Determination of total phosphorus and nitrogen in turbid waters by ...

Analytica Chimica Acta 463 (2002) 283–293

Determination of total phosphorus and nitrogen in turbid waters by oxidation with alkaline potassium peroxodisulfate and low pressure microwave digestion, autoclave heating or the use of closed vessels in a hot water bath: comparison with Kjeldahl digestion W. Maher a,∗ , F. Krikowa a , D. Wruck b , H. Louie c , T. Nguyen b , W.Y. Huang c a

Ecochemistry Laboratory and CRC for Fresh Water Ecology, Division of Science and Design, University of Canberra, Bruce, Canberra, ACT 2601, Australia b Queensland Health Scientific Services, 39 Kessels Road, Coopers Plain, Qld 4108, Australia c AGAL, Pymble Sydney, NSW, Australia Received 11 October 2001; received in revised form 1 March 2002; accepted 22 April 2002

Abstract The evaluation of the use of alkaline peroxodisulfate digestion with low pressure microwave, autoclave or hot water bath heating for the determination of total phosphorus and nitrogen in turbid lake and river waters is described. The efficiency of these digestion procedures were compared to a Kjeldahl digestion procedure with sulphuric acid–potassium sulfate and copper sulfate. The final solution before digestion was 0.045 M in potassium peroxodisulfate and 0.04 M in sodium hydroxide. Procedures were evaluated by the analysis of suspensions of two reference materials, National Institute of Environmental Science, Japan, no. 3 Chlorella and no. 2 pond sediment and natural turbid waters. Best recoveries of phosphorus and nitrogen by microwave heating were obtained when solutions were digested at 95 ◦ C for 40 min. Quantitative recoveries of phosphorus from Chlorella suspensions up to 1000 mg/l were obtained by all three heating procedures, but incomplete recoveries of nitrogen occurred above 20 mg N/l in the digested sample. Good recoveries of phosphorus and nitrogen from suspended sediment suspensions were obtained only from solutions containing 150 mg/l) and are only suitable for the analysis of very turbid samples when the turbidity is due to organic matter (algal cells, plant detritus). Underestimation of nitrogen occurs when samples contain more than 20 mg N/l. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Phosphorus; Nitrogen; Turbid waters; Alkaline peroxodisulfate digestion; Low pressure microwave heating; Autoclave heating; Water bath heating; Kjeldahl digestion; Flow injection analysis; Air-segmented continuous flow analysis; NEIS no. 3 Chlorella; NEIS no. 2 pond sediment

∗ Corresponding author. Tel.: +61-26201-2531; fax: +61-26201-5305. E-mail address: [email protected] (W. Maher).

0003-2670/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 0 2 ) 0 0 3 4 6 - X

284

W. Maher et al. / Analytica Chimica Acta 463 (2002) 283–293

1. Introduction

2. Methods

Eutrophication is recognised as a major water quality problem worldwide [1]. Reliable measurements of total phosphorus and nitrogen are needed as total phosphorus is used for predicting algal biomass [2,3] and total nitrogen to total phosphorus ratios used to predict algal species composition [4]. In Australia, many rivers and lakes are very turbid because of the erodible nature of the continents soils. Digestion with alkaline potassium peroxodisulfate has become the preferred method to release phosphorus and nitrogen from particles for total phosphorus and nitrogen analysis [5], because of its simplicity and ease of use and APHA/AWWA/WEF standard methods for the examination of water and waste water now contains a total nitrogen method based on alkaline persulfate digestion [6]. Procedures incorporating the heating of samples using autoclaves are extensively used [5] while a few methods using microwave heating have been reported [7–10]. Several published papers have compared the use of alkaline persulfate and Kjeldahl digestion procedures for waters [11,12], but their use for turbid waters has not been evaluated. Previously, we have evaluated an alkaline peroxodisulfate digestion procedure for the determination of total phosphorus in turbid waters using autoclave heating and high pressure microwave heating [10]. Use of microwave heating was found to be superior in releasing phosphorus from soil particles. However, relatively few samples could be digested simultaneously because of the size of the commercially available microwave vessels and the need to rigorously clean vessels between digestions. In this paper, we describe a low pressure microwave digestion procedure utilising alkaline peroxodisulfate in which 36 samples can be digested simultaneously. The procedure uses 50 ml polycarbonate tubes and digestions are performed at atmospheric pressure. For comparison, we have evaluated digestion with alkaline peroxodisulfate using microwave heating, autoclave heating, hot water bath heating and digestion by the traditional Kjeldhal procedure with sulphuric acid–potassium sulfate and copper sulfate.

2.1. Experimental design 2.1.1. Microwave heating: recoveries from reference material suspensions, effect of temperature and time The recovery of phosphorus and nitrogen from suspensions of two reference materials, National Institute of Environmental Science, Japan, no. 3 Chlorella (16 ± 1 mg P/g, 8.5 mg N/g) and no. 2 pond sediment (1.3 ± 0.1 mg P/g, 4.3 ± 0.3 mg N/g), were used to determine the best microwave digestion conditions. Suspensions of the two reference materials were prepared by adding these materials to deionised water (pH 6.5) to give a final concentration of 1000 mg/l. Homogenisation was achieved by the use of shaking, sonification and stirring. Suspensions were continually stirred and subsamples taken for dilution to lower concentrations. Suspensions containing ∼100 ␮g P/l and ∼50 ␮g N/l were used for recovery studies. Suspended sediments concentrations were measured as out lined in standard methods for water and waste water [13]. Digestion tubes were removed from the microwave cavity at 5 min intervals and cooled rapidly by refrigeration. The conditions under which best recovery of phosphorus and nitrogen occurred were determined using a two-way analysis of variance (Statistical Analysis Systems, general linear models procedure) with the temperature and time as factors. Where significant differences occurred a Tukey’s mutiple comparisions test was used to rank differences. 2.1.2. Recoveries from reference materials and natural suspended sediment suspensions The recoveries of phosphorus and nitrogen from a range of suspended sediment suspensions were determined using alkaline peroxodisulfate with microwave, autoclave and hot water bath heating as well as the Kjeldahl digestion procedure. Suspensions of two reference materials, NIES no. 3 Chlorella and NIES no. 2 pond sediment (7–1000 mg/l) were prepared as previously described. Suspensions were analysed without dilution. A natural turbid water sample (6790 mg/l) was also serially diluted with deionised


water (pH 6.5) to obtain a range of suspended solids (34–6790 mg/l). 2.1.3. Recovery of added phosphorus and nitrogen compounds Stock solutions (1000 ␮g P or N/l) of potassium dihydrogen phosphate, glucose-6-phosphate, phytic acid, 2-amino ethyl phosphonic acid, dl-␣-glycerophosphate, phosphonoformic acid, o-phosphonyl ethanol, phospho(enol) pyruvate, sodium hexa-metaphosphate, sodium tripoly-phosphate, calcium phosphate, aluminium phosphate, ethylenediaminetetraacetic acid (EDTA), nicotinic acid, glutamic acid, urea and hexamine were prepared by dissolution in distilled deionised water (pH 6.5). These compounds were added individually to distilled water (100 ␮g P/l, 2 mg N/l) and total phosphorus or nitrogen measured. To determine recoveries, the initial concentrations of phosphorus or nitrogen in the distilled water were determined and subtracted from the phosphorus or nitrogen concentrations found in the solutions into which the compounds had been added. Recoveries were then calculated by dividing this value by the original amount of phosphorus or nitrogen added. Differences in phosphorus and nitrogen recoveries were determined using a two-way analysis of variance (Statistical Analysis Systems, general linear models procedure) with heating method and phosphorus compound as factors. Where significant differences occurred a Tukey’s mutiple comparisions test was used to rank differences. 2.1.4. Analysis of natural turbid water samples Water samples from rivers and lakes throughout the ACT, NSW and Qld were collected to compare the three alkaline peroxodisulfate and the Kjeldahl digestion procedures. A litre of water was collected 300 cm below the surface of each water body and frozen in polyethylene bottles until analysed. This procedure has been shown to be satisfactory for the storage of samples up to 6 months [14]. Differences in phosphorus and nitrogen concentrations measured by digestion procedures were determined using regression analysis.

285

2.2. Equipment A model MDS-5000 (CEM, Indian Trail, NC, USA) microwave oven rated at 600 W was used. Pressure and temperature within the vessels were monitored continuously using a pressure gauge and an optical fibre thermocouple. A circular rack with two rows of tubes, 18 on the inside and 18 on the outside was used. Autoclave digestion: an Atherton steam steriliser operated at 15 psi was used for all autoclaving. Pressure and temperature within the autoclave were monitored continuously using the fitted pressure gauge and a thermocouple. Hot water bath digestion: a water bath with top plate modified to fit 50 ml polycarbonate tubes was used. Kjeldahl digestion: a calibrated digestion block (Technicon BD40, USA) capable of maintaining temperatures up to 360 ◦ C was used together with 75 ml Pyrex digestion tubes. Total phosphorus and nitrogen in all alkaline-peroxodisulfate digests were measured using a Lachat flow injection analyser. Total phosphorus and nitrogen in Kjeldahl digests were measured using a Skalar segmented flow analyser. 2.3. Digestion procedures 2.3.1. Alkaline peroxodisulfate digestion All alkaline peroxodisulfate digestions were carried out in 50 ml screw capped polycarbonate tubes (Lab Supply, Australia). The stock alkaline peroxodisulfate digestion reagent contained 0.27 M potassium peroxodisulfate (Ajax-Univar, Australia) and 0.24 M of sodium hydroxide (BDH-AnalaR, Australia) in deionised water. For digestions, 2 ml of digestion reagent was added to 10 ml of sample. Using this ratio of digestion reagent to sample volume, i.e. 1:5 (v/v) the final concentration of potassium peroxodisulfate and sodium hydroxide in solution prior to heating were 0.045 and 0.04 M, respectively. When microwave heating was used, samples were heated at 95 ◦ C for 40 min. When autoclaving heating was used, samples were heated at 120 ◦ C for 60 min

286


while samples in the water bath were heated at 100 ◦ C for 60 min. 2.3.2. Kjeldahl digestion The stock Kjeldahl digestion reagent contained 2.4 M sulphuric acid, 0.39 M potassium sulfate and 0.023 M copper sulfate. For digestions, 10 ml of the digestion reagent was added to 25 ml of sample in a digestion tube with 25 carborundum chips. Samples were digested at 160 ◦ C for 90 min and then at 360 ◦ C for 120 min. Digested samples were cooled, 25 ml of water added, vortexed to dissolve solidified salts and diluted to 75 ml for analysis. 2.4. Phosphorus and nitogen determination 2.4.1. Lachat flow injection analyser Phosphorus as orthophosphate was determined by the formation of a reduced phosphoantimonylmolybdenum complex with ammonium molybdate, potassium antimony tartrate and ascorbic acid and measurement at 880 nm [15]. Nitrogen as nitrite was determined after cadmium column reduction by the formation of a red-purple azo dye by reaction with sulphanilamide coupled to N-1-napthylethylene dihydrogen chloride and measurement at 520 nm [16]. 2.4.2. Skalar segmented flow analyser Kjeldahl phosphorus as orthophosphate was determined by the formation of a reduced phosphoantimonylmolybdenum complex with ammonium molybdate, potassium antimony tartrate and ascorbic acid and measurement at 880 nm [17]. Kjeldahl nitrogen was measured as ammonia using salicylate (in lieu of phenol), hypochloride and nitroprusside at 660 nm [18]. Total nitrogen was calculated by adding nitrogen as nitrate/nitrite and Kjeldahl nitrogen. 3. Results and discussions 3.1. Microwave heating: recoveries from reference material suspensions, effect of temperature and time Preliminary experiments showed that the use of microwave digestion temperatures >95 ◦ C resulted in the polycarbonate tubes leaking and high evaporation rates (>1%) and temperatures above this were not further investigated.

Fig. 1. Effect of microwave temperature and time on the recovery of: (a) phosphorus and (b) nitrogen from suspensions of Chlorella n = 3, and (c) change in pH.

3.1.1. Phosphorus The effect of digestion temperature and time on the recovery of phosphorus from suspensions of Chlorella and pond sediment are shown in Figs. 1a and 2a, respectively. A two-way analysis of variance showed that temperature and time caused significant differences in the recoveries of phosphorus from suspensions (Table 1). The Tukey’s multiple comparison


287

pH is reduced to 2 as acidic conditions are required to hydrolyse some organic phosphorus compounds and polyphosphates [5,17,19,20]. We also believe that acid extraction of occluded phosphorus from particles is also occurring at low pHs. 3.1.2. Nitrogen The effect of digestion temperature and time on the recovery of nitrogen from suspensions of Chlorella and pond sediment are shown in Figs. 1b and 2b, respectively. A two-way analysis of variance showed that temperature and time caused significant differences in the recoveries of nitrogen from suspensions (Table 2). The Tukey’s multiple comparison tests showed that consistent high recoveries of nitrogen (95–103%) from both Chlorella and pond sediment suspensions were obtained when heating was maintained at 95 ◦ C for 25–45 min. For further investigations a microwave digestion temperature of 95 ◦ C and a digestion time of 40 min was used. 3.2. Recoveries from reference materials and natural suspended sediment suspensions

Fig. 2. Effect of microwave temperature and time on the recovery of: (a) phosphorus and (b) nitrogen from suspensions of pond sediment n = 3, and (c) change in pH.

tests showed that the best recoveries of phosphorus (92–100%) were obtained from both Chlorella and pond sediment suspensions when heating was maintained at 95 ◦ C for 30–40 min. The pH during digestion also varied with the temperature and time of digestion (Figs. 1c and 2c). A final pH of 2 was reached after 20 min for both Chlorella and pond sediment at 95 ◦ C. It is important that the

The recoveries of total phosphorus from Chlorella suspensions by all digestion procedures were similar (Fig. 3a) with regressions of y = 0.015x + 0.0347, r 2 = 0.9982; y = 0.0147x − 0.0639, r 2 = 0.9946; and y = 0.0143x − 0.094, r 2 = 0.9899 for the microwave, autoclave, hot water bath and Kjedhal digestions, respectively. Solution of regression equations when Chlorella concentration equals 1000 mg/l gives total phosphorus concentrations of 14.3–15.3 mg P/g which are 90–96% of the non-certified concentration of 16.1 ± 0.1 mg P/g. Thus, if samples are turbid and turbidity is due to organic matter, i.e. algal cells and plant detritus, then procedures using alkaline peroxodisulfate give good estimates of total phosphorus. Incomplete recoveries of nitrogen were obtained using the alkaline peroxodisulfate digestion techniques when digested solutions contained >20 mg N/l (Fig. 3b). Below this value, recoveries of the peroxodisulfate and Kjeldahl procedures were complete and similar with regessions of y = 0.08333x + 0.5643, r 2 = 0.9975; y = 0.085x +0.2484, r 2 = 0.9999; y = 0.086x −0.444, r 2 = 0.9954 and y = 0.091x −0.062, r 2 = 0.9988 for the microwave, autoclave, hot water

288


Table 1 Optimisation of microwave digestion for total phosphorus determination Source of variation

Chlorella DF

Temperature Time

Time × temperature

Pond sediment F-value

Prob > F

DF

Tukey’s test

F-value

Prob > F

2.80 8.80

12.72 3.28

0.00015 0.004

2.80 8.80

2.43 5.78

0.0975 0.0001

16.80

0.8

0.7020

16.80

2.41

0.0083

Chlorella

Pond sediment 80 ◦ C

95 > 90 = At 95 ◦ C, 25 = 30 = 35 = 40 > 20 > 5 = 10 = 15 = 45 min

95 = 90 = 80 ◦ C 20 = 25 = 30 = 35 = 40 > 5 = 10 = 15 = 45 min

Table 2 Optimisation of microwave digestion for total nitrogen determination Source of variation

DF Temperature Time

Time × temperature a

Pond sedimenta

Chlorella

2.80 8.80

16.80

F-value

Prob > F

2.85 3.12

0.0605 0.0057

0.7838

0.7

DF

Tukey’s test

F-value

Prob > F

Chlorella

Pond sediment

2.44 8.44

9 1.61

0.009 0.1973

95 = 90 = 80 ◦ C 10 = 15 = 20 = 25 = 30 = 35 ≥ 40 = 45 > 1045 min

95 > 90 = 80 ◦ C At 95 ◦ C, 25 = 30 = 35 > 15 =20 = 40 > 5 = 45 min

16.44

2.29

0.0480

Analyses only performed on 25, 30, 35, 40 and 45 min data.

bath and Kjeldahl digestions, respectively. Solution of regression equations when Chlorella concentration equals 1000 mg/l gives total nitrogen concentrations of 8.3–9.1 mg P/g which are 98–107% of the expected value of 8.5 mg P/g. Given the complete recovery of phosphorus, the alkaline peroxodisulfate present is sufficiently oxidising to digest organic material to release phosphorus.

The low recoveries of nitrogen are attributed to insufficient oxygen to oxidise ammonium to nitrate as oxygen supplied by the persulfate reagent may be depleted by suspended and dissolved organic carbon present in suspensions [21]. Care needs to be taken, if samples to be analysed are of a sewage origin, as these will contain high concentrations of ammonia and organic matter.

Fig. 3. Effect of suspended solids concentration on the recoveries of: (a) phosphorus and (b) nitrogen from Chlorella suspensions.


Fig. 4. Effect of suspended solids concentration on the recoveries of: (a) phosphorus and (b) nitrogen from pond sediment suspensions.

Recoveries of total phosphorus and nitrogen from suspensions of pond sediments using alkaline peroxodisulfate were significantly lower than those obtained using the Kjeldhal procedure at high suspended sediment concentrations (Fig. 4). High recoveries of total phosphorus were obtained for suspended sediments below 150 mg/l. At these lower suspended sediment concentrations all three alkaline peroxodisulfate procedures have similar recoveries (regressions were: for microwave, y = 0.00096x + 0.0056, r 2 = 0.9881; for autoclave, y = 0.001x + 0.003, r 2 = 0.9970; for hot water bath, y = 0.001x + 0.0051, r 2 = 0.9596). Solution of regression equations when pond sediment concentration equals 1000 mg/l gives total phosphorus concentrations of 0.097–1.1 mg P/g which are 75–85% of the non-certified concentration of 1.3 ± 0.1 mg P/g. The Kjeldahl digestion procedure recoveries were quantitative in the suspended sediment range 0–1000 mg/l (y = 0.0013x −0.0053, r 2 =

289

Fig. 5. Effect of suspended solids concentration on the recoveries of phosphorus and nitrogen using peroxydisulfate oxidation from a serially dilute natural turbid water sample.

0.9976). High recoveries of total nitrogen were obtained for suspended sediment concentrations below 250 mg/l with the microwave (y = 0.004x − 0.018, r 2 = 0.9958) and autoclave (y = 0.0041x − 0.049, r 2 = 0.9985) alkaline peroxodisulfate digestion procedures giving similar recoveries. Solution of regression equations when pond sediment concentration equals 1000 mg/l gives total nitrogen concentrations of 4–4.1 mg P/g which are 93–95% of the non-certified concentration of 4.3 ± 0.3 mg P/g. Using hot water bath heating gave a lower regression (y = 0.0035x − 0.0188, r 2 = 0.9973) corresponding to recoveries of 81%. The Kjeldahl digestion procedure recoveries were quantitative in the suspended sediment range 0–1000 mg/l (y = 0.0045x − 0.0212, r 2 = 0.9986). These results were confirmed by the analysis of a natural turbid water sample that had been serially diluted (Fig. 5a), that showed decreased recoveries at high suspended sediment concentrations were ob-

290


tained using peroxodisulfate with all three heating methods. At lower suspended sediment concentrations good recoveries were obtained (Fig. 5b). Agglomeration of particles at high suspended sediment concentrations, insufficient acid produced by the alkaline peroxodisulfate reagent or the low temperatures used in the alkaine peroxodisulfate procedures may prevent the extraction of occluded phosphorus and nitrogen from particles. Other reports have suggested that re-adsorption of phosphorus by particles may also be occurring [22], but this is not likely at pH of 2. 3.3. Recoveries of added phosphorus and nitrogen compounds 3.3.1. Phosphorus Recoveries of phosphorus from all compounds except aluminium orthophosphate and sodium-hexameta-poly-phosphate by the three heating procedures with alkaline peroxodisulfate (Table 3) were quantitative (94–108%) and not significantly different (DF, 2.27; F = 8.32; P = 0.075). The use of autoclave and hot water bath heating resulted in quantitative recoveries of phosphorus from sodium-hexa-meta-poly-phosphate (96 ± 4% and 114 ± 6%, respectively) while microwave heating only gave recoveries of 34 ± 8%. The incomplete recoveries from polyphosphates, are in practice, not a problem as significant quantities of polyphosphates are rarely found in turbid river and lake water samples [23,24]. Microwave, autoclave and hot water

bath heating all gave low recoveries of phosphorus for aluminium orthophosphate (Table 3) probably due to its low solubility. In other studies of digestion procedures, complete recoveries of easily hydrolysable compounds containing C–O–P bonds, such as orthophosphate, adenosine phosphate and glucose phosphate have been reported (5). The recovery of phosphorus from compounds such as phytic acid that are expected to be present in soil particles [25] are less frequently reported. This study shows that the complete release of phosphorus from a range of phosphates including phytic acid (C–O–P bonds) and phosphonates (C–P bonds) was achieved (Table 1). Recoveries of phosphorus from poly phosphates (P–O–P bonds) were low. The recovery of phosphorus from polyphosphate compounds is more difficult and dependent on the hydrolysis temperature and time of hydrolysis [26]. High recoveries of phosphorus from poly phosphates are usually only obtained using temperatures above 100 ◦ C and long heating times, i.e. 30 min [5]. 3.3.2. Nitrogen Recoveries of all nitrogen compounds examined by the three heating procedures with alkaline peroxodisulfate (Table 4) were near quantative (94–102%) and not significantly different (DF, 2.14; F = 7.13; P = 0.10). These results are in agreement with published studies that have shown that alkaline peroxodisulfate quantitatively releases nitrogen from nitrogen compounds containing C–N bonds [9,22,27,28].

Table 3 Recovery of phosphorus from phosphorus compounds in deionised water Phosphorus compound

Structure

Recovery (%) Microwave

Glucose-6-phosphate Phytic acid dl-␣-Glycerophosphate Phospho(enol)pyruvate 2-Aminoethylphosphoric acid Phosphoformic acid o-Phosphonyl ethanolamine Sodium-hexa-meta-phosphate Aluminium phosphate Mean ± S.D., n = 4.

C–O–P C–O–P C–O–P C–O–P C–P C–P C–P P–O–P Al–P

107 94 108 99 102 106 106 34 17

± ± ± ± ± ± ± ± ±

3 1 6 2 3 2 7 8 6

Autoclave 113 101 108 103 104 106 109 114 23

± ± ± ± ± ± ± ± ±

6 2 4 7 2 2 2 6 5

Hot water bath 106 100 107 101 103 104 107 96 8

± ± ± ± ± ± ± ± ±

3 7 2 1 3 4 3 4 6

W. Maher et al. / Analytica Chimica Acta 463 (2002) 283–293 Table 4 Recovery of nitrogen from nitrogen compounds in deionised water Nitrogen compound

Structure

EDTA Nicotinic acid Glutamic acid Urea Hexamine

C–N C=N–C C–N C–N C–N

Recovery (%) Microwave 99 97 100 99 96

± ± ± ± ±

3 2 2 5 4

Autoclave 99 96 103 98 96

± ± ± ± ±

4 2 2 5 4

Hot water bath 102 101 112 94 96

± ± ± ± ±

3 3 4 2 8

Mean ± S.D., n = 3.

3.4. Analysis of natural turbid water samples Water samples collected from turbid water bodies from south east Australia representing a range of suspended sediments were also analysed by the alkaline peroxodisulfate and Kjeldahl digestion procedures (Figs. 6 and 7).

291

When natural turbid water samples containing 60–6500 mg/l of suspended sediments were analysed underestimation of total phosphorus by the peroxodisulfate digestion procedures at high suspended sediment concentrations was evident (Fig. 6a). However, total nitrogen concentrations measured using the peroxodisulfate procedures were a good approximation of the total nitrogen concentrations at all suspended sediment concentrations (Fig. 7a). In natural turbid waters, most of the phosphorus is of soil origin and will be adsorbed to particles and as shown in this paper not able to be extracted at high suspended sediment conentrations. We have found that filterable reactive phosphorus is about 2–50% of total phosphorus in turbid waters (Krikowa, Maher and Wruck, unpublished data). About 20–50% of nitrogen in turbid waters is present as ammonia/nitrate/nitrate (Krikowa, Maher and Wruck, unpublished data). However, often little TN is of soil origin and nitrogen is present in organic forms. Hence, underestimation

Fig. 6. Comparison of digestion procedures for total phosphorus analysis of turbid water samples: (a) comparison of peroxodisulfate and Kjeldahl digestion procedures; (b) comparison of microwave and autoclave digestion procedures; (c) comparison of microwave and hot water bath digestion procedures.

292


Fig. 7. Comparison of digestion procedures for total nitrogen analysis of turbid water samples: (a) comparison of peroxodisulfate and Kjeldahl digestion procedures; (b) comparison of microwave and autoclave digestion procedures; (c) comparison of microwave and hot water bath digestion procedures.

of nitrogen associated with sediment particles is less of a problem unless the organic carbon present consumes the oxygen generated by the decomposition of peroxodisulfate [21]. Regression analysis showed there was no significant difference between total phosphorus and nitrogen measurements at the 95% confidence interval by all three digestion procedures using alkaline peroxodisulfate at suspended sediments below 150–200 mg/l (Figs. 6b, c and 7b, c). This confirmed that the use of the low pressure microwave procedure gives comparable results to the autoclave and hot water bath procedures. 3.5. Precision The precision of the three alkaline peroxodisulfate and Kjeldahl digestion procedures as given by relative standard deviations (R.S.D.) were similar at low suspended sediment concentrations, i.e.