20203e

Table2.2.2.-5.-ReferencesolutionsGY

Volumesinmillilitres

ReferencesolutionGY1

GY2GY3GY4GY5GY6GY7

StandardsolutionGY

25.015.08.55.03.01.50.75

Hydrochloricacid(10g/lHCl)75.0

85.091.595.097.098.599.25

ThepotentiometricdeterminationofpHismadeby

measuringthepotentialdifferencebetween2appropriateelectrodesimmersedinthesolutiontobeexamined:1oftheseelectrodesissensitivetohydrogenions(usuallyaglasselectrode)andtheotheristhereferenceelectrode(forexample,asaturatedcalomelelectrode).

Apparatus.Themeasuringapparatusisavoltmeterwithaninputresistanceatleast100timesthatoftheelectrodesused.ItisnormallygraduatedinpHunitsandhasasensitivitysuchthatdiscriminationofatleast0.05pHunitoratleast0.003Vmaybeachieved.

Method.Unlessotherwiseprescribedinthemonograph,allmeasurementsaremadeatthesametemperature(20-25°C).Table2.2.3.-2showsthevariationofpHwithrespecttotemperatureofanumberofreferencebuffersolutionsusedforcalibration.Forthetemperaturecorrection,whennecessary,followthemanufacturer’sinstructions.Theapparatusiscalibratedwiththebuffersolutionofpotassiumhydrogenphthalate(primarystandard)and1otherbuffersolutionofdifferentpH(preferablyoneshowninTable2.2.3.-2).ThepHofathirdbuffersolutionofintermediatepHreadoffonthescalemustnotdifferbymorethan0.05pHunitfromthevaluecorrespondingtothissolution.Immersetheelectrodesinthesolutiontobeexaminedandtakethereadinginthesameconditionsasforthebuffersolutions.

Whentheapparatusisinfrequentuse,checksmustbecarriedoutregularly.Ifnot,suchchecksshouldbecarriedoutbeforeeachmeasurement.

Allsolutionstobeexaminedandthereferencebuffersolutionsmustbepreparedusingcarbondioxide-freewaterR.

PREPARATIONOFREFERENCEBUFFERSOLUTIONS

Table2.2.2.-6.-ReferencesolutionsR

Volumesinmillilitres

ReferencesolutionR1

R2R3R4R5R6R7

StandardsolutionR

100.075.050.037.525.012.55.0

Hydrochloricacid(10g/lHCl)0.0

25.050.062.575.087.595.0

Storage

ForMethodI,thereferencesolutionsmaybestoredinsealedtubesofcolourless,transparent,neutralglassof12mmexternaldiameter,protectedfromlight.

ForMethodII,preparethereferencesolutionsimmediatelybeforeusefromthestandardsolutions.

Potassiumtetraoxalate0.05M.Dissolve12.61gof

01/2008:20203C4H3KO8,2H2Oincarbondioxide-freewaterRanddiluteto

1000.0mlwiththesamesolvent.

2.2.3.POTENTIOMETRICDETERMINATIONOFpH

ThepHisanumberwhichrepresentsconventionallythehydrogenionconcentrationofanaqueoussolution.Forpracticalpurposes,itsdefinitionisanexperimentalone.ThepHofasolutiontobeexaminedisrelatedtothatofareferencesolution(pHs)bythefollowingequation:

Potassiumhydrogentartrate,saturatedat25°C.ShakeanexcessofC4H5KO6vigorouslywithcarbondioxide-freewaterRat25°C.Filterordecant.Prepareimmediatelybeforeuse.

Potassiumdihydrogencitrate0.05M.Dissolve11.41gofC6H7KO7incarbondioxide-freewaterRanddiluteto1000.0mlwiththesamesolvent.Prepareimmediatelybeforeuse.

Potassiumhydrogenphthalate0.05M.Dissolve10.13gofC8H5KO4,previouslydriedfor1hat110±2°C,incarbondioxide-freewaterRanddiluteto1000.0mlwiththesamesolvent.

Potassiumdihydrogenphosphate0.025M+disodiumhydrogenphosphate0.025M.Dissolve3.39gofKH2PO4and3.53gofNa2HPO4,bothpreviouslydriedfor2hat120±2°C,incarbondioxide-freewaterRanddiluteto1000.0mlwiththesamesolvent.

Potassiumdihydrogenphosphate0.0087M+disodiumhydrogenphosphate0.0303M.Dissolve1.18gofKH2PO4and4.30gofNa2HPO4,bothpreviouslydriedfor2hat120±2°C,incarbondioxide-freewaterRanddiluteto1000.0mlwiththesamesolvent.

Disodiumtetraborate0.01M.Dissolve3.80gof

Na2B4O7,10H2Oincarbondioxide-freewaterRanddiluteto1000.0mlwiththesamesolvent.Storeprotectedfromatmosphericcarbondioxide.

inwhichEisthepotential,expressedinvolts,ofthecellcontainingthesolutiontobeexaminedandEsisthepotential,expressedinvolts,ofthecellcontainingthesolutionofknownpH(pHs),kisthechangeinpotentialperunitchangeinpHexpressedinvolts,andcalculatedfromtheNernstequation.

Table2.2.3.-1.–Valuesofkatdifferenttemperatures

Temperature(°C)

1520253035

k(V)0.05720.05820.05920.06010.0611

24Seetheinformationsectionongeneralmonographs(coverpages)

EUROPEANPHARMACOPOEIA6.02.2.5.Relativedensity

Table2.2.3.-2.–pHofreferencebuffersolutionsatvarioustemperatures

Temperature

(°C)

Potassiumtetraoxalate0.05M

Potassiumhydrogentartratesaturatedat25°C

Potassiumdihydrogencitrate0.05M

Potassiumhydrogenphthalate0.05M

Potassiumdihydrogenphosphate0.025M

+disodiumhydrogenphosphate0.025MKH2PO4+Na2HPO4

6.906.886.876.856.84−0.0028

Potassiumdihydrogenphosphate0.0087M

+disodiumhydrogenphosphate0.0303MKH2PO4+Na2HPO4

7.457.437.417.407.39−0.0028

Disodiumtetraborate0.01M

Sodiumcarbonate0.025M

+sodiumbicarbonate0.025M

Calciumhydroxide,saturatedat25°C

C4H3KO8,2H2O

1520253035

1.671.681.681.681.69+0.001

C4H5KO6C6H7KO7

3.803.79

C8H5KO4

4.004.004.014.024.02+0.0012

Na2B4O7,10H2O9.289.239.189.149.10−0.0082

Na2CO3+NaHCO310.1210.0610.019.979.93−0.0096

Ca(OH)212.8112.6312.4512.2912.13−0.034

3.563.553.55−0.0014

3.783.773.76−0.0022

(1)pHvariationperdegreeCelsius.

Sodiumcarbonate0.025M+sodiumhydrogencarbonate0.025M.Dissolve2.64gofNa2CO3and2.09gofNaHCO3incarbondioxide-freewaterRanddiluteto1000.0mlwiththesamesolvent.Storeprotectedfromatmosphericcarbondioxide.

Calciumhydroxide,saturatedat25°C.ShakeanexcessofcalciumhydroxideRwithcarbondioxide-freewaterRanddecantat25°C.Storeprotectedfromatmosphericcarbondioxide.

STORAGE

Storebuffersolutionsinsuitablechemicallyresistant,tightcontainers,suchastypeIglassbottlesorplasticcontainerssuitableforaqueoussolutions.

ReactionNeutral

pHIndicatorColourYellow

6.0–8.0MethylredsolutionR

PhenolredsolutionR(0.05ml)

Orange-redNeutraltomethyl4.5–6.0MethylredsolutionR

red

<8.0NeutraltoPhenolphthaleinColourless;pinkorred

solutionR(0.05ml)afteradding0.05mlphenolphtalein

of0.1Mbase

<6OrangeorredAcidMethylredsolutionR

Bromothymolblue

solutionR1

4.0–6.0MethylredsolutionR

Bromocresolgreen

solutionRCongoredpaperR

YellowOrangeGreenorblueGreenorblue

Faintlyacid

Stronglyacid<4

01/2008:20204

01/2008:20205

2.2.4.RELATIONSHIPBETWEENREACTIONOFSOLUTION,

APPROXIMATEpHANDCOLOUROFCERTAININDICATORS

To10mlofthesolutiontobeexamined,add0.1mloftheindicatorsolution,unlessotherwiseprescribedinTable2.2.4.-1.

Table2.2.4.-1

ReactionAlkaline

pH>8

IndicatorLitmuspaperredR

ColourBlueGreyorviolet-blueColourlessorpink

GreyRedViolet-blue

2.2.5.RELATIVEDENSITY

Therelativedensityofasubstanceistheratioofthemassofacertainvolumeofasubstanceattemperaturet1tothemassofanequalvolumeofwaterattemperaturet2.

isUnlessotherwiseindicated,therelativedensity

used.Relativedensityisalsocommonlyexpressedas.Densityρ20,definedasthemassofaunitvolumeofthesubstanceat20°Cmayalsobeused,expressedinkilogramspercubicmetreorgramspercubiccentimetre(1kg·m−3=10−3g·cm−3).Thesequantitiesarerelatedbythefollowingequationswheredensityisexpressedingramspercubiccentimetre:

ThymolbluesolutionR(0.05ml)

Slightlyalkaline8.0–10.0Phenolphthalein

solutionR(0.05ml)ThymolbluesolutionR(0.05ml)

>10StronglyalkalinePhenolphthalein

paperRThymolbluesolutionR(0.05ml)Relativedensityordensityaremeasuredwiththeprecisiontothenumberofdecimalsprescribedinthemonographusingadensitybottle(solidsorliquids),ahydrostaticbalance(solids),ahydrometer(liquids)oradigitaldensitymeterwithanoscillatingtransducer(liquidsandgases).

GeneralNotices(1)applytoallmonographsandothertexts25