RO_ph Measurements Rev.0.9 07.04.2009

8/13/2019 RO_ph Measurements Rev.0.9 07.04.2009

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pH MeasurementElementsHanna Instruments

R&D Baciu

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Definition of pH

- pHis a measure of relative amount of hydrogenand hydroxide ions in a aqueous solution:

H2O --->H+ + OH-

- is another way of expressing the hydrogen ionconcentration

pH = -log H+

!

- is really a measure of ion activity

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Principle of measurement

"erst #quation:

E = Eind

- Eref

= E0- 2.3!R " # $n$ lo% a

H

where:

# - measured voltage %m&$#'

- reference potential including other sta(le and fixed potentials

) - gas constant %*,.. /01mol$3 - a(solute temperature %1$4 - 4araday5s 6onstant %78.*9 6$

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Measurement setup

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Measurin% electrode

3he essential element ofthe glass electrode is a pH

sensitive glass mem(rane;n electrical potentialdevelops at glass liquidinterfaces

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Reference electrode

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3he reference electrode

supplies a constant potentialvalue against which wemeasure the potential of thepH electrode


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"'pes of electrodes


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"'pes of electrodes

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(in%le +unction electrode Dou*le +unction electrode


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"'pes of electrodes

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Reference electrode

)alomel electrode: consists of a pool mercury with its surface

completely covered with mercurous chloride

(il,er (il,er )loride%;g ;g6l$: formed (y electolytic

deposition of silver chloride on a silver wire

"alamid electrode/is essentially a metal in contact with asaturated solution of its chloride salt %thallous chloride$

Mercur' Mercurous sulpate: is exactly li?e a calomel electrode(ut is covered with solid mercurous sulphate

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Ho does te measurement ta1eplace

- in acidic solution theH+ diffuse in and apositive charge isesta(lished on theouter side of gel layer

- in al1aline solutionthe H+ diffuse out anda negative charge isesta(lished on theouter side of gel layer

"e total mem*rane potential is a result of te difference *eteente inner and outer car%e.

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imitation of pH electrodes

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4l1aline error

High concentrations of sodium ions interfere with readingsin al?aline solutions 3he pH at which the interferencestarts to (e significant depends upon the composition ofthe glass 3his interference is called al?aline error and

causes the pH to (e underestimated Hanna@s glassformulations have the indicated characteristics


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"'pical Electrode ife

;m(ient 3emperature , years

7' A6 %7. A4$ Bess than . months

2' A6 %2.* A4$ Bess than month


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Elements tat compete in pHmeasurement

"emperature effect

pH value changes with temperature 3he slope of the electrode changes with the

temperature

Interference Effect

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Isopotential pH

4requently the cali(rationtemperature and the processtemperature are different3herefore the cali(ration slope

which depends on temperatureis not appropriate to the sample

Definition: the isopotential pHis the pH at which the cell

voltage does not change whenthe temperature changes

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(olutions used for pHmeasurement

$illin% solution: is a solution containing the anion to which the

reference electrode of the operational pH cellis reversi(le eg 6hloridefor ;g- ;g6l electrode

(tora%e solution/ is the solution which is in the protective cap in whichis always ?eep the glass (ul( and the Cunction of the pH electrode

)leanin% solution/ is the solution used for cleaning the glass (ul( ofthe electrode once a day at least once a wee? to prevent Cunction

clogging and to maintain the accuracy

)onditionin% solution/ is the solution in which the electrode must (eimmerse to activate the glass selective mem(rane

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Buffer solutions

;re formed most usually from a ea1 acid and hissalt formed it a stron% *ase.

#D: 6H,6OOH06H,6OO"a

3he main characteristic of (uffers is the *ufferin%capacit'.

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pH *uffer temperature dependence

3emperature has an effecton pH 3he cali(ration (uffersolutions are affected (ytemperature changes to alower degree than normal

solutions


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)ali*ration

WHY...?

pH cells are not perfect 3herefore te' must *ecali*rated

)ali*ration tries to compensate any deviation fromideal (ehavior

)ali*ration is made with solutions having exactly the?nown pH

"e cali*ration process is generally performed (ymeasuring in two different (uffer solutions

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(teps for te )ali*ration Process

Measurin% solutions

Rinsin% solutions 5

2 3 6

7

8

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)ali*ration Pro*lems

6ontamination of (uffer solutions

#lectrode internal elements degradation

#lectrode aging

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)ontamination of *uffer solutions

#xpiration date

)oom temperature

Esed (uffer


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Electrode internal elementsde%radation

Biquid Cunctions with fi(ers or ceramic pins

(loc?ed due to

crystalliFation

precipitation

3rapped air (u((les

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Electrode 4%in%

G 3he sum of internal elements degradation andeffects of all external physico-chemical actions%temperature pressure interference etc$

G roper clean of electrode oil solutions

G #lectrode slope offset response time quality of

glass change over time

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)al )ec1

Most users *elie,e tat if te' cali*rate often9 te' ill%et accurate readin%s.

3his is only true in certain cases for example:

6ontaminated (uffers o(viously lead to (ad readings

Enconditioned or dirty electrodes also lead to (ad readingsas the performance of the electrode will change aftercali(ration

)ali*ration )ec1"M is the Hanna name for a class of

techniques for automatically detecting these pro(lems andimproving the accuracy of pH measurements

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) l ) 1


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)al )ec1

pH Electrode 4%in% E:periment

Ie have done experiments ?eeping electrodes for over . months in:milk, juice, pH 1, pH 13, oilmilk, juice, pH 1, pH 13, oil

6onclusions of experiments:

3he actual electrode slope and offsetcan%e slol' over time

If not properl' cleaned the slope and offset can changedramatically over time even over a short period of time

It is ,er' difficult to clean the electrodes properly %particularly

for the oil solution$ and to tell whether the electrodes have (eencleaned sufficiently

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)al )ec1pH )leanin% electrode e:periment

Jtart with , new conditioned electrodes


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)al )ec1pH )leanin% electrode e:periment

-':''

-':'9

-':-'

-':-9

-':2'

-':29

-':,'

' ':9 - -:9 2 2:9 ,

"ime !.ours;

pHR

eadin%

3est 2

3est -

6lean

Lmmediately after cali(rationall electrodes measureaccurately

3he dirty electrodes getcleaned with time andapproach the real offset andslope

However the cali(rationcontinues to compensate forthe dirty parameters quic?lyleading to errors

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Most important pointMost important point/#ven if the user thin?s he is doing a good Co( during cali(rationnormal meters give no warnings when the cali(ration can (e (adas the cali(ration parameters have changed more than expected

)ali*ration )ec1 features can significantly improve theaccuracy of pH readings (y alerting the user that something can(e wrong

;pplication-specific cleaning solutions need to (e used to cleanelectrodes properly

)al )ec1

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4pplications

ess common applications/

G Melatin and glue manufacturing

G /am and Celly manufacturing

G #lectrical equipments

G Beather

G rinting

G 3ropical fish (reeding

G erfume industry

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pH meter features


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pH meter features

Neasured parameters: pH m& )elative m& LJ# andtemperature

Nanual selection automatic and semiautomatic pHcali(ration in up to five points and custom (uffers %up to 9custom (uffers$

Nanual Jelection and 6ustom Jtandard LJ# cali(ration inup to five points with standard %up to five standardsolutions for each measurement unit$ and customsolutions %up to 9 custom solutions$ with or without

temperature compensation ;utoHold feature to freeFe first sta(le reading on the B6


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pH meter features

3wo selecta(le alarm limits %for pH m& LJ#$

3hree selecta(le logging modes: ;utomatic Nanual;utoHold logging

Ep to '' logging lots for automatic or manual modes andup to '' LJ# methods reports

Jelecta(le area and sampling period feature for automaticlogging

MB feature Operator L< this option allows you to edit the name of the operator Lnstrument L< this option allows you to edit an identification

name0num(er for the instrument 6ompany "ame this option allows you to edit the company name ;dditional Lnfo ;dditional Lnfo 2 for general purpose notations

pH meter features


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pH meter features

rofile

3emperature compensation: ;36 N36

Jta(ility 6riteria: 4ast

Nedium

;ccurate

Online and offline graph

Opto-isolated 6 interface via )J2,2 respectively EJP


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Thank you for your

attention!!!

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RO_ph Measurements Rev.0.9 07.04.2009

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