drinking water.

Determination •

Zinc ions react with ferrocyanide in acid medium to produce a white precipitate of zinc ferrocyanide

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This precipitate can be measured turbidimetrically or by comparison with the precipitate formed by using standard zinc solution.

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Iron, lead and copper ions interfere with the test.

(3) Copper 0.03 mg/l (0.03 ppm) The average acceptable concentration of zinc in potable water is 0.03 ppm. Sources: Copper may be present in water due to: 4. Solvent effect of acid water in copper pipe 5. Copper sulfate added to control algae and bacteria in water. Effects: •

Traces of copper are necessary for normal body metabolism and its absence causes nutritional anemia in children.

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Large oral doses of copper can cause emesis and may eventually result in liver damage.

Determination of copper a) Diethyldithiocarbamate method: b) ferrocyanide method: c) Hydrogen sulfide method: d) Dithizone method:

a) Diethyldithiocarbamate method: •

Copper form a yellowish brown colloidal chelate compound with sodium diethyldithiocarbamate.

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In the presence of traces amount of copper a golden yellow color is obtained. The reagent is not specific for copper.

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Interference caused by the presence of iron can be eliminated by: 1. extracting the formed complex with chloroform (iron complex is insoluble in chloroform), 2. precipitating iron as hydroxide. 3. improving the selectivity of the reagent by using masking agent particularly EDTA.

Continued C2H5 C2H5

S N

C

+ Cu+2

SNa

diethyldithiocarbamate

C2H5 C2H5

S

S N

C

Cu S

S

C

N

C2H5 C2H5

Yellowish brown colloidal chelate

b) ferrocyanide method: •

Copper react with ferrocynide in dilute acid solution with the formation of brown precipitate.

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In the presence of traces of copper the precipitate appears as yellowish brown color.

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Interference can arise from the presence of iron and zinc.

c) Hydrogen sulfide method: •

Copper ions form a black precipitate with hydrogen sulfide.

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If traces of copper is present in the solution a brown or yellow colloidal precipitate will appear.

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Interference from iron can be eliminated by the addition of acetic acid.

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Lead ions will produce similar precipitate with hydrogen sulfide. Lead sulfide is insoluble in potassium cyanide, while, copper sulfide is soluble forming cupper cyanide complex.

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Therefore the determination is carried out in absence and presence of KCN, any difference in the color will be

d) Dithizone method: •

Copper gives an orange complex with dithizone in acid medium.

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The complex is extractable in chloroform. Phe NH NH Phe S N N Phe

+

Cu2+

S C

NH N N

N Phe

Dithizone

Phe Cu

N

N

N NH Phe

orange complex

C S

(4) LEAD 15 µg/l (15 ppb) •

Lead is a poison whose effects are cumulative. Drinking water should not exceed 15 ppb.

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Natural water contain very low level of lead due to its tendency to be precipitated by a large number of substances.

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When groundwater contains a higher level, it may indicate contamination from the discharges of smelting or mining operations, or leachate from municipal sewage sludge fertilizer.

The source of lead in drinking water •

Materials that contain lead which have frequently been used in the construction of water supply distribution systems and plumbing systems, also

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Water storage tanks which have been painted with leadbased paint.

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Lead in these materials can contaminate drinking water as a result of the corrosion that takes place when water comes into contact with those materials.

Lead effects in human health •

Lead can cause a variety of adverse health effects in humans.

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At relatively low levels of exposure, these effects may include 1. interference in red blood cell chemistry, 2. delays in normal physical and mental development in babies and young children, 3. slight deficits in the attention span, hearing, and learning abilities of children, 4. and slight increases in blood pressure of some adults.

Steps to be taken to minimize your exposure to lead 1.

Flush your plumbing to counteract the effects of "contact time." Flushing involves allowing the cold faucet to run until a change in temperature occurs (minimum of one minute).

2.

Do not consume hot tap water.

3.

Steps should be taken to make water non corrosive. Water treatment devices for individual households include calcite filters and other devices to lessen acidity.

4.

Insist on lead-free materials for use in repairs and newly installed plumbing.

5.

Lead can be removed from your tap water by installing point-ofuse treatment devices now commercially available, which include: ion-exchange filters, reverse osmosis devices, and

Determination a) Hydrogen sulfide method: •

Lead + hydrogen sulfide  a black precipitate.

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Interference from copper can be removed by the addition of potassium cyanide which produce a soluble complex with copper.

d) Dithizone method: •

The reagent in ammoniacal medium gives a red complex with lead. The selectivity of the method may be enhanced by the addition of potassium cyanide.

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Sodium citrate may be added to prevent the precipitation of iron as hydroxide in the ammoniacal medium.

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The structure of the complex is similar to that of copper

Manganese 0.05 mg/l • (5) Manganese is present in ground water(0.05 as the ppm) divalent ion (Mn2+) due to the lack of surface oxygen. Surface water may contain combination of various oxidation states as soluble complexes or as suspended particles. •

Manganese is a minor constituent of many rocks and soils. It present in slight amount in water particularly acid water.

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The presence of manganese in public water is of economic problem rather than a potential health hazards. Manganese causes dark stain in laundry and imparts an objectionable taste to beverages such as coffee and tea. Level of 0.1 mg/L is sufficient to cause taste and staining problems.

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The maximum allowable level of manganese in public water is 0.05 mg/L (0.05 ppm) with the total iron and manganese content not to exceed 0.3 mg/L.

Determination •

The determination is based on the oxidation of manganese ion into permanganate using strong oxidizing agent such as persulfate and periodate.

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Silver nitrate is used as a catalyst in the case of persulfate.

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Interference from chloride can be avoided by the addition of mercuric ion which form poorly ionized mercuric chloride (HgCl2). Mn2+ + K2S2O8  MnO4- +

Fluoride •

Fluoride is found in some ground water.

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A level of 1 mg/L is maintained normally in public drinking water supplies for the prevention of dental caries.

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Increased amount of fluoride causes objectionable discoloration of tooth enamel called fluorosis.

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Level of 8 mg/L is found to be physiologically harmful for human.

Determination

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Fluoride in water is determined by colorimetric method.

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It is based on the reaction of fluoride with red zirconium alizarin.

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Fluoride forms a colorless complex anion (ZrF6) and liberates free alizarin sulphonic acid which have yellow color in acid medium.