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Weighing & Preparation of Solutions of Different

strengths & their Dilution

ANDHandling Techniques of

Solutions

Presented by:

Ekta Belwal

HHM-2013

M.Sc FN (I yr.)

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We will discuss..

1. Solutions2. Measuring Chemicals3. Different chemical concentrations4. Dilution of stock solutions5. Labelling6. Lab safety7. Conclusions

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• Solution uniform homogenous mixture of two or more substances i.e, solute and solvent.

Solution= solute + solvent• Standard solution: very precise solution, usually to 3-4 significant

figures, used in quantitative analysis or an analytical procedure.

• Saturated solution: a solution that contains the maximum amount of a particular a solute that will dissolve at that temperature

• Supersaturated solution: a solution that contains more solute than equilibrium condition allow; it is unstable & the solute may precipitate upon slight agitation or addition of single crystal

Solutions

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Preparing Solutions

Solutions of known concentration can be prepared in a number ofdifferent ways depending on the nature of the analyte and/or theconcentration required:

• Weighing out a solid material of known purity, dissolving it in a suitable solvent and diluting to the required volume

• Weighing out a liquid of known purity, dissolving it in a suitable solvent and diluting to the required volume

• Diluting a solution previously prepared in the laboratory

• Diluting a solution from a chemical supplier.

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Weight Measurements

Basic protocol 1: measuring mass using a top-loading Balance1. Turn on balance and wait for display to read 0.0 g.2. Place weighing vessel on the balance pan (e.g., creased weighing

paper, weigh boat)3. Press tare button so that display reads 0.0g.4. Gently add the substance being weighed to the weighing sample.

5. Record mass.6. Remove weighed sample.

7. Clean spills off balance with brush or absorbent laboratory tissue. Discard any disposable weighing vessel.

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Contd...

• Basic protocol 2: measuring Mass using an analytical Balance

1. Turn on balance and wait for display to read 0.0000 g.2. Check the level indicator & do not lean on table while

weighing.3. Place weighing vessel on the balance pan (e.g., creased

weighing paper, weigh boat)4. Close the sliding doors & wait for stability light indicator,

indicating that the weight is stable.5. Press tare button so that display reads 0.0g.6. Gently add the substance being weighed to the weighing

sample.7. Record mass.8. Remove weighed sample.9. Clean spills off balance with brush or absorbent laboratory

tissue. Discard any disposable weighing vessel

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Volume Measurements

Micropipettes

Volumetric or transfer pipettes

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Volumetric Containers

1.Beakers & Erlenmeyer flasks

2. Volumetric flasks

3. Graduated cylinders

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Procedure for preparing a solution of known concentration from a known amount of a solid material

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Procedure for preparing a solution of known concentration by dilution

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Common Practical Units forReporting Concentration

Name Units SymbolMolarity Moles of solute / litres of solution M

Normality Number of EWs solute / Litre of solution N

molality Moles of solute / Kg of solvent m

Weight % g of solute / 100 g of solution % w/w

Volume % mL of solute / 100 mL of solution % v/v

Weight-to-Volume % g of solute / 100 mL of solution % w/v

•Weight per unit volume e.g., g/L, mg/ml

•Parts per million(ppm) or ppb

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1. Molar solutions• Molarity is number of moles of a solute that are dissolved per liter of total

solution.

• A 1 M solution contains 1 mole of solute per liter total volume.

Example:

A 1M solution of H2SO4 contains 98.06 g of sulfuric acid in 1 liter of total solution.

"mole" is an expression of amount

"molarity" is an expression of concentration.

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Contd..• "Millimolar", mM, millimole/L.

– A millimole is 1/1000 of a mole.

• "Micromolar", µM, µmole/L.

– A µmole is 1/1,000,000 of a mole.

HOW MUCH SOLUTE IS NEEDED FOR A SOLUTION OF A PARTICULAR MOLARITY AND

VOLUME?

(g solute ) X (mole) X (L) = g solute needed

1 mole L

or

FW X molarity x volume = g solute needed

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TO MAKE SOLUTION OF GIVEN MOLARITY AND VOLUME

1. Find the FW of the solute, usually from label.

2. Determine the molarity desired.

3. Determine the volume desired.

4. Determine how much solute is necessary by using the formula.

5. Weigh out the amount of solute.

6. Dissolve the solute in less than the desired final volume of solvent.

7. Place the solution in a volumetric flask or graduated cylinder. Add solvent until exactly the required volume is reached, Bring To Volume, BTV.

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2. Normal Solutions• Normality is defined as the gram Eq.Wt. of the

solute per L of the solvent.1N sol. = 1 EW solute / 1L of sol.

• Conc. Of acids and alkalis are usually expressed in this unit.

• gram Eq.Wt. is the M.W divided by the no. of H+ or OH- ions released from 1 molecule of the acid or base, respectively in solutions.Eq. Wt. = MW of the substance / replaceable no. of H+ or OH-

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Example:1N Sulphuric Acid

M.W of H2SO4 = 98 g

Each molecule of acid releases 2 H+ ions in solutions.Eq. Wt. = 98/2

= 49So, 1L of 1N H2SO4 solution contains 49 g of H2SO4

Chemical M.W Eq. Wt. 1N of solution contains

NaOH 40 1 40 g

KOH 56 1 56g

Na2CO3 106 2 53g

HCl 36.45 1 45g

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3. Molal solutions

• Molality expresses the no. of moles per 1000 g or 1 Kg of solvent.

• It is dependent on the density of solvent.• It is different from Molarity as the later refers to

volume of the solution, which is temperature dependent.

• Molal solutions are not usually used in biochemical exp.

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4. Percent solution• Mass percent solutions are defined based on the grams of solute per 100

grams of solution. Example: 20 g of sodium chloride in 100 g of solution is a 20% by mass solution.

• Volume percent solutions are defined as ml of solute per 100 mL of solution. Example: 10 mL of ethyl alcohol + 90 ml of H2O (making approx. 100 mL of solution) is a 10% by volume solution.

• Mass-volume percent solutions are also very common. These solutions are indicated by w/v % & are defined as the grams of solute per 100 mL of solution.Example: 1 g of phenolphthalein in 100 mL of 95% ethyl alcohol is a 1 w/v % solution.

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5. PPM and PPB

ppm: The number of parts of solute per 1 million parts of total solution.

ppb: The number of parts of solute per billion parts of solution.

Example

5 ppm chlorine = 5 g of chlorine in 1 million g of solution,

Or

5 mg chlorine in 1 million mg of solution,

Or

5 pounds of chlorine in 1 million pounds of solution

CONVERSIONS

To convert ppm or ppb to simple weight per volume expressions:

5 ppm chlorine = 5 g chlorine = 5 g chlorine 106 g water 106 mL water

= 5 mg/1 L water

= 5 X 10-6 g chlorine/ 1 mL water

= 5 micrograms/mL

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A COMPARISON OF METHODS OF EXPRESSING THE CONCENTRATION OF ASOLUTE

CONCENTRATION OF SOLUTE AMOUNT OF SOLUTE AMOUNT OF WATER(Na22SO44)

1 M 142.04 g Na2SO4 BTV 1 L with water

1 m 142.04 g Na2SO4 Add 1.00 kg of water

1 N 71.02 g Na2SO4 BTV 1 L with water

1 % 10 g Na2SO4 BTV 1 L with water

1 ppm 1 mg BTV 1 L

CONCENTRATION OF SOLUTE AMOUNT OF SOLUTE AMOUNT OF WATER(Na22SO44)

1 M 142.04 g Na2SO4 BTV 1 L with water

1 m 142.04 g Na2SO4 Add 1.00 kg of water

1 N 71.02 g Na2SO4 BTV 1 L with water

1 % 10 g Na2SO4 BTV 1 L with water

1 ppm 1 mg BTV 1 L

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PREPARING DILUTE SOLUTIONS FROM CONCENTRATED ONES

• Concentrated solution = stock solution• Use this equation to decide how much stock

solution you will need:C1V1=C2V2

Where, C1 = concentration of stock solutionC2 = concentration you want your dilute solution to beV1 = how much stock solution you will needV2 = how much of the dilute solution you want to

make

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EXAMPLE

• How would you prepare 1000 mL of a 1 M solution of Tris buffer from a 3 M stock of Tris buffer?

– The concentrated solution is 3 M, and is C1.

– The volume of stock needed is unknown, ?, and is V1.

– The final concentration required is

1 M, and is C2.

– The final volume required is 1000 mL and is V2.

SUBSTITUTING INTO THE EQUATION:

C1 V1 = C2 V2

3 M (?) 1 M (1000 mL)

? = 333.33 mL

So, take 333.33 mL of the concentrated stock solution and BTV 1 L.

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Preparation of exact 1N HCL• Dilute 100 ml of HCl with water to 1 L. Mix well.

• Prepare exact 1N sol. Of Na2CO3 by dissolving 5.30g anhydrous Na2CO3 in 100 ml H2O.

• Phenolphthalein indicator– Dissolve 250 mg indicator in 50 ml of 50% alcohol.

• Titration– Take 10ml of acid sol & 10ml H2O in a small beaker– Add 2-3 drops of indicator – Titrate with Na2CO3 sol from a 25ml burette till a faint red colour

is obtained– Note the vol.(x ml) of base consumed at the end point

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Contd..

• Calculate the exact normality of the acid by formula– Normality of base X vol. of base = normality of

acid X vol. of acid– So, the normality of HCl = 1x X/10– Normality of base is 1 vol of base is x ml.

• After calculating the exact normality of the acid, it is proportionately diluted with water to obtain to exact 1 normal sol.

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Exact 1N NaOH solution• Eq. Wt. Of NaOH is 40g

So 40g dissolved in 1L of H2O fo approx. 1N sol. & used.

• But for exact normality it is titrated against Oxalic acid sol.(6.3g in 100ml water)

• Take 10ml oxalic acid + 10ml water in a beaker, add 2-3 drops of phenolpthalein indicator. Titrate against the NaOH from a burette till a faint red colour is obtained.

• Calculate exact normality of sodium hydroxide sol as in case of acid & dilute proportionately with water to obtain exact 1N sol.

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• Do not use chemicals from unlabeled containers

• Do not place labels on top of one another.

• Label chemicals clearly and permanently.

Become A Label Fanatic

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You make it- you label it

1. identity of contents2. concentration3. your name4. date of preparation5.Hazard alert (if applicable)

An unlabeled container will become tomorrow’s“Mystery Substance”

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Do NOT × eat, drink or smoke in the

laboratory .× pipette by mouth× leave equipment using

water, gas or electricity on overnight

× Never add water to conc. Sulphuric acid

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ALWAYS  Keep your working area clean

and tidy.

Open bottles near window where ventilation is available.

Handle conc. Acids & liquor Ammonia with care.

label containers & solutions.

secure the tops of reagent bottles immediately after use

wear a lab coat & appropriate eye protection

wash hands after using any substances hazardous to health, on leaving the laboratory.

keep broken glassware & sharps separate from other waste & dispose of in the appropriate containers

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TO ASSURE THE QUALITY OF A SOLUTION

Know the solutions & different conc. to represent them.

Documentation, labeling & recording what was done

TraceabilitySOPs & SPsMaintenance and calibration of instrumentsStability and expiration date recordedProper storage

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ReferencesGallagher Sean R.; A.Wiley Emily; Current protocols, essential laboratory techniques; 2nd Ed. ; Wiley-Blackwell a John Wiley & sons, Inc.

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