an ignition method for determination of total phosphorus in lake ...

glatt.r Re,c,~rtl~ VO[. 10. pp. 32q to 331. Perg,irnt, n Pr~.~s 1'~'6 Printed in Great Britain.

AN I G N I T I O N M E T H O D FOR D E T E R M I N A T I O N O F TOTAL P H O S P H O R U S IN LAKE SEDIMENTS J. M. ANDERSEN Freshwater Biological Laboratory. University of Copenhagen. 51 Helsingorsgade. DK 3400 Hillerod, Denmark (Receired l January 1975: in recised form 22 August 1975)

Abstract--A method is described for determination of total phosphorus in lake sediments or biogenic material. The organic matter is destructed by ignition. Material remaining after ignition is boiled in 1 N HCI and orthophosphate determined after dilution. The method generally gave lower results than the perchloric acid digestion method (97.7, 98.7. 94.4. 100-5 and 97-3°,,) for four sediment samples and one sample of dried leaves. The reproducibility of the ignition method was slightly less than that observed with the perchloric acid method. Recovery after addition of known amounts of hydroxylapatite to the sediment samples was 98.5-102.67g.

INTRODUCTION

Determination of total phosphorus in soil, sediment, or biogenic matter involves a preliminary digestion to convert the phosphorus into orthophosphate, which can be readily determined. Comparative studies of different methods for determination of total phosphorus in sediment have been made by Sommers et al. (1970); Nordforsk (1974). Digestion with perchloric acid (Jackson, 1958; Standard Methods, 1971) is the most common and generally accepted procedure. A more rapid method for determination of total phosphorus in water samples by digestion with persulfate was introduced by Koroleff (1968), but this method has not been widely used for sediment samples. Preliminary measurements of phosphorus in lake sediments at this laboratory using the persulfate digestion method gave considerably lower values than the perchloric acid method. Determination of total phosphorus in lake sediments by ignition of samples in a muffle furnace at 550~C, boiling of the residue from ignition in 1 s HCI, and subsequent determination of orthophosphate gave approximately the same values as the perchloric acid digestion. Therefore the ignition method was investigated more closely for comparison with the perchloric acid method. The ignition principle is recommended by Stainton et al. (1974) for determination of particulate phosphorus in water after the particulate matter has been retained on a glass fiber filter, and by Saunders and Williams (1955) for determination of organic phosphorus in soils. Andersen (1974) used this method for sediment samples but he did not compare his results with those obtained by other methods. METHODS

AND MATERIALS

The uppermost 20cm of sediment was dried at 103°C and ground and mixed in a mortar. Ten determinations of total phosphorus were made with the perchloric acid method and ten with the ignition method on each type of dried sediment. The two methods were also compared using plant material (dried leaves of Glyceria maxima Holmb.). Some characteristics of the sample material are shown in Table 1. Perchloric acid method (Standard Methods, 1971) Ten millilitres of demineralized water and 2 ml of concentr.'lted HNO3 were added to 0"15-0.2 g of dry sediment or plant material in a 100ml erlenmeyer flask. After a preliminary oxidation by evaporation of water and HNO3 on a hot plate 2 ml of conc. HCIO,~ were added, and the sample was boiled until clear. After cooling the sample was diluted to 100 mt in a volumetric flask, and an aliquot was withdrawn for orthophosphate determination by the ascorbic acid reduction method of Murphy and Riley 11962). Blanks and standards were treated as samples. Ignition method Dry sediment or plant material (0"15-0-2 g) was ignited in a muffle furnace in a porcelain crucible (550°C for 1 h). After cooling the residue was washed into a 100 ml erichmeyer flask with 25 ml 1 N HCI and boiled for 15 rain on a hot plate. The sample was diluted to 100 ml in a volumetric flask, and orthophosphate was determined as in the perchloric acid method. Standards and blanks were not ignited.

Table 1. A characterization of the investigated lakes

,ake E. . . . . .

Fureso ~h~o Kvind ~o

Sediment samples from four lakes of different type were collected with a K.'tiak bottom sampler [Kajak et al.. 19651. ul.,,.,i,~iea~e~ 329

Maximum depth ( ml

32 Eutrophic Alkaline 36 Eutrophic Alkaline II Humic Acid 2"5 Pollut,xt by sev.a~¢ Alkaline

Sediment

Loss on ignition of dry weight (*'o)

Gyttja

35

98

Calcareous gyttja Dy gyttja

lI

176

29

4

G~,ttia

35

6

93

5

Caloium dry weight (mg I - =1

330

J.M.

ANDERSEN

The average of the 10 determinations together with the standard deviation of a single determination, the standard error of the mean. and the 95"~ confidence limit of the mean were calculated according to Croxton 11959). Recoxery of the ignition method was determined after addition of different amounts 15-100 mgl of hydroxylaparite [approximately Cat~IPO++)~,IOH).,] to 0-5-10g of dried sediment. Phosphorus content of the hydroxylapatite ~as determined directly after dissolution in I N HCI. In both methods the aliquot for orthophosphate deterruination is strongly acid. Usually this does not cause any trouble, as the aliquot has to be diluted before orthophosphate determination. In this investigation the samples are diluted about 10 times, which means that the reagent added to the sample by the orthophosphate analysis contains roughly 20 times as much acid (as equivalents) as the linal solution which is analyzed, and hence there should be no problems with the acidity of the sample. However. if phosphorus content of samples are very low, a neutralization may be necessary. RESULTS AND DISCUSSION The 95",, confidence limits vary from +0-5 to _+ 1.4",,, of the average for the perchloric acid method and from +1-0 to +2-1 for the ignition method (Table 2). Thus, the reproducibility of the two methods is similar, but a little better for the perchloric acid method. In two cases (Fureso and Kvind so) there are not significant differences between the average total phosphorus content found by the two methods. In the other three cases (Esrom So, Gribso and Glyceria leaves) the 95"],, confidence limits of the two averages do not overlap, which indicates-a significant difference. The sizes of these differences relative to the P contents found are: Esrom So 2.3%, Gribso 5"6yo, and Glycerin leaves 2"7~o. The differences found for Esrom So and Glycerin leaves are small and can eventually be due to system-

atic errors at other steps in the analytical procedure than the ignition. However. for Gribso the difference between the two methods is 5-6°0, and this can hardly be attributed to anything but an effect of ashing. One possibility for the lower results obtained by the ignition method is that organically bound phosphate is liberated as P,Os. which may sublimate ISublimation point 347 :C) if it does not react with alkaline minerals (e.g. CaCO3). To avoid a possible sublimation of P205 by the ignition the total phosphorus content of the Gribso sediment was deterrfiined once again after addition of 0.5 g Na2CO3 to each crucible before ignition. The P content observed was not affected by this treatment, and therefore the cause of the low P content of Gribso sediment found by the ignition method is probably not due to a sublimation of P_,Ov Table 3 shows the results of the recovery experiment with Gribso and Kvind so sediment after addition of hydroxylapatite with a phosphorus content of 19"30"0. Recoveries varied from 98.5 to 102"6~0 (average 101.~,~,). It is relevant to use apatite in the recovery experiment as it is an important constituent of many lake sediments, but it must be noticed that other phosphorus containing compounds will not necessarily give the same recovery. Phosphate analysis after digestion by a straight acid hydrolysis or persulphate oxidation was performed on sediment from Kvind so to test whether these direct methods gave results similar to the ignition method. However, the two methods only yielded 63 and 69~, respectively compared to the ignition method. Concerning the effect of the time for which the ignited residue is boiled with HCI an experiment with sediment from Kvind so was made. Recoveries of 97,

Table 2. Total phosphorus contents of dried sediment and plant material determined by the perchloric acid digestion and by ignition. Ten determinations on each sample (rag P g- t dry matter) F~rom S*s I)crchloric acid Ignition A~cragc ". of the perchloric acid method Standard deviation Standard error 95",, coniidcnce limit DO",, of average

Fures~ Pcrchloric acid Ignition

Gribs*l Perchloric acid Ignition

Kvind so Perchloric acid Ignition

Dried leaves of Glyt~ria Perchloric acid Ignition

2.109

2.061

1.387

1-369

1.356

1.280

2-495

2.507

3" 188

3.101

0-031 0.010

97.7 0-034 0.011

0.017 0.005

98.7 0-0,.11 0.013

0.027 0.009

94.4 0.019 0.006

0.0..26 0.008

100.5 0-048 0.015

0.023 0-007

97.3 0.055 0.017

0..022 1-0

0.023 1-1

0.012 0-9

0.029 2.1

0-019 1"4

0.013 I-0

0.018 0.7

0.033 l'3

0-016 0-5

0"038 1-2

Table 3. Recovery of P by adding apatite [Cato(PO,)6(OH)2] to Gribso and Kvind so sediment (ignition method) Gribs,

Kvind s~s

P added as sediment

P added as apalite

P found

ling)

lmg)

(mgl

0.805 0"830 0.950 I"i37 I"095 1"198

1"090 1-370 2"526 3-787 7"12 20"85

1'900 2.240 3'512 4'85 8"42 22"20

Recovery

P added as .~dimcnt

P added as apalite

P found

q'ol

qrngl

Imgl

(mg)

(?~)

10C'3 101"8 101-0 98"5 102"5 100"7

1.799 1"859 1-324 1-890 I ",/,40 2.09

0"855 1-623 3-242 4"57 10"90 1867

2"656 3"532 4-648 6"45 12"66 21-05

100.1 101.4 [01"8 99"8 102'6 101-4

Recovery

Total phosphorus in lake sediments 98. 98. 99 and 100°o were obtained after I. 2, 5. 15 and 30 min of boiling. SU~IM.~RT AND CONCLt'SIONS An ignition method for determination of total phosphorus in lake sediments or biogenic matter has been compared with the standard perchloric acid digestion method. Destruction of the sample takes place by ignition in a fumace 155OC for l h), and after boiling of the ignition residue in 1 N HCI and diluting to a known volume, the total phosphorus content of the original sample is determined as orthophosphate by the molybdate--ascorbic acid method. It can be concluded that a determination of total phosphorus in sediments or plant material gives the same value or slightly smaller value than the perchloric acid method. No explanation for this possible difference has been found. The reproducibility of the two methods is similar, but a little better for the perchloric acid method ~see Table 2). The ignition method does not seem to be well suited for sediments with a high content of humic matter (5"6'~0 below the perchloric acid method). For the other types of sediments and for the dried leaves considered in this investigation the method is accept7o/ lower than able although the results are up to "L _-,,o by the perchloric acid destruction. The main advantage of the ignition method is the convenient destruction by ignition in a furnace. Moreover the loss on ignition is determined during many sediment investigations, and hence the residue from this determination can be used for the total P determination simply by boiling in IN HCI and diluting to a known volume. The ignition method may not be suitable for deter-

331

mining phosphorus in all types of particulate matter. A comparison of results obtained with the perchloric acid method is recommended before widespread application of the technique. REFERENCES

Andersen J. M. 119741 Nitrogen and phosphorus budgets and the role of sediments in six shallow Danish lakes. Arch. Hvdrobiol. 74~4k 528-550. Croxton 1~. E. 119591 Elementary Stati.stics, 379 pp. New York. Jackson M. L. 11958J Soil Chemical .qn~llysis, 498 pp. Englewood Cliffs. Kajak Z.. Kacprzak K. & Polkowski R. 119651Tube bottom sampler for taking samples of micro and macro benthos and for sampling of undisturbed structures of mud samples for experimental purposes. Ekol. Pol. (B) I I. 159-65.

Koroleff F. ~1970} Determination of total phosphorus in natural waters by means of persulfate oxidation. International Council for the Exploration of the Sea (ICES~. Report No. 3.

Murphy J. & Riley J. P. (19621 A modified single solution method for the determination of phosphate in natural waters. Analyt. Chim. Acttt 12, 162-76. Nordforsk (1974) lnterkalibrering av sedimentkemiska analysmetoder. Nordforsk, Mi/ji'wfirdssekretariatet. Saunders W. M. H. & Williams E. G. 119551 Observations on the determination of total organic phosphorus in soils. J. Soil. Sci. 6(2). 254--67. Sommers L. E., Harris R. F., Williams J. D. H.. Armstrong D. E. & Syers J. K. 11970) Determination of total organic phosphorus in lake sediments. Limnol. Oceanoyr. 15, 301--4. Stainton M. P., Capel M. J. & Armstrong F. A. J. 119741 The chemical analysis of fresh water. Department of the Environment (Canada). Fisheries and Marine Service. Miscellaneous special publication No. 25. Standard Methods (1971) Standard Methods for the Examintttion ¢~ Water and Waste 14later. 13th edition, pp. 1-874. New York.