ISO 10378:2005

INTERNATIONAL ISO
STANDARD 10378
Second edition
2005-07-01
Copper, lead and zinc sulfide
concentrates — Determination of gold
and silver — Fire assay gravimetric and
flame atomic absorption spectrometric
method
Concentrés sulfurés de cuivre, de plomb et de zinc — Dosage de l'or et
de l'argent — Méthode gravimétrique par essai au feu et spectrométrie
d'absorption atomique dans la flamme

Reference number
©
ISO 2005
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ii © ISO 2005 – All rights reserved

Contents Page
Foreword. v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Principle. 2
3.1 Fusion . 2
3.2 Cupellation . 2
3.3 Parting. 2
3.4 Retreatment . 2
3.5 Correction for blank contamination. 2
4 Reagents. 2
5 Apparatus . 4
6 Sample . 5
6.1 Test sample . 5
6.2 Test portion . 5
7 Procedure . 5
7.1 Number of determinations . 5
7.2 Trial fusion. 5
7.3 Blank tests. 5
7.4 Charge preparation. 6
7.5 Primary fusion. 7
7.6 Cupellation . 7
7.7 Retreatment of residues. 8
7.8 Determination of gold in the primary bead . 8
7.9 Determination of gold and silver in secondary beads and blanks, and of silver in prills. 9
7.10 Determination of silver in the parting solution. 11
8 Expression of results . 11
8.1 Mass fraction of gold. 11
8.2 Mass fraction of silver. 12
9 Precision. 13
9.1 Expression of precision . 13
9.2 Method for obtaining the final result (see Annex H) . 14
9.3 Precision between laboratories. 15
9.4 Check of trueness. 17
9.4.1 General. 17
9.4.2 Type of certified reference material (CRM) or reference material (RM) . 17
10 Test report . 18
Annex A (normative) Procedure for the preparation and determination of the mass of a predried
test portion . 19
Annex B (normative) Trial fusion. 21
Annex C (normative) Blank determination . 22
Annex D (normative) Inquartation .23
Annex E (normative) Determination of vaporization loss of silver during the cupellation process . 24
Annex F (normative) Sulfuric acid - Parting . 25
Annex G (normative) Determination of impurities in parting solutions and washings. 27
Annex H (normative) Flowsheet of the procedure for the acceptance of analytical values for test
samples (see 9.2) . 31
Annex I (informative) Flowsheet of the method. 32
Annex J (informative) Roasting method. 33
Annex K (informative) Guide to the preparation of dilutions for the determination of silver in
parting solutions and residues. 34
Annex L (informative) Derivation of precision equations. 35
Bibliography . 52

iv © ISO 2005 – All rights reserved

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 10378 was prepared by Technical Committee ISO/TC 183, Copper, lead, zinc and nickel ores and
concentrates.
This second edition cancels and replaces the first edition (ISO 10378:1994), which has been technically
revised.
Introduction
This International Standard describes a method for the determination of the mass fraction of gold and silver in
copper, lead and zinc sulfide concentrates. This International Standard was prepared to enable laboratories to
determine the mass fraction of gold and silver in suitable samples using instrumental methods.

vi © ISO 2005 – All rights reserved

INTERNATIONAL STANDARD ISO 10378:2005(E)

Copper, lead and zinc sulfide concentrates — Determination of
gold and silver — Fire assay gravimetric and flame atomic
absorption spectrometric method
WARNING — This International Standard may involve hazardous materials, operations and equipment.
It is the responsibility of the user of this International Standard to establish appropriate health and
safety practices and determine the applicability of regulatory limitations prior to use.
1 Scope
This International Standard specifies a fire assay gravimetric and flame atomic absorption spectrometric
method for the determination of the mass fraction of gold and silver in copper, lead and zinc sulfide
concentrates as follows:
 Copper concentrates:
The method is applicable to the determination of mass fractions of gold from 0,5 g/t to 300 g/t and of
mass fractions of silver from 25 g/t to 1 500 g/t in copper sulfide concentrates containing mass fractions of
copper from 15 % to 60 %.
 Lead concentrates
The method is applicable to the determination of mass fractions of gold from 0,1 g/t to 25 g/t and of mass
fractions of silver from 200 g/t to 3 500 g/t in lead sulfide concentrates containing mass fractions of lead
from 10 % to 80 %.
 Zinc concentrates
The method is applicable to the determination of mass fractions of gold from 0,1 g/t to 12 g/t and of mass
fractions of silver from 10 g/t to 800 g/t in zinc sulfide concentrates containing mass fractions of zinc up to
60 %.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 385-1:1984, Laboratory glassware — Burettes — Part 1: General requirements
ISO 648:1977, Laboratory glassware — One-mark pipettes
ISO 1042:1998, Laboratory glassware — One-mark volumetric flasks
ISO 3696:1987, Water for analytical laboratory use — Specification and test methods
ISO 4787:1984, Laboratory glassware — Volumetric glassware — Methods for use and testing of capacity
ISO 9599:1991, Copper, lead and zinc sulfide concentrates — Determination of hygroscopic moisture in the
analysis sample — Gravimetric method
3 Principle
Fire assaying for the determination of gold and silver comprises a series of steps to separate firstly the
precious metals from most of the associated metals, followed by separation of the gold from silver and other
metals preconcentrated into a precious-metal alloy.
The stages that comprise the determinations are described in 3.1 to 3.5 inclusive.
3.1 Fusion
The samples are fused in a crucible after mixing with a litharge-based flux which, under reducing conditions,
collects the precious metals in a metallic lead button.
3.2 Cupellation
The base metals present in the lead button are substantially separated from the precious metals by oxidizing
fusion. Cupellation produces a bead largely comprising a silver-gold alloy with small quantities of other metals.
3.3 Parting
Gold is separated from the primary bead by treatment with nitric acid. The gold prill is weighed. Gold prills
having a mass less than 50 µg are dissolved in aqua regia and the gold is determined by atomic absorption
spectrometry (AAS). Silver is determined in the parting solution by AAS.
3.4 Retreatment
All residues are retreated to maximize the recovery of gold and silver. The addition of collectors for either gold
or silver is not required, as both metals are present in sufficient amounts to be readily visible after the
cupellation stage. The second bead is dissolved in acids followed by analysis of both metals by AAS.
3.5 Correction for blank contamination
Contamination by gold and silver impurities in the reagents is corrected for by fusing the reagents without the
test portion.
4 Reagents
During the analysis, use only reagents of recognized analytical grade and water that complies with grade 2 of
ISO 3696.
4.1 Sodium carbonate, anhydrous.
4.2 Litharge (PbO), assay grade having a mass fraction of gold of less than 0,01 g/t and a mass fraction of
silver of less than 0,2 g/t.
4.3 Silica, precipitated.
4.4 Potassium nitrate or sodium nitrate
NOTE If sodium nitrate is used, the masses specified for potassium nitrate will have to be modified:
85,0
gof KNO×= gof NaNO
101,1
4.5 Flour
2 © ISO 2005 – All rights reserved

4.6 Borax, fused anhydrous sodium tetraborate (borax glass powder).
4.7 Nitric acid, concentrated (ρ 1,42 g/ml), chloride concentration < 0,5 µg/ml.
4.8 Nitric acid, diluted 1+1.
Slowly add 500 ml of concentrated nitric acid (4.7) to 500 ml of water, while stirring.
4.9 Lead, foil, having a mass fraction of gold of less than 0,01 g/t and a mass fraction of silver of less than
0,2 g/t.
4.10 Silver, of minimum purity 99,99 %.
4.11 Hydrochloric acid (ρ 1,16 g/ml to 1,19 g/ml).
4.12 Thiourea, 10 g/l solution.
Add 1 g of thiourea to 100 ml of water.
4.13 Aqua regia
Mix 3 parts of hydrochloric acid (4.11) with 1 part of nitric acid (4.7). Prepare freshly as required.
4.14 Standard solutions
Standard solutions should be prepared at the same ambient temperature as that at which the determinations
will be conducted.
4.14.1 Silver, standard stock solution A (500 µg of Ag/ml).
Weigh 0,500 0 g of silver metal to the nearest 0,1 mg. Transfer to a 100 ml beaker, add 20 ml of diluted nitric
acid (4.8) and warm to dissolve. Cool and add 20 ml of concentrated nitric acid (4.7). Transfer to a 1 000 ml
volumetric flask, fill up with water nearly to the mark, mix and cool to room temperature; then fill up exactly to
the mark and mix again.
4.14.2 Silver, standard solution B (50 µg of Ag/ml).
Pipette 10,00 ml of silver standard stock solution A (4.14.1) into a 100 ml volumetric flask, fill up with water
nearly to the mark, mix and cool to room temperature; then fill up exactly to the mark and mix again.
Prepare a fresh solution per batch.
4.14.3 Gold, standard solution (1000 µg of Au/ml).
Weigh 1,000 g of gold metal to the nearest 0,1 mg. Transfer to a 200 ml beaker, add 25 ml of aqua regia
solution (4.13) and warm to dissolve. Cool and transfer to a 1 000 ml volumetric flask. Add 75 ml of
hydrochloric acid (4.11), fill up nearly to the mark with water, mix and cool to room temperature; then fill up
exactly to the mark and mix again.
4.14.4 Gold and silver, standard solution (100 µg of Au/ml + 50 µg of Ag/ml).
Pipette 10,00 ml of silver standard stock solution A (4.14.1) into a 100 ml volumetric flask. Add 40 ml of
hydrochloric acid (4.11). Pipette 10,00 ml of gold standard solution (4.14.3) into the volumetric flask. Fill up
nearly to the mark with water, mix and cool to room temperature; then fill up exactly to the mark and mix again.
4.15 Calibration solutions
Calibration solutions should be prepared at the same ambient temperature as that at which the determinations
will be conducted.
4.15.1 Gold/silver calibration solutions
Pipette 0,0 ml, 1,00 ml, 2,00 ml, 5,00 ml and 10,00 ml of gold and silver standard solution (4.14.4) into a
series of 100 ml one-mark volumetric flasks.
Add 40 ml of hydrochloric acid (4.11) to each flask, fill up nearly to the mark with water, mix and cool to room
temperature; then fill up exactly to the mark and mix again.
These solutions contain 0,0 µg of Au/ml, 1,00 µg of Au/ml, 2,00 µg of Au/ml, 5,00 µg of Au/ml and 10,00 µg of
Au/ml; and 0,0 µg of Ag/ml, 0,50 µg of Ag/ml, 1,00 µg of Ag/ml, 2,50 µg of Ag/ml and 5,00 µg of Ag/ml and
shall be freshly prepared.
4.15.2 Silver calibration solutions
Pipette 0,0 ml, 1,00 ml, 2,00 ml, 4,00 ml, 6,00 ml, 8,00 ml and 10,00 ml of silver standard solution B (4.14.2)
into a series of 100 ml volumetric flasks. Add 10 ml of nitric acid (4.7), fill up nearly to the mark with water, mix
and cool to room temperature; then fill up exactly to the mark and mix again.
These solutions contain 0,0 µg of Ag/ml, 0,50 µg of Ag/ml, 1,00 µg of Ag/ml, 2,00 µg of Ag/ml, 3,00 µg of
Ag/ml, 4,00 µg of Ag/ml and 5,00 µg of Ag/ml, and shall be freshly prepared.
Contamination by gold and silver impurities in the reagents is corrected for by fusing the reagents without the
test portion.
5 Apparatus
5.1 Assay crucible furnace, with a maximum required operating temperature of 1 200 °C.
5.2 Muffle furnace, with a maximum required operating temperature of 1 100 °C. Temperature indication,
automatic temperature control and controlled air flow are preferable.
5.3 Assay crucibles, made of fire clay, of nominal capacity 200 ml to 600 ml, capable of withstanding
corrosion by the samples and fluxes at 1 100 °C. The crucible shall be of such a size that the charge does not
fill the crucible to a depth greater than 3/4 the depth of the crucible.
5.4 Cupels, made of magnesium oxide, or bone-ash cupels having a nominal capacity of 50 g of molten
lead. The inside bottom of the cupel shall be concave, as recommended in the fire assay texts referred to in
the Bibliography.
5.5 Conical mould, made of cast iron, of sufficient capacity to contain all of the molten lead plus slag from
the crucible fusion.
5.6 Analytical balance, sensitive to 1 mg.
5.7 Microbalance, sensitive to 1 µg or less.
5.8 Ordinary laboratory glassware, washed free of chlorides.
5.9 Volumetric glassware, of class A complying with ISO 385-1, ISO 648 and ISO 1042, and used in
accordance with ISO 4787.
5.10 Atomic absorption spectrometer (AAS), equipped with background correction and a glass bead in
the spray chamber.
5.11 Inductively coupled plasma (ICP) atomic emission spectrometer
5.12 Pulverizer
5.13 Hotplate
4 © ISO 2005 – All rights reserved

6 Sample
6.1 Test sample
Prepare an air-equilibrated test sample in accordance with ISO 9599.
NOTE A test sample is not required if predried test portions are to be used (see Annex A).
6.2 Test portion
Taking multiple increments, extract a test portion from the test sample in such a manner that it is
representative of the whole contents of the dish or tray. Weigh to the nearest 1 mg approximately 10 g to 20 g
of the test sample. At the same time as test portions are being weighed for analysis, weigh test portions for
the determination of hygroscopic moisture in accordance with ISO 9599.
Alternatively, the method specified in Annex A may be used to prepare predried test portions directly from the
laboratory sample.
If a mass fraction of arsenic above 2 % is present in the sample, this element should be removed by following
the procedure in Annex J; otherwise, interference with the cupellation stage may occur.
NOTE If the mass fraction of copper is greater than 30 %, a 10 g or 15 g test portion is preferable (see the fourth
paragraph of 7.4).
For lead concentrates, the test portion should be 10 g to ensure an adequate supply of lead.
7 Procedure
7.1 Number of determinations
Carry out the determinations at least in duplicate, as far as possible under repeatability conditions, on each
test sample.
NOTE 1 Repeatability conditions exist where mutually independent test results are obtained with the same method on
identical test material in the same laboratory by the same operator using the same equipment, within short intervals of
time.
NOTE 2 In the case where the ratio of silver to gold does not exceed 2,5 to 1 and the procedure specified in 7.10 is
carried out for the silver determination, separate determinations for gold and silver will be necessary (see Annex D). Four
test portions, therefore, are required, i.e. two for determinations of gold and two for silver.
7.2 Trial fusion
Carry out a trial fusion as described in Annex B, to ensure that the mass of the lead button is between 30 g
and 45 g.
7.3 Blank tests
Carry out a reagent blank test as described in Annex C in parallel with the analysis, using the same quantities
of all reagents, with the addition of sufficient flour (4.5) to the flux to give a lead button of between 30 g and
45 g. Omit the test portion and the potassium nitrate. The total blank should not exceed 5 µg of gold or 100 µg
of silver.
7.4 Charge preparation
Determine the mass of potassium nitrate (4.4) and flour (4.5) required in the charge, as indicated by the trial
fusion (see Annex B), and include this reagent in the flux mixture. Typical masses of the flux components for
copper, lead and zinc concentrates are shown in Tables 1 to 3 respectively.
Thoroughly mix the test portion with a flux.
Intimate mixing of flux components and the test portion is very important. All flux components should be in a
finely divided state with a preferred particle size of less than 0,5 mm.
Table 1 — Typical masses of flux components for copper concentrates
Mass
Flux components
g
Sodium carbonate (4.1) 30
Litharge (4.2) 210
Silica (4.3) 25
Potassium nitrate (4.4) —
Flour (4.5) —
Test portion 20
NOTE If the mass fraction of copper is greater than 30 %, the mass of litharge should be 30 times that of copper plus
35 g for the lead button. Alternatively, a 10 g or 15 g test portion can be used while retaining the flux composition given in
Table 1. If there are difficulties experienced in achieving a fluid melt, the amount of silica recommended in Table 1 can be
reduced to 19 g, while including 6 g borax.
Table 2 — Typical masses of flux components for lead concentrates
Mass
Flux components
g
Sodium carbonate (4.1) 30
Litharge (4.2) 100
Silica (4.3) 10
Borax (4.6) 10
Potassium nitrate (4.4) —
Flour (4.5) —
Test portion 10
Table 3 — Typical masses of flux components for zinc concentrates
Mass
Flux components
g
Sodium carbonate (4.1) 30
Litharge (4.2) 120
Silica (4.3) 10
Borax (4.6) 10
Potassium nitrate (4.4) —
Flour (4.5) —
Test portion 20
Place the mixture in an assay crucible (5.3).
6 © ISO 2005 – All rights reserved

7.5 Primary fusion
Place the crucibles in the furnace (5.1) preheated to approximately 900 °C.
If oil-fired or gas-fired furnaces are used, the fuel should be turned off immediately before opening the furnace.
Slowly raise the furnace temperature to 1 000 °C to 1 050 °C. Maintain this temperature until the fusion has
been tranquil for at least 10 min.
The optimum furnace setting temperature depends on the furnace structure and the position of the
thermometer. If unfused material remains in the bottom of assay crucibles, the setting temperature of the
furnace should be raised and the assay repeated.
To minimize crucible corrosion and build-up of impurities in the lead button, the overall fusion time should not
exceed 40 min. However, should the fusion take an extended time to settle, the fusion time may need to
exceed 40 min.
It is recommended that a fire-clay lid or a layer of salt or borax approximately 12 mm thick be used with this
fusion to prevent loss of material by dusting or ebullition. If borax is used as a cover, the amount used in the
flux may be reduced accordingly. If some ‘shotting’ of the lead is noted in the slag (this is common for zinc
concentrates), it is recommended that the extra borax be retained.
Pour the mixture into a dry conical mould (5.5), taking care that no loss of lead or slag occurs. Reserve the
crucible for retreatment fusion.
Allow the mixture to cool and carefully separate the lead button from the slag. Hammer the lead button as
necessary to remove any small particles of adhering slag. Reserve the slag for retreatment.
Weigh the lead button. If the button weighs less than 30 g or more than 45 g, discard the button and slag and
repeat the assay after appropriate adjustment of the oxidizing agent (see Annex B).
NOTE Buttons weighing less than 30 g may show poor collection efficiencies, whereas those exceeding 45 g may
contain higher amounts of copper and other base metals.
7.6 Cupellation
Place the lead button obtained in 7.5 into a preheated cupel (5.4) in a muffle furnace (5.2) at 900 °C. Allow the
cupellation to proceed at the lower muffle temperature of approximately 860 °C with a steady air flow.
Variations depend on the cupel type and furnace conditions. In the case of cupels made of bone ash, a
cupellation temperature of 820 °C is recommended.
NOTE 1 In the case where only gold is being determined, it could be effective to raise the furnace temperature to
900 °C to finish the cupellation after the visible lead melt on the cupel is approximately 10 mm in diameter [approximately
80 % (mass fraction) of lead absorbed].
High cupellation temperatures will cause higher silver losses and low temperatures can cause “freezing” of the
bead and incomplete cupellation. It is recommended that loss of silver during the cupellation process be
determined, to decide upon the furnace conditions (see Annex E).
Remove the cupel from the furnace and cool.
Carefully extract the primary bead and remove any adhering cupel material with a brush. Flatten the bead
slightly and place in a 30 ml porcelain crucible.
NOTE 2 A test tube can be used instead of a porcelain crucible.
NOTE 3 If the ratio of silver to gold in the primary bead is greater than 2,5 to 1, the silver can be determined by the
gravimetric method instead of the procedure specified in 7.10. For the gravimetric method, weigh the primary bead, in
micrograms, to the nearest 1 µg (m ), carry out the parting by the procedure specified in 7.8, and determine the impurities
in the parting solutions and washings by the procedure specified in Annex G.
NOTE 4 If it is difficult to recover the bead because of its small size, 1 mg of palladium can be added before fusion. In
this case, the palladium bead is dissolved and determined by the procedure specified in 7.9.
Reserve the cupel for retreatment of residues.
7.7 Retreatment of residues
Place both the cupel and the slag in a pulverizer (5.12) and pulverize for about 20 s to reduce the material to
minus 150 µm.
If magnesium cupels are used, it is recommended that the slag and the cupel be retreated separately.
NOTE 1 Longer grinding can cause caking of the material and heating of the grinding barrel.
NOTE 2 The pulverizer may be cleaned between samples by grinding small portions of broken glass or quartz.
Thoroughly mix the ground residues with a flux. Typical composition of the flux is shown in Table 4.
Table 4 — Typical masses of flux components for the retreatment of residues
Mass
g
Flux components
Magnesium oxide
Bone ash cupel
cupel
Sodium carbonate (4.1) 50 to 60 40
Litharge (4.2) 50 to 60 45
Silica (4.3) 50 to 60 20
Flour (4.4) 4 2 to 3
Borax (4.6) 30 to 50 15
The mass of flour shown in Table 4 is typical. The mass should be sufficient to produce a 30 g to 45 g lead
button.
The combined mass of slag and cupel of the primary fusion, in addition to the flux components given in
Table 4, may exceed the capacity of the assay crucibles, or the re-fusion may be so reactive that the fusions
may froth over. In these cases, it is permissible to split the residues into equal halves and fuse separately in
two crucibles. The lead buttons obtained should be cupelled separately, or be scorified together and the
resultant lead button cupelled.
Place the mixture in the original assay crucible.
Carry out the fusion as detailed in 7.5 and discard the crucible and slag.
Cupel the lead button as detailed in 7.6 to obtain a second bead and discard the cupel.
7.8 Determination of gold in the primary bead
Add 10 ml of dilute nitric acid (4.8) to the primary bead in the porcelain crucible prepared in 7.6 and heat
gently on a hotplate (5.13) for 20 min or until the reaction ceases.
NOTE 1 It is essential that chloride be absent during parting; otherwise, some of the gold may dissolve.
When the bead is treated with hot dilute nitric acid, silver will start to dissolve provided that the ratio of silver to
gold in the bead exceeds 2,5 to 1. The rate of dissolution increases with increasing mass fraction of silver of
8 © ISO 2005 – All rights reserved

the bead. Rapid attack of the bead should be avoided by further dilution and slow heating to prevent
disintegration of the gold. Should the ratio of silver to gold be less than 2,5 to 1, as shown by failure to part in
hot dilute nitric acid, the bead should be inquarted (see Annex D).
If there is danger of the gold sponge crumbling during the parting operation, it is recommended that the
operation be carried out with sulfuric acid (see Annex F).
Carefully pour the solution into a 200 ml beaker by decantation to avoid losses.
Add 15 ml of warm dilute nitric acid (4.8) to the porcelain crucible and continue heating gently until parting is
complete. This should take approximately 25 min.
Carefully pour the solution into the 200 ml beaker by decantation to avoid losses. Wash the crucible and gold
with four 15 ml volumes of hot water. Collect all the washings in the same 200 ml beaker. Reserve the solution
for the determination of silver as specified in 7.10.
NOTE 2 The possibility of gold particles occurring in the collected parting and washing solutions can be determined by
evaporating the solutions slowly down to 2 ml to 3 ml, then continuing with the determination as specified in 7.9.
Dry the gold sponge in the porcelain crucible on the hotplate.
Place the crucible in the muffle furnace (5.2) to anneal the gold at dull red heat for approximately 5 min.
Cool and weigh the resultant gold prill, in micrograms, to the nearest 1 µg (m ).
If the mass of the gold is less than 50 µg, it is recommended that the gold be dissolved and determined by the
procedure specified in 7.9.
If the mass of the gold is less than 50 µg, repeat the fusion and cupellation, then dissolve the prepared bead
and determine the gold and silver concentration as specified in 7.9 without the parting operation. This
alternative procedure is recommended where there is a danger of the gold sponge crumbling during the
parting operation. The procedure, however, cannot be applied if the product of mass of test portion by mass
fraction of silver, i.e. mass in test portion, is larger than 7 500 µg.
NOTE 3 If the sensitivity of the microbalance is 0,1 µg, the applicable range of the gravimetric method can be extended
to 5 µg of gold. In such a case, weigh the gold prill, in micrograms, to the nearest 0,1 µg (m ).
Reserve the gold prill to determine silver in the prill. The prill is dissolved and the silver concentration is
determined as specified in 7.9. Several of the weighed prills of the same laboratory sample can be combined
for the determination.
Platinum and palladium are removed from the prill during parting with nitric acid. If the determination of these
elements remaining in the prill is considered necessary, determine these by the procedure specified in 7.9,
followed by the addition of platinum and palladium to the standard solutions in relevant proportions. If a
sufficient detection limit for AAS or ICP cannot be obtained on a single prill basis, a large number of prills of
the same laboratory sample should be combined.
7.9 Determination of gold and silver in secondary beads and blanks, and of silver in prills
For blanks and samples determined by the procedure specified in the fourth last paragraph of 7.8 without the
parting operation, the primary and secondary beads should be combined and treated together.
Transfer the bead(s) or prill(s) to a test tube or a porcelain crucible. Add 2 ml of nitric acid (4.7) and warm in a
heating block or a sand bath set at approximately 98 °C. Add 6 ml of hydrochloric acid (4.11) and heat again
to dissolve the gold. If necessary, add a further 2 ml of nitric acid (4.7). Take the above solution, or that
prepared according to Annex C, and heat almost to dryness.
The solution should not be allowed to evaporate to dryness; otherwise, metallic gold will form.
Remove the test tube or the crucible from the heating block or the sand bath and allow to cool. Add 10 ml of
hydrochloric acid (4.11) and mix or swirl to dissolve any salts. Transfer quantitatively to a 50 ml volumetric
flask, add 10 ml of hydrochloric acid (4.11), make up to the mark with water and mix well.
Depending upon the mass fraction of silver (see Annex K), it may be necessary to make dilutions so that the
concentration of silver in the test solutions is in the range covered by the silver calibration solutions (4.15.1).
Hydrochloric acid (4.11) should be added so that 40 ml of the acid is contained per 100 ml of the diluted test
solutions.
Aspirate the test solutions and gold/silver calibration solutions (4.15.1) into the atomic absorption
spectrometer (5.10) and measure the absorbance. As a guide, the atomic absorption settings shown in
Table 5 are recommended; however, the instrument should be optimized to be free from any interference and
to give maximum sensitivity and as near as practical to a linear relationship between absorbance and
concentration.
Table 5 — Recommended atomic absorption settings
Parameter Gold Silver
Flame air/acetylene(oxidizing)
Wavelength 242,8 nm 328,1 nm
Lamp current 4 mA 5 mA
Background corrector on off
Aspiration rate optimize for maximum signal
Integration time 3 s
Number of integrations 5
0,17 0,55
Absorbance of 5 µg/ml calibration solution

Perform three measurements on each test solution and calibration solution. Calculate, to three significant
figures, the mean absorbance for each solution, provided that the range of values does not exceed 0,003
absorbance units. If this range is exceeded, repeat the measurement.
In order to clean out the nebulizer system, it is recommended to aspirate a cleaning solution, which is, for
instance, prepared by carefully adding 500 ml of hydrochloric acid (4.11) and 100 ml of concentrated nitric
acid (4.7) to 400 ml of water, between measurements.
Plot a calibration graph of absorbance versus concentration and determine the gold and/or silver
concentrations, in micrograms per millilitre, in the test solutions, followed by calculation of the mass (m ), in
a
micrograms, of the gold and/or silver using the following equation:
m = ρ × TDF (1)
a
where
ρ is the mass concentration of gold and/or silver;
TDF is the total dilution factor.
Alternatively, an ICP atomic emission spectrometer (5.11) can be used for the determination of gold and silver
at the appropriate wavelength. Typical wavelengths are 242,8 nm for gold and 328,1 nm for silver; however,
the instrument should be optimized to be free from any interference and to give maximum sensitivity and as
near as practical to a linear relationship between absorbance and concentration. In order to improve the
precision, it is recommended that a simultaneous internal standard correction be adopted. Yttrium is generally
used as an internal standard.
10 © ISO 2005 – All rights reserved

During all AAS or ICP determinations, the test solutions and calibration solutions should have the same
temperature, as well as the same acid concentration.
7.10 Determination of silver in the parting solution
NOTE 1 This determination is applicable where the ratio of silver to gold is greater than 2,5 to 1 and the silver has been
parted from the gold as indicated in 7.8.
NOTE 2 The silver can be determined by the gravimetric method instead of the procedure specified in this subclause.
For the gravimetric method, determine the impurities in the parting solutions and washings by the procedure specified in
Annex G.
Take the parting solutions and washings, which were reserved in 7.8. Heat to evaporate to approximately
20 ml, cool, and then add 2 ml of nitric acid (4.8). Transfer quantitatively to a 100 ml volumetric flask, make up
to the mark with water and mix well.
If the solution turns cloudy, add 1 % of thiourea solution (4.12) drop by drop, while stirring until the solution is
clear. Add 2 ml in excess, then dilute to volume.
Depending upon the mass fraction of silver (see Annex K), it may be necessary to make dilutions so that the
concentration of silver in the test solutions is in the range covered by silver calibration solutions (4.15.2). Nitric
acid (4.7) should be added so that 10 ml of the acid is contained per 100 ml of the diluted test solutions.
Aspirate the test solutions and silver calibration solutions (4.15.2) into the atomic absorption spectrometer
(5.10) and measure the absorbance. As a guide, the atomic absorption settings shown in Table 5 are
recommended; however, the instrument should be optimized to be free from any interference and to give
maximum sensitivity, and as near as practical to a linear relationship between absorbance and concentration.
Perform three measurements on each test solution and calibration solution. Calculate, to three significant
figures, the mean absorbance for each solution, provided that the range of values does not exceed 0,003
absorbance units. If this range is exceeded, repeat the measurement.
Plot a calibration graph of absorbance versus concentration and determine the silver concentrations, in
micrograms per millilitre, in the test solutions, followed by calculation of the mass (m ), in micrograms, of the
a
silver using Equation (1).
Alternatively, an ICP atomic emission spectrometer (5.11) can be used for the determination of silver at the
appropriate wavelength. A typical wavelength is 328,1 nm; however, the instrument should be optimized to be
free from any interference and to give maximum sensitivity and as near as practical to a linear relationship
between absorbance and concentration. In order to improve the precision, it is recommended that a
simultaneous internal standard correction be adopted. Yttrium is generally used as an internal standard.
During all AAS or ICP determinations, the test solutions and calibration solutions should have the same
temperature, as well as the same acid concentration.
8 Expression of results
8.1 Mass fraction of gold
The mass fraction of gold of the test portion (w ), expressed in grams per tonne, is given by the following
Au
equation:
mm+−()m+m −m+m 100
12 B BR 5 W
w=× (2)
Au
mH100 −
where
m is the mass, in micrograms, of gold obtained in the primary bead (as weighed or as
determined by AAS);
m is the mass, in micrograms, of gold in the secondary bead;
m + m is the mass, in micrograms, of gold in the primary blank bead combined with the mass of
B BR
gold in the secondary blank bead;
m is the mass, in micrograms, of silver remaining in the prill (in the case of AAS being used to
obtain m , m = 0);
1 5
m is the mass, in micrograms, of gold in the parting and washing solutions;
w
m is the mass, in grams, of the test portion;
H is the hygroscopic moisture content, in percent, of the test portion (in the case of a predried
test portion being used, H = 0).
8.2 Mass fraction of silver
If the silver is determined by the procedure specified in 7.10, the mass fraction of silver of the test portion
(w ), expressed in grams per tonne, is given by the following equation:
Ag
mm+−()m+m +m
34 B BR 5
w=× (3)
Ag
mH100 −
where
m is the mass, in micrograms, of silver in the parting and washing solutions;
m is the mass, in micrograms, of silver in the residue recovered;
m + m is the mass, in micrograms, of silver in the blank primary bead combined with the mass of
B BR
silver in the blank determination of the residue recovered;
m is the mass, in micrograms, of silver remaining in the prill;
m is the mass, in grams, of the test portion;
H is the hygroscopic moisture content, in percent, of the test portion (in the case of a predried
test portion being used, H =
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