higher physics topical investigation skin prevention and ...€¦ · national 5 chemistry practical...

Higher Physics Topical Investigation Skin Cancer—Prevention and Cure

2

Investigation Brief Suntan creams stop harmful UV radiation

reaching the skin. Manufacturers’ products

are rated with a Sun Protection Factor

(SPF). Suntan creams can have SPF values

from 6 to over 50.

UV radiation monitors normally measure

National 5 Chemistry

Chemical Analysis

Teacher/Technician Guide


Practical Assignment Chemical Analysis: Teacher/Technician

Contents

Teacher’s Guide

Page 3 Introduction and background

Page 4 Investigation A1 – “Calcium analysis of water”

Page 12 Investigation A2 – “Calcium analysis of milk”

Page 16 Investigation B – “Iron in tea and cereals”

Page 21 Investigation C – “Chloride in seawater”

Technicians’ guide

Page 25 Investigation A – calcium investigations

Page 28 Investigation B – “Iron in tea and cereals”

Page 30 Investigation C – “Chloride in seawater”

Page 32 Risk assessments



Introduction and Background

The ability to know the precise composition of a

substance is always going to be important: whether

it be finding the percentage of metal in an ore to

see if it is suitable for mining or analyzing the level of

pollutants in drinking water to ensure public health.

These days most of the analysis is highly automated using complex (and very

expensive) equipment such as mass spectrometers or gas chromatographs which

are out of the reach of most schools.

Traditional analytical techniques still have their place though, especially in the field

as a way of getting initial values before taking samples back for further analysis.

Additionally, the analyses are a useful showcase for some good chemical

techniques.

These activities can be used to:

• provide evidence for the National 5 Chemistry assignment

• provide an opportunity for learners to become familiar with the use of

titrations as an analytical technique. The data produced can be used to

provide a context within which learners can practice calculations involving

the mole and balanced equations.

A simple introduction is given for each of the

experiments in the candidate guide. This sets the

scene for each experiment and contains very limited

background information.

Prior knowledge of redox and compleximetric titrations is not required at National 5

level. To make the chemistry accessible, each titration reaction is represented by a

simplified balanced equation. This allows candidates to access interesting

experimental chemistry at a level appropriate to National 5.

Before starting these activities it would be very useful if candidates had experience

of doing titrations.

Where does it fit?

The Chemistry

Why is Chemical analysis

topical?



Investigation A1 - “Calcium in water”

Background

Drinking water contains small amounts of salts and minerals dissolved from rocks that

the water has passed through. Across Britain there is considerable variation in the

concentration of different ions present in tap water.

Calcium ions, Ca2+, in drinking water can supplement the calcium in our diet and may

be beneficial to our health. Some popular bottled waters are advertised as being high

in dissolved minerals.

In high concentrations, Ca2+ ions can be a cause of “water hardness”. Hard water is

not a health hazard but can form an unpleasant scum with soap as well as causing

washing machines, irons and heating boilers to break down. The determination of

water hardness is a useful test that provides a measure of quality of water for

households and industrial uses. Originally, water hardness was defined as the

measure of the capacity of the water to precipitate soap. Soap scum is formed when

the calcium ion binds with the soap. This causes an insoluble compound that

precipitates to form the scum you see. Soap actually softens hard water by removing

the Ca2+ ions from the water.

The concentration of calcium ions can be measured by titrating a sample of water

using a chemical known as EDTA.

Ca2+ + Na2C10H14N2O8 Ca C10H14N2O8 + 2Na+

calcium ion EDTA calcium compound sodium ions

The calcium ion concentration can be determined by titration with a chelating agent,

ethylenediaminetetraacetic acid (EDTA), usually in the form of disodium salt. The

titration reaction is:



The Ca2+(aq) ion is determined at a high pH, by adding NaOH solution to precipitate

any Mg2+(aq) ions present in the water as Mg(OH)2(s).

Murexide indicator is used which changes from pink to purple when the endpoint is reached. As the titre of EDTA is directly proportional to the concentration of calcium ions, candidates can compare the calcium ion levels in different samples without the need to carry out concentration calculations. Titre values can be used to rank the samples in order of increasing calcium ion concentration. This can be compared with the order found using literature/internet data.

Possible Investigations

There is a variety of different factors candidates can investigate. For instance,

the calcium content can be compared:

• in different brands of mineral water

• in water samples from around the UK*

• in samples passed through different domestic water filters

*Details on how to prepare simulated water samples from different

UK locations are provided in the technician guide.



Media Items

1. A video providing an explanation of how calcium ions enter water supplies.

https://www.youtube.com/watch?v=ebygQes5Wig

2. Scottish water have a very useful breakdown of calcium content of water across

the country.

http://www.scottishwater.co.uk/-/media/Domestic/Files/You-and-Your-

Home/Water-Quality/ScottishWaterHardnessData2015.pdf?la=en

3. Map of England showing calcium carbonate levels

http://www.dwi.gov.uk/consumers/advice-leaflets/hardness_map.pdf

4. Information on EDTA titration of calcium ions

http://www.titrations.info/EDTA-titration-calcium

5. World Health Organisation document about calcium in drinking water

http://www.who.int/water_sanitation_health/dwq/chemicals/hardness.pdf

6. The average calcium content (along with other minerals) for the different bottled

waters that they sell are provided on supermarket websites such as Tesco,

Sainsburys etc.

7. How water filters work

http://www.explainthatstuff.com/howwaterfilterswork.html

https://www.youtube.com/watch?v=ebygQes5Wig

http://www.scottishwater.co.uk/-/media/Domestic/Files/You-and-Your-Home/Water-Quality/ScottishWaterHardnessData2015.pdf?la=en

http://www.scottishwater.co.uk/-/media/Domestic/Files/You-and-Your-Home/Water-Quality/ScottishWaterHardnessData2015.pdf?la=en

http://www.dwi.gov.uk/consumers/advice-leaflets/hardness_map.pdf


http://www.who.int/water_sanitation_health/dwq/chemicals/hardness.pdf

http://www.explainthatstuff.com/howwaterfilterswork.html



The Experiment

You will need

0·01 mol l-1 EDTA solution (if your water

sample is very pure, you may need to

use a 0.001 mol l-1 solution)

Murexide indicator

1 mol l-1 sodium hydroxide solution

(NaOH)

Funnel

Clamp and stand 3 cm3 dropper or 5/10 cm3 measuring

cylinder

50 cm3 burette 25 cm3 pipette and safety filler

100 cm3 conical flask

See technician’s guide for details of the reagents. * In the Pupil booklet it suggests 0.01 mol l-1 EDTA solution but for water samples very low in calcium, you may need to use 0·001 mol l-1 EDTA solution. ** A solution will give greater consistency of colour intensity but it is easier to use the powder ground with sodium/potassium chloride added on a spatula tip – this gets around any issues of stability – though that is not much of a problem with murexide.



Method

1. Using the funnel, fill a 50 cm3 burette with 0·01 mol l-1 EDTA solution, making

sure the tip is full and free of air bubbles.

2. Using a pipette, add 25·0 cm3 of your water sample into a 100 cm3 conical

flask.

3. Add 2 cm3 of 1 mol l-1 sodium hydroxide to the flask using a dropper or a small

measuring cylinder.

4. Add a spatula tip of murexide indicator powder

5. Remove the funnel from the top of the burette and note the reading on the

burette.

6. Titrate the water sample using the 0·01 mol l-1 EDTA solution until the colour

changes from pink to purple and then read the burette to the nearest 0·1 cm3.

7. Repeat the titration until your titres agree to within 0·2 cm3.



Extension

Total Hardness Determination.

As was mentioned in the introduction, permanent hardness of water is due to the

presence of calcium and/or magnesium ions in the water; almost always both but in

varying proportions.

The total hardness, therefore is a combination of the two concentrations. It may be of

interest to compare two water samples of equal ‘hardness’ to see if they are actually

the same, or indeed to see if two samples with the same calcium concentration have

the same level of ‘hardness’.

To do this, we need to determine the amount of magnesium in the water. It is not

possible (or at least not straightforward) to do this directly but it is fairly easy to

determine the total hardness. Subtracting the calcium hardness from this will give

the concentration of magnesium.

For the calcium determination, the pH of the solution is raised to pH 12 or above.

This causes the magnesium salts to precipitate out as insoluble magnesium

hydroxide.

The total hardness titration is carried out at a lower pH, about pH 10 produced by an

ammonia buffer, and using a different indicator, Eriochrome black T (aka solochrome

black)

Preparation

Ammonia Buffer

1. Dissolve 17·5g of ammonium chloride (NH4Cl) in 142 cm3 of concentrated

ammonia (0·880).

2. Dilute to 250 cm3 with distilled water.

Eriochrome Black T preparation

The easiest method is for the Eriochrome Black T to be ground with potassium or sodium chloride as described for the murexide in the calcium titration. A spatula-tip of the powder can then be added.

If a liquid indicator is desired, for instance to ensure a consistent colour intensity, it can be prepared as follows:

1. Put on gloves and protective eyewear and weigh out approximately 0·5 g of solid

Eriochrome Black T, (EBT) on a balance and transfer it to a small beaker or flask.

Add about 50 cm3 of 95 percent ethanol (IDA) and swirl the mixture until the EBT

has fully dissolved.



2. Weigh out 4·5 g of hydroxylamine hydrochloride on a balance and transfer it to

the beaker or flask containing the EBT. Swirl until the hydroxylamine

hydrochloride has fully dissolved.

3. Transfer the solution containing the EBT and hydroxylamine hydrochloride to a

100 cm3 graduated cylinder. Add enough 95 percent ethanol (IDA) to bring the

total volume to exactly 100 cm3

4. Transfer the EBT solution from the 100 cm3 graduated cylinder to a dropper

bottle and label the bottle "0·5% Eriochrome Black T in ethanol."

Tips & Warnings

• EBT indicator solutions typically exhibit very short shelf lives. Always prepare

a fresh EBT solution when performing complexometric titrations.

• Hydroxylamine hydrochloride is highly toxic and corrosive to skin and mucous

membranes. Avoid direct skin contact. Wear rubber gloves and protective

eyewear at all times when handling this compound.

• Ethanol is flammable. Avoid working near open flames or other possible

sources of ignition.

You will need

0·01 mol l-1 EDTA solution (if your water

sample is very pure, you may need to

use a 0.001 mol l-1 solution)

Murexide indicator


(NaOH)

Funnel


cylinder


100 cm3 conical flask

Method

1. Fill a 50 cm3 burette with 0·01 mol l-1 EDTA solution, making sure the tip is

full and free of air bubbles.

2. Add 25·0 cm3 of an unknown hard water solution into a 100 cm3 beaker.



3. Add 5 cm3 of ammonia buffer to the beaker.

4. Add 0·5 cm3 of Eriochrome Black T indicator.

5. Titrate with the 0·01 M EDTA until the colour changes from wine red to pure

blue. Read burette to +/- 0·1 cm3.

6. Repeat the titration until the final volumes agree to +/- 0·2 cm3.



Investigation A2 - “Calcium in milk”

Introduction

Milk, and other dairy produce are extremely important sources of calcium in the diet. It is very important for:

helping build strong bones and teeth

regulating muscle contractions, including heartbeat

making sure blood clots normally

A lack of calcium could lead to a condition called rickets in children and osteomalacia or osteoporosis in later life.

The same technique as for water analysis, EDTA titration, can be used to determine the concentration of calcium in milk, though using a higher concentration of EDTA to reflect the higher concentration of calcium.

As the titre of EDTA is directly proportional to the concentration of calcium ions, candidates can compare the calcium ion levels in different samples without the need to carry out concentration calculations. Titre values can be used to rank the samples in order of increasing calcium ion concentration. This can be compared with the order found using literature/internet data.

Possible investigations

There is a variety of different factors candidates can investigate..

For instance,

the calcium content can be compared:

• in milks from different sources (cow, goat, soya etc)

• in treated milk (skimmed, homogenized, semi-skimmed, UHT etc)

• in baby milks

• in powdered milks

Media Items



1. A table of values for calcium in various milks and other foods

https://www.iofbonehealth.org/osteoporosis-musculoskeletal-

disorders/osteoporosis/prevention/calcium/calcium-content-common-foods

2. A leaflet from the British Dietitians Association that gives information about milk

and its dietary importance.

https://www.bda.uk.com/foodfacts/Calcium.pdf

3. The average calcium content (along with other minerals) for the different bottled

waters that they sell are provided on supermarket websites, such as Tesco,

Sainsburys etc.

4. Detailed data about the nutritional content can be found in tables from Public

Health England, here.

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/416

932/McCance___Widdowson_s_Composition_of_Foods_Integrated_Dataset.xlsx

(This gives you a large dataset in spreadsheet form – Open the tab (on the

bottom) labelled inorganics. All sorts of mineral values are given, including

calcium)

5. A BBC Good Food article on milk and nutrition

https://www.bbcgoodfood.com/howto/guide/which-milk-right-you

6. Information on EDTA titration of calcium ions


The Experiment

You will need

0·1 mol l-1 EDTA solution Murexide indicator


(NaOH)

Funnel


cylinder


100 cm3 conical flask 100 cm3 measuring cylinder

Distilled water White tile

https://www.iofbonehealth.org/osteoporosis-musculoskeletal-disorders/osteoporosis/prevention/calcium/calcium-content-common-foods

https://www.iofbonehealth.org/osteoporosis-musculoskeletal-disorders/osteoporosis/prevention/calcium/calcium-content-common-foods

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/416932/McCance___Widdowson_s_Composition_of_Foods_Integrated_Dataset.xlsx


https://www.bbcgoodfood.com/howto/guide/which-milk-right-you




Preparation

A convenient way to use murexide indicator is by trituration.

A small amount of indicator, 0·1 g, is ground in a pestle and mortar with 20 g of

potassium (or sodium) chloride until it is fully mixed. A spatula tip of the powder can

then be added to the solution to titrate.

Method

1. Using a funnel, fill the burette with 0·1 mol l-1 EDTA solution, making sure the

tip is full and free of air bubbles.

2. Using a pipette, add 10·0 cm3 of milk to the 100 cm3 conical flask.

3. Using the measuring cylinder, add 40 cm3 of distilled water to the flask.

4. Add 5 cm3 of 1 mol l-1 sodium hydroxide using a 3 cm3 Pasteur pipette or a

small measuring cylinder.

5. Add a spatula tip of murexide indicator powder.


burette.

7. Titrate with the 0·1 mol l-1 EDTA until the colour changes from ‘salmon’ pink to

‘orchid’ purple*. Read the burette to the nearest 0·1 cm3.

8. Repeat the titration until the titres agree to within 0·2 cm3.

* The colour change is not as clear as it is for water samples but is still clear enough to see.



Investigation B – “Analysis of Iron in foods”

Introduction

In this experiment the sample is dissolved in nitric acid which oxidises the iron to the

ferric-state, Fe3+. Addition of excess iodide under mildly acidic conditions results in

quantitative iron reduction to the ferrous-state, Fe2+, and simultaneous oxidation of

the iodide to iodine.

2Fe3+

+ 2Iˉ 2Fe2+

+ I2

Iodine produced in the iron reduction is titrated with standard thiosulfate to a starch end-point.

I2 + 2S2O32-

2Iˉ + S4O62-

If students are simply comparing the levels of calcium in different samples, as long

as the same sodium thiosulfate solution is used in each experiment its concentration

does not need to be accurately known so it can be simply taken as made up.

If, however, you wish to use this experiment to determine actual concentrations of

calcium ions, for example with Higher or AH students, as sodium thiosulphate is not

a primary standard it will have to be standardised before use. This can be done by

using your thiosulphate solution to titrate the iodine produced when an unmeasured

excess of potassium iodide is added to a known volume of an acidified standard

potassium iodate solution (iodate is a primary standard). The amount of iodine is

known and thus the concentration of thiosulphate can be determined.

There are a few foods that will work using this method but we have only tested tea

and breakfast cereal – there is no reason, however, why other readily ‘ashable’

foods could not be chosen too.

From a practical point of view, it might be preferable if the ‘ashed’ samples are

prepared by technicians though there is no specific reason why pupils should not

carry this out if it is wished.

As the titre of thiosulfate is directly proportional to the mass of iron present in each sample, candidates can use their titre values to rank the foods without having to calculate the mass of iron present. This can be compared with the order found using literature/internet data.




There is a variety of different factors you can investigate. For instance: Iron levels

could be determined

• in different types of tea

• in teas from different countries

• in breakfast cereals made from different crops (wheat, oat, corn or rice)

• in organic, branded or own-label products

Media Items

1. Information from the NHS about iron in the diet.

http://www.nhs.uk/Conditions/vitamins-minerals/Pages/Iron.aspx

2. Detailed data about the nutritional content can be found in tables from Public

Health England, here.

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/41

6932/McCance___Widdowson_s_Composition_of_Foods_Integrated_Dataset.xls

x

(This gives you a large dataset in spreadsheet form – Open the tab (on the

bottom) labelled inorganics. All sorts of mineral values are given, including iron)

3. Research paper with data on iron (and other mineral content) of various teas.

http://www.agriculturejournals.cz/publicFiles/50276.pdf

4. Information on iron intake and content in various foods

http://www.uhs.nhs.uk/Media/Controlleddocuments/Patientinformation/Digestiona

ndurinaryhealth/Adviceforimprovingyourironintake-patientinformation.pdf

5. SSERC documents about iron and manganese in tea, including some sample

data

http://www.sserc.org.uk/advanced-higher-revised/3069-iron-and-manganese-in-

tea

6. Information on iodometric titrations

http://www.titrations.info/iodometric-titration

http://www.nhs.uk/Conditions/vitamins-minerals/Pages/Iron.aspx




http://www.agriculturejournals.cz/publicFiles/50276.pdf

http://www.uhs.nhs.uk/Media/Controlleddocuments/Patientinformation/Digestionandurinaryhealth/Adviceforimprovingyourironintake-patientinformation.pdf

http://www.uhs.nhs.uk/Media/Controlleddocuments/Patientinformation/Digestionandurinaryhealth/Adviceforimprovingyourironintake-patientinformation.pdf

http://www.sserc.org.uk/advanced-higher-revised/3069-iron-and-manganese-in-tea

http://www.sserc.org.uk/advanced-higher-revised/3069-iron-and-manganese-in-tea

http://www.titrations.info/iodometric-titration



7. RSC Classic Chemistry Experiments – Iron in breakfast cereal

http://www.rsc.org/learn-

chemistry/resource/download/res00002108/cmp00000462/pdf

8. A Times of India article on the prevalence of iron filings in tea.

http://timesofindia.indiatimes.com/city/pune/Zero-iron-filings-in-tea-powder-is-not-

possible/articleshow/18600259.cms

The experiment

You will need

Preparing the solution

Sample of food or tea 2 mol l-1 nitric acid solution

Access to a balance (2dp) crucible

Bunsen burner, tripod and pipe-clay

triangle

100 cm3 beaker

25 or 100 cm3 measuring cylinder 50 cm3 volumetric flask

Funnel and filter paper

The titration

20 cm3 pipette and safety filler 100 cm3 flask

funnel 0·01 mol l-1 sodium thiosulfate solution

1% starch solution burette and stand

Dropper (for adding starch) white tile

Method


1. Accurately weigh about 2·0 g of tea/breakfast cereal into a crucible and roast

it in a fume cupboard for several minutes until all the tea has turned to ash

and no more smoke is coming off.

A significant amount of smoke is likely to be produced – It may be that the

technician will prepare the extracts (or at least do the burning). If the pupils

are doing it then there will need to be good ventilation or use of a fume

cupboard.

http://www.rsc.org/learn-chemistry/resource/download/res00002108/cmp00000462/pdf

http://www.rsc.org/learn-chemistry/resource/download/res00002108/cmp00000462/pdf

http://timesofindia.indiatimes.com/city/pune/Zero-iron-filings-in-tea-powder-is-not-possible/articleshow/18600259.cms

http://timesofindia.indiatimes.com/city/pune/Zero-iron-filings-in-tea-powder-is-not-possible/articleshow/18600259.cms



2. Allow the ash to cool and wash it into a beaker using 2 mol l-1 nitric acid.

[CORROSIVE]

3. Add a further 20 cm3 of 2 mol l-1 nitric acid [CORROSIVE] is added and boil

the mixture for 5 minutes.

4. Let the mixture cool again and then filter it (to make sure any unburned

carbon that could possibly remain in the mixture and affect the result is

removed).

5. Place the filtrate is then placed in a 50 cm3 standard flask and made up to the

mark using distilled water.

The titration

1. Using a funnel, fill the burette with 0·01 mol l-1 sodium thiosulfate solution,

making sure the tip is full and free of air bubbles.

2. Using a pipette and safety filler, add 20·0 cm3 of the food extract to a conical

flask.

3. Add 1·0 g of potassium iodide. The solution should now go brown.


burette.

5. Titrate the solution in the conical flask using the 0·01 mol l-1 sodium thiosulfate

in the burette.

6. When the yellow colour has almost gone, add 1 cm3 of starch solution to

produce a dark blue/black solution.

7. Continue titrating until the solution goes clear and colourless (and remains

clear and colourless for at least 1 minute). Read the burette to the nearest 0·1

cm3.

8. Repeat the titration until the titres agree to within 0·2 cm3.



Investigation C – “Chloride in sea water”

Introduction

This method determines the chloride ion concentration of a solution by titration with

silver nitrate. As the silver nitrate solution is slowly added, a precipitate of silver

chloride forms.

Ag+(aq) + Clˉ(aq) → AgCl(s)

The end point of the titration occurs when all the chloride ions are precipitated. Then

additional silver ions react with the chromate ions of the indicator, potassium

chromate, to form a red-brown precipitate of silver chromate.

2 Ag+(aq) + CrO42–(aq) → Ag2CrO4(s)

This method can be used to determine the chloride ion concentration of water

samples from many sources.

As the titre of silver nitrate is directly proportional to the concentration of chloride ions, candidates can compare the chloride ion levels in different samples without the need to carry out concentration calculations. Titre values can be used to rank the samples in order of increasing chloride ion concentration. This can be compared with the order found using literature/internet data.


There is a variety of different factors you can investigate. For instance: The level of

chloride ions could be determined:

• In samples of water from different seas

• In water sampled at different points in an estuary

Media Items

1. A simple explanation of the oceans’ salinity

http://oceanservice.noaa.gov/facts/whysalty.html

2. Average composition of seawater and salinity of various seas.

http://dardel.info/IX/other_info/sea_water.html

3. A list of the salinity of various bodies of water.

https://en.wikipedia.org/wiki/List_of_bodies_of_water_by_salinity

http://oceanservice.noaa.gov/facts/whysalty.html

http://dardel.info/IX/other_info/sea_water.html

https://en.wikipedia.org/wiki/List_of_bodies_of_water_by_salinity



4. How salinity varies as you travel up an estuary.

https://books.google.co.uk/books?id=pXwSDAAAQBAJ&pg=PA8&lpg=PA8&dq=

varying+salinity+in+forth+estuary&source=bl&ots=HSH0dWU7ok&sig=yZZZOYf

cHV5VSEL2vEVfaaOXIYs&hl=en&sa=X&ved=0ahUKEwj91YLp8sbTAhVpBsAK

HYZ0BU8Q6AEIXzAI#v=onepage&q=varying%20salinity%20in%20forth%20est

uary&f=false

5. A guide to the Mohr method for determination of chlorides

http://www.titrations.info/precipitation-titration-argentometry-chlorides-Mohr

6. A World Health Organisation about chlorides in drinking water.

http://www.who.int/water_sanitation_health/dwq/chloride.pdf

The Experiment

Equipment Needed

Preparing dilute samples of seawater

20 cm3 pipette and safety filler 100 cm3 volumetric flask

Titration

diluted sea water sample 250 cm3 conical flasks

10 cm3 and 100 cm3 measuring

cylinders

0·1 mol l-1 silver nitrate

1 mol l-1 potassium chromate indicator burette and stand

white tile funnel

Solutions Needed

Silver nitrate solution: (0·1 mol l−1)

• If possible, dry 5·0 g of AgNO3 for 2 hours at 100°C and allow to cool.

• Accurately weigh about 4·25 g of solid AgNO3 and dissolve it in 250 cm3 of

distilled water in a conical flask.

• Store the solution in a brown bottle.

Potassium chromate indicator solution: (approximately 0·25 mol l-1)

• Dissolve 1·0 g of K2CrO4 in 20 cm3 distilled water.

https://books.google.co.uk/books?id=pXwSDAAAQBAJ&pg=PA8&lpg=PA8&dq=varying+salinity+in+forth+estuary&source=bl&ots=HSH0dWU7ok&sig=yZZZOYfcHV5VSEL2vEVfaaOXIYs&hl=en&sa=X&ved=0ahUKEwj91YLp8sbTAhVpBsAKHYZ0BU8Q6AEIXzAI#v=onepage&q=varying%20salinity%20in%20forth%20estuary&f=false





http://www.titrations.info/precipitation-titration-argentometry-chlorides-Mohr



Sample Preparation

If the seawater contains traces of solid matter such as sand or seaweed, it must

be filtered before use.

Dilute seawater by pipetting a 20 cm3 sample into a 100 cm3 volumetric flask and making it up to the mark with distilled water.

Titration

1. Pipette a 10·0 cm3 aliquot of diluted seawater into a conical flask and add

about 50 cm3 distilled water and 1 cm3 of chromate indicator

2. Titrate the sample with 0·1 mol l-1 silver nitrate solution. Although the silver

chloride that forms is a white precipitate, the chromate indicator initially gives

the cloudy solution a faint lemon-yellow colour. Before the addition of any

silver nitrate the chromate indicator gives the clear solution a lemon-yellow

colour.

3. The endpoint of the titration is identified as the first appearance of a red-

brown colour of silver chromate

4. Repeat the titration with further aliquots of diluted seawater until concordant

results (titres agreeing within 0·2 cm3) are obtained.



Additional Notes

1. This titration should be carried out under conditions of pH 6·5 – 9·0. At higher pH

silver ions may be removed by precipitation with hydroxide ions, and at low pH

chromate ions may be removed by an acid-base reaction to form hydrogen

chromate ions or dichromate ions, affecting the accuracy of the end point.

If you are analysing samples of water as described then this will not be a

problem.

2. It is a good idea to first carry out a “rough” titration in order to become familiar

with the colour change at the end point.

3. The Mohr titration is sensitive to the presence of both chloride and bromide ions

in solution and will not be too accurate when there is a significant concentration

of bromide present as well as the chloride. However, in most cases, such as

seawater, the bromide concentration will be negligible.



Technician Guide

Investigation A - Calcium

Each group will need

EDTA solution* Murexide indicator**

1 mol l-1 NaOH 1x Burette

Clamp and stand 1 x 100 cm3 beaker for topping up

burette with EDTA

100 or 250 cm3 flasks for titrations - 1

(to be washed out after each titration) or

more

Small funnel for topping up burette.

Spatula for adding indicator 3 cm3 pasteur pipette (or 5/10 cm3

measuring cylinder) for adding NaOH

Samples of different milks Samples of different waters***

Preparation

* EDTA solution

If possible, dry the disodium salt of EDTA for several hours or overnight at 80°C, allow to cool.

For calcium in water, this should be 0·01 mol l-1 BUT – if the water is very low in calcium then a lower concentration such as 0.001 mol l-1 will be needed

Weigh 1·86 g of the dried EDTA salt and dissolve it in 500 cm3 of distilled water in a volumetric flask.

(for waters that are very low in calcium, it may be necessary to dilute the EDTA further (1:10) to get a reasonable titre.

For calcium in milk, it should be 0·1 mol l-1

Weigh 4·65 g of the dried EDTA salt and dissolve it in 500 cm3 of distilled water in a volumetric flask.

**Murexide preparation

The easiest way to do this is a method called trituration. In a pestle and mortar add 0·1g of indicator powder to 20g or potassium or sodium chloride and grind thoroughly.



To use – add a spatula-tip of the salt/indicator powder to the solution.

*** Water preparation

In Scotland, most tap waters are low in calcium.

The easiest way to get round this is to purchase various mineral waters – they tell you the mineral content, including the calcium content, on the label. You can decant the water and suggest for instance that they are waters from different springs.

Alternatively, you can make artificial hard water

Add 0·7g of calcium sulphate-2-water to 1 litre of water in a bottle. Leave overnight to dissolve.

This gives you a solution that has 360 ppm of calcium in it – equivalent to very hard water areas like York and Lincoln.

To get water samples representative of other parts of the UK, dilute as follows:

Hard water eg Leicester 250ppm 69 cm3 made up to 100 cm3

Moderately hard eg Cheltenham 150 ppm 42 cm3 made up to 100 cm3

Slightly hard eg Blackpool 100 ppm 27 cm3 made up to 100 cm3

Or for Scotland

moderately sofy eg Moffat 24.5 6.8 cm3 made up to 100 cm3

moderately hard eg Shetland 52.1 14.6 cm3 made up to 100 cm3

Hard (eg Tiree) 110 30.5 cm3 made up to 100 cm3

(Note that Scottish Water uses ‘Hard’ and ‘Soft’ at slightly different levels.

Calcium sulphate produces what is known as permanent hardness.

If the experiment is looking at the effect of boiling water on calcium concentration, you will probably want to make up some temporary hard water.

• Take 445 cm3 of freshly made limewater

• Bubble carbon dioxide through the solution so that the calcium carbonate

precipitates.

• Continue bubbling it until the solution goes clear again.

• Dilute the solution to 1 litre.

Assuming all the calcium has ended up as calcium hydrogen carbonate, this will give you a concentration of 360 ppm.

If you want, you can then make up dilute solutions as above.



Calcium hydrogencarbonate is not stable, it will slowly return to CO2 and calcium carbonate.



Investigation B – “Analysis of Iron in foods”

In this experiment the sample is dissolved in nitric acid which oxidises the iron to the

ferric-state, Fe3+.

Sodium thiosulphate is not a primary standard so it will have to be standardised

before use.

There are probably lots of foods that will work using this methods but we have only

tested tea and breakfast cereal.


Access to a balance (2dp) crucible

Bunsen burner, tripod and pipe-clay

triangle*

100 cm3 beaker

Funnel and filter paper 100 cm3 flask

50 cm3 volumetric flask Burette and stand

pipette

2 mol l-1 nitric acid** 0·01 mol l-1 sodium thiosulphate

solution

1% starch solution

* A significant amount of smoke is likely to be produced – It may be that the

technician will prepare the extracts (or at least do the burning). If the pupils are doing

it then there will need to be good ventilation or use of a fume cupboard.


1. Accurately weigh about 2·0 g of tea/breakfast cereal into a crucible and roast

it in a fume cupboard for several minutes until all the tea has turned to ash

and no more smoke is coming off.

2. Allow the ash to cool and wash it into a 100 cm3 beaker using 2 mol l-1 nitric

acid. [CORROSIVE]

3. Add a further 20 cm3 of 2 mol l-1 nitric acid [CORROSIVE] is added and boil

the mixture for 5 minutes.

4. Let the mixture cool again and then filter it (to make sure any unburned

carbon, that could possibly remain in the mixture and affect the result, is

removed).



5. Place the filtrate in a 50 cm3 standard flask and make up to the mark using

distilled water.

** 2 mol l-1 nitric acid is corrosive. Goggles to BS EN166 3 will be needed.



Investigation C – “Chloride in sea water”

This method determines the chloride ion concentration of a solution by titration with

silver nitrate. As the silver nitrate solution is slowly added, a precipitate of silver

chloride forms.

The end point of the titration occurs when all the chloride ions are precipitated. Then

additional silver ions react with the chromate ions to form a red-brown precipitate of

silver chromate.


burette and stand 10 and 20 cm3 pipettes/measuring

cylinders.

100 cm3 volumetric flask 250 cm3 conical flask(s). If they are in

short supply, pupils can wash theirs out

between titrations.

10 cm3 and 100 cm3 measuring

cylinders

0·1 mol l-1 silver nitrate

1 cm3 pasteur pipette 20 cm3 pipette and filler*

0·25 mol l-1 potassium chromate

indicator

* If this is not easily accessible, the fact that the density of seawater is so close to

that of distilled water, 1·025 compared to 1·000, means the aliquot can be measured

by mass. 20 cm3 of seawater has a mass of 20·5g

Preparation

Silver nitrate solution: (0.1 M)

• If possible, dry 5·0 g of AgNO3 for 2 hours at 100°C and allow to cool.

• Accurately weigh 4·25 g of solid AgNO3 and dissolve it in 250 cm3 of distilled

water in a conical flask.

• Store the solution in a brown bottle.

Potassium chromate indicator solution: (approximately 0·25 mol l-1 )

• Dissolve 1·0 g of K2CrO4 in 20 cm3 distilled water.



Water

• If the seawater contains traces of solid matter such as sand or seaweed, it

must be filtered before use.

Seawater can be prepared artificially by

EITHER

Purchasing marine salts from an aquatic centre

OR

Making up your own

Just make up solutions of sodium chloride

Dead sea – a 29% solution

Red sea – a 4·1% solution

North sea – a 3·4% solution

Black sea – a 2% solution

Baltic sea – a 0·8% solution

Estuaries, if you are unable to get samples from an actual estuary, you can

make up representative samples for the different zones:

Mouth 3·4%

Lower estuary 2·7%

Middle estuary 2·1%

Inner estuary 1·2%

Upper estuary 0·25%

• Dilute the seawater by pipetting a 20 cm3 sample into a 100 cm3 volumetric

flask and making it up to the mark with distilled water.

Alternative microscale titration

Prepare the solutions as above

As well as those you will need equipment for a microscale titration – see the SSERC website for details

Activity assessed Testing water for calcium/magnesium

Date of assessment 26th July 2013

Date of review (Step 5)

School

Department

Step 1 Step 2 Step 3 Step 4 List Significant hazards here:

Who might be harmed

and how?

What are you already doing? What further action is

needed?

Action

by

whom?

Action

by

when?

Done

EDTA is a skin, eye and

respiratory irritant

Technician preparing

solutions.

Wear gloves and eye

protection. Avoid raising

dust.

Sodium hydroxide is

corrosive

1M sodium hydroxide

solution is corrosive


solutions

Technician, teacher or

pupils by splashes

Wear gloves and goggles (BS

EN166 3).

Wear goggles (BS EN166 3).

Ammonia .880 is

corrosive and the fumes

are toxic (Cat 3)

The ammonia buffer is

corrosive and gives off

toxic fumes (Cat 3)


buffer solution.


pupils by splashes or

inhaling fumes


EN166 3). Handle in a fume

cupboard

Wear goggles (BS EN166 3).

Work in a well-ventilated

areas and keep lid off bottle

for as short a time as possible.

SSERC Risk Assessment (revised version November 2009) (based on HSE ‘5 steps to risk assessment’)

2 Pitreavie Court, South Pitreavie Business Park, Dunfermline KY11 8UB tel : 01383 626070 fax : 01383 842793

e-mail : [email protected] web : www.sserc.org.uk

mailto:[email protected]

http://www.sserc.org.uk/

Step 1 Step 2 Step 3 Step 4 Murexide indicator

(ammonium purpurate)

has no significant hazard

Eriochrome black T is an

eye irritant

Ethanol is flammable

Hydroxylamine

hydrochloride is harmful

by ingestions/skin

contact, a skin/eye

irritant, a skin sensitiser

a category 2 carcinogen

and can damage organs

on repeated exposure.

Eriochrome Black T

indicator solution is a

skin sensitiser and a

category 2 carcinogen.


solution.


solution.


solution.


pupils by splashes

Wear eye protection. Avoid

raising dust.

Keep away from sources of

ignition. Wear gloves and eye

protection.


EN166 3).


EN166 3).

The reaction mixture is

of no significant hazard.

Description of activity:

Water samples are titrated against EDTA solution. Using murexide and eriochrome black T indicators. The solution is made alkaline by pH 10

ammonia buffer for the total hardness or sodium hydroxide for the magnesium.

Activity assessed Analysis of Iron in tea/cereal

Date of assessment 28th April 2017


School

Department


Who might be harmed

and how?


needed?

Action

by

whom?

Action

by

when?

Done

Burning Tea/cereal

produces irritating

smoke

Anyone nearby by

inhalation of the smoke.

If more than a very small

amount, carry out in a fume

cupboard.

Sulphuric acid is

extremely corrosive

Technician making up

dilute solution

Wear gloves and face shield

(or chemical resistant goggles

EN 166 3 if the quantity is

not large). Always add acid to

water.

Additional comments:






Step 1 Step 2 Step 3 Step 4 1M sulphuric acid is

corrosive

Pupil/teacher by

splashes during

experiment

Wear gloves and chemical

resistant goggles EN 166 3

Nitric acid is highly

corrosive and oxidizing


dilute solution

Wear gloves and face shield

(or chemical resistant goggles

EN 166 3 if the quantity is

not large). Keep away from

flammables and reducing

agents.

2M Nitric acid is

corrosive

Pupil/teacher by

splashes during

experiment

Wear gloves and chemical

resistant goggles EN 166 3

potassium manganate

VII is a powerful

oxidiser (and harmful if

swallowed)


dilute solution

Keep away from flammables

and reducing agents. Avoid

raising dust.

0.01M potassium

manganate VII has no

significant hazard.

Potassium iodide is an

eye irritant

Pupil (or technician)

weighing out solid

Wear eye protection. Avoid

raising dust.

Iodine – the

concentration of iodine

in the solution is low

enough to be of no

significant hazard

Sodium thiosulphate is

of no significant hazard.

Description of activity: Tea/cereals (or other foods) are burned and the ash boiled with 2M nitric acid to convert all the Iron to Iron III. The solution, diluted with water

has potassium iodide added which reacts with Iron III to produce iodine. This is titrated with sodium thiosulphate using a starch indicator near the

end point.

Activity assessed Mohr titration of chloride (Silver nitrate)

Date of assessment 28th April 2017


School

Department


Who might be harmed

and how?


needed?

Action

by

whom?

Action

by

when?

Done

Silver nitrate is an

oxidising agent and is

corrosive to skin and

eyes.

Technician by splashes

while preparing

solutions

Avoid raising dust. Keep

away from flammables and

reducing agents. Wear gloves

and goggles EN 166 3.







Step 1 Step 2 Step 3 Step 4 Potassium chromate is a

mutagen and carcinogen.

It is also a skin/eye and

respiratory irritant and a

skin sensitiser.

The 1M solution has the

same properties.

Technician while

making up solution and

pupils/teacher by

splashes when using.

Avoid raising dust. Wear

gloves and goggles EN 166 3.

Seawater is of low

hazard but if genuine

seawater is used it is best

to boil the sample before

use to destroy any

potentially harmful

micro-organisms.

The reaction mixture is

still classed as mutagenic

and carcinogenic due to

the chromate.

Description of activity:

Samples of seawater (real or artificial) are titrated against silver nitrate using potassium chromate as an indicator.


The chromate is very hazardous to the environment. To dispose, filter the reaction mixture and keep the residue (a mixture of silver chloride and

silver chromate) for disposal by registered contractor.

If the filtrate is yellow, meaning there is unreacted chromate, acidify to approximately pH 2 and add sodium hydrogensulphite to reduce to

Cr(III). Precipitate the Cr3+ as hydroxide, filter and keep for disposal by a licensed contractor.

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