‘THE CONCEPT OF VOLUMETRIC ANALYSIS

Volumetric analysis is a quantitative analysis involving the measurement of different solutions ‘These solutions are made to reaet completely and the completion of the reaction is indicated by certain substances called indicators. The quantitative composition of the solution is then determined.

Important steps of volumetric analysis include:

• Weighing,
• Preparation of the solution:
• Titration; and
• Calculation

In volumetric analysis, we deal with volumes of solutions, That is why this quantitative termination of solutions of substances is called volumetric analysis The amount of a substance present in a solution is given in terms of its volume and its concentration, The volume of 2 solution is usually given in litres (dm’). The concentration of a

solution is given in moles per litre (mol/dm*) or grams per litre (g/dm’)

Volumetric analysis is a means of finding the concentration of an unknown solution For

example, the concentration of an unknown solution of an aeid can be found if itis reacted with @

standard solution ofan alkali. A standard solution is one whose concentration is well known and does not change with time. In volumetric analysis, the reaction is carried out in a carefully controlled way. The volumes are measured accurately using a pipette and burette. The method is to add a solution of one reactant to the solution of another reactant until the reaction is complete. When the reaction is complete, we say the end-point has been reached. If the reactants are acids and bases, completion (end-

point) is determined by the change in colour of an acid-base indicator. The method is called titration. In other reactions, completion is determined by a colour change of reactant(s) The concentration of one of the reactant solutions must be known in order to be able to find the concentration of unknown solution

Significance of Volumetric Analysis

1, Volumetric analysis is used to quantify the amount of substances present in solutions by analytical procedure, which involves precise measurements of volumes of solutions and messes of solids

• Volumetne analysis helps in the determination of the accurate volumes and concentrations of the

reacting substances, oflen solutions

• Volumetric analysis (tittation) helps in the preparations of standard solutions
• Volumetric analysis knowledge helps in the standardization of acids and bases.

VOLUMETRIC APPARATUS

Use volumetric apparatus We have seen that volumetric analysis involves determinations of quantities of substances, usually acids and alkalis, present in volumes of solutions. This is usually done by using measuring apparatus.

Apparatus used in volumetric analysis is based on volume measurements and since the analysis demands high accuracy, the apparatus has to be calibrated with the highest possible accuracy. It is for this reason that all apparatus for volumetric analysis are specifically for this and not other purposes.

Apparatus used for volumetric analysis include, burette, pipette, burette stand, white tile, conical flask, filter funnel, reagent bottle, watch glass, beaker, measuring eylinder and measuring flask (or volumetric flask). For approximate measurements, measuring eylinders may be used. For accurate measurements of volumes, volumetric flasks are used Burette This is a long glass tube with a narrow lower part, which is ited with a tap that controls the

amount of solution let out of the burette. This instrument is calibrated from 0 to 50 em’ Before measuring the solution, rinse the burette with distilled water, then with the solution i is going to hold. tt has to be filled to the tip and all gas bubbles removed. Thus, the burette is an apparatus used for transferring the solution tothe titration vessel (normally a flask) Pipette This apparatus has a wider middle part with narrow parts at either ends, The upper narrow part

has a mark which marks the volume of all the space below it. If, say, the pipette is one that is marked 25 em’, we can say that a solution, when filled in the pipette up to this mark, will have @ volume of 25 em?

The pipette is used in transferring a standard solution to the titration flask. There are many types of pipettes depending on their volume capacity. The common ones are the 25-cm'and 20- cm’ capacity pipettes. Less common ones are the 10-cm capacity. Before measuring the solution, rinse the pipette several times with distilled water and then with the solution to he measured, suck the rinsing solution above the graduated mark, then discard the rinsing

The pipette is commonly filled by mouth suction but the use of pipette fillers is highly recommended. When using a pipette, never blow out the last drop.

• A pipette (b) A pipette and pipette filler (used to fll and empty pipettes)

Measuring (Volumetric) flask The flask is made oF glass and has mark at the upper part of the nartow tube. The space in the flask up to this mark represents a certain volume. If a solution is filled up to this mark, the volume of the solution is equal to the volume indicated by inscriptions on the Mask e.g 50 em’, 100 em’, 150 em’, 250 em’, S00 em’, ete Filter funnel A filter funnel is required for effective transfer of the weighed solid, liquid or solution into the

volumetric flask or burete, A filter funnel Wash bottle Wash botile contains water and when squeezed, water squarts out. This is used in washing down the remains of the weighed solid into the volumetric flask A wash bottle A weighing bottle This is used in weighing the solute. It is s stoppered bottle. A watch glass can also be used to serve the same purpose, Retort stand A burette stand is used for holding the hurette in place while carrying out volumetric analysis

experiments, A burette stand Dropper A dropper is used to ad the indicator dropwise into the solution White tile or paper ‘A white tle or piece of paper is placed under the flask to give a clear background for accurate observation of the colour change atthe end of the reaction (end point),

Standard Solutions

‘The Steps for Preparation of Standard Solutions of Common Acids Explain she steps for preparation of standard solutions of common acids A standard solution is a solution of known concentration. For example, a solution containing 15g of sulphuric acid in dm of solution isa standard solution.

It has now been approved that volumetric work should be based upon the molar (M) solution AJ molar (1M) solution ofa compound is a solution which contains one mole of that compound In I dm’ of the solution, For example, 58 5g of sodium chloride (NaC!) dissolved in dim* of the

solution makes 2 molar solution of sodium chloride (IM NaCl). Likewise, 1062 of sodium

carbonate (NazCO3) in 1 dm? of the solution gives a molar solution of sodium carbonate Therefore, a molar sodium carbonate solution contains 106g of the salt in I dm’ of the solution.

1 malar solution of some compounds commonly used in titration contain the following masses of the compounds in 1 dm’ of solution: Compound Relative molecular weight (1 mole) Sodium hydroxide, NaOH 40g Potassium hydroxide, KOH 560 Sulphuric acid, HsSO, 98g" Hydrochloric acid, HCI 365g Sodium carbonate, NaxCO, 106g Sodium bicarbonate, NaHCO; 84g Derivative concentrations are also used e.g, 0.1M, 0.5M. 2M, ete.

Preparation of standard solutions

AA standard solution is required as a starting point for volumetric analysis. We learned early that im order to find the unknown concentration of a substance in volumetric analysis, the concentration of one of the solutions must be known A small range of substances are suitable for direct preparation of accurately standard solutions Substances that cannot be used for direct preparation of standard solutions include sodium

hydroxide, potassium hydroxide and concentrated sulphuric acid. These substances absorb water Vapour from the air and hence cannot be weighed out precisely without taking extra precautions, Apart from absorbing water vapour from the air, sodium and potassium hydroxides react with carbon dioxide of the air tn form respective carbonates 2NaOH + COag)+ NaxCOny * HOH 2KOH a) + COn1g+ KC Os) + HO Some solutions are volatile in nature and so ate Tikely to change slowly in concentration during

ordinary use, These include concentrated hydrochloric acid and ammonia. A compound commonly used for preparation of a precisely standard solution is anhydrous sodium carbonate, It is best prepared from highly pure sodium carbonate. This is achieved by heating sodium bicarbonate to constant mass to make sure the compound is fully decomposed 2NaHICO so) -»NasCOse) + HO + COr) The sodium carbonate so formed is suitable for preparation of a standard solution and ean be

weighed without undergoing any appreciable change in composition. Precautions to be observed while preparing a standard solution

• Transference of the substance from the weighing bottle to the beaker or flasks should he done

with outmost care so that not a single particle ofthe substance is lost

• Undissolved substance should not be transferred to the measuring flask, Make sure all the solid

dissolves into solution before transferring the solution to flask.

• During making up of the volume, the last drop of the water should be added carefully. Do not

bow out the final drop.

Standard Solutions of Bases

Prepare standard solutions of bases

Preparation of 0.1M sodium carbonate solution

‘The molecular mass of sodium carbonate (NayCOs) is 106g. Therefore, 2 molar (1M) solution of sodium carbonate contains 106g in 1 dm* (1000 em") of solution. In order to prepare 01M solution of the carbonate, we have to weigh 10 6g of the carbonate and put i into a volumetric flask, which has a capacity of 1000 em.

However, normally 250 em’ flasks are used. This means, in a 250 em' flask we have to add

‘The same procedure can be followed when preparing 0.25M, 0.5M, 2M, etc. ofthe solutions Procedure

• Weigh exactly 2 65g of sodium carbonate using @ common balance and put it onto a watch glass
• Transfer it slowly into a beaker of 500-cmé capacity containing about SO cm? of hot distilled

# Wash down the watch glass with a jet of hot distilled water from a wash bottle and allow the washings to fall into the beaker (figure $.7). Make sure all the sodium carbonate is washed into the beaker

• Stirwith a glass rod until all the solid is completely dissolved, and then cool the solution to room

temperature, Leave the rod standing in the solution

• Pour the solution carefully down the glass rod into a 250 em’ measuring flask.
• Wash the beaker out at least twice with jets of cold distilled water directed round the slides and

pour the washings down the plass rod into the measuring flask (figure 5.8).

• Shake the flask gently and fill it up with cold distilled water almost to the mark.
• Add more distilled water drop by drop from a pipette until the meniscus is on the graduation

mark (figure 5,9),

• Stopper the measuring flask and shake well. The liquid should then be exactly 0.1M sodium

carbonate solution

Preparation of standard solutions of other bases of different molarities e.g. 0.2M, 0.5M, 1.0M,

2.0M, ete. can be achieved by using the above procedures. The only variable will be the weight of the solids and volume of water as stated early. LWA Ve Filling the washings into the flask ] meniscus ee graduation mark Correct reading of the liquid volume

Acid-base Titration Experiments

Carry out acid-base titration experiments

Preparation of 0.1M sulphuric acid solution

AA standard solution of sulphurie acid cannot be prepared directly because concentrated sulphuric acid is hygroscopic in nature (it tends to absorb water vapour from the air diluting itself) and is never reliably pure. A solution a little above 0.1M is prepared and then standardized and diluted with distilled water to exactly 0.1M.A molar solution of H:SO,contains 98g of pure acid in 1 dm’. Therefore, the 01M acid contains 9.82 of the acid in dm? of the solution "The pure

concentrated acid has a density (concentration) of about 1.8g/em’. So, 9.8g of it occupy about

The preparation of a standard solution of sulphuric acid involves two stages: 1 Diluting 2 concentrated solution of the acid 1o an approximate molarity

• Finding the exact concentration of the acid (standardizing it) by titrating it against a standard

solution of a base (previously prepared).

Dilution of concentrated sulphuric ncid Caution: Make sure you wear safety goggles and gloves before carrying out this experiment Procedure

• Cautiously, because the acid is very eorrasive, take $5 ~ 6.0 em’ of concentrated sulphuric acid

ina small measuring cylinder.

• Pour the acid carefully, with stirring, into a 250-cm’ volumetric flask containing about 100

cm of cold distilled water

• Pour this solution into, say, 700em’ of cold distilled water in a measuring flask of capacity

1000ern?

• Wash out the acid solution remaining in the measuring cylinder with cold distilled water twice

and add the washings into the measuring flask

• Then add distilled water approximately to the mark on the measuring flask, stopper it, and shake

well This should give sulphuric acid of concentration a little above 0.1M. ‘The diluted acid is now standardized with the 0.1M sodium carbonate solution prepared above Determination of the molarity (standardization) of sulphuric acid solution by titrating it against 0.1M sodium carbonate solution The estimation of the concentration of a solution of an acid by reacting the acid with a standard alkali solution is known 2s tivation, The end-point of an acid-base reaction is commonly

determined by using a substance known as an indicator Procedure

• Measure 25 em’ of 0.1M sodium carbonate solution and transfer it into a conical Mask, using

pipette. Add a few drops of meth! orange indicator. This will tum the sodium earbonate solution yellow

• Setup the apparatus as shown in figure 5.10
• Pour the acid into a S0-cm’ burette. Read and note the level ofthe acid in the burete

a i Burette i Burette.§ —— 7 stand Flask a Titration setup

• By-means of a tap at the base of the burett, drip the acid slowly into the conical flask, swirling

the flask continuously until the colour of the liquid in the Mask turns orange. This is the end- ‘point of titration. Record the new level of acid in the burette

• Repeat the titration three to four times, noting the initial and final reading of the burette each

time

• Find the volume of the acid as shown below

Specimen readings Titration Rough titration (Pilot) Tired Titre? Final burette reading, 2420 23.65 23:55 Initial burette reading 0.00 0.00 0.00 Volume of acid added 2420 23.65 23:55 Neglecting the first (rough) trial nan, the average titration is 23.60 cm* Cateutation The first step in caleulating the molarity of any solution from the results of an acid-base titration is to write the equation forthe reaction, From the equation, find the number of reacting moles of

the acid and base NaxCO iat mis H2SO su nt NODSOsn)+ COnasy + HO Now we have the following data

• Volume of acid, Va = 23.60 em’
• Volume of base, Vb = 25.00 em (this is the average amount of the base that was added to the

‘ask in titration)

• Molarity of acid, Ma ="?
• Molarity ofbase, Mb 0.1M
• Number of moles of acid, Na= 1
• Number of moles of base, Nb = 1

The molarity ofthe acid can be calculated from the following general formula VaMa_VbMb

Na Nb

NaxVbx Mb

Ma=——

Vax Nb

23.6×1

The concentration of sulphuric acid is 0.106M. sodium carbonate solution (0.1M), 23.6 em’of the acid must be diluted to 25 cm’, that is, 1.4 cmn’of distilled water must be added to 23.6 cm‘of the acid Remember it was stated carly that in order to prepare 0.1M sulphuric acid solution, you need to dissolve 9.8g of the acid in 1000 cm’ (1 dm') of distilled water. Assume that some of the acid was wasted through spillage and mishandling and that only 920 cm of the acid was left. If, say,

5Scm’of distilled water. This gives exactly 0.1M of the acid. This is the same as saying that, if 23.6 cm’ of the acid were diluted with .4cm‘ of distilled water, then 920cm* of the acid would be tuted with

If for instance, the volume of acid left was, say, 850 cm’, the amount of distilled water to be

In principle, the amount of distilled water to be added is always calculated based on the amount ofthe aid left ae exerted above sodium hydroxide, potassium hydroxide, hydrochloric acid, nitric acd, ete. You may dilute any standardize it by stir procedures, Choice of indicators in acid-base titration We teamed early that the estimation of the concentration of a solution of an acid or base by reaction with a standard alkali or acid solution respectively, is known as titration. The end-point

of an acid-base titration is commonly determined using substances known 2s indicators, which usually portray certain characteristic colours when in alkaline or acid solutions, The indicators in acid-base titrations must be chosen carefully because the choice of an inappropriate indicator would lead to an incorrect result. The choice of an indicator is based on the strength of an acid or base involved in the reaction There are three common indicators which are used in titration experiments involving acids and

bases namely, methyl orange, litmus and phenolphthalein. The other indicators in less common use areas included in the table below. ‘The table shows the colours which cach of these indicators take up in acid or alkaline sotution.

Colour of indicators in acid and alkaline solutions Indicator Colour of indicator ‘acid solution alkaline sotution Methyl orange pink yellow Litmus red blue Phenolphthalein colourless pink Malachite green yellow blue/green Thymol blue red yellow Bromocresol green yellow blue Bromothymol blue yellow blue Indicators suitable for particular types of acid-base reactions are as given in the table below: Indicators suitable for different acid-base reactions

Acid-base titration Example Choice of indicator Strong acidistrong base HySO, and NaOH Aay indicator Weak acid’stong base CH;COOH (ethanoie acid) and KOH Phenolphthalein Strong acid/weak base HCI and NH ‘Methyl orange Weak acid’weak base CH;COOH and NH No satisfactory indicator available

Volumterie Calculations

Common Mineral Acids

‘Standardize common mineral acids Data for calculations of volumetric analysis problems are obtained ffom volumetic analysis experiments. For any volumetric analysis problem, at least one standard solution is required. A.

correctly balanced reaction equation (from which moles ratios can be derived) is a prerequisite for all these calculations. This is because the mole ratio 1s an integral part of the general expressionused for all volumetric analysis calculations, The general expression is given by

VM, Ny, Vebls Vals

V,M, N, Ny, N,

where;

• Va= Volume of acid
• Ma= Molarty of acid
• Va=Volume of base
• Mp =Molaity of base
• N= Number of moles of acid
• Np=Number of moles ofbase

The Relative Atomie Mass of Unknown Element in an Acid or Alkali Find the relative atomic mass of unknown element in an acid or alkat

Example 1

12.5 cm’ of 0.5M sulphuric acid neutralized 50 cm’ of a given solution of sodium hydroxide What is the molarity ofthe alkali?

Solution

Reaction equation is HySO dng) + 2NAOHy) —*Ne2SO ay + 21209) From the equation, 1 mole of sulphuric acid solution reacts with 2 moles of sodium hydroxide

solution. So, the number of moles of the acid, Na~ 1 and the number of moles of base, Nu~

2.The other data areas follows:

ee 5 VM, Ny

¥,M, N, 2.5×0.5 2×12.5×0.5 125405 1 nd Hig OS. 95 30xMp 2 30

Example2

20 em? of a solution containing 7g/dm’ of a metal hydroxide, XO, were exactly neutralized with 25 em’ of 010M hydrochloric acid

• Write « balanced chemical equation for the neutralization of the metal hydroxide, XOH, with

hydrochloric acid.

• Caleulate the concentration of the metal hydroxide in moles per de
• (Calculate the molar mass of NOH (i) Kdentfy element X

Solution

<l-[endif]->HCliay) + XOHiaay> XClag) + HsQqy

VM, _N, V,M, N; 25×0.1 _1 20xM, 1 25×0.1 M;= oe

sein tN dine

‘Molarity(mol / dm) _ gdm? 0.125mol /dm*

Therefore, element X is potassium (K) The Percentage Purity of an Acid oF an Alkali

Example3

5.1g of impure sodium carbonate solution was dissolved in water to make 500 cm’ of solution 20 cm® of this solution was titrated against 20.45 cm’ of 0.04M hydrochloric acid, Calculate the pereeniage purity ofthe sodium carbonate solid

Solution

’Reaction equation is: NaC + HCl) ~* 2NAChig) + HO + COng)

Given.

VM, _ Ny _ 2045×0.04_2

V,M, N, 20xM, 1

20.45% 0.04 x1 ia) ext 20×2

To calculate the concentration of pure base (Na:COs solution) in g dm”: Concentration = Molartty » Molar mass

Concentration of ure sample Pesceneae pacity Ona pemo nang ng Concentration of impure sample 2.1677

The Number of Molecules of Water of Crystalliraton of Substance Find the number of molecules of water of crystallization of a substance Beample4 0.465¢ of a hydrated form of sodium carbonate exactly reacts with 75 cm? of 0.10M Ijdrochlri acd Caleuiate the umber of molecules of water of erytalliztion presen in one mole of the hydrated salt

Solution

NaxCOy + 2HClay) > 2NaChac) + CO) + H2Ogy

Moles of acid N,_2 pug Molevas acl _ Na 2 Moles of base Ny 1 0.0075 2 Moles of base 1 0.0075

Now, let the formula of the hydrated salt be NayCOs.XH2O, where X is the number of moles of water erystallization Then 2420. _ Mass of water of erstalization Na,CO, ‘Mass of anhydrous salt 18x _ 0.0675 106 0.3975 yx 106x 0.0675 11803975 Therefore, the formula of the salt is NaxCOz H20

Application of Volumetric Analysis

Explain the application of volumetric analysis in eal life situations Volumetric analysis has a variety of laboratory and industrial applications in everyday life. The following are just afew ofthe applications (uses) of volumetric analysis in daly ie known solution) i added to a known quantity of analyte (unknown solution) and a reation takes unknown substance

• Use im environmental and water safety: Titration is important in enviconmental chemistry.

where scientists ean use it to analyze acid rain or contaminants in surface water samples, Environmental studies usually involve an analysis of precipitation and its response to pollution.

To quantify the degree of contamination in natural rainwater or snow, titration is used. The process is quick and results are reliable Since most titration processes do not require expensive ‘or specialized equipment, the test ean be performed often and in different areas with relatively litle effort.The safety of water is ased on its chemical ingredients, By analyzing wastewater tho extent of contamination and the requirements for filtering and cleaning can be determined.

Titration is a key mechanism in this analysis. Often, more specialized titration equipment is used in this application, which measures ammonia levels im combination with other reactants to ‘quantify other chemicals present

• Use im food and beverage industry: In the food and beverage industry, manufacturers must

ensure their products moct certain quality criteria or contain standard concentrations of specific additives, so titration is often used to analyze the products before sale. Wine is often affected by its degree of acidity. It 1s possible to improve wine production by measuring acidity using titration Simple, inexpensive titration kits are available to winemakers for this purpose. The results of a titration test on wine can suggest if additional ingredients are necessary to maintain

its quality.In general, all brewing industries and distilleries apply the knowledge of volumetric analysis (titration) to determine the acidity and alcohol contents of their beers and other alcoholic beverages The process also finds ample use in food industry. The compounds which make up food products help determine their nutritional implications. Titration is one technique that assists in these studies. The acidity of orange juice, for example, is easily determined using a standard

titration process. In this process, an electrode is added to a solution made up of orange juice and deionized water The titrant catalyst then measures the acidity of the juice. Manufacturers can use the technique to vary this quality to satisfy customers or those with special nutritional needs.

• Use im agriculture: Volumetric analysis technique 1s used to determine the soil pH. This is

important because, if the pH of a certain soil is found to be extremely low or high, comtective measures are taken by adding the correct quantity of agrieultural limes or other chemicals to make the soil suitable for plant growth. The method is also used by agronomists and farmers to analyse the kind and amount of plant nutrient elements present in a particular sample of sol, the knowledge of which helps determine soil fertility

INDUSTRIAL AND LABORATORY SKILLS OF VOLUMETRIC ANALYSIS

Compare industrial and laboratory skills of volumetric analysis The knowledge of volumetric analysis (titration) is used in hospitals and medical laboratories to carry out such duties as preparation of solutions and suspensions, blood analysis, and diagnosis of certain diseases and health problems. For example, when dissolving a solid drug to make a

solution for mjection, uimost precision is required to measure the correct volume of liquid to be

used to dissolve a correct amount of solid drug to prepare the solution of a given concentration to inject to a patient Also titration is very important in the pharmaceutical industry, where precise measurements of ‘quantities and concentrations are essential throughout the manufacturing process. Titration is thus an important part of the pharmaceutical industry to ensure quality control. Many variations of the titration technique are used, and specialized equipment for pharmaceutical titration is often

developed to make the process more efficient