SPECIAL

Quantitative Analysis of Copper Sulfides
by a New Electrochemical Method
Shigeyoshi NAKAYAMA* and Tokiko UMEMOTO

A new voltammetric method using a strongly alkaline electrolyte (6 M KOH + 1 M LiOH) as the supporting electrolyte
was applied for simultaneous determination of copper oxides and sulfides. It was found that the reduction peak for Cu2S
was well separated from those of copper oxides and appeared at a slightly higher potential than that of Cu2O. On theother hand, Cu2O was reduced prior to the reduction of Cu2S in 0.1 M KCl, a commonly used electrolyte in conventional
methods. An electrolyte of 6 M KOH + 1 M LiOH, having a special ability to suppress the reduction of Cu2O, enabledreversal of the reductive potentials of Cu 2 O and Cu 2 S. As a result, the three peaks of Cu 2 O, Cu 2 S and CuO were
sufficiently separated. In the voltammetric measurement of a copper plate in 6 M KOH + 1 M LiOH containing S 2-, theexistence of Cu 2 S was confirmed by the emergence of its redox peak pair. In addition, Cu 9 S 5 was detected by its
reduction peak appearing at a potential slightly higher than that of Cu2S.
Keywords: voltammetry, 6 M KOH + 1 M LiOH, copper sulfides, copper oxides, quantitative analysis

1. Introduction
In April 2011, the authors were awarded the best paperfor “Quantitative Analysis of Copper Sulfides by Voltammetry Using a Strongly Alkaline Solution”(1) by the Japan Society of Corrosion Engineering (JSCE). The paper was
published in Zairyo-to-Kankyo, a journal covering techniques for corrosion and corrosion protection of metallic
materials. In the paper, a new method was described thatuses voltammetry to quantitatively determine copper corrosion products. The reduction mechanism was also clarified. Since both copper oxides and sulfides can be
determined by a single measurement, this method can beapplied for copper corrosion analysis.

2. Background of Investigation of a
New Analytical Method
The color of copper surfaces changes depending on
the thickness of copper oxide or copper sulfide(2). Tarnishing frequently reduces the commercial value of industrial
products. Problems, such as defective wire bonding(3),stress corrosion cracking(4) and contact resistance of connectors(5), have also been reported. Each chemical species
formed on copper surfaces has different properties. Therefore, qualitative and quantitative analyses of copper corrosion products are important.
The qualitative performance of surface analytical techniques, such as X-ray photoelectron spectroscopy (XPS)and X-ray diffraction (XRD), has been progressing in recent years. However, quantitative analysis of copper corrosion products continues to be an important task. On theother hand, in the 1930s, chemical state analysis of copperoxide films by chronopotentiometry (CP), one of various
electrochemical methods, was reported(6), and this method
was later applied for determination of both copper oxides
and chalcocite, Cu2S(2), (7). The supporting electrolytes usedin CP measurements were neutral or weak alkaline electrolytes such as 0.1 M KCl (M = mol dm-3). Researchers in
the field of copper corrosion have been interested in copper sulfide films in addition to copper oxide films. Recently, corrosion tests have been carried out by flowing
mixed gas containing H2S(8), (9).
In the U.S., the CP method using 0.1 M KCl for copperoxides was standardized by the American Society for Testing and Materials (ASTM). However, there is no method
for copper sulfides. The contents of the latest edition(10)are basically the same as those of the first report(6). It wasstated that the reduction of Cu2O occurs first, followed by
the reduction of CuO. However, no convincing evidence
has been presented. Although this order is extremely important for chemical state analysis of copper oxide films,
detailed re-examinations have not been carried out for
more than 70 years. In recent years, the opposite viewpoint
concerning the order has been reported(11), (12).
In our laboratory, copper plates having both Cu2O andCuO films were prepared and then measured by the ASTM
method. As a result, it was revealed that the separation ofcopper oxides was difficult. Considerable interest has beenshown in the influence of copper oxides on stress corrosion
cracking. Since quantitative values of Cu2O and CuO wererequired, investigation of a new analytical method wasneeded. The method we selected was voltammetry, inwhich peak-like curves were obtained under appropriate
analytical conditions. As a result of detailed investigation,a mixed electrolyte of 6 M KOH + 1 M LiOH as the supporting electrolyte was concluded to be the best electrolyte
for selective determination of copper oxides. The research
paper(13) was published in the Journal of the Electrochemical Society. A mixed electrolyte of 6 M KOH + 1 M LiOH
allowed perfect separation between two cathodic peaks dueto the reduction of Cu2O and CuO. It was also found thatin the course of voltammetric reduction, CuO was first reduced to Cu in one step, followed by the reduction of Cu2O
to Cu. The reduction of Cu2O is selectively suppressed due
not only to the alkalinity of the solution but also to the ef-

8 · Quantitative Analysis of Copper Sulfides by a New Electrochemical Method

fect of Li+ ions. In 0.1 M KCl, the reduction order was thesame as the above order of CuO → Cu2O(15). In Japan, theCopper Alloy Data Book (1st edition) by the Japan Copperand Brass Association (JCBA) shows the reduction orderof Cu2O → CuO in CP measurements. However, the reduction order was corrected to CuO → Cu2O in the 2nd edition(16) using the same technical paper referenced in the
1st edition.
After the new analytical method was investigated, numerical measurements were carried out for various copper
corrosion products. Although substantial results were obtained by this method, peaks at a shifted potential fromthat of Cu2O were occasionally detected. Based on researchof the history of such samples, detected peaks were assumed to show a reduction of copper sulfides. Investigation
was also conducted focusing on the reduction of copper
sulfides (mainly Cu2S). As a result, 6 M KOH + 1 M LiOH,
which was optimized for selective determination of copperoxides, could fortunately be applied for both copper oxides and Cu2S(1).
At the conference presentation, there were many questions concerning the extraordinary electrolyte of 6 M KOH+ 1 M LiOH. Recently, many researchers in the field of copper corrosion have been quoting our papers in Japan andoverseas.

3. Summary of Analytical Method
for Copper Sulfides
Details of our newly developed electrochemical method
are described in a textbook(17). Several important points concerning analytical operations are also shown in a paper(18).
Voltammetry, a well-known method of electrochemical
analysis, was applied for selective determination of copper
corrosion products. There are two main analytical modes:

linear sweep voltammetry (LSV) and cyclic voltammetry(CV). In LSV measurements, the potential of copper samples was swept from the rest potential to the potential forhydrogen evolution in the cathodic direction. After that
operation, in CV, the potential was swept back to the rest
potential. The reduction peaks could be separated wellwhen the sweep rate of the potential was selected appropriately for the amounts of corrosion products. The sweep
rate of figures in this paper is 10 mV s-1.
Electrochemical impedance spectroscopy (EIS) measurements were made over the frequency range from 10
kHz to 10 mHz with application of 1 mA DC current superimposed with 0.1 mA AC current. In the course of themeasurement, copper oxide films or copper sulfide films
had been reduced.
3-1 Reduction behavior of copper oxides andcopper sulfides
We obtained current-potential curves for powder standard samples (i.e., Cu2O, CuO and Cu2S) in two different
types of electrolytes. Although powder standard samples
are commercially available, it is difficult to measure them
directly by electrochemical methods. Therefore, prior tomeasurement, carbon paste was used for mixing with thepowder sample(19).
As shown in Fig. 1 A, reduction peaks for Cu2O andCuO were clearly separated in 6 M KOH + 1 M LiOH. However, in 0.1 M KCl (Fig. 1 B), peak separation between
Cu2O and CuO was not clear. On the other hand, the reduction peak for Cu2S was not greatly affected by the typeof electrolyte. In the voltammogram for a CuS powder sample, two peaks of comparable peak areas were detected(data not shown)(1).
The peak for Cu2S emerged at a slightly higher potential level than that of Cu2O in 6 M KOH + 1 M LiOH. Thisorder of the potential is thermodynamically abnormal. The
redox reaction formulas are expressed as follows:

A

B

0

0

Cu2O
Cu2S

Cu2S
I / mA-1

I / mA

-1

-2

CuO
-2

Cu2O

CuO

-3

-3
-1.5

-1.0

(E vs. Ag/AgCl) / V

-1.5

-1.0

(E vs. Ag/AgCl) / V

Fig. 1. Current-potential curves recorded by LSV for powder samples. Electrolyte solutions: (A) 6 M KOH + 1 M LiOH, (B) 0.1 M KCl. Scan rate: 10 mV s-1

SEI TECHNICAL REVIEW · NUMBER 74 · APRIL 2012 · 9

Cu2O + H2O +2e- → 2Cu + 2OH- E 0 = -0.365 V .... (1)Cu2S + 2e- → 2Cu + S2-

E 0 = -0.898 V .... (2)where E 0 is the normal electrode potential.
E 0 in Eq.2 has a more cathodic potential than that in
Eq.1. Its value (-0.898 V) is close to the actual reduction
potential (E) of Cu2S in Fig. 1. On the other hand, E in
Eq.1 depends on the pH value as follows(20):E = 0.471 – 0.0591 pH ............................................. (3)Even if a pH value of 15 is assigned to Eq.3, E shows amore positive value than -0.898 V. However, Cu2S was reduced prior to Cu2O. This specific behavior can not be
evaluated by thermodynamical theory. On the other hand,
Cu2O was reduced prior to Cu2S in neutral or weak alkaline
electrolytes(2),(7)-(9). The reduction order in Fig. 1 B is thesame as the reported one.
EIS measurements were carried out for detailed investigation of the reduction behavior of copper sulfides andcopper oxides. Test samples were Cu2S/Cu(1), Cu2O/Cu(21)and CuO/Cu(21) (Dumet wire: JIS H 4541, Sumitomo Electric Industries, Ltd., ø0.71 mm). The Cu2S/Cu sample wasobtained by immersing the copper plates in a 5 mM Na2S
solution at 25˚C for 8 h. Cu2S is known to be formed by thereaction between copper and S2- ions in an aqueous solution(22).
Figure 2 shows the electrochemical impedances for reductions of Cu2O/Cu and Cu2S/Cu in two types of electrolyte. These figures are called “cole-cole plots.” As seenin Fig. 2 A, the impedance behavior of Cu2O reduction varied appreciably depending on the type of electrolyte. In 6
M KOH + 1 M LiOH, the charge transfer resistance Rct(represented by the diameter of each semicircle in thehigh-frequency range) was incomparably greater than thatin 0.1 M KCl. Since the measurements were carried out
with reduction of Cu2O, the amplitude of Rct was inversely
proportional to the reduction rate of Cu2O. That is, the reduction of Cu2O was strongly suppressed in 6 M KOH + 1
M LiOH. In addition, an inductive loop was detected in the
low-frequency range. It is generally acknowledged that the
emergence of a loop suggests the presence of intermediate
reaction products(23).
Meanwhile, Rct representing Cu2S reduction (Fig. 2 B)
was practically comparable in the two types of electrolytes.
This behavior is compatible with the small difference in reduction potential for Cu2S (Fig. 1). Since the reduction of
Cu2O was selectively and strongly suppressed in 6 M KOH+ 1 M LiOH, the reduction order of Cu2O and Cu2S was inverted depending on which electrolyte was used.
Although Rct for CuO was slightly changed by the typeof electrolyte, the degree of change was small compared to
Rct for Cu2O. Accordingly, the ability to suppress the reduction of Cu2O contributes greatly to the separation of Cu2O
and CuO.
3-2 Sulfidation reaction of copper
Sulfur sources are necessary for sulfidation of copper.
Copper sulfides can form easily on copper surfaces whencopper plates are exposed to an atmosphere containing
H2S gas or immersed in a solution containing sulfide ions.
Copper plates were first ground using SiC #2000 abrasive-coated paper and then immersed in 5 mM Na2S for various periods. Figure 3 shows 100-magnified photos ofcopper samples observed with a stereomicroscope. The
color of the copper surface changed in the order of red →blue → gray by coherency of light. Figure 4 shows currentpotential curves of copper plates immersed in 5 mM Na2S
for 1, 10 and 60 min. The peak around -0.8 V to -0.9 V isthe reduction peak for Cu(OH)2(24). The reduction peakincreased at lower potential with increased treatment time.
Besides Cu(OH)2, two chemical species seemed to coexist

AB
Cu2O/Cu-500

Cu2S/Cu-100

C
Im (Z ) / Ω

Im (Z ) / Ω

Im (Z ) / Ω

0.1 M KCl

CuO/Cu

-100

0

-50

6 M KOH+1 M LiOH

0

6 M KOH + 1M LiOH

-50

0.1 M KCl

050

Re (Z ) / Ω100

0.1 M KCl

6 M KOH+1 M LiOH
0500

Re (Z ) / Ω

0

0

50

100

Re (Z ) / Ω

Fig. 2. Electrochemical impedances of (A) a Cu2O/Cu sample, (B) a Cu2S/Cu sample and(C) a CuO/Cu sample. Electrolyte solutions were 6 M KOH + 1 M LiOH and 0.1 M KCl. Current density: -1.0 mA cm-2. Imposed current: 0.1 mA

10 · Quantitative Analysis of Copper Sulfides by a New Electrochemical Method

× 1,000

Cu

1

1 min

20 minCu2S

10 min1h

15 h

Intensity / cps

Before immersion

Cu9S5

0.5

Fig. 3. Photos of copper plates immersed in 5 mM Na2S at 25˚C
for various periods
0
20

0

40

60

2 θ / deg

Fig. 5. XRD patterns obtained by the grazing incident method fora copper plate immersed in 5 mM Na2S at 25˚C for 8 h

-2

1 min

I / mA

0

Cu9S5

-2

10 min0

Cu2S

10 min0

I / mA-2

60 min
-1.5

-1.0

Cu2S

-10

Cu(OH)2

-0.5

5 min
-20

(E vs. Ag/AgCl) / V

Fig. 4. Current-potential curves recorded by LSV for copper plates
immersed in 5 mM Na2S at 25˚C for 1, 10 and 60 min.
Electrolyte solution: 6 M KOH + 1 M LiOH. Scan rate: 10 mV s-1

Cu2O

0 min
CuO

-30

CuO/Cu2O/Cu
-1.5

-1.0

(E vs. Ag/AgCl) / V

because this peak was split into two yielding peaks.
Figure 5 shows the XRD pattern obtained for the copper plate immersed in 5 mM Na2S at 25˚C for 8 h. A weak
Cu9S5 phase was detected besides Cu2S. Thus, the reduction
peak at the lower potential was found to refer to a reductionof Cu2S, whereas that at the higher potential refers to a reduction of Cu9S5. Using another analytical method, it is concluded that the obtained amounts of Cu2S could be
calculated using Faraday’s Law and the peak area of Cu2S
based on Eq.2(1). After immersing the copper plates in 5
mM Na2S for 1 min, the average thickness of the sulfide film
deposited on Cu was estimated to be around 10 nm.
Cu2S can also be formed from Cu2O by chemical conversion(22).
Cu2O + SH- → Cu2S + OH- ...................................... (4)Figure 6 shows current-potential curves for CuO/

Fig. 6. Current-potential curves recorded by LSV for CuO/Cu2O/Cu
samples immersed in 5 mM Na2S at 25˚C for various periods.
Electrolyte solution: 6 M KOH + 1 M LiOH. Scan rate: 10 mV s-1

Cu2O/Cu samples(15) immersed in 5 mM Na2S at 25˚C forvarious periods. A CuO/Cu2O/Cu sample has a copper
film with a thickness of about 1.4 µm. In only a few minutes, both a decrease in thickness of the Cu2O film andgrowth of the Cu2S film occurred. The thickness of the
CuO film also decreased in the course of immersion in 5
mM Na2S. Thus, chemical conversion of copper oxides to
Cu2S occured easily. CuO was assumed to have a large influence on the reaction because the rate of conversion was
relatively slow in the case of using a Cu2O/CuO sample(21).
3-3 Estimation of present or absence of Cu2SAs shown in Fig. 1 A, the reduction peaks for Cu2O
and Cu2S emerged at nearby potential levels in 6 M KOH+ 1 M LiOH. It was assumed that the distinction between

SEI TECHNICAL REVIEW · NUMBER 74 · APRIL 2012 · 11

these two peaks could become difficult for copper samples
with certain levels of relative ratios of Cu2O and Cu2S. Referring to available CV performance data reported in thecase of sulfide ion injection into an alkaline solution(25), (26),similar monitoring was undertaken in 6 M KOH + 1 M
LiOH to review reduction performances of Cu2O and Cu2S.
In a high-alkaline solution, Na2S would yield S2- ions.
Figure 7 A shows current-potential curves by LSV for
copper plates. A preliminary cathodic potential sweep wasdone from the rest potential of about -0.7 V to -1.7 V. A potential sweep was then done up to +0.7 V. In the solution
containing S2- ions in 7 A(b), an oxidation peak referring
to formation of Cu2S emerged at -1.0 V. In 6 M KOH + 1 M
LiOH in 7 A(a), the P1 peak was identified to represent
Cu2O formation on the top layer of the Cu surface. The P2
peak that appeared at -0.2 V was speculated to represent
the formation of Cu(OH)2(27).
Figure 7 B shows CV monitoring results for copper
plates. A primary potential sweep was done from the rest
potential to -1.6 V. Then, a potential sweep was done from
-1.6 V up to -0.7 V and then back to -1.6 V. This operation
was carried out twice. In 6 M KOH + 1 M LiOH without S2-,
a reduction current referring to H2 gas evolution reaction
alone was detected (Fig. 7 B(a)). On the other hand, in 6
M KOH + 1 M LiOH containg S2-, an oxidation peak at -1.0
V (indicated by the arrow) emerged during anodic potential sweeping, and a reduction peak emerged reproduciblyat -1.3 V during cathodic potential sweeping in the first andsecond potential sweep processes. These peaks are respectively considered to correspond to the following reactions:
2Cu + S2- → Cu2S + 2e- ............................................ (5)Cu2S + 2e- → 2Cu + S2- ............................................ (6)That is, in the highly alkaline electrolyte containing
S2-, Cu2S formed during the oxidation process and S2- wasgiven up from Cu2S into the electrolyte during the reduction process. Under the chosen test condition, a certain
level of S2- was present in the electrolyte, and the intensityof the oxidation and the reduction peaks remained unchanged from the initial one in the second process of potential sweeping.
To verify the series of mechanisms shown above, CV
measurement for Cu2S/Cu samples immersed in 5 mM
Na2S for 60 min was carried out. A primary potential sweep
was done from the rest potential to -1.6 V. Then, from -1.6
V, a potential sweep was carried out up to the rest potential.
This operation was carried out twice. As shown in Fig. 7B(b), a weak Cu2S formation peak emerged at -1.0 V during
the anodic potential sweep. The oxidation peak at -1.0 V
was considered to represent the Cu substrate with S2- yieldedfrom the dissolution of Cu2S. In the conditions used to obtain the results shown in Fig. 8, S2- concentration near theCu substrate was considered to be lower than in the conditions used to obtain the results shown in Fig. 7 B(b). Accordingly, the intensity of the peak at -1.0 V in Fig. 8 was
weaker than in Fig. 7 B(b).
By performing an anodic potential sweep after a preliminary cathodic potential sweep, the presence or absenceof Cu2S over a Cu substrate surface can be estimated. A
nanometer-level thickness of Cu2S can also be determinedby separation from coexisting Cu2O. The proposed
method is useful for copper product management because
the commercial value of some copper products is lost bymere tarnishing.

A

B

100

(a)

P2

0

(a)

50

P1

-5

100

(b)

P2

I / mAI / mA

0

0

(b)

50

P1
-5
0
-1.0

0.0

(E vs. Ag/AgCl) / V

-1.5

-1.0

(E vs. Ag/AgCl) / V

Fig. 7. (A) Current-potential curves recorded by LSV. The potential was first swept from the rest potential to -1.7 V, and then a single potential sweep was
performed to +0.7 V(B) Current-potential curves recorded by CV. The potential was first swept from the rest potential (around -0.7 V) to -1.6 V, and then two successive
cyclic potential sweeps were performed between -1.6 V and the rest potential. Electrolyte solutions:(a) 6 M KOH + 1 M LiOH and (b) 6 M KOH + 1 M LiOH + 5 mM Na2S. Scan rate: 10 mV s-1

12 · Quantitative Analysis of Copper Sulfides by a New Electrochemical Method

toya, technical advisor at Japan Copper and Brass Association, and Associate Professor Toshiyuki Osakai at Kobe
University, for valuable advice and discussions during thepast decade.

0

Cu(OH)2

I / mA

-2

Cu2S

-4

Cu9S5
1st

References
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p.57(4) A.J. McEvily, Jr and A.P. Bond, J. Electrochem. Soc., 112 (1965) p.131(5) H. Abbott, Mater. Perform, 24 (1985) p.46
(6) H.A. Miley, J. Am. Chem. Soc., 59 (1937) p.2626(7) J.B. Dyess and H.A. Miley, Trans. Amer. Inst. Min. Met. Eng., 133(1939) p.239(8) M. Reid, J. Punch, L.F. Garfias-Mesias, K. Shannon, S. Belochapkine
and D.A. Tanner, J. Electrochem. Soc., 155 (2008) C147
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Philadelphia, PA, USA.
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Electrochem. Soc., 148 (2001) B467
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154 (2007) C1(16) Copper and Copper–based Alloy Products Data Book, Japan Copperand Brass Association, 2nd edition, p.130 (2009).
(17) T. Notoya, S. Nakayama and T. Osakai, Benzotriazole and Corrosion
Inhibitors for Copper and Its Alloys, Japan Association of Corrosion
Control (2008).
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JRICu, 43 (2004) p.235(20) M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, 2nd English ed., p.386 (1974).
(21) S. Nakayama, M. Shibata, T. Notoya, and T. Osakai, Bunseki Kagaku,
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(1)

0

-1

2nd
-1.5

-1.0

(E vs. Ag/AgCl) / V

Fig. 8. Current-potential curves recorded by CV for a copper plate
immersed in 5 mM Na2S for 1 h at 25˚C. Two successive cyclic
potential sweeps were performed between the rest potential and-1.6 V. Electrolyte solution: 6 M KOH + 1 M LiOH. Scan rate:
10 mV s-1. Arrows indicate the small anodic peak for the formationof Cu2S on the copper surface

4. Conclusion
A new voltammetric method for estimation of copper
sulfides was described. This method was investigated as anapplication of state analysis of copper oxides (Cu2O andCuO). The following features concerning reduction behavior of copper sulfides were clarified.
1. The peak for Cu2S emerged at a slightly higher potential level than the peak for Cu2O in 6 M KOH+ 1 M LiOH. This order of potential is thermodynamically abnormal.
2. Since the peak for Cu2S emerged by sufficient separation from that of Cu2O, the main copper corrosion products (i.e., Cu2S, Cu2O and CuO) were
determined by a single measurement.

5. Acknowledgements
This paper summarizes a series of studies on the characterization of corrosion products formed on copper andits alloys. The studies have been conducted since 1998 by
the Analysis Technology Research Center, Sumitomo Electric Industries, Ltd., Osaka, Japan. The authors wish to
thank their colleagues, Professor Susumu Kuwabata at the
University of Osaka, and Mr. Masahiro Shibata at Tokai
Rubber Industries, Ltd., who contributed to parts of thiswork, and Mr. Mitsuaki Nishie, fellow at Sumitomo ElectricIndustries, Ltd., for encouraging us to perform the studies.
The authors are especially indebted to Dr. Takenori No-

SEI TECHNICAL REVIEW · NUMBER 74 · APRIL 2012 · 13

Contributors (The lead author is indicated by an asterisk (*).)

S. NAKAYAMA*• Senior Specialist
Senior Assistant General Manager, Analysis
Technology Research CenterHe has been engaged in the developmentof technologies for inorganic chemical
analysis and electrochemical analysis as well as in theprovision of analysis services.

T. UMEMOTO• Analysis Technology Research Center

14 · Quantitative Analysis of Copper Sulfides by a New Electrochemical Method