12810 Chem. Commun., 2011, 47, 12810–12812 This journal is c The Royal Society of Chemistry 2011

Cite this: Chem. Commun., 2011, 47, 12810–12812

CR@BaSO4: an acid rain-indicating materialw

Hong-Wen Gao* and Xin-Hui Xu

Received 1st September 2011, Accepted 6th October 2011

DOI: 10.1039/c1cc15435d

The CR@BaSO4 hybrid was synthesized, characterized and

used as an acid rain-indicating (ARI) material. A painted ARI

umbrella was discolored after exposure to simulated acid rain of

pH 5 or less and returned to the initial color after the rain

stopped. Such a functionalized material may make acid rain

visual to remind people in real-time.

Acid rain was first observed in the mid 19th century, when

some people noticed that forests located downwind of large

industrial areas showed signs of deterioration.1 It occurs when

large amounts of gases such as sulphur dioxide and nitrogen

dioxide are released into the air. Acid rain (pH o 5.6) affects

us in many different ways.2 Breathing and lung problems in

children and adults who have asthma have been linked to acid

air pollution.3 It can also damage non-living things. For

example, it destroys forests and biodiversity,4 pollutes lakes

and soil5 and corrodes buildings.6 At worst, it can damage

non-replaceable statues, and sculptures that are part of our

nation’s heritage.7 Some of the most acidic rain ever recorded

fell in the UK in 1974 and was measured to have a pH of 2.4.8

In December 1982, a sample of fog taken at Corona del Mar in

Southern California had a pH of 1.69. This extremely high

acidity developed after a two-day ground-level temperature

inversion in the Los Angeles basin.9 Places showing significant

impact of acid rain around the globe include most of eastern

and north-western Europe e.g. Poland, Scandinavia, Sweden

and Norway, northern and eastern America and south-eastern

Canada.10 In China, most of the acid rain effected areas are

located between the south of the Yangtze River and the east of

the Qinghai–Tibet Plateau, where the total area is approximately

1 200000 square kilometers. Early on the 15th of November 1990,

President Bush signed Amendments to the Clean Air Act. Title IV

of the Amendments authorized the Environmental Protection

Agency to establish an Acid Rain Program, the overall goal of

which is to reduce sulphur dioxide and nitrogen dioxide emissions.9

During the next two decades, many governments have introduced

laws to reduce these emissions.11 Over the years the scope and

intensity of acid rain has only been known by the relevant

government and research institutions. It is difficult for most people

to distinguish the acidity of rainfall on-site. It is necessary to

develop a way of visually detecting acid rain. This work aims

to prepare a potential acid rain-indicating material, which is

filled into the acrylic-based emulsion paint used to paint

common rain gear to visually remind people of acid rain in

real-time.

Congo red (CR, C.I. 22120) chemically named benzidinediazo-

bis-1-naphthylamine-4-sulfonic acid with a bright red color and

low toxicity is widely used in textiles, printing and paper-

making, etc. CR is still usually used in chemical laboratories as

a conventional acid–base indicator. It dissociates into a mixture

of H2L (blue), HL� (red) and L2� (red) with a transition range

of pH 3–5.2, which just corresponds to the scope of acid rain. It

is not readily reactive with the conventional ions in rain e.g.

Ca2+, Mg2+, Na+, Cl�, SO42�, CO3

2� etc. Barium sulfate

(BaSO4) isn’t dissolved in acidic aqueous media. It is usually

used as a filler in paint, paper, rubber and plastic in order to

increase the hardness and whiteness. The hybridization of CR

into BaSO4 obeyed the Langmuir sorption isotherm when

freshly formed (Fig. S1 A, ESIw). It indicated that the CR

was bound to the BaSO4 particles’ surface in a monolayer via

the affinity of Ba with –SO3�.12 The saturation mole ratio (N)

of CR to BaSO4 was calculated to be 1/60 and their binding

constant (K) to be 2.61 � 105 M�1. Therefore, CR was

immobilized firmly onto BaSO4. The reaction rate of CR is

less than 40% when the initial mole ratio of CR to BaSO4 is

more than 0.02 (Fig. S1 B, ESIw). According to the recom-

mended procedure, the CR@BaSO4 hybrid formed contained

1.6% C and 0.4% N by elemental analysis. The mole ratio of

CR to BaSO4 is calculated to be 1/85 in the hybrid. Weight loss

(about 5%) of CR@BaSO4 appeared obviously at around

720 1C, which is much higher than the decomposition temperature

of CR e.g. 360 and 410 1C (Fig. S2, ESIw). The thermal stability

of CR embedded in the hybrid obviously increased. CR was

calculated to occupy 3.3% of the hybrid, i.e. the mole ratio 1/83

of CR to BaSO4, which corresponds with the above analysis.

The CR@BaSO4 hybrid (40%) aqueous liquid is a thick

dark-red semifluid (Fig. 1Aa). Many rough elliptic particles/

sheets (50–200 nm of size) layered together to form the

irregular particles of 1–10 mm (Fig. 1A) (Fig. S3, ESIw). Theyare similar to MB@BaSO4 but different from CR@CaCO3.

13

From TEM, some regular thin layers were found inside the

CR@BaSO4 hybrid (Fig. 1Ba–b and Ca), where the thickness

of CR and BaSO4 layers were both approximately 1 nm. There

are two –SO3� groups located at the head and tail of CR so

that CR prostrated between adjacent BaSO4 layers to form a

State Key Laboratory of Pollution Control and Resource Reuse,College of Environmental Science and Engineering, Tongji University,Shanghai 200092, China. E-mail: hwgao@tongji.edu.cn;Fax: 86-21-65988598; Tel: 86-21-65988598w Electronic supplementary information (ESI) available: Experimentaldetails and Fig. S1 to S9. See DOI: 10.1039/c1cc15435d

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This journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 12810–12812 12811

firm complex (Fig. 1Cb). Such an interlayer stacked structure

will prevent CR leaching in the aqueous solution.

Owing to a large number of –SO3� groups embedded in the

CR@BaSO4 hybrid (Fig. 1Cb), the hybrid carried a great

deal of negative charge confirmed by the z-potential analysis(Fig. S4, ESIw), e.g. �34.1 mV at pH 5.6, �28.5 mV at pH 4,

and �20.6 mV at pH 3. Without doubt, the CR@BaSO4

hybrid may adsorb H+. From the acid dissociation constant

(Ka) of CR, pKa 4.1,14 the [H2L]2/[HL�][L2�] ratio was

calculated to be 13 at pH 3, resulting in a blue hue, 1.3 at

pH 4, a bluish-purple hue and only 0.03 at pH 5.6, a red hue.

The CR@BaSO4 hybrid is also insoluble in aqueous media.

This was confirmed from an immersion experiment on the

hybrid (Fig. S5, ESIw), where the color was not stripped when

the hybrid was immersed in aqueous solution for 330 days and

acidic media for 24 h. Therefore, it is feasible to use this hybrid

as an acid rain-indicating material.

The simulated acid rains flowed across the ARI paint plates

and the color changed from red to blue (Fig. S6, ESIw). Also,

the blue color darkened with a decrease in pH from 5.6 to 3.5.

The blue color of the ARI paint plate also deepened with an

increase of the water flow time. Though the most obvious color

change of the ARI paint plates appeared after 15 min, the color

difference was clearly distinguishable at 5 min. Without doubt,

the ARI paint may indicate acid rain in real-time.

The absorption by the ARI paint of visible light between

450 and 550 nm decreased with a decrease of pH and that of

light between 600 and 750 nm increased (Fig. S7, ESIw). Thus,the reflection of blue-green light from the ARI paint plates

increased and that of the red light decreased. In accordance

with the visual observation above.

The quantitative color difference (DE) of the ARI

paint may be calculated by the relation

DE ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiða0 � a1Þ2 þ ðb0 � b1Þ2 þ ðL0 � L1Þ2

q,15 where the

symbol a is redness–greenness, b yellowness–blueness and

L lightness–darkness. The subscripts 0 and 1 refer to the ARI

paint plate measured before and after exposure to rain. A DE of

7 occurs when the critical color difference is distinguished by

100% of observers.16 When the rain ceased for 1 h, DE values of

all of the plates were determined to be less than 4, i.e. they

returned to the initial red (Fig. S6, ESIw). Therefore, CR was

immobilized firmly into BaSO4 and not leached during rain.

CR is one kind of azo dye that is easily decomposed under

ultraviolet light irradiation. When an ARI paint plate was

exposed to strong sunshine for 18 h, DE was determined to

be 10. However, DE changed from 0.4 to 4.5 when the plate

was placed indoors for 10 to 270 days. Therefore, the ARI

paint is suitable for painting on the surface of rain gear but

unfit for painting on the external wall of a building.

The ARI umbrella test indicated that the four sectors coated

with ARI paint changed from red to blue with a reduction of

pH (Fig. 2B–E). During exposure to pH 5.6 simulated acid

rain, the sectors painted with the ARI paint changed little and

could not be readily differentiated (Fig. 2B). The DE of sector a

was determined to be 3.5 against the dry umbrella (Fig. 2A).

When the pH of simulated acid rain is less than 5, the color

change of the ARI paint sectors was obviously distinguishable

in comparison with the adjacent sectors (Fig. 2C–E). The DE of

sector a was determined to be 9 at pH 5, 15 at pH 4.5 and 23 at

pH 4 (Fig. 2F). Thus, the ARI umbrella testing indicated that

the ARI paint is suitable for indicating acid rain of pH r 5.

The raining duration indicated that the color change of the

ARI paint sectors was easily differentiated after exposure to

rain for 5 min with pH 4.5 of simulated acid rain (Fig. S8,

ESIw). The blue color darkened slightly with the passing of

time, which is similar to the ARI plate’s appearance above.

Thus, the ARI umbrella may indicate acid rain rapidly.

In order to investigate the repeatable use of the ARI

umbrella, it was exposed to three cycles of pH 4.5 simulated

acid rain. The results indicated that the repeated raining

seldom influenced the acid rain-indicating function of the

Fig. 1 Morphology and structure of the CR@BaSO4 hybrid. A: SEM image, Aa: the CR@BaSO4 hybrid liquid (40%); B and C: TEM images,

Ba, Bb and Ca: layer structure area inside the hybrid, Cb: cartoon illustration of CR binding to the BaSO4 layers.

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12812 Chem. Commun., 2011, 47, 12810–12812 This journal is c The Royal Society of Chemistry 2011

ARI umbrella (Fig. S9, ESIw). In addition, the ARI paint

sectors returned to the initial color after the rain had ceased

for 1 h. Therefore, the ARI umbrella can be used repeatedly

during rainfall.

In conclusion, acid rain as a form of serious air pollution17 is

recognized as one of the most serious global environmental

problems. It has brought great losses to the global economy and

civilization. This work developed a facile preparation of an ARI

material, i.e. a CR@BaSO4 hybrid synthesized by conventional

organic–inorganic hybridization. When characterizing the

structure and morphology, the CR@BaSO4 hybrid contained

a great deal of negative charges and the thermal stability of

CR when firmly embedded increased up to 720 1C. The ARI

paint was prepared by mixing the CR@BaSO4 hybrid with

acrylic-based emulsion paint and was then coated on an

umbrella. The ARI umbrella discolored sensitively after

5 min of exposure to simulated acid rain of pH o 5 and

returned to the initial color after the rain ceased for 1 h.

During airing an ARI umbrella seldom discolored, i.e. the

color difference (DE) was less than 7 when placed indoors for

9 months. Such a new-type of functionalized material provides

the public with an on-site method of observation for acid rain.

Surely, with every citizen all over the world acting forcefully,

air pollution and acid rain problems may eventually be solved.

The authors acknowledge financial support from the National

Key Technology R&D Program of China (Grant No.2008-

BAJ08B13) and the Key Project of State Key Laboratory of

Pollution Control and Resource Reuse (2011–2014).

Notes and references

1 J. H. Seinfeld and S. N. Pandis, Atmospheric Chemistry andPhysics: From Air Pollution to Climate Change, J. Wiley & Sons,New York, 2nd edn, 2006, p. 954.

2 (a) D. Malakoff, Science, 2010, 330, 910; (b) J. K. C. Nduka,O. E. Orisakwe, L. O. Ezenweke, T. E. Ezenwa, M. N. Chendo and

G. Ezeabasili, Sci. World J., 2008, 8, 811; (c) Nicola Griffith, NewSci., 2009, 203, 49; K. Thorburn and A. Darbyshire, Crit. Care,2009, 13, 1008.

3 US EPA, Effects of Acid Rain - Human Health, 2009-5-13.4 (a) G. E. Likens, C. T. Driscoll and D. C. Buso, Science, 1996,272, 244; (b) T. Ruuhola, L. M. Rantala, S. Neuvonen, S. Y. Yangand M. J. Rantala, Basic Appl. Ecol., 2009, 10, 589; (c) J. Kovacik,B. Klejdus, M. Backor, F. Stork and J. Hedbavny, Ecotoxicology,2011, 20, 348; (d) A. Singh and M. Agrawal, J. Environ. Biol., 2008,29, 15; (e) X. Zheng and D. C. Jin,Appl. Entomol. Zool., 2011, 46, 265.

5 (a) US EPA, Effects of acid rain - surface waters and aquaticanimals, http://www.epa.gov/acidrain/effects/surface_water.html;(b) H. Rodhe, F. Dentener and M. Schulz, Environ. Sci. Technol.,2002, 36, 4382.

6 Y. F. Fan, Z. Q. Hu, Y. Z. Zhang and J. L. Liu, Constr. Build.Mater., 2010, 24, 1975.

7 (a) D. Mareci, R. Chelariu, I. Rusu, N. M. Puica and D. Sutiman,Eur. J. Sci. Theol., 2010, 6, 57; (b) E. Bernardi, C. Chiavari,B. Lenza, C. Martini, L. Morselli, F. Ospitali and L. Robbiola,Corros. Sci., 2009, 51, 159.

8 A. Tiwary and J. Colls, Air Pollution: measurement, modelling andmitigation, Routledge Press, 3rd edn, 2010, p.219.

9 A. Nixon,T. Curran,Government of Canada, Science andTechnology Division, Acid Rain, http://dsp-psd.pwgsc.gc.ca/Collection-R/LoPBdP/CIR/7937-e.htm.

10 (a) Ed. Hatier, Acid Rain in Europe. United Nations EnvironmentProgramme, GRID Arendal, 1993; (b) US EPA, Clean AirMarkets, 2008 Highlights.

11 (a) N. Oreskes and E. M. Conway, Nature, 2010, 466, 815;(b) I. Lange, Energy Policy, 2010, 38, 1251.

12 (a) J. Lin and H. W. Gao, J. Mater. Chem., 2009, 19, 3598;(b) H. W. Gao, J. Lin, W. Y. Li, Z. J. Hu and Y. L. Zhang,Environ. Sci. Pollut. Res., 2010, 17, 78.

13 D. H. Zhao, Y. L. Zhang, Y. P. Wei and H. W. Gao, J. Mater.Chem., 2009, 19, 7239.

14 T. Erban and J. Hubert, J. Insect Sci., 2010, 10, 42.15 ASTM. Standard Practice for Calculation of Color Tolerances

and Color Differences from Instrumentally Measured ColorCoordinates, West Conshohocken, PA, USA, 2007, Vol. D2244-07.

16 P. Doray, X. Wang, J. Powers and J. Burgess, J. Prosthodontics,1997, 6, 183.

17 K. H. Chang, F. T. Jeng, Y. L. Tsai and P. L. Lin, Atmos. Environ.,2000, 34, 3281.

Fig. 2 Change of the ARI umbrellas after 5 min of rain with various simulated acid rains. (A) Before raining; (B to E) pH 5.6, 5.0, 4.5 and 4.0; (F)

change in DE of the above ARI umbrellas (B to E, area a) during raining.

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