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13 Preparation of One Molar (1M) Solutions of Concentrated Acids

13 Preparation of One Molar (1M) Solutions of Concentrated Acids

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16



2



Expression of Concentration



Table 2.3 Preparation of 1M solutions of concentrated acids

Acids



Molarity



Percent



S.G.



Volume of concentrated

acid per litre



Acetic acid (CH3COOH)

MW ¼ 60.05



17.4 M



99



1.05



58 mL



Hydrochloric acid (HCl)

MW ¼ 36.46



11.6 M



36



1.18



86 mL



Nitric acid (HNO3)

MW ¼ 63.01

Sulphuric acid (H2SO4)

Hydrobromic acid (HBr)

MW ¼ 20.01



16.4 M



69



1.42



61 mL



17.6 M

36 N



95



1.84



56 mL/1 (1 M)

28 mL/(1 N)



27 M



48



1.49



11.1 mL



Hydriodic acid (HI)

MW ¼ 127.90



7M



57



1.70



143 mL



Perchloric acid (HClO4)

MW ¼ 100.46



11.7 M

9.5 M



70

61



1.71

1.66



85.5 mL

106 mL



Phosphoric acid (H3PO4)

MW ¼ 97.99



45 N

15 M



88



1.69



22.5 mL (1 N)

68 mL (1 M)



Ammonia solution

MW ¼ 35.05



14.5 M

15 N



28



0.88



71 mL (1 M)



2.14



Formula to Calculate the Volume of Stock Solution

Required to Prepare Solution of Desired Normality



100  desired normality  molecular weight  desired volume

1; 000  Basicity  percentage of stock solution  specific gravity of stock solution



2.15



Formula to Calculate Volume of Stock Solution

Required to Prepare Solution of Desired Percentage

Desired percentage  desired volume

Percentage of stock solution



(Where the percentage is weight/volume)



2.17



2.16



Preparation of Standard Solution of Acids and Bases



17



Formula to Calculate the Dilution Factor

Used in Spectroscopic Estimations

Absorbance  dilution factor

1; 000



(Where concentration is per litre)

Calculation of dilution factor

1  Initial volume  volume made for spetrophotometric analysis

Weight of sample  initial volume taken for dilution

Example

Weight of sample taken ¼ 5 g

Initial volume made ¼ 50 mL

Initial volume taken for dilution ¼ 1 mL

Volume made for spectrophotometeric analysis ¼ 10 mL

Dilution factor ¼



2.17



1 Â 50 mL Â 10 m

5 g  1 mL



Preparation of Standard Solution of Acids and Bases



The preparation of standard acid solution, viz. H2SO4, HCl, HNO3 and alkalis, viz.

KOH, NaOH have difficulty because they cannot be pipetted/weighed very accurately. Moreover alkalis gather moisture while weighing and assay percentage of acids

varies. Therefore, oxalic acid, which can be accurately weighed is used as primary

standard to standardize NaOH which then is used to standardize acids using indicators

as end points. If the strength of one component is known exactly, the strength of the

other can be experimentally found by using the under mentioned equation.

N1 Â V1 ¼ N2 Â N2

N and V indicate normality and volume of acids and bases respectively.

Reagents: 0.1 N H2SO4, 0.1 N HCl, 0.1 N HNO3, 0.1 N NaOH, 0.1 N oxalic acid,

1% phenolphthalein.

Glassware/equipments: Conical flask (250 mL), burette (25 mL), burette stand,

volumetric flask and beakers, funnel, droppers.



18



2



Table 2.4 Common

solutions used in analytical

procedures



Expression of Concentration



Primary standards

N/7 Na2CO3

N/10 Oxalic acid

2.04 N Na2CO3



Per cent solutions

2% HNO3 (v/v)

3% KNO3 (w/v)

2% Boric acid (w/v)

40% NaOH (w/v)

20% Ammonium molybdate (w/v)



Secondary standards

N/7 H2SO4

N/7 HNO3

N/7 NaOH

N/10 KMnO4

2.04 N H2SO4

2.50 N NaOH



Ratio solutions (v/v)

1:4 H2SO4

1:2 HCl

1:4 Liquor ammonia

Saturated solution

Saturated ammonium oxalate



Procedure: Standardization of NaOH: Pipette out 10 mL of NaOH into a 250 mL

conical flask. Add a drop of indicator phenolphthalein. Fill the burette with 0.1 N

oxalic acid and set at zero mark. Titrate till pink colour changes to colourless and

note the volume of acid used. Repeat the same process thrice.

Observation: Volume of 0.1 N oxalic used to neutralize 10 mL of NaOH

S.No. Initial reading

1

2

3

Mean volume ¼ a



Final reading



Volume used (mL)



Calculation of normality of NaOH

Applying

N1 Â V1 ¼ N2 Â V2

0:1 Â a ¼ N2 Â 10

N2 ¼ 0:1 Â a=10 say it ¼ x

The primary and secondary standardized solutions of known normally along

with different ratio and percentage solutions required for the analysis of various

proximate principles are given in Table 2.4.



2.17.1



Normal Solutions



2.17.1.1



Primary Standards



Primary standards are used for the standardization of solutions of unknown strength.



2.17



Preparation of Standard Solution of Acids and Bases



19



Preparation of N/7 Na2CO3

Reagent

– Anhydrous sodium carbonate.

– As sodium carbonate is highly hygroscopic, dry it in hot air oven at 100 Ỉ 2 C

for 3–4 h to make it anhydrous.

Calculations

Molecular weight of Na2CO3 ¼ 105.989;

i.e. [(2 Â 22.9898) + 12.0112 + (3 Â 15.9994)]

Equivalent weight ¼



Molecular weight 105:989

¼

¼ 52:9945

Acidity

2



(Acidity of Na2CO3 ẳ 2, based on the reaction,

Na2 CO3 ỵ 2HCl ! 2NaCl ỵ H2 O ỵ CO2 )

Gram-equivalent weight ¼ 52.9945 g

Therefore,

1,000 mL of 1 N Na2CO3 contains 52.9945 g

(or) 1,000 mL of N/7 Na2CO3 contains 7.5706 g

Procedure

To prepare 250 mL of N/7 Na2CO3, weigh exactly 1.8927 g of oven-dried

and desiccator-cooled anhydrous Na2CO3 in a clean dry beaker. Dissolve in distilled

water and make up the volume by transferring the beaker contents with repeated

washings into a 250 mL volumetric flask and mix by gentle shaking. Label it as

N/7 Na2CO3. It is utilized as primary standard for standardizing N/7 H2SO4.



Preparation of N/10 Oxalic Acid

Reagent

Oxalic acid [(COOH)2∙2H2O]

Calculation

Molecular weight of [(COOH)2∙2H2O] ¼ 126.0666

i.e. [2(12.0112 + 31.9988 + 1.0080) + 2(2.0159 + 31.9988)]

Equivalent weight ¼

Gram-equivalent weight ¼ 63.0333 g



126:0666

¼ 63:0333

2



20



2



Expression of Concentration



Therefore,

1,000 mL of 1 N oxalic acid contains 63.0333 g

(or) 1,000 mL of N/10 oxalic acid contains 6.3033 g

Procedure

Weigh exactly 1.5758 g of oxalic acid in a clean dry beaker and make up to 250 mL

in a volumetric flask with distilled water, shake gently and label as N/10 oxalic acid.

The strength of N/10 oxalic acid can be checked by titrating against N/10 Na2CO3

using phenolphthalein as indicator. It can serve as a primary standard for preparing

N/10 KMnO4.



Preparation of 2.04 N Na2CO3

Reagent

Anhydrous Sodium carbonate

Calculations

Molecular weight ¼ 105.989

Gram-equivalent weight ¼ 52.9945 g

Therefore, 1,000 mL of 1 N Na2CO3 contains 52.9945 g

(or) 1,000 mL of 2.04 N Na2CO3 contains 108.1088 g

Procedure

To prepare 250 mL, weigh accurately 27.0272 g of oven-dried desiccator-cooled,

anhydrous Na2CO3 in a clean dry beaker, dissolve in small quantity of distilled water,

and make up the volume to 250 mL with repeated distilled water washings in a

volumetric flask, shake gently and label. It serves as a primary standard for the

preparation of 2.04 N H2SO4.

2.17.1.2



Secondary Standards



Preparation of N/7 H2SO4

Reagents

1. Sulphuric acid (AR) (Sp. gr., 1.84; Purity, 98%)

2. N/7 Na2CO3 solution

3. Methyl orange indicator

Calculations

Molecular weight of H2SO4 ¼ 98.0776; i.e. [(2 Â 1.0080 + 32.064 + 4(15.9994)]



2.17



Preparation of Standard Solution of Acids and Bases



Equivalent weight ¼



21



98:0776

¼ 49:0388

2



Gram-equivalent weight ¼ 49.0388 g

Basing on Sp. gr. ¼ Mass/volume, 49.0388 g of H2SO4 ¼ 49.0388/1.84 ¼ 26.65 mL.

Taking purity as 98%, 26.65 mL of pure H2SO4 will be present in 27.19 mL.

(or) 1,000 mL of N/7 H2SO4 contains 3.885 mL.

Procedure

Add slowly 4 mL of H2SO4 from the side of a beaker containing half the quantity of

required distilled water, with the help of measuring cylinder and make up the

volume to 1,000 mL by transferring the beaker content to a volumetric flask after

cooling and by repeated washings to the beaker. It is standardized against 10 mL of

N/7 Na2CO3 in a conical flask in the presence of 1–2 drops of methyl orange with

the help of a burette. The change of colour from orange–yellow to orange–red will

indicate the quantity of sulphuric acid solution of unknown strength consumed

to neutralize 10 mL of N/7 Na2CO3. Calculate the strength of unknown H2SO4

solution using the equation N1V1 ¼ N2V2 to add either distilled water or concentrated sulphuric acid depending upon the increase or decrease in the strength

of the solution, respectively. Titration are repeated until alteast three consecutive

readings are obtained. The standardized N/7 H2SO4 is then stored in a clean reagent

bottle after labelling.

On the basis of this procedure, sulphuric acid of different normalities can be

prepared as per the requirement.



Preparation of N/7 NaOH

Reagents

1. Sodium hydroxide pellets (AR)

2. N/7 H2SO4

3. Phenolphthalein indicator

Calculations

Molecular weight of NaOH ¼ 39.9972; i.e. (22.9898 + 15.9994 + 1.0080)

Equivalent weight ¼ 39.9972/1 ¼ 39.9972

Gram-equivalent weight ¼ 39.9972 g

Therefore, 1,000 mL of 1 N NaOH contains 39.9973 g

(or) 1,000 mL of N/7 NaOH contains 5.7139 g

Procedure

Weigh around 6 g of NaOH quickly (highly hygroscopic) in a clean dry beaker,

add small quantity of distilled water and transfer the contents to 1 L volumetric



22



2



Expression of Concentration



flask after cooling. Make up the volume by giving repeated washings to beaker.

Mix the contents by gentle shaking. Find the strength of unknown NaOH by

titrating from a burette against 10 mL of N/7 H2SO4 in a conical flask in the

presence of 1–2 drops of phenolphthalein till the contents turn pink. Add calculated quantity of either distilled water or NaOH as per the observed strength till

three consecutive readings are obtained. Store in a clean reagent bottle and label

as N/7 NaOH.



Preparation of N/7 HNO3

Reagents

1. Nitric acid (AR) (Sp. gr., 1.42; Purity, 69%)

2. N/7 NaOH

3. Phenolphthalein indicator

Calculations

Molecular weight of HNO3 ¼ 63.0129; i.e. [1.0080 + 14.0067 + 3(15.9994)]

Equivalent weight ¼ 63.0129/1 ¼ 63.0129

Gram-equivalent weight ¼ 63.0129 g

Basing on Sp. gr. ¼ Mass/volume,

63.0129 g of HNO3 ¼ 63.0129/1.42 ¼ 44.3756 mL

Considering purity as 69%, 44.3753 mL of pure HNO3 will be present in

64.3120 mL

Therefore, 1,000 mL of 1 N HNO3 contains 64.3120 mL

(or) 1,000 mL of N/7 HNO3 contains 9.1874 mL

Procedure

Pour slowly about 10 mL of HNO3 from the sides of the beaker containing half the

quantity of required distilled water with the help of a measuring cylinder and make up

the volume to 1,000 mL by transferring the contents of the beaker after cooling and

with repeated washings. Mix the contents gently and standardize HNO3 solution of

unknown strength by titrating with burette against 10 mL of N/7 NaOH in a conical

flask in the presence of 1–2 drops of phenolphthalein till the contents turn colourless.

After calculating actual strength of the HNO3 solution on hand, add required quantity

of either distilled water or acid accordingly and recheck the strength by getting three

consecutive readings. Store in a clean reagent bottle and label as N/7 HNO3.



Preparation of N/10 KMnO4

Reagents

1. Potassium permanganate crystals (AR)



2.17



Preparation of Standard Solution of Acids and Bases



23



2. N/10 oxalic acid

3. 1:4 H2SO4

Calculations

Molecular weight of KMnO4 ¼ 158.0376; i.e. [39.102 + 54.9380 + 4(15.9994)]

Gram-equivalent weight in acidic medium ¼ 31.6 g (refer equivalent weight for

calculations as already described)

Therefore, 1,000 mL of 1 N KMnO4 contains 31.6 g

(or) 1,000 mL of N/10 KMnO4 contains 3.16 g

Procedure

Dissolve about 3.5 g of KMnO4 in 1,000 mL distilled water in a round bottom flask

and boil the contents for 10–15 min to remove the traces of organic matter, if any

present in the distilled water to avoid reduction of KMnO4 and allow it to stand in

dark coloured (preferably amber coloured) container for a few days. Filter the

contents through glasswool. Titrate KMnO4 of unknown strength with burette

against 10 mL each of N/10 oxalic acid and 1:4 H2SO4 in conical flask, when

contents of the flask are at about 70–80 C (roughly when the first bubble appears

at the time of heating the contents of flask) till a pink colour persists. Find the

strength of unknown KMnO4 and add required quantity of either distilled water

(preferably boiled and cooled) or KMnO4 according to the calculated concentration

of solution in hand. Check the strength to 1/10 normality till three consecutive

readings are obtained and store in a clean dry amber coloured reagent bottle. Label

it as N/10 KMnO4.



Preparation of 2.04 N H2SO4

Reagents

1. Sulphuric acid (AR) (Sp. gr., 1.84; Purity, 98%)

2. 2.04 N Na2CO3

3. Methyl orange indicator

Calculations

Molecular weight of H2SO4 ¼ 98.0776

Equivalent weight ¼ 49.0388

Gram-equivalent weight ¼ 49.0388 g

Basing on Sp. gr. ¼ Mass/volume, 49.0388 g of H2SO4 ¼ 26.65 mL.

Considering 98% purity,

26.65 mL pure H2SO4 will be present in 27.19 mL

Therefore, 1,000 mL of 1 N H2SO4 contains 27.19 mL

(or) 1,000 mL of 2.04 N H2SO4 contains 55.4676 mL.



24



2



Expression of Concentration



Procedure

Gently add 56 mL of H2SO4 from the sides of a beaker containing half the required

quantity of distilled water. Make up the volume to 1,000 mL in a volumetric flask

after cooling by transferring the beaker contents and repeated washings. Standardize against 2.04 N Na2CO3 in the presence of 1–2 drops of methyl orange

(orange–yellow to orange–red) by addition of required amount of either distilled

water or H2SO4 depending upon the concentration of unknown solution till three

consecutive readings are obtained. Store in a clean dry reagent bottle and label as

2.04 N H2SO4.



Preparation of 2.5 N NaOH

Reagents

1. Sodium hydroxide pellets (AR)

2. 2.04 N H2SO4

3. Phenolphthalein indicator

Calculations

Molecular weight of NaOH ¼ 39.9972

Equivalent weight ¼ 39.9972

Gram-equivalent weight ¼ 39.9972 g

Therefore, 1,000 mL of 1 N NaOH contains 39.9972 g

(or) 1,000 mL of 2.50 N NaOH contains 99.993 g

Procedure

Dissolve approximately 100 g of NaOH in half of the required quantity of distilled

water in a beaker and make up the volume to 1,000 mL in a volumetric flask by

transferring the beaker contents after cooling and by repeated washings to the

beaker. Standardize by titrating NaOH solution of approximate strength from a

burette against 10 mL of 2.04 N H2SO4 in conical flask in the presence of 1–2 drops

of phenolphthalein till the pink colour appears. Calculate the strength of unknown

to add required quantities of either distilled water or NaOH depending upon actual

strength. Ten millilitre of 2.04 N H2SO4 should neutralize 8.16 mL of 2.50 N

NaOH. Repeat the titration until three consecutive readings are obtained. Store in a

clean dry reagent bottle and label as 2.50 N NaOH.



2.18

2.18.1



Percentage Solutions

Reagents



1. Nitric acid (AR) (Purity, 67%)

2. Potassium nitrate (AR)



2.19



Ratio Solutions



25



3. Boric acid (AR)

4. Sodium hydroxide

5. Ammonium molybdate (AR)



2.18.2



Preparation



1. 2% HNO3 (v/v):

Calculate the quantity of nitric acid required to prepare desired quantity taking

purity into consideration and mix the acid slowly from the sides of a beaker and

make up the total volume. Cool and store. E.g. To prepare 1,000 mL of 2%

HNO3 take 30 mL of 67% HNO3 and 970 mL of distilled water.

2. 3% KNO3 (w/v):

Mix the required quantity of potassium nitrate in known volume of distilled

water and make up the final volume with the help of a measuring cylinder. E.g.

To prepare 1,000 mL of 3% KNO3, dissolve 30 g in approximately 970 mL of

distilled water to finally measure 1,000 mL.

3. 2% Boric acid (w/v):

Dissolve the calculated amount of boric acid in distilled water by constant stirring

and slight warming, as boric acid is sparingly soluble at room temperature. E.g.

To prepare 1,000 mL, take 20 g boric acid and approximately 980 mL of distilled

water. It is required for preparing Tashiro’s indicator as described earlier.

4. 40% NaOH (w/v):

Dissolve weighed quantity of crude or commercial sodium hydroxide flakes

in ordinary tap water by constant stirring. Store after cooling. E.g. To prepare

1,000 mL, dissolve 400 g of NaOH flakes in approximately 600 mL of tap water.

5. 20% Ammonium molybdate (w/v):

Dissolve the required quantity of ammonium molybdate in equal amounts of

distilled water and liquor ammonia solution by constant stirring. Store in a clean

bottle. E.g. To prepare 100 mL take 20 g of ammonium molybdate and approximately 40 mL each of distilled water and liquor ammonia solution.



2.19



Ratio Solutions



Reagents

1. Sulphuric acid (AR) (Purity, 98%)

2. Hydrochloric acid (AR) (Purity, 35%)

3. Liquor ammonia solution

Preparation

1. 1:4 H2SO4 (v/v):

Add one volume of acid to four volumes of distilled water slowly; cool, label and

store in a bottle. E.g. For 1,000 mL of 1:4 H2SO4, take 200 mL H2SO4 and

800 mL distilled water.



26



2



Expression of Concentration



2. 1:2 HCl (v/v):

Add one volume of acid to two volumes of distilled water slowly; cool, label and

store in bottle. E.g. For 1,000 mL of 1:2 HCl, take 333 mL acid and 666 mL

distilled water.

3. 1:4 Liquor ammonia (v/v):

Mix one volume of liquor ammonia solution to four volumes of distilled water,

label and store in a bottle. Care should be taken to cool liquor ammonia bottle

while opening the cork, especially in summer months, to avoid sudden spurting

of accumulated ammonia gas in the bottle. E.g. To prepare 1,000 mL, take

200 mL of liquor ammonia solution and 800 mL distilled water.



2.20



Titration



The process of bringing a measured volume of a solution of known concentration

into reaction with the desired constituents (or its equivalent of another substance) is

called titration. In other words adding of a solution of known strength to another in

order to complete the reaction is known as titration. Since volume of a solution of

unknown strength is measured, it is also known as volumetric analysis.

1. Titre

The titre is the weight of solute contained in a mL of solution or the weight of

any substance which will react with or equivalent to 1 mL of solution.

As described earlier a solution of accurately known concentration is called as

standard solution, which may be prepared directly (primary standard) or by

standardization (secondary standard) through reaction with a primary standard.

The end point of a titration occurs when chemically equivalent amounts of

reactants are brought together, as indicated by an abrupt change in colour as

shown by an indicator, at the stoichiometric point of the reaction.

2. Stoichiometric point

The stoichiometric point is an equivalence point at which an equivalent of the

reacting substance has been added, irrespective of the type of reaction involved.

3. End point

The end point in an acid–alkali titration is that point at which the titration is

stopped, being shown by the colour change of the particular indicator used.

The suitability or otherwise of an indicator in any given titration depends

upon the pH value at which the indicator shows its specific colour change.

4. Indicator

Indicator is a substance that indicates the physico-chemical status of a reaction.

They may be internal, external or self indicators. They are mostly organic

compounds of high molecular weight. When dissolved in water or any suitable

solvent, they behave either as a weak acid or weak base. Basic indicator possess

a coloured cation and acidic indicator possess coloured anion. The internal

structural rearrangement is responsible for colour change. Important indicators

used in titrimetric analysis are given below (Table 2.5):



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