—A B B M E A S U R EM EN T & A N A LY T I C S | O PER AT I N G I N S T R U C T I O N | I M/8 0 0 2 R E V. M

8002Ammonia probe

Measurement made easy

The Company

We are an established world force in the design and manufacture of instrumentation for industrial process control, flow measurement, gas and liquid analysis and environmental applications.

As a part of ABB, a world leader in process automation technology, we offer customers application expertise, service and support worldwide.

We are committed to teamwork, high quality manufacturing, advanced technology and unrivaled service and support.

The quality, accuracy and performance of the Company’s products result from over 100 years experience, combined with a continuous program of innovative design and development to incorporate the latest technology.

1

CONTENTS

1 INTRODUCTION .................................... 2

2 GENERAL INFORMATION .................... 32.1 Requirements ................................ 32.2 Accessory Equipment ................... 32.3 Probe Assembly Instructions ........ 32.4 Storage ......................................... 52.5 Internal Filling Solution .................. 5

3 PROBE MEASUREMENTS ................... 53.1 General Procedure ........................ 53.2 Buffer Solution – Sample pH ........ 63.3 Probe Calibration .......................... 83.4 Probe Range and Response ......... 93.5 Temperature Effects ...................... 93.6 Interference ................................. 103.7 Continuous flow analysis ............ 103.8 The Known Addition Method ...... 123.9 Modification of the Internal

Filling Solution ............................. 123.10 Analysis of Kjeldahl Digests ........ 13

3.10.1 Method ......................... 143.10.2 Standard Solutions ....... 143.10.3 Probe Filling Solution .... 15

3.11 Drift ............................................. 153.12 Ageing of the Glass Electrode .... 153.13 Checking the Performance

of the Glass Electrode ................ 163.14 Aging of the Reference

Element ....................................... 16

4 THEORY ............................................... 16

5 SPECIFICATION .................................. 17

6 SPARES & ACCESSORIES ................. 17

APPENDIX ................................................... 18A.1 Procedure for Chloridising the

Reference Element ...................... 18A.1.1 Requirements ............... 18A.1.2 Cleaning ....................... 18A.1.3 Chloridisation ............... 18

REFERENCES ............................................. 19

NOTES ......................................................... 20

2

The Ammonia Probe, Model 8002, is a highsensitivity gas sensing probe developed byABB. It measures the partial pressure, andhence concentration of ammonia in aqueoussolution. Ammonium concentrations can bemeasured after the sample has been treatedwith alkaline buffer to generate ammonia. Theprobe has a Nernstian response up to 1000mgNH3l

–1 or 10,000mg NH3l–1 with modification of

the internal filling solution. The lower limit has notbeen determined as it is set by the purity ofavailable water.

Model 8002 should be used in conjunction withan expanded scale pH meter such as Model7046; for convenience the direct readingconcentration scales may be used with thefunction switches set to monovalent anion. Theprobe is supplied with membranes and fillingsolution and a flow-through cap which enablescontinuous analyses of up to 60 samples perhour, when incorporated into a suitable flowsystem.

The probe may also be used as a sensor inindustrial monitors, for example the Model 8232Ammonia Monitor, an industrial continuous flowanalyser incorporating automaticstandardisation.

Model 8002 is widely used for the determinationof ammonia in boiler water, river water, treatedand untreated sewage, etc., (see References 2,3, 5, 6, 7, 9 and 10). Total nitrogen in the forms ofammonia in the residue from the Kjeldahldigestion may also be measured quickly andeasily with the probe without the need for theusual final distillation step, (see References 4and 8). The internal filling solution of the probemust be modified in this application and in themeasurement of other samples containing ahigh concentration of dissolved species.

1 INTRODUCTION

Fig. 1.1 Contents of Kit

Made in England

PART No 8002 265

INTERNAL

FILLING SOLUTION

AMMONIA PROBE

Colour change from yellow to

voilet when solution made alkaline

PART No. 8002-240

MADE IN ENGLAND

SPARES KIT

t

3

2.1 RequirementsThe following will be required in addition to theprobe:

a) An expanded scale pH meter, such as ABBModel 7046 (which has a direct readingconcentration scale).

b) Standard solutions of an ammonium salt.

c) Alkaline buffer solution (see Section 3).

d) Magnetic stirrer.

2.2 Accessory Equipment

Note. See list of spares and accessories.

Flow-through cap, Part No. 8002-830. This itemis supplied with the kit. It allows samples to bepumped directly onto the sensing membrane ofthe probe. A simple closed flow system may beconstructed using the cap. Further details aregiven in Section 3.

2.3 Probe Assembly InstructionsThe probe takes only a few minutes toassemble, but for the best results theinstructions must be followed carefully.

a) Unscrew the end cap from the probe body.Rinse out the probe body with distilled ordeionised water and allow to drain.

b) Remove the teat from the glass electrode andrinse the electrode with distilled or deionisedwater. Dry with a paper tissue.

c) Insert the glass electrode into the body andscrew it in until the top of the electrode is flushwith the end face of the probe. Note thenumber on the electrode cap in line with themark on the probe body. Unscrew theelectrode cap four full turns.

d) Take one of the washers and a membranefrom the spares kit. Carefully peel off thebacking material from the membrane. Dropthe washer into the end cap, then place themembrane on to the washer.

e) Screw the end cap firmly onto the body: boththe body seal and the membrane sealingwasher should be under compression. Theend cap should not be screwed on so tightlythat the membrane wrinkles.

Note. If the probe is to be used to measureammonia in sea water, it is recommendedthat 1.7 g of ammonium chloride and 1 drop(approximately 0.05 ml) of 0.1M silver nitrateis added to the contents of the 50 ml bottle.

f) Hold the probe upright and fill the probethrough the filling hole with the internal fillingsolution provided until there is a 50 to 60mmdepth of solution inside. Wipe any excessfilling solution from the body.

g) Tap the end of the probe with a finger todislodge any air bubbles which may havebecome trapped between the end of theglass electrode and the membrane. Thenscrew the glass electrode down four turns tothe flush position again and then a further 1/3to 1/2 turn. The tip of the electrode will now bepressing against the membrane. In somecircumstances, particularly if the electroderesponse is found to be unusually slow, thescrew may be given a further 1/4 turn.

Note. Screwing the electrode down too farmay puncture the membrane.

h) Push the probe cap onto the top of theprobe body.

2 GENERAL INFORMATION

4

pH Electrode

ReferenceElement

FillingSolution

FillingSolutionAperture

MembraneWasher

Body

Body Seal

Washer

Membrane

End Cap

Frommixing

coil

Exhaust

Clear PVC tubing3 mm bore x 4.5 mm O/D

Clear PVC tubing1.5 mm bore

3.2 to 3.5mm O/D

Alternative assembly with flow-throughcap illustrating incorporation

into a continuous flow system

…2 GENERAL INFORMATION

Fig. 2.1 Model 8002 Ammonia Probe

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2.4 StorageStore the assembled probe with the end dippedinto a 0.1M ammonium chloride solution in orderto keep the osmotic pressure of the solutions oneither side of the membrane equal – see Section4. After storage the probe should be well rinsedin distilled or deionised water. Satisfactoryresults will not be obtained if the probe is storedin distilled or deionised water.

On no account should the end of the probe beallowed to dry out, should this accidentallyoccur the performance of the probe maysometimes be restored by loosening andtightening the electrode retaining nut to allowmore filling solution to flow between the glasselectrode and the membrane. If this procedure isnot successful the membrane and filling solutionneed replacement.

If the probe is to be returned to its box it must bedismantled: firstly the glass electrode should beunscrewed, then the rest of the probe emptied,rinsed and drained. The glass electrode shouldbe stored with its end in a teat containing aneutral buffer solution, taking care not to coverthe reference element.

2.5 Internal Filling SolutionThe internal filling solution incorporates acoloured indicator which is normally yellow. Theindicator changes to blue if the probemembrane, or membrane seal, allows alkalinereagent to leak into the probe. If this occurs themembrane and the filling solution should bereplaced. Take care to tighten the end capsufficiently to provide a good seal. The solutionkeeps indefinitely.

3.1 General ProcedureFor measurement of either the ammonium ion or‘total’ ammonia concentration in samples, thesamples must be alkaline when measured sothat ammonia species are present as freeammonia. As most samples are not alkaline it isnecessary to treat them with a buffer solution,the choice of which is explained in the followingsection.

Alkaline samples slowly lose ammonia someasurements in open beakers are not alwayssatisfactory. Thus it is suggested that thesamples are contained in 100ml conical flasks toreduce losses. The probe body will fit closely intothe top of such a flask but losses may be furtherreduced by slipping the larger of the ‘O’ ringsfrom the spares kit over the probe stem so that itmakes a seal with the neck of the flask.

All samples should be stirred duringmeasurement; this may conveniently be doneusing a magnetic stirrer. With some stirrers it willbe necessary to put a sheet of thermal insulatingmaterial, such as polystyrene sheet, underneaththe beaker to prevent heating of the sample bythe stirrer motor.

After immersing the tip of the probe in a stirredsample check that no air bubbles are trapped onthe end of the probe. Allow time for the probepotential to reach a steady value; theequilibration time will be dependent on theconcentration of the ammonia in the sample. Ata sample concentration of 10–3M (17mg NH3l

–1)the time taken by the probe to reach itsequilibrium potential should be under twominutes, although it may take longer with certaintypes of sample.

2 GENERAL INFORMATION 3 PROBE MEASUREMENTS

6

The probe is sensitive to the osmotic pressure ofsamples and special attention needs to be paidto this in the analysis of samples containing ahigh total concentration of dissolved species(i.e. the sum of the concentrations of all anions,or cations and non-dissociated species).If this total concentration in the buffered samplepresented to the probe is greater than 0.25M,the probe potential will drift. To overcome thisproblem either the samples should be diluted orthe internal filling solution of the probe modified,as explained in a later section.

The probe is sensitive to changes in bothambient temperature and the temperature ofsamples, hence it is recommended that theprobe is not used in direct sunlight or in a seriesof samples or standards at differenttemperatures. For the most accurate andreproducible results the probe should be kept ata constant temperature by use of a jacketed cellor by immersion of the lower half of the probe ina water bath (using the probe, fitted with a flow-through cap, in a flow system).

In addition to the measurement of ‘total’ammonia in samples, the probe may also beused directly in untreated samples to measurethe ‘free’ ammonia concentration. This isparticularly useful in complex equilibria studiesor in the measurements of strong ammoniasolutions at fixed pH.

3.2 Buffer Solution – Sample pHThe two functions of the buffer solution are toadjust the pH of the samples to greater than 11so that all the ammonium ions are converted toammonia and to fix the total concentration ofdissolved species in the buffered sample atapproximately 0.2M. In order to ensure virtuallycomplete conversion of the ammonium ions it isnecessary to have a sufficient excess of hydroxylions over the ammonium ions: to ensure thecorrect pH the final hydroxyl concentrationshould be at least two or three times themaximum ammonium ion concentrationexpected (both concentrations expressed asmolarities).

Thus a 1M sodium hydroxide solution (NaOH)added to samples at a volume ratio of 1 : 10, isa suitable buffer for samples containing up to 5 x10–2M NH4

+ (ca. 1000mgl–1), provided thesamples do not contain a high concentration ofdissolved species. However for a sample whichcontains less than 5 x 10–3M NH4

+ (ca.100mgl–1) but has a total concentration of dissolvedspecies of 0.2M, a suitable buffer for addition ata volume ratio of 1 : 10 is 0.1M NaOH, as thisproduces the correct pH while maintaining thecorrect level of dissolved species.

A correlation between % conversion and pH isshown in Fig. 3.1.

If samples contain small concentrations of metalions, such as copper, nickel or mercury ions,these will complex part of the ammonia to bemeasured and cause a low result. Hence if suchinterferents are likely to be present it is advisableto replace the usual 1M NaOH solution with abuffer of 1M NaOH + 0.1M Na2 EDTA.

For the analysis of samples containing greaterthan 0.25M total dissolved species or more than5 x 10–2M NH4

+, see later section.

…3 PROBE MEASUREMENTS

7

7 9 1086 11 12 13 14

0

10

20

30

40

50

60

70

80

90

100

% F

ree

Am

mon

ia

pH

pK of NH4+

-100

0

0.01

mV

mgl–1[NH3]0.1 10

M

-200

100 1000

10-6 10-5 10-4 10-3 10-2 10-1

1

3 PROBE MEASUREMENTS…

Fig. 3.1 Ammonia Content vs pH (25C)

Fig. 3.2 Typical Ammonia Probe Calibration Graph

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3.3 Probe CalibrationThe probe should be calibrated regularly. After thecalibration graph has been prepared, thecalibration should be checked at a single pointevery two or three hours. The concentrations of thestandard solutions used to calibrate the probeshould be chosen to bracket the expected rangeof concentration of the samples.

The standard solutions for calibration of theprobe may conveniently be prepared by serialdilution of fresh standard 0.1M or (if working inmgl–1 units) 2000 mgl–1 ammonium chloridesolution. The diluted standards are treated witha buffer as discussed in the previous section;typically 1 volume of 1M sodium hydroxide isadded to every 10 volumes of standard. Recordthe potential of the probe in each of the bufferedstandards and prepare a calibration graph of theprobe by plotting the potentials against thelogarithm of the concentration of ammoniumions in the standards. This procedure issimplified if the graph is plotted on semi-logarithmic graph paper with the concentrationalong the logarithmic axis. A typical calibrationgraph is shown in Fig. 3.2.

If the level of ammonia contamination in thewater and reagents is sufficiently low withrespect to the lowest concentration of ammoniain standards and samples for the probe to give aNernstian response in the requiredmeasurement range, then the calibrationprocedure may be simplified by using a pHmeter with a direct reading concentration scale.Consult the pH meter instruction manual for theprocedure. Use the monovalent anion setting onthe function switch.

For accurate results the composition of thestandards should be as similar as possible tothat of the samples. Thus when the compositionof the samples is well defined, standards shouldbe prepared from the model samples withknown amounts of added ammonium ion. In thisway the constancy of the Henry’s Law constant(see Section 4) between standards and samplesis assured.

In common with other gas sensing membraneprobes and many ion selective electrodes, theammonia probe reaches equilibrium morequickly after an increase in determinandconcentration than after a decrease.Accordingly it is recommended that the probe iscalibrated in the standard solutions in ascendingorder of concentration.

The probe may be used to measure the ‘free’ammonia in equilibrium with ammonium ions insamples. In such applications it is usually notsatisfactory to pretreat the samples beforemeasurement as this may upset the equilibria inthe samples. The calibration of the probe forthese measurements should be done withstandards of known ammonia concentrationafter buffering; for example, one volume of astandard 10–4M NH4

+ buffered with one volumeof 0.2M NaOH is a 5 x 10–5M NH3 standard.Thus in this case the probe is calibrated againstammonia standards in place of the usualammonium ion standards.

…3 PROBE MEASUREMENTS

9

3 PROBE MEASUREMENTS…

C HNp/Vm 3 C HNp/Vm 3

5015102

91.5581.6571.7571.85

52035304

61.9551.0641.1631.26

3.4 Probe Range and ResponseThe probe has a Nernstian response up to 5 x10–2M (approximately 1000mg NH3l

–1); thisrange may be extended by modification of theinternal filling solution. The lower limit ofNernstian response is set by the purity ofreagents and water, as these inevitably containsmall traces of ammonia; if no special care istaken the level of ammonia in distilled ordeionised water usually produces a lowerNernstian limit of approximately 0.1 to 0.2mgNH3l

–1 (approximately 10–5M).

The response times of the probe have beenreported in Reference 1. The typical time takenfor the probe to reach 1mV from the finalequilibrium potential after a tenfold increase inammonia concentration is 30 to 35 seconds.

The probe will drift if used continually near itsupper limit; for the analysis of samplescontaining more than 10–2M (approximately200mg NH3l

–1) it is advisable to rinse the probebetween measurements in 0.1M NH4Cl solution,or briefly in distilled water, to preventaccumulation of ammonia in the bulk of the fillingsolution.

A further temperature dependent parameter isthe Henry’s Law Constant; thus the partialpressure of ammonia in a solution of fixedconcentration varies with temperature.

The effect of temperature on the probe isdiscussed in more detail in Reference 1.

3.5 Temperature EffectsThe temperature coefficient of the ammoniaprobe is approximately +1 to +2mV/Cdepending on the conditions of measurement.Thus the probe should not be used in directsunlight or in an area of rapidly changingtemperature. Also the temperature of standardsand samples should be held as constant aspossible.

The temperature sensitivity of the probe arisesfrom the temperature dependence of both of theelectrodes’ potentials, the osmotic equilibriaand the positions of the equilibria of the chemicalreactions.The Nernst factor also varies with temperatureand so the slope of the calibration graph as wellas the normal potential of the probe vary. Thetheoretical values of the Nernst factor at differenttemperatures are given in Table 3.1.

Table 3.1 Variation of Nernst Factor withTemperature

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3.6 InterferenceThe probe is virtually free from interference. Asthe gas permeable membrane is hydrophobic,neither anions nor cations can interfere with theresponse. However, interference does resultfrom volatile or filming amines in the solution,such as hydrazine, cyclohexylamine oroctadecylamine, (see References 2 and 3);these produce an apparent increase in theammonia concentration in the samples.

3.7 Continuous flow analysisThe response of the probe is sufficiently rapid forit to be used for the continuous flow analysis ofdiscrete samples. For this purpose, the flow-through cap is fitted onto the probe body inplace of the standard end cap. A suitable flowsystem is shown in Fig. 3.3. Samples areseparated by appropriate wash solution in theusual way. The flow rate through the cap shouldbe approximately 2 to 5 ml per minute. Caremust be taken to flush out air bubbles from thecap; this may be assisted by using narrow boretubing (approximately 1mm) up to the cap inletand only a short length of tubing on the outlet.

An example of a trace obtained with such asystem is shown in Fig. 3.3 where ammoniasamples were analysed at the rate of 60ml/hour.A higher sampling rate of 120/hour has beenused and although the precision of the resultswas somewhat poorer, such rates wouldcertainly be practical in many cases. The mixingcoil in the Fig. 3.3 is necessary to ensure that thesample and buffer are thoroughly mixed beforereaching the probe.

An ammonia probe has been used in this way fortotal nitrogen analysis by the Kjeldahl method asdiscussed in a later section and in Reference 4.After the digestion step the acidic digest is madealkaline and its ammonia content determineddirectly with the probe. The sample becomeshot after the alkali addition and the mixing coiland probe body were therefore immersed in athermostatted water bath to ensure thermalstability; there is no danger of loss of ammoniabecause the alkali addition is made in a closedsystem.

…3 PROBE MEASUREMENTS

11

3 PROBE MEASUREMENTS…

Traces Obtained from an Automatic Flow System

AmmoniaProbe Model8002

Chart100mV f.s.d.60cm/h

80%

70%

60%

50%

40%

30%

20%

10%

0%

2mg/l

10mg/l

1mg/l1mg/l

90%

Wash

Mixer

Pump

Air

Buffer

Waste

Probe

Sampler

Fig. 3.3 Flow System

12

3.8 The Known Addition MethodThere are some advantages in using the KnownAddition Method of analysis for occasionalsamples with ammonia concentrations greaterthan the lower Nernstian limit in the experimentalconditions used. This method reduces thenumber of standard solutions required to one;the concentration of this standard solutionshould be between ten and a hundred times thatof the sample. The experimental slope factor (SmV/pNH3) of the probe should be known.

To analyse a sample by this method, pipette analiquot, Vs ml, of alkaline sample into a beakerand measure the potential of the probe in thissolution, E1 mV. Add a known volume, Va ml, ofthe standard solution (where 10Va VS) ofconcentration Ca mol l

–1 and after stirringmeasure the new potential of the probe, E2 mV.It should be so arranged that the potentialchange is several millivolts. The concentration ofammonia in the original alkaline sample, Cs moll–1, may then be calculated from the formula:

Cs =

CaV aV a+V s

V s

(V a +V s )E 2-E 1

S[ [antilog10

3.9 Modification of the Internal FillingSolutionFor analysis of samples containing very highconcentrations of ammonia the internal fillingsolutions must be modified to prevent theosmotic transfer of water across the membrane(see Reference 1). The internal filling solutionprovided in the kit consists principally of 0.1Mammonium chloride saturated with silverchloride.

In the case of measurements in strong ammoniasolutions the molarity of ammonium chloride inthe internal filling solution should be increasedso that it is at least two or three times themolarity of the strongest ammonia solution to beanalysed. Sufficient silver nitrate solution shouldbe added to ensure that the solution is saturatedwith silver chloride. The buffer for treating thesamples should be chosen so that the bufferedsamples have pH greater than 11 and the totalconcentration of dissolved species matches thatin the new filling solution.

For measurements of samples containing a hightotal concentration of dissolved species,sufficient inert electrolyte, e.g. potassium nitrate,must be added to the filling solution so that theconcentrations in the internal filling solution andbuffered samples are equal. Such modificationof the filling solution is necessary in the analysisof, for example, seawater or Kjeldahl digests. Anexample of the calculation of the correctconcentration of inert electrolyte to add is givenin the subsequent sections.

Note. A simple method for normalseawater analysis is to add 1.7 g ofammonium chloride and 1 drop(approximately 0.05 ml) of 0.1M silver nitrateto the contents of the 50 ml filling solutionbottle.

…3 PROBE MEASUREMENTS

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3.10 Analysis of Kjeldahl DigestsThe Ammonia probe can be used for the directdetermination of the ammonia content of theresidue from Kjeldahl digestions, thuseliminating the distillation step required before acolorimetric or titrimetric finish (Reference 4).

Warning. The normal safety procedurescarried out during Kjeldahl digestions shouldalways be observed.

After digestion of the sample the residue iscooled, diluted with ammonia free water and analiquot is made alkaline for ammoniameasurements with the probe. The exactprocedure varies with the application but theimportant requirements are as follows:

a) The initial dilution of the residue afterdigestion should be as large as possible tosimplify the procedure. If the dilution factoris such that the total concentration ofdissolved species in the solution presentedexceeds 0.25M then the probe fillingsolution should be modified to preventosmotic transfer of water through themembrane.

b) Loss of ammonia during and after additionof alkali to the diluted residue must beprevented. Losses of ammonia can beminimised by efficient cooling to dissipatethe heat of reaction.

c) Standard solutions should be similar incomposition to the diluted residue to avoidchanges in the Henry’s Law constant. Ingeneral it will be most convenient to usestandard solutions containing the sameconcentration of sulphuric acid as thediluted residue.

d) The temperature of all solutions presentedto the probe should be essentially the same(see b above).

e) If the digestion mixture contains a mercurycatalyst, sodium iodide should be added tothe alkaline buffer to prevent interference;for a copper catalyst EDTA should beadded to prevent precipitation.

Note. Many of these requirements may bemet by the use of a continuous flow system,as already noted.

3 PROBE MEASUREMENTS…

14

3.10.1 MethodThere are many variations of the Kjeldahldigestion process, but all result in a very acidresidue of high dissolved solids content. Withthe above points in mind, a typical procedurecan be outlined.

a) Place sample containing 0.5 to 5.0 mgl–1Nin Kjeldahl flask.

b) Add 10ml H2SO4 (1.84 s.g.) and anycatalyst and/or other additives required.

c) Carry out digestion.

d) After cooling make digest up to 100ml withammonia free water.

e) Pipette 25ml diluted digest into 100mlconical flask.

f) Add 25ml buffer solution (4.0M NaOH),stopper the flask, Swirl and cool rapidly tothe working temperature.

g) Remove stopper, place magnetic stirrer barin flask and insert probe containingmodified filling solution.

h) Stir the solution gently and note instrumentreading immediately a stable potential isobtained.

The procedure in this section produces a finalsolution for measurement having theapproximate composition:

0.90M Na2SO4

0.20M NaOH

and the standard solution and probe fillingsolution compositions can be calculated fromthis.

3.10.2 Standard SolutionsThese should contain N in the ranges 5 to50mgl–1 in the form NH4Cl. In addition, theyshould contain sulphuric acid, catalyst and otheradditives used for digestion; 1 litre of standardsolution should contain ten times the amount ofeach of these used in step b) of the procedure inSection 3.10.1.

They should be treated in exactly the same wayas the diluted digest, i.e. taken through steps e)to h) of Section 3.10.1.

…3 PROBE MEASUREMENTS

15

3.10.3 Probe Filling SolutionThe total concentration of dissolved species, ci, should be the same as that in the finalsolution, i.e.

ci = (Na+) + (SO4

2–) + (OH–)

= ([2 x 0.90] + 0.20) + 0.90 + 0.20M

= 3.10M

The standard filling solution is 0.1M NH4Cl andthis should be modified by the addition of solidNa2SO4 to increase ci from 0.2 to 3.1M; this isachieved by the addition of 3.44g anhydrousNa2SO4 to 25ml standard filling solution.

If the increase in ci is greater than 3.0M thenNaCl should be used in place of Na2SO4.

The above calculation assumes that thesolutions exhibit ideal behaviour, but this will notbe the case in practice. Thus, in general, somedrift in probe potential will still be observable andfurther adjustment of the probe filling solutionmay be necessary. If the probe potential drifts inthe direction of lower ammonia concentration,then the filling solution requires more Na2SO4and vice versa. It is recommended that ci forthe filling solution be changed in 5% incrementsuntil the drift is adequately small.

The matching of total concentration of dissolvedspecies on either side of the probe membrane ismore critical as the solutions become moreconcentrated; thus some preliminary dilution ofthe residue after digestion is advantageous.

3.11 DriftDrift of the potential of the probe at a rate greaterthan 1mV in 12 hours should not beexperienced. If greater drift does occur it isprobably due to:

• inadequate temperature control;

• osmotic effects, i.e. total concentration ofdissolved species in samples too high;

• loss of ammonia from the sample;

• use of the probe in very strong ammoniasolution immediately prior to measurement;

• puncture of the membrane (usually visible);

• interference;

• ageing of the glass electrode – see Section3.12.

3.12 Ageing of the Glass ElectrodeAfter the probe has been in use for somemonths, the performance of the glass electrodemay eventually deteriorate due to its continueduse in weakly buffered solutions at near neutralpH. The response of the probe becomessluggish and the response slope drops. Theelectrode may frequently be restored to its initialcondition by soaking for 12 hours in 0.1Mhydrochloric acid. The performance of theelectrode may be checked as described below.

3 PROBE MEASUREMENTS…

16

3.13 Checking the Performance ofthe Glass ElectrodeThe glass electrode may be checkedindependently of the probe with a laboratorycalomel reference electrode (e.g. Model 1370210) by testing the pair in pH buffer solutions.

The glass electrode should only be immersed toa depth of 5 to 10mm. There should be no liquidcontact with the reference element.

Connect the electrodes to a pH meter andcalibrate with pH buffers in the usual way. Theglass electrode may be found to be somewhatslower in response than a conventional bulbtype electrode, but adequate scale lengthshould be obtained, typically better than 90% ofthe theoretical slope value.

3.14 Aging of the Reference ElementThe silver chloride coating of the referenceelement gradually dissolves after many monthsuse. If drift is experienced in an old electrode itcould be due to this effect. Either replace theinner electrode (Part No. 8002-650) orCAREFULLY follow the chlorodisationprocedure in Section 7.1.

The probe responds to the partial pressure ofammonia in the sample solution. When theprobe is immersed in a sample, ammonia istransferred across the gas-permeablemembrane until the partial pressure of the gas inthe thin film of solution between the glasselectrode and the membrane is equal to thepartial pressure in the sample. The internal fillingsolution contains ammonium ions. The ratio ofthe amount of ammonia to the activity of theseions in the thin film gives rise to a characteristicpH which is measured by the glass electrode/reference electrode system. The value of this pHis a function of the chemical reactions in thesolution. As a result of the equilibria the probeexhibits a potential in ammonia solutions whichmay be represented by:

[NH3 ]log102.3RT

F–E = E

Ammonia obeys Henry’s Law in dilute solutions,so the partial pressure of ammonia in thesolutions is directly proportional to itsconcentration, i.e.

PNH3 = H[NH3]aq

where PNH3 is the partial pressure of ammonia,H is the Henry’s Law constant and [NH3] aq theconcentration of ammonia in the aqueoussolution. The constant, H, is as previouslydiscussed, a function of temperature but also ofthe total concentration of dissolved species inthe solution. Thus the temperature and totalconcentration of dissolved species in samplesshould be as closely similar as possible to thoseof the calibration standards.

The effect of the osmotic pressure of samples onthe probe response is discussed in Reference 1.

4 THEORY…3 PROBE MEASUREMENTS

17

Concentration Range0.1 mgl–1 to 1000mgl–1 NH3(5 x 10–6M to 5 x 10–2M)

pH Range:7 to 14pH

Temperature Range:5 to 40C

Reproducibility:Better than 2% of concentration

Minimum Sample Size5ml in a 50ml beaker

StorageProbe should be stored in 0.1M ammoniumchloride solution.

Caution. Do not store dry.

LifeUnder normal operating conditions the probe lifeis at least 60 days without renewal of membrane

DimensionsTotal Length 152mmLength below cap 105mmStem diameter 17.5mm

Item Part No.Combination electrode 8002 650assembly

Flow-through cap 8002 830

Internal filling solution 8002 240(50ml bottle)

End cap 8002 820

Spares kit 8002 260comprising:

10 membranes1 silicone washer1 pH electrode O-ring2 O-rings (end cap & top cap)1 O-ring (for use in flowcell)

5 SPECIFICATION 6 SPARES & ACCESSORIES

18

APPENDIX

A.1 Procedure for Chloridising theReference Element

A.1.1 Requirements

1) Chemicals:a) Plating solution Hydrochloric acid 0.1M

(500ml)

Note. Do not use acid preserved withmercuric ions.

b) Plating solution

c) Cleaning solutions:Ammonia solution 50% vol./vol. (200ml)

To prepare, dilute 100ml A.R. concentratedammonia solution, s.g. 0.88, with100ml distilledwater and stir.

Nitric acid, 25% vol./vol. (200ml)

To prepare, cautiously pour 50ml A.R.concentrated nitric acid, s.g. 1.42, into 150mldistilled water, stirring continuously. Allow tocool before use.

Caution. Strong ammonia solutions andstrong nitric acid are both very poisonousand corrosive. Care should be taken not toinhale the fumes of either or to get any onskin or clothing. Any slight spillage on skin,clothing or working surface should bewashed away with plenty of water.

2) Hardware:Constant current supply, 2mA d.c. output

Silver wire (counter electrode)

1 beaker

A.1.2 CleaningImmerse the lower 5cm of the combinationelectrode (i.e. so that the silver referenceelement is covered) in the ammonia solution forabout one minute. Remove and rinse withdistilled water.

Immerse the electrode to the same depth in the25% nitric acid until the silver element is auniform creamy white. This process usuallytakes about one minute but the element must beinspected frequently as prolonged immersionis detrimental.

If the element is not uniform in colour repeat theammonia/nitric acid process.

When clean, rinse with distilled water andimmediately transfer to the plating solution suchthat the reference element is completelyimmersed.

A.1.3 ChloridisationAttach the screen of the combination electrodecable to the negative terminal of the constantcurrent supply and immerse the lower 5cm ofthe electrode in the plating solution so that thereference element is covered. Immerse a silvercounter electrode in the plating solution andconnect to the positive terminal of the supply.

Pass a current of 2mA for approximately 30seconds. Carefully tap the electrode to removeall bubbles and then reverse the connections tothe supply (i.e. the electrode to be plated isconnected to the positive terminal).

Pass a current of 2mA for 30 minutes after whichtime the electrode will be a dark brown or greycolour. A more uniform coating may be obtainedby employing moderate stirring during theprocess

Remove the electrode from the plating solution,rinse with /distilled water and dry with a tissue.

19

REFERENCES

1. Performance characteristics ofGas Sensing Membrane Probesby P.L. Bailey and M. RileyAnalyst, 1975, 100, 145.

2. Determination of Ammonia inCondensed Steam and BoilerFeed-water with aPotentiometric Ammonia Probeby D. Midgley and K. TorranceAnalyst, 1972, 97,626.

3. Manual determination of Ammoniain Fresh Waters using anAmmonia sensitive membraneelectrodeby M. J. Beckett and A. L. WilsonWater Research, 1974, 8, 333.

4. Estimation of Nitrogen with anAmmonia Probeby G. K. BuckeeJournal of the Institute of Brewing,1974, 80, 291.

5. Determination of Ammonia levels inWater and Waste water with anAmmonia Probeby W. H. Evans and B. F. PartridgeAnalyst, 1974, 99, 367.

6. Electrometric AmmoniaDeterminationin Plasmaby G.T.B. Sanders and W. ThorntonClinica Chimica Acta, 1973, 46, 465.

7. A simple Method for the Estimationof Plasma Ammonia using an IonSpecific Electrodeby N. J. Park and J.C.B. FentonJournal of Clinical Pathology, 1973,26, 802.

8. Use of the Ammonia Electrodefor Determination of Nitrogen inMeat Productsby P. M. ToddJournal of the Science of Food andAgriculture, 1973, 24, 488.

9. Continuous Determination of Ammonia inCondensed Steam and High purity BoilerFeed-water by Using a PotentiometricAmmonia Probe.by D. Midgley and K. TorranceAnalyst, 1973, 98, 217.

10. On-Stream Determination of Ammonia inBoiler Feed-water with an Ammonium IonSelective Electrode and an Ammonia Probeby J. Mertens, P. Van den Winkel and D. L.MassartBull. Soc. Chim. Belg., 1974, 83, 19.

11. Semi-automated Ammonia ProbeDetermination of Kjeldahl Nitrogen in Freshwatersby R. J. StevensWater Research, 1976, 10, 171.

12. Total Nitrogen Estimation in Herbage andSoils using the Ammonia Gas sensingElectrodeby A. J. ClarkLaboratory Practice, 1979, 28, No.10.

20

NOTES

NOTES

21

IM/8

00

2 R

ev. M

0

2.20

19

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1 INTRODUCTION 2 GENERAL INFORMATION 2.1 Requirements 2.2 Accessory Equipment 2.3 Probe Assembly Instructions 2.4 Storage 2.5 Internal Filling Solution

3 PROBE MEASUREMENTS 3.1 General Procedure 3.2 Buffer Solution – Sample pH 3.3 Probe Calibration 3.4 Probe Range and Response 3.5 Temperature Effects 3.6 Interference 3.7 Continuous flow analysis 3.8 The Known Addition Method 3.9 Modification of the Internal Filling Solution 3.10 Analysis of Kjeldahl Digests 3.10.1 Method 3.10.2 Standard Solutions 3.10.3 Probe Filling Solution

3.11 Drift 3.12 Ageing of the Glass Electrode 3.13 Checking the Performance of the Glass Electrode 3.14 Aging of the Reference Element

4 THEORY 5 SPECIFICATION 6 SPARES & ACCESSORIES APPENDIX A.1 Procedure for Chloridising the Reference Element A.1.1 Requirements A.1.2 Cleaning A.1.3 Chloridisation

REFERENCES NOTES