simultaneous determination of silicate and phosphate in boiler water at power plants based on series...

Analytica Chimica Acta 455 (2002) 315–325

Simultaneous determination of silicate and phosphatein boiler water at power plants based on series flow

cells by using flow injection spectrophotometry

Yong-Sheng Li∗, Yu Muo, Hou-Mei XieDepartment of Applied Chemistry, Northeast China Institute of Electric Power Engineering,

No. 169 Changchun Road, Jilin City, Jilin Province 132012, China

Received 26 March 2001; received in revised form 3 December 2001; accepted 13 December 2001

Abstract

A new flow injection spectrophotometric method is described for the simultaneous determination of silicate and phosphate.Effects on the sensitivity of the method of the wavelength, temperature, length of reaction coils, pump rates, acidity, samplingvolume, concentration of the chromogenic reagent, etc. were also investigated. The optimum conditions were ascertained.

The principle of the method is that total concentration of silicate plus phosphate is determined when a injected sampleplug is passing through the first flow cell and then the concentration of silicate is serially) determined at a second flow cell ofthe same detector after continuously masking the yellow molybdophosphate in the sample zone. Finally, the concentration ofphosphate is obtained by difference.

Silicate and phosphate are determined in boiler water at power plants; 60–120 samples h−1 be analyzed. Determinationranges are 0.05–22 mg l−1 for silicate and 0.1–24 mg l−1 for phosphate. Relative standard deviations for metasilicate andorthophosphate were≤1.2 and 1.3%, respectively. Recovery ranges of silicate and phosphate in the samples are 98–103%.© 2002 Elsevier Science B.V. All rights reserved.

Keywords: Flow injection; Phosphate; Silicate; Simultaneous determination; Power plant; Boiler water

1. Introduction

Boiler feedwater at power plants is kept slightly al-kaline (pH ca. 9.0) as an anticorrosion measure. Phos-phate is widely used as an agent for adjusting the pHof the water in medium, high and ultrahigh pressureboilers. It also serves as a softening agent that convertscalcium and magnesium water scale and other insol-uble salts that form in boilers. However, if there wasan excessive dose of phosphate it also tends to pro-duce NaFePO4 and Fe3(PO4)2. The formation of this

∗ Corresponding author.E-mail address: [email protected] (Y.-S. Li).

type of scale is apt to lead to the swelling and eventualbursting of the boiler water pipes. On the other hand,when too little phosphate is added to the feedwater, thecalcium ion concentration is not lowered sufficiently,resulting in the formation of CaSO4, CaSiO3 and othertypes of scale, which also become the cause of latentdamage to the heating equipment and turbines. Be-sides, an excess concentration of SiO3

2− in the hollerwater tends to cause silica scale to form on the insidesurfaces of the water pipes under high boiler loads. Itcan also cause an increase in the silica carryover inthe steam. This is apt to lead to corrosion of the heat-ing equipment and turbines and thus greatly affectsturbine operating efficiency.

0003-2670/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0003-2670(01)01609-9

316 Y.-S. Li et al. / Analytica Chimica Acta 455 (2002) 315–325

Therefore, to avoid such problems and to secure theefficient operation of the power generating units, it isessential to monitor closely the concentration of phos-phate and silicate in the boiler water. At present, an-alytical methods being employed to determine thesespecies are generally based on reactions that reachequilibrium [1]. Under certain conditions, however,they will be not suitable for automatic monitoring be-cause the analysis rate will be to slow and unaccept-able lags can occur. If flow injection analysis (FIA)[2] technology is applied to management and onlinemonitoring of the water quality, it could overcome thedrawbacks of the online analyzers now being used andcompensate for the deficiencies in conventional ana-lytical techniques.

Upto now, many research results have been re-ported on simultaneous determinations of phosphateand silicate on the basis of FIA. In 1981, Fogg andBsebsu [3] first reported a method of flow injectionvoltammetric determinations of phosphate (6–10 M),silicate (7–10 M) and arsenate (5–10 M) glassy Celectrode by injecting their performed heteropolyacids into an eluent serving as a carrier stream. Onthe basis of this method, Fogg et al. [4] had also con-ducted researches for the determination of total phos-phate (involving hydrolysis of polyphosphates) andsoluble silicate in commercial anionic detergents andon the effect of increasing ethanol and acetone con-centrations on the differential pulse voltammograms.Linares et al. [5] proposed FIA method for fluoro-metric differential-kinetic determination of SiO32−and PO4

3− in water, based on the different rates offormation of their molybdate heteropoly acids. Thefluorometrically monitored product is thiochromeformed by oxidation of thiamine by the heteropolyacid. The FIA manifolds allow two measurements atdifferent times on each sample injected. The methodpermits the determination of anions at concentrationsof 30–600�g l−1 in ratios from 1:10 to 10:1 and canbe applied to running and bottled water. The samplingfrequency is 60 h−1.

Li [6] has conducted the development of simulta-neous determination methods and established an si-multaneous determination of SiO3

2− and PO43−. The

method made use of the reversed FIA (rFIA) [7] andthe FIA-T1-721 type flow injection spectrophotome-ter developed by them [8]. This method is capable ofanalyzing 60 samples h−1, with determination ranges

of 50–6000�g l−1 for SiO32− and 2–30 mg l−1 for

PO43− and relative standard deviations (RSDs) of

<1% [9]. Jacintho et al. [10] reported another flowinjection spectrophotometric method for alternatedetermination of PO43− and SiO3

2− in river water.The flow system used a commutator (correspondingto a two-channel synchronous injection valve and aswitch valve) which permits the implementation oftwo different methods in the same manifold. Thesample analysis rate is 60 h−1. RSDs were 1% for2.5–15 mg l−1 for Si and 0.25–1.5 mg l−1 for P.

Shen and Miao [11] have determined phosphorusand silicon in carbon and low-alloy steels based on themolybdenum blue method by rFIA. The sample solu-tion is treated with NH4F to eliminate the interferenceof iron(III) molybdate. The sample solution is usedas the carrier and mixed with ammonium molybdatesolution to form the heteropoly acid. After a reducingagent is injected, molybdenum blue forms for deter-mination of phosphorus and silicon. Imakita et al.[12] also conducted researches on the spectrophoto-metric determination of silicon or phosphorus in ironand steel by FIA, based on the formation of hetero-molybdenum blue. Silicon in the range 0.005–0.5%in iron and steel could be determined at a samplingrate of 30 samples h−1. Phosphorus in the range0.00l–0.08% in low alloy steels could be determined at20 samples h−1.

Lacy et al. [13] used the molybdenum blue reac-tion for the determination of phosphate as a model toillustrate the extension of the use of hydrophobic sor-bents in FIA for the pre-concentration of an anion andfor on-column detection, i.e. optosensing. Optosens-ing provides for real time monitoring of the rate ofsignal change (dA/dT), so that the rate of color de-velopment during the reduction step of the analysiscan be measured. The synergistic relationship betweenthe rates of formation of the phosphate and silicateheteropoly complex is examined. A kinetic optosens-ing method was developed in which the difference inreduction rate for the heteropolyacid species allowssimultaneous determination of�g l−1 phosphate andmg l−1 silicate. Narusawa et al. [14] described a simul-taneous determination of silicon and phosphorus withan anion-exchange column using FIA spectrophotom-etry. Detection limits were 3.1 and 0.55 ng for P andSi, respectively. Bovine liver, chlorella and pepperbushwere analyzed for both elements by the method.

Y.-S. Li et al. / Analytica Chimica Acta 455 (2002) 315–325 317

Mas et al. [15] had reported a method for the si-multaneous determination of PO4

3− and SiO32− in

water by FIA, based on the different rates of forma-tion of the respecting heteromolybdic acids. Mea-surements are based on spectrophotometry of the ionpairs formed from the molybdic acids and RhodamineB. The method allows the determination of the twoanions at a rate of 20 samples h−1 over the range0.05–2.5 mg l−1 PO4

3− and 0.8–15 mg l−1 SiO32−

which allows application to a variety of waters. Kanget al. [16] has also utilized the rFIA method for in-vestigating nutrients in seawater and for continuouslydetermining PO43− and SiO3

2− in seawater. Themethod has detection limits of 0.05 and 0.51 mM,respectively. Shpigun et al. [17] have also reportedsome results on using rFIA in marine chemical in-vestigations. New modifications of rFIA manifoldsfor the determination of dissolved silicate, PO4

3−,SiO3

2−, sulfide and Mn(II) in seawater samples andnormal FIA methods for the determination of total al-kalinity (SO4

2−) and main nutrient-type constituentsin interstitial water samples are described.

Although many reports on simultaneous determi-nations of phosphate and silicate based on FIA havebeen made, as mentioned above, these cannot beused directly for power plants because the manifolddesigns of their FIA systems, the reagents or thedetermination ranges are inappropriate for manufac-turing an online analyzer or for online monitoring.Besides, compared with the usual molybdenum bluemethod, the molybdophosphate yellow and molyb-dosilicate yellow methods are simple, economical(need no reducing agent), the reaction product formedis quite stable and can decrease the complexities in themanufacture of online analyzers. So after evaluatingprevious work, we selected the colorimetric molyb-denum yellow method to realize the simultaneousdetermination of silicate and phosphate.

The analysis principle is that the total concentra-tion of silicate and phosphate is determined when aninjected sample zone is passing through the first flowcell of the detector and the concentration of silicate isdetermined at a second flow cell of the same detector.Finally, the concentration of phosphate is obtained bycalculating the difference between the two results. Inthis way, we have successfully developed a simple andrapid method able to be used for monitoring boilerwater at power plants.

2. Experimental procedure

2.1. Preparation of regent and standardsample solutions

All chemicals were of analytical-reagent grade.Ultra purified water was used for the preparation ofsolutions. All flasks and glassware were filled witha mixture of HNO3:H2O2:H2O (2:1:2 v/v/v), leftovernight and washed with water. All solutions werestored in polyethylene vials.

Silica stock solution: 0.4737 g of Na2SiO3·9H2Owas dissolved in water and diluted to 1000 ml. Thesilica concentration was 100 mg l−1 SiO2.

Phosphate stock solution (1000 mg l−1 PO43−):

1.4334 g of anhydrous potassium dihydrogenphos-phate (dried at 110◦C for 2 h) was weighed, dissolvedin water and diluted to exactly 1000 ml.

Molybdenum(VI) solution (8.0% w/v): The solutionwas prepared by dissolving 80.0 g of hexaammoniumheptamolybdate tetrahydrate in 200 ml water, adding36.0 ml of concentrated sulfuric acid (98%) in 400 mlwater, gently mixing the reagents cooling and com-pleting to 1.0 l [18].

Oxalic acid solution (8.0% w/v): This was preparedby dissolving 80.0 g of oxalic acid and 20 ml of con-centrated sulfuric acid (98%) in water and completingto 1.0 l.

Working standard solution of 50.0 mg l−1 (SiO2):The solution was prepared by taking 25.30 mlof silicate stock solution into a 500 ml polyethy-lene calibrated flask and diluting to the mark withwater.

Working standard solution of 100.0 mg l−1 phos-phate (PO4

3−): The solution was prepared by tak-ing 50.0 ml of phosphate stock solution in a 500 mlpolyethylene calibrated flask and completing the vol-ume to the mark with water.

2.2. Apparatus

The flow injection analyzer used was a FIA-T1model equipped with a 721-type single-beam spec-trophotometer [8]. The recorder is a XWT-100 model(Instrument Corp., Shanghai Dahua). The lines of theFIA manifold used 1.0 mm bore PTFE tubing, exceptfor the reaction coils (RC1 and RC2), which were of0.5 mm bore PTFE tubing.


Fig. 1. FIA manifold for simultaneous determination of silicate and phosphate. C: Ultrapure water; R1: molybdenum ammonium (coloringreagent); R2: masking reagent (oxalic acid solution); P: pump; W: waste; RC1: first reaction coil (i.d. 0.5 mm); RC2: second reaction coil(i.d. 0.5 mm); I: injecting position; S: sampling position; rpm: revolution per minute of pump; D1: the first flow cell; D2: the second flowcell; peak1: the signal from D1; peak2: the signal from D2.

2.3. FIA system and procedures

The FIA system for simultaneous determination ofsilicate and phosphate is shown in Fig. 1. It consistsof a peristaltic pump, injection valve, reaction coils, awater bath to control the reaction temperature and a721-type spectrophotometer fitted with two flow cells,10 mm path length.

The procedures are as follows. When the injectionvalve is switched to the sampling position (S), thesample solution containing silicate and phosphate isfilled into the loading loop. The valve is switched tothe injection position (I) and the injected sample zonegoes into RC1 after merging with the color reagent(molybdate solution). The yellow color develops, theabsorbance of the product is measured at 405 nmwhile it is passing through the first flow cell and thusthe total concentration of silicate and phosphate isobtained, then the sample zone passes through RC2and reacts with oxalic acid to mask (decolorize) thephosphate product. Thus, only silicate gives a re-sponse as the solution passes through the second flowcell, so the concentration of silicate can be obtained.The concentration of phosphate can be obtained bycalculating the different between the two results.

3. Results and discussion

3.1. Examination of wavelength

The effects of the wavelength on the sensitivelywere examined from 400 to 415 nm [19]. The testsamples used were 24 mg l−1 phosphate and 22 mg l−1

silicate. The results obtained are shown in Fig. 2.The peak1(P) and peak2(P) are two response signalsobtained from detectors D1 and D2 by injecting onephosphate standard sample (24 mg l−1); peak1(Si) andpeak2(Si) are two response signals obtained fromD1 and D2 by injecting one silicate standard sam-ple (22 mg l−1). This shows that the absorbance ismaximal at 405 nm, which was selected for use.

3.2. Acidity effect of reaction withammonium molybdate

The experimental conditions were as in 3.1, exceptthat an oxalic acid solution (8% w/v), containing20 ml l−1 concentrated sulfuric acid (98%) was used.When the concentration of ammonium molybdatewas fixed at 10% (w/v), the effect of the acidity onthe sensitively was examined. Because the solution


Fig. 2. Effect of the wavelength on the absorbance. Pump speed: 30 rpm; RC1: 300 cm; RC2: 1100 cm; sample volume: 500�l; temperatureof water bath: 30◦C; coloring reagent: 10% (w/v) ammonium molybdate. The oxalic acid solution (masking reagent, R2) was not beintroduced into the FIA system here.

of ammonium molybdate readily forms a precipi-tate due crystallization between 8 and 30 ml l−1 ofthe concentrated sulfuric acid [18] the examinationof the effect of acidity was conducted in the range32–56 ml l−1.

The results obtained are shown as Fig. 3. Peak1(P) isthe response signal obtained by injecting the phosphatestandard sample (24 mg l−1) (note: peak2(P) has beenmasked by the oxalic acid); peak1(Si), and peak2(Si)are two response signals obtained by injecting thesilicate standard sample (22 mg l−1); peak1(Si+P) andpeak2(Si+P) are two response signals obtained by in-jecting the mixed standard samples of silicate andphosphate (Si+ P = 22+ 24 mg l−1) with oxalic acidintroducing (R2).

Under the conditions used, response curves forphosphate and silicate decrease in sensitivity with in-crease in acidity. The greatest sensitivity is achievedwhen the concentration of the concentrate sulfuricacid is at 36 ml l−1. Therefore, this concentration ofH2SO4 was selected for adjusting the acidity.

In the process of preparing solutions of various con-centrations of ammonium molybdate we discoveredthat precipitation will readily occur if 10% (w/v) am-monium molybdate solution was left for a long time.For this reason, the concentration was selected as 8%(w/v) for use in online analysis.

3.3. Effect of reaction temperature

The experimental conditions were as in Section 3.2,except that an ammonium molybdate solution (8%w/v) containing 36 ml l−1 concentrated sulfuric acidwas used. The samples used were the silicate and phos-phate standard solutions.

The effects of reaction temperature on the ab-sorbance from silicate (with oxalic acid solutionintroduced (R2)) and phosphate (no oxalate (with-out R2)) were examined in the range 30–70◦C. Theresults obtained are shown in Fig. 4. It can be seenthat the effect of temperature on the determinationof silicate is greater than that of phosphate and that


Fig. 3. Effects of the acid volume in the molybdenum solution for the simultaneous determination of silicate and phosphate. Conditionsas in Fig. 1, except for 8% (w/v) C2H2O4·2H2O + 20 ml l−1 concentrated H2SO4; wavelength: 405 nm.

the absorbance (sensitivity) for determining silicateincreases with increase of temperature. The optimaltemperature was selected as 50◦C.

3.4. Effect of reaction coil length

If RC2 is too shorter, it will lead overlapping ofthe first and second peaks and cause carryover. There-fore, the length of RC2 must be optimized. Experimentshow that complete peak separation is achieved whenRC2 is 920 cm. Consequently, the length was fixed at920 cm is achieved.

The effects of the length of RC1 on the sensitivitiesfor silicate and phosphate were examined from 100 to270 cm. The experimental results are given in Fig. 5.They show that the length of RC1 does not have a greateffect on the sensitivity for phosphate, but it does onthe sensitivity for silicate.

When using a silicate standard as the test sample, theheight of peak1(Si) and peak2(Si) are both signals forsilicate; peak1(P) and peak2(P) are both for phosphate

this means that the reaction of molybdate yellow is notcomplete when the injected sample is passing throughRC1 and first flow cell.

The height of peak2(Si) does not have much varia-tion. This means that the molybdosilicate yellow reac-tion has finished when the sample has passed throughRC1 and RC2 (920 cm) and arrived at the second flowcell.

When using a phosphate standard, the heights ofpeak1(P) and peak2(P) do not vary much with increaseof length of RC1.

For simultaneous determinations of silicate andphosphate, peak1 is required indirectly measure phos-phate and peak2 to directly measure silicate. So peak1and peak2 should be as high as possible. Conse-quently, the length of RC1 was selected as 140 cm.

3.5. Effect of acidity of oxalic acid solution

The ability of the oxalic acid to mask molyb-dophosphate yellow formation increases with increase


Fig. 4. Effects of reaction temperature on simultaneous determination of phosphate and silicate conditions as in Fig. 3, except for RC1:140 cm; RC2: 920 cm; test samples were 24 mg l−1 phosphate and 22 mg l−1 silicate standards.

Fig. 5. Effects of the length of RC1 on the peak height for phosphate and silicate. Conditions as in Fig. 4, except temperature of waterbath: 50◦C; test samples were 24 mg l−1 phosphate and 22 mg l−1 silicate standards. The oxalic acid solution (R2) was not be introducedinto the FIA system.


of oxalic acid concentration [14], but in the processof preparing the solution, we discovered that a pre-cipitate will readily form if 10% (w/v) oxalic acidsolution was left for 24 h. Therefore, the concentra-tion was selected as 8% (w/v) to prevent the FIAtubing becoming closed with the precipitate. Also,because the molybdenum yellow reaction takes placeunder acidic conditions, the acidity of the oxalicacid solution also match with that of the ammoniummolybdate solution. Therefore, the effect of the acidaddition volume in the oxalic acid solution on thesensitivity was investigated in the range 15–36 ml l−1

of concentrated sulfuric acid. From the results ob-tained, the acidity change did not much affect on theability of the oxalic acid to mask molybdophosphateyellow. Consequently, the acid addition volume wasselected at 20 ml l−1 of concentrated sulfuric acid.

3.6. Effect of sampling volume

The effect of the sampling volume on the peakheight was examined in the range 200–600�l. The ab-sorbance increased with increase in sampling volumeupto 500�l, above which it is constant. So, 500�l wasselected in the FIA system.

3.7. Effect of pump speed

The effect of the pump revolution speed on the ab-sorbance was examined in the range 15–45 rpm. Theabsorbance highest at 30 rpm (3.62 ml min−1), conse-quently, this pump speed was selected.

4. Results of determination

4.1. Calibration and analysis of boiler waters

The optimum conditions used in the method are asfollows. The sampling volume is 500�l; the pumprevolution speed is 30 rpm; the temperatures of RC1and RC2 both are 50◦C, the lengths of RC1 and RC2are 140 and 920 cm (i.d. 0 5 mm), respectively; thewavelength is 405 nm; the chromogenic reagents is8% (w/v) ammonium molybdate and 36 ml l−1 H2SO4(98%), the masking reagent consists of 8% (w/v) ox-alic acid and 20 ml l−1 H2SO4 (98%) (see above).

Under the optimum conditions, a series of sili-cate standard solutions were determined by using the

Table 1Results for simultaneous determination of silicate and phosphatein boiler water at power plants

Sample no. PO43− (mg l−1) SiO32− (mg l−1)

Boiler 11 12.59 0.482 9.67 0.353 3.28 4.97

Boiler 21′ 9.68 02′ 8.48 03′ 3.96 5.31

system shown in Fig. 1, mixed standard solutions ofsilicate and phosphate were analyzed and final stan-dard solutions of phosphate were determined. Theresulting graphs are shown in Fig. 6. The recorderoutput is shown in Fig. 7. The calibration is good overthe whole determination range. The 10σ determina-tion for silicate is 0.05 mg l−1 and that for phosphateis 0.10 mg l−1.

Boiler water samples from some power plants wereanalyzed. The results are listed in Table 1.

4.2. Recovery tests

Recovery tests were made with six boiler watersamples and two silicate and phosphate standards bya standard addition procedure. The results, shown inTable 2, indicate recoveries in the range 98–103%, forsilicate and phosphate.

4.3. Calculation of the sample concentrations

The concentration values for simultaneous determi-nation of silicate and phosphate in the samples shouldbe calculated by the following equations:

Cm(Si+P) = Am(Si+P) − bm(Si+P)

km(Si+P)

(1)

C2(Si) = A2(Si) − b2(Si)

k2(Si)(2)

C3(P) = Cm(Si+P) − C2(Si)

= Am(Si+P) − bm(Si+P)

km(Si+P)

− A2(Si) − b2(Si)

k2(Si)(3)

whereCm(Si+P) refers to the mixed concentration ofsilicate and phosphate (from peak1); bm(Si+P), km(Si+P)


Fig. 6. Calibration graphs for simultaneous determination of phosphate and silicate by the FIA serial spectrophotometric method under therecommended conditions.

and Am(Si+P) refers to the intercept, slope and ab-sorbance of the calibration graph from the mixed stan-dard solution of silicate and phosphate, respectively.C2(Si) is the concentration of silicate;b2(Si), k2(Si) andA2(Si) refers to the intercept, slope and absorbance

Table 2Recovery test for determination silicate or phosphatea

Sample no. Sample concentration(mg l−1) (25 ml)

Added concentration(mg l−1) (25 ml)

Found conc. (mg l−1) Recovery (%)

Boiler 11 0.241 (SiO2) 3.0 (SiO2) 3.239 99.92 0.177 (SiO2) 3.0 (SiO2) 3.013 98.03 2.483 (PO43−) 4.0 (PO4

3−) 6.661 103.0

Boiler 21′ 0 (SiO2) 3.0 (SiO2) 2.949 98.32′ 0 (SiO2) 3.0 (SiO2) 3.076 103.03′ 2.653 (PO4

3−) 4.0 (PO43−) 6.492 97.6

a The sample solutions for the recovery test were prepared by mixing equal volumes of the standards and boiler water samples.

of the silicate calibration graph (from peak2), respec-tively. C3(P) is the concentration of phosphate. Thereal data use were:

Cm(Si+P) = Am(Si+P) − 0.0253

0.0158(4)


Fig. 7. Recorded peaks obtained for the simultaneous determination of silicate and phosphate.

C2(Si) = A2(Si) − 0.0031

0.0079(5)

C3 =∣∣∣∣

A2(Si) − 0.0253

0.0158

∣∣∣∣−

∣∣∣∣

A2(Si) − 0.0031

0.0079

∣∣∣∣

(6)

4.4. Precision determination

The experimental conditions are the same above.Ultra purified water was used as the carrier and themixed solution and standard solutions of silicate andof phosphate were used as the samples. Each deter-mination was done in duplicate. Results in RSDs ob-tained from determining silicate phosphate and mixedstandard solutions were 1.3% (0.155 mg l−1, n = 10),1.2% (0.087 mg l−1, n = 12) and 0.9% (0.233 mg l−1,n = 16+ 14).

5. Conclusions

The simultaneous determination of silicate andphosphate in water is achieved by injecting one sam-ple plug. The approach is convenient, its analyticalfrequency is high and the FIA manifold is simple,

not requiring a reducing reagent. About 60–120samples h−1 can be analyzed. Determination rangesare 0.05–22 mg l−1 for SiO2 and 0.1–24 mg l−1 forPO4

3−. Recovery ranges for SiO2 and PO43− in the

samples are 98–103%.The method meets requiredcontrol standards [1,14] for steam-water quality atpower plants and is suitable for water quality onlinemonitoring in middle, high, superhigh and subcriticalpressure boilers. The method is ca. 15 times fasterthan that of conventional online phosphate or silicaanalyzers.If the method is involved in the manufactureof an online monitoring analyzer, one analyzer couldhave dual function of monitoring PO43− and SiO2.Moreover, the apparatus could monitor and controlthe water quality for 5–20 boilers simultaneously in1 h. That would be revolutionary improvement overthe performance of conventional online monitoringanalyzers.

Acknowledgements

The author gratefully thanks Mr. Muo Yu andMs. Xie Huo-Mei and Jing Zhou, for his helpful


discussions in the experiment, also thanks the sectionof Science and Technology of the Northeast ChinaInstitute of Electric Power Engineering, for theirfinancial help.

References

[1] Water Treatment Committee, Chinese Electrical PowerEngineering Association, Methods for Testing and AnalyzingSteam at Thermal Power Plants, Hydroelectric PowerPublishing Co., Beijing 1984, pp. 1513–5721.

[2] Y.-S. Li, W.-C. Cheng, Flow Injection Analysis, BeijingUniversity Press, Beijing, 1987, p. 71 (in Chinese).

[3] A.G. Fogg, N.K. Bsebsu, Analyst (London) 106 (1269) (1981)1288.

[4] A.G. Fogg, G.C. Cripps, B.J. Birch, Analyst (London)108 (1293) (1983) 1485.

[5] P. Linares, M.D.L. De Castro, M. Valcarcel, Talanta 33 (11)(1986) 889.

[6] Y.-S. Li, GuoluShuichuli 1 (3) (1989) 4.[7] K.S. Johnson, R.S. Petty, Anal. Chem. 54 (1982) 1185.[8] Y.-S. Li, W.-C. Cheng, Fengxi Yiqi 69 (3) (1986) 46.[9] Y.-S. Li, Fengxi Shiyanshi 10 (2) (1991) 41.

[10] A.O. Jacintho, E.A.M. Kronka, E.A.G. Zagatto, M.A.Z.Arruda, J.R. Ferreira, J. Flow Inject. Anal. 6 (1) (1989) 19.

[11] J.-H. Shen, F.-Q. Miao, Yejin Fenxi 9 (3) (1989) 30.[12] T. Imakita, M. Taniguchi, K. Narita, Anal. Prog. Chem. Iron

Steel Ind. 185 (1992) 25.[13] N. Lacy, G.D. Christian, J. Ruzicka, J. Anal. Chem. 62 (14)

(1990) 1482.[14] Y. Narusawa, T. Katsura, F. Kato, Z. Fresenius, Anal. Chem.

332 (1988) 162.[15] F. Mas, J.M. Estela, V. Cerda, Int. J. Environ. Anal. Chem.

43 (2/3) (1991) 71.[16] D.-W. Kang, L. Chen, X.-K. Lu, Haiyang Xuebao 13 (1)

(1991) 60.[17] L.K. Shpigun, I.Ya. Kolotyrkina, Yu.A. Zolotov, Anal. Chim.

Acta 261 (1/2) (1992) 307.[18] Y.-S. Li, Dianli Jishu 23 (1) (1990) 21.[19] Y.-S. Li, Jilin Dianli Jishu 80 (2) (1989) 10.

simultaneous determination of silicate and phosphate in boiler water at power plants based on series...

Documents