3. Digestion with Liquid Reagents
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Transcript 3. Digestion with Liquid Reagents
Principles of Digestion Technology
Contents
1. Purpose and Objective
3. Digestion using Liquid Reagents
3.1 Introduction
3.2 Theory of Digestion Process
4. Digestion in Practice
4.1 Open Digestion at the Reflux
4.2 Pressure Digestion in Steel Tanks
4.3 Pressure Digestion by Microwave
5. Synopsis
1. Purpose and Objective
Purpose
Quantitative detection of elements in solids
usually by subsequent spectroscopic analysis
1. Purpose and Objective
Objective
Clear dissolution of solid
Complete destruction of matrix
avoiding disturbances in subsequent analysis
Avoidance of loss
3. Digestion with Liquid Reagents - Principle
Chemical digestion of sample matrix
Heating accelerates rate of reaction
Max. temperature in open digestion limited by boiling point of solution
Pressure build-up in closed vessels permits higher temperatures
3. Digestion with Liquid Reagents - General Aspects
General procedure
1. Homogenisation of sample
2. Weigh-in of a representative aliquot
3. Addition of digestion reagent
4. Supply of energy (usually heat)
3. Digestion with Liquid Reagents - General Aspects
Reagents
Acid
(e.g. with HCl, H2SO4...)
Base
(e.g. with NaOH, NH3...)
Oxidising
(e.g. with HNO3, H2O2, K2S2O8...)
Reductive
(e.g. with HJ, HBr...)
Complexing agent (e.g. H3BO3...)
3. Digestion with Liquid Reagents - General Aspects
Choice of reagents
Organic matrices
usually oxidising substances or mixtures
(HNO3, H2O2, K2S2O8 and possibly H2SO4)
Inorganic matrices
usually mixtures with HNO3, HCl, (also aqua regia), HF and possibly H2SO4
pure metals: HCl, aqua regia, HCl/HF
Oxides: H2SO4/HCl, H3PO4/HCl, mixtures containing HF
3. Digestion with Liquid Reagents - General Aspects
Organic matrices - choice of reagents
HNO3 (65%)
Universally used
For readily oxidisable samples (food, wood, fat, oil)
Nitrate or nitrogen must not interfere with analysis
Mixture of HNO3 (65%) / H2O2 (30%) approx. 4:1
Improved quality of digestion
No improvement for samples that are difficult to digest (e.g. plastics)
3. Digestion with Liquid Reagents - General Aspects
Organic matrices - choice of reagents
Aqueous samples (waste water)
Digestion with H2O2 (30%) / H2SO4 (1:1) mixture
Difficult-to-digest samples (e.g. plastics)
Digestion with HNO3 / H2SO4 (1:1) mixture
Carbon in matrix made more readily corridible by dehydration
Higher digestion temperatures due to lower vapour pressure of mixture
3. Digestion with Liquid Reagents - General Aspects
Inorganic matrices - choice of reagents
Pure metals
Digestion with HCl, aqua regia or HCl / HF mixtures
Oxides, including Al2O3 in particular
Digestion in H2SO4 / HCl or H3PO4 / HCl or HF mixtures
High proportion of high-boiling acid (approx. 80%) needed in order to
achieve highest digestion temperatures at moderate pressures
3. Digestion with Liquid Reagents - Theory
Digestion process - critical parameters
Digestion temperature
Digestion time
Chemical potential of digestion reagents
3. Digestion with Liquid Reagents - Theory
Digestion temperature
High digestion temperatures shorter reaction time
Digestion temperatures are limited by:
vapour pressure of digestion acids
temperature resistance of container/vessel materials
pressure resistance of containers/vessels
3. Digestion with Liquid Reagents - Theory
Pressure buildup in closed digestion
Total pressure p
p = p(CO2) + p(acid)
p(CO2)
p(acid)
= partial pressure of CO2 produced
= partial pressure of acid mixture
CO2 pressure:
dependent on carbon content of sample and weigh-in
p(CO2) = 6.9 * mc [g] * T/V [K/ml]
Example:
V = 30 ml, 0.2 g carbon, 200°C p(CO2) = 22 bar
V = 80 ml, 0.2 g carbon, 200°C p(CO2) = 8 bar
3. Digestion with Liquid Reagents - Theory
Example (60 ml vessel at 200°C):
500 mg carbon develops 930 ml CO2
partial CO2 pressure of 26 bar
Acid pressure for HNO3 at 200°C of approx. 10 bar
Total pressure approx. 36 bar (60 ml vessel at 200°C)
3. Digestion with Liquid Reagents - Theory
Vapour-pressure curves/graphs of pure acids
a. Aqua regia
b. HCl 36%
c. HNO3 91%
d. HCl 22.9%
e. Water
f. Boling point HNO3 100%
g. Boiling point H2SO4 100%
h. Boiling point H3PO4 96%
(Panholzer, LaborPraxis, Oct. 1994, 32)
T [°C]
3. Digestion with Liquid Reagents - Theory
Digestion time
Short digestion times recommended
greater throughput of samples
But good control of the process takes priority, e.g.:
Slow heating in warm-up phase avoids spontaneous reactions
Over-vigorous process control
unnecessary wear and tear on material
3. Digestion with Liquid Reagents - Theory
Chemical potential depends on
Concentration of reagents employed in the solution
Interaction of reagents
Interaction of reagents with sample water
Goal:
The concentration of acids should not reduce greatly
during digestion.
4. Practical Conduct of Digestion
Open method at reflux
Max. temperature limited by boiling point of acid mixture
( conc. H2SO4)
Allows high weigh-ins
Quality of digestion not always sufficient
Loss of volatile elements (e.g. Hg, lead salts)
4. Practical Conduct of Digestion
Pressure digestion in steel vessels
Pressure resistance 200 bar
Temperature max. 230°C (briefly 260°C)
Digestion times from about 2 hours to several days
Free from contamination due to PTFE-TFM lining
Different internal volumes (25-250 ml) and therefore weigh-in quantities
Outstanding quality of digestion
No loss of volatile elements (e.g. Hg, lead salts)
High degree of safety, easy operation
4. Practical Conduct of Digestion
Pressure digestion in steel vessels - specimen application
M a trix
W e ig h -in
A c id
C e llu lo se /sta rch
1000 m g
HNO3
1 6 0 °C
2h
F lo u r/g ra in /le a ve s
1000 m g
H N O 3 /H F
1 8 0 °C
2h
T issu e /live r
1000 m g
HNO3
1 7 0 °C -1 9 0 °C
2h
F a t/o il
1000 m g
H N O 3 (p o ss. H 2 O 2 )
2 0 0 °C
4h
P la stics
500 m g
H N O 3 /H 2 S O 4
2 0 0 °C
3 -4 h
C a rb o n /re sin
500 m g
HNO3
2 2 0 °C
6h
C e ra m ics/o xid e s
500 m g
HF or HCl
2 3 0 °C
2 -8 h
S te e l
500 m g
H N O 3 /H C l
2 0 0 °C
4h
Digestion Vessel DAB-3 (250 ml)
T e m p e ra tu re
T im e
4. Practical Conduct of Digestion
Pressure digestion with microwave heating
Pressure resistance dependent on type of container/vessel (40-100
bar)
Free from contamination through use of PTFE-TFM containers
Different interior volumes (10-100 ml) and therefore weigh-ins
Quality of digestion mostly sufficient
No loss of volatile elements (e.g. Hg, lead salts)
High throughput of samples due to short digestion times (10-60 mins.)
4. Practical Conduct of Digestion
Pressure digestion with microwave heating
It is primarily the sample that is heated
Container material (plastic) is only indirectly heated
Relatively high digestion temperatures can be reached for short periods
(30-40 minutes)
4. Practical Conduct of Digestion
Container materials
PTFE
maximum 260º C
PTFE-TFM
maximum 260º C
PFA
maximum 200º C
Quartz (silica) glass
maximum 1,000º C (theoretically)
4. Practical Conduct of Digestion
Temperature control
The most important aspect of controlling microwave digestion
Rate of reaction depends on temperature
The temperature in the various vessels may vary as a function of the
type of sample and the weigh-in quantity
Temperature control is required in all vessels
All samples can be subjected to non-contact IR temperature
measurement
speedwave MWS-3+
Temperature Control - Principle
IR-measurement at wavelength, where TFM does not absorb IR-radiation
Thermal radiation of the vessel is filtered out
Heat radiation of
vessel surface
Heat radiation
of sample
TFM
Mikrowave
radiation
IR-Sensor
Filter
IR-radiation
5. Synopsis
Work in closed vessel wherever possible
higher reaction temperature
better quality of digestion
Parameters determining digestion temperature, limited by:
pressure resistance of container/vessel
material of vessel
Digestion in steel vessel for “most difficult samples” or lower sample throughput
Digestion under microwave especially for high sample throughput