AMMONIA User Manual
AMMONIA_User 9/09 5
Some'typical'applications'using'ammonia'sensors'are:'
-municipal'water'plant'chloramination'monitoring
-wastewater'of'gas'scrubbers
-wastewater'monitoring
-ammonia'leaks'in'cooling'towers
-the'food'sector,'e.'g.'for'wine'and'beer
-laboratories.
The'measurement'can'be'made'directly'in'the'liquid'medium.'This'means'that'it'is'not'necessary'to'carry'out'time-
consuming'and'complicated'sample'preparation.'Colored'or'cloudy'samples'can'also'be'measured.''The'pH'value'
must'be'greater'than'7.3'or'the'chemical'equilibrium'of'the'ammonium'ions'shifts'towards'the'ammonia'state.'
If'the'sample'contains'substances'that'could'damage'the'sensor'membrane,'such'as'oils,'fats'or'tensides,'then'the'
ammonia'can'still'be'measured,'using'the'technique'known'as'“head'space”.'In'this'case,'damage'to'the'sensor'is'
ruled'out,'because'there'is'no'direct'contact'between'the'sensor'and'the'sample.'Measurement'is'performed'in'a'
gas-tight'space'above'the'substance'being'measured.''This'gas-tight'space'above'the'substance'being'measured'is'in'
equilibrium'with'the'liquid'as'far'as'ammonia'is'concerned.'
The'simple'sample'preparation'makes'determination'of'the'ammonia'concentration'a'fast'and'economical'operation'
that'can'be'applied'with'high'precision'over'a'wide'range'of'concentration.'''
Principles of analyzer operation
The'ammonia'sensor'consists'of'a'glass'pH'electrode'and'a'reference'electrode'which'are'surrounded'by'a'common'
electrolyte'that'is'separated'from'the'liquid'being'measured'by'a'hydrophobic,'gas-permeable'membrane.'
A'thin'layer'of'electrolyte'is'between'the'hydrophobic'membrane'and'the'glass'pH'electrode.'Its'pH'value'increases'
when'NH3'gas'makes'its'way'through'the'membrane.'The'chemical'processes'in'the'electrolyte'layer'are'described'
by'the'following'reaction'equation.'
NH3'+'H2O'à'NH4+'+'OH-(1)'
This'means'that'NH3'functions'as'a'base'and'thus'increases'the'pH'in'the'thin'electrolyte'layer'in'front'of'the'glass'
pH'electrode.'Since'there'is'only'a'very'small'volume'of'liquid'between'the'hydrophobic'PTFE'membrane'and'the'
glass'electrode,'the'electrode'reacts'very'sensitively'to'the'smallest'amount'of'ammonia.'
A'strongly'simplified'derivation'leads'to'the'following'relationship'between'the'sensor'signal'and'the'concentration'
of'ammonia'in'the'substance'being'measured:'The'reaction'(Equation'1)'is'described'by'the'law'of'mass'action:'
K'='[NH4+]'[OH-]'/'[NH3] (2)'
K'='equilibrium'constant'
[x] ='concentration'component'x
