Application of the indophenol blue method to the determination of ammonium in silicate rocks and minerals

Applied Geochemistry, Vol. 8, pp. 101-105, 1993

0883-2927/93 $5.00+ .00 © 1992 Pergamon Press Ltd

Printed in Great Britain

Application of the indophenol blue method to the determination of ammonium in silicate rocks and minerals A . HALL Department of Geology, Royal Holloway and Bedford New College (University of London), Egham, Surrey TW20 0EX, U.K.

(Received 11 March 1992; accepted in revised form 21 May 1992) A b s t r a c t - - A colorimetric method is given for determining ammonium in silicate rocks and minerals at concentrations down to 1 ppm. The sample is digested in cold HF, and ammonia is then separated by distillation and measured colorimetrically as the indophenol blue complex. Using the reagents and procedures described here, the problems of colour instability and poor reproducibility encountered by previous workers have been overcome.

INTRODUCTION RECENT studies on the geochemistry of a m m o n i u m have shown its importance as a guide to hydrothermal activity (KYDD and LEVINSON, 1986; RIDGWAY et al., 1990; HALL et al., 1991). Unfortunately, the existing techniques for measuring a m m o n i u m at low concentrations are rather elaborate, and not easily adapted for the routine analysis of a large number of samples. This paper describes a comparatively simple colorimetric method which requires less apparatus than existing methods and less supervision during the stage of sample digestion, and is very suitable for the study of hydrothermally altered rocks. The most widely used technique for determining a m m o n i u m in non-geological materials is the colorimetric method which utilizes Nessler's reagent (alkaline potassium mercuric iodide), following a distillation to separate a m m o n i a from interfering constituents. This has been applied to rock analysis by URANO (1971). This method works satisfactorily for rocks with a m m o n i u m contents of at least 10 ppm, but lacks the sensitivity needed to measure a m m o n i u m at the lower levels present in many igneous rocks, and is relatively time-consuming because of the care needed in the initial sample digestion by hot HF. The indophenol blue method (Berthelot's method) is an alternative colorimetric method which potentially offers greater sensitivity, but has a reputation for unreliability and has not been much used for rock analysis. Many previous authors have c o m m e n t e d on its lack of reproducibility (BOLLETERet al., 1961; RILEY, 1975; PATTON and CROUCH,1977) and proneness to colour-fading, and have made divergent recommendations on the optimum temperature, the order in which reagents are to be added, and even on the best wavelength for colour measurement. However, recent studies of the process of colour formation have clarified the reaction mechanism involved, and emphasized the importance of p H for color

development and stability, as well as suggesting catalysts to promote color development (RILEY, 1975; PATRON and CROUCH, 1977; N6O et al., 1982). The m e t h o d described here has two main features: (1) the use of a cold H F digestion in a closed tube to minimize ammonia loss during sample digestion; and (2) the colorimetric reagents and procedure have been tested to achieve the optimum p H for intensity and stability of color development. In addition, there are several small details of the procedure which are of crucial importance in achieving high reproducibility. The method has been developed for determining the total a m m o n i u m content of rocks, but it may easily be adapted to determining exchangeable ammonium, or fixed a m m o n i u m by difference.

RECOMMENDED METHOD

This procedure utilizes a distillation apparatus consisting of a 300 ml Kjeldahl flask connected to a water-cooled condenser. No steam generator or spray trap is required. Samples are conveniently processed in batches often, and in so far as the results can be predicted it is desirable to process low-ammonium and high-ammonium samples in separate batches to minimise the risk of cross-contamination. All measurements of ammonium at low concentrations are vulnerable to contamination because of the ubiquitous presence of ammonia or ammonium in the environment. The necessity of cleaning the apparatus with deionized water before use is obvious, but this precaution is not sufficient for measurements at the 1/~g level. Small amounts of ammonia or ammonium ion are adsorbed onto glass and plastic surfaces and are not easily removed. To bring distillation blanks down to an acceptable level it is essential to carry out a pilot distillation at the start of each batch and discard the distillate, before the apparatus is used for actual samples. Reagent blanks must be processed along with the samples to be analysed. Two types of reagent blank are required, which may be distinguished as the "digestion blank" and the "colorimetrie blank". For the digestion blank, 2 ml aliquots of HF are measured into sample digestion tubes alongside the samples being analysed, and are subjected to the same subsequent digestion and distillation procedure. Their purpose is to correct for NH + contamination in the analytical 101

102

A. Hall

reagents and apparatus; the biggest contribution to this comes from the KOH reagent used in distillation. For the colorimetric blank, 10/~g aliquots of NH~ in solution are added to 50 ml volumetric flasks in the same way as the sample distillates. Their purpose is to correct for NH~ in the colorimetric reagents. In practice, it is possible to reduce the number of distillations required by running several digestion blanks with the first batch of samples, and then using the same corrections for as long as the same batches of HF and KOH are in use. Colorimetric blanks must be included with every batch of samples.

Reagents Deionized water is used for making up all the reagents and for rinsing all glassware. Distilled water is not suitable, because distillation does not remove dissolved ammonia and other N-bearing substances from water. Hydrofluoric acid, 40%, analytical grade. Potassium hydroxide solution (25% w/v). Dissolve 100 g of KOH in deionized water and make up to 400 ml. Phenol reagent. Weigh out 7.0 g of crystalline phenol into a 200 ml beaker. Add 0.02 g of sodium nitroprusside, followed by 20 ml of 25% KOH solution (as above), and 60 ml of deionized water. Stir to dissolve, and make up to 100 ml. Sodium hypochlorite reagent. Dilute 20 ml of commercial sodium hypochlorite solution containing - 5 % available C1 to 100 ml with deionized water. Sodium hypochlorite solutioin deteriorates during storage, and should be periodically replaced by a new supply. The diluted solution has a pH of -11.8 when the NaOCI reagent is fresh, decreasing to -10.5 as its chlorinating capacity becomes almost exhausted. Powdered pumice. Grind about 20 g of granular pumice (commercial reagent: Rhone-Poulenc) to a powder (<0.1 mm grain size) in an agate mortar, and bake at 650°C in a clean crucible for 24 h. Analyse a portion of this material to confirm that it is ammonium-free. Standard ammonium solution. Dissolve 0.7412 g of ammonium chloride in 250 ml of deionized water. This solution contains 1000/~g/ml of ammonium ion. Dilute 5 ml of this solution to 1 litre to make a standard solution containing 5 pg/ml of NHJ-.

Procedurefor the determination of total ammonium 1. Weigh exactly 0.25 g of rock powder into a 30 ml polythene specimen tube. If the NH~- content of the sample is expected to exceed 100 ppm, the sample weight should be reduced accordingly. 2. Add 2 ml of 40% HF. Cap the tube, and leave the sample to digest for 1 week, gently swirling the tube every day or two to ensure complete decomposition of the rock powder. 3. Prepare the distillation apparatus. Add -0.01 g of prepared pumice to the distillation flask to prevent bumping. Immerse the open end of the delivery tube in 15 ml of 0.04 N HeSO 4 in a 50-ml conical flask ( - 1 drop of 50% H2SO 4 in 15 ml He0 ). 4. Make the rock solution alkaline by adding 20 ml of 25% KOH solution to the digestion tube. 5. Transfer the solution immediately to the distillation flask, making sure that any insoluble residue is also transferred. 6. Boil the solution until 15 ml of distillate has been collected ( - 4 - 5 rain). 7. Transfer the distillate to a 50-ml volumetric flask. 8. Add 2.5 ml of phenol reagent from a burette, and gently swill the flask.

9. Add 2.5 ml of NaOCI reagent from a burette, and gently swill the flask. 10. Add deionized water to make up to 50 ml. 11. Similarly treat 50 ml flasks containing 15 ml of 0.04 N n 2 s o 4 with and without additions of 10/tg NH~- to act as the standard and as the colorimetric blank. For the highest accuracy more than one standard may be prepared. 12. Leave solutions to stand for 2 h to allow complete color development. Turn on the spectrophotometer to stabilize during this period. 13. Immediately before measurement, shake the flasks briefly to ensure homogeneity. 14. Measure the optical absorbance at 630 nm in a 2-cm cell.

EXPLANATION OF THE RECOMMENDED PROCEDURE

Sample digestion T h e p r o c e d u r e for total a m m o n i u m starts with a cold digestion of the sample in HF. This differs from the m e t h o d of U ~ y o (1971), in which the sample is digested in hot H F - H 2 S O 4. T h e r e were two reasons for preferring a cold digestion in a closed vessel, O n e was to reduce c o n t a m i n a t i o n by a t m o s p h e r i c amm o n i a which is easily a b s o r b e d by an acid digestion m e d i u m . Several s t a n d a r d p r o c e d u r e s used in geochemical laboratories involve the h a n d l i n g of amm o n i u m hydroxide or c a r b o n a t e , so an u n c e r t a i n a m o u n t of N H 3 could potentially be p r e s e n t in the l a b o r a t o r y a t m o s p h e r e . A i r b o r n e dust also contains organic particles which could be hydrolysed to N I ~ . T h e second r e a s o n is t h a t the U r a n o m e t h o d involves heating the Pt crucible to dryness to c o n v e r t fluorides to sulphates, a n d loss of NH~- at this stage was c o n s i d e r e d to be a risk unless the crucible t e m p e r a ture could be constantly m o n i t o r e d . In the cold m e t h o d , the p e r i o d of digestion is not critical. Most silicate minerals, including the potassium minerals likely to contain most of the NH~-, are d e c o m p o s e d in a few hours, but the digestion tubes can be left for any length of time before the distillations are carried out because t h e r e is no d a n g e r of any NH~- being lost. It is possible that some of the NH~- released by the sample digestion may be i n c o r p o r a t e d into relatively insoluble fluoroaluminates, but these are readily d e c o m p o s e d by alkalis, and this is why step 5 of the r e c o m m e n d e d p r o c e d u r e insists o n any insoluble digestion residue b e i n g t r a n s f e r r e d to the distillation flask.

Distillation T h e t e c h n i q u e for s e p a r a t i o n of a m m o n i a by distillation differs in two main respects from that a d o p t e d by previous authors: (1) the use of a simple distillation a p p a r a t u s r a t h e r t h a n a s t e a m distillation apparatus to separate a m m o n i a from the o t h e r constituents of the rock; and (2) the use of K O H r a t h e r t h a n

Determination of ammonium in silicate rocks and minerals NaOH to make the solution alkaline for distillation of ammonia. Steam distillation is not essential to the complete recovery of ammonia by distillation, its main function being to moderate the ebullience of the boiling solution by introducing a stream of bubbles. This is desirable both on safety grounds and to ensure that no alkaline spray is carried over into the receiving flask. The purpose of the pumice that is added to the distillation flask is to prevent the bumping which normally occurs when solutions of NaOH or KOH are boiled. With this modification it is possible to dispense with the steam generator usually employed in the distillation of ammonia. Natural pumice is a very effective anti-bumping agent, but it is essential that the pumice used is completely free of ammonium. As a glassy volcanic rock, formed from a melt at a high temperature and low pressure, it should not contain any ammonium in the first place, and the supply used by the author did not contain any detectable NH~-, but pumice is easily altered by groundwater and some pumices are recorded as containing traces of ammonium, so the preliminary heat treatment is carried out to ensure that any ammonium is removed. The use of KOH rather than NaOH to make the solution alkaline for distillation is dictated by the presence of HF in the digested sample solution. If NaOH were to be used for neutralizing a solution containing HF an undesirable degree of dilution would be required, because NaF is relatively insoluble compared with KF, and its precipitation causes the H F - N a O H reaction to be unacceptably violent. The volume of KOH solution added to the sample solutions is greater than is needed just to make them alkaline; the extra volume serves also to keep down the temperature of the tubes while their contents are being transferred to the distillation apparatus, thereby minimizing any possible loss of ammonia by vaporization at this stage. In contrast to NaOH, analytical grade KOH normally contains a significant trace of NI-I~, so the KOH solution added to the sample solution before distillation must be measured out precisely in order to make an accurate correction for the reagent blank.

The colorimetric method

According to PAITONand CROUCH(1977) the principal intermediate compound in the formation of the indophenol blue complex is monochloramine (NH2C1), and the effect of pH on the formation and decomposition rates of this compound are the main reason why pH control is so crucial to the reproducibility of the Berthelot method. The maximum rate of formation of monochloramine occurs at pH 8, but at this pH the compound is very unstable. Decomposition of monochloramine is rapid below pH 10.5, but as the pH increases the rate of monochloramine

103

formation becomes slower and slower, so the best range for monochloramine formation is between 10.5 and 11.5. The rate of formation of monochloramine can be increased by the use of a catalyst, e.g. sodium nitroprusside, but if more than a very small amount of this catalyst is used the wavelength of maximum absorption is altered, implying that a colored complex other than indophenol is being formed. Figure 1 shows how the rate and intensity of color development is affected by the pH of the final solution in the pH range 9.3-11.6. The pH was controlled by varying the amount of KOH used in making up the phenol reagent. Further experiments showed that between pH 11.0 and 12.0 the final color intensity is almost constant, but as the pH rises above 12.0 the fully developed color becomes weaker and weaker. The optimum range for colorimetry is therefore 11.012.0. If the reagents are made up as specified for this method, the pH of the final solution is in the range 11.5-12.0, the uncertainty being due to variations in the pH of the sodium hypochlorite reagent as it deteriorates on keeping. So long as the diluted NaOC1 reagent has a pH between 10.0 and 11.5, the pH of the final solution is well within the optimum range, but the dilution of the NaOC1 reagent solution may need to be varied to maintain its available CI content. Some chemical suppliers offer commercial NaOC1 solutions of different strength expressed as available C1, and these may be used at an appropriate dilution so long as the pH of the diluted reagent solution is kept between 10.5 and 11.8. The way in which the color-forming reagents are added to the distillates affects the process of color development. For maximum color intensity and reproducibility, the phenol and NaOCI reagents must not only be added in that order, but with the minimum delay between addition of the two reagents. Moreover the contents of the flasks must be homogenized at each step, so that as far as possible the concentrations of reactants are identical throughout the solution volumes of all the standards and unknowns. In the course of testing the colorimetric method, a problem was discovered which has not been mentioned by previous authors. Unlike some colored complexes measured by spectrophotometry, the indophenol blue complex forms a true solution and not a suspension, so the possibility of settling during color development was not considered. However, heterogeneity does develop in the indophenol blue solution, and can be measured by extracting portions of solution from the volumetric flasks by means of a syringe. In an undisturbed volumetric flask the solution occupying the neck of the flask has a color intensity (optical absorbance) greater than that in the body of the flask by up to 20% after 2 h of color development. There is not enough variation in the body of the flask to be visible to the naked eye, and there is no cloudiness or other indication of the

104

A. Hall

-50

I

pH 11.6 pH 10"8

•

pH 10.6

•

pH

9'8

pH

9"25

.40 c

3 "30 .D

-20 --

"10

0

"I-

0

I

I

I

50

100

150

Time

(minutes)

Fl6.1. Relation between colour development and pH of the indophenol solution. Each solution contained 10pg NH+ in 50 ml; colour development was at 20°C. Table 1. Recovery of ammonium from the digestion and distillation procedure Solution distilled Zero NH~- + HF + KOH 10/~g NH~- + HF + KOH Net NH~- recovered:

NH~- measured in distillate 1.26pg (mean of 1.28, 1.19, 1.31 pg) ll.06pg (mean of 10.94, 10.75, 11.48/~g) 9.80/~g

heterogeneity. The source of the heterogeneity is not certain, but a possible explanation is upward diffusion of dissolved gas (C12 or 02) resulting from the slow dissociation of hypochlorite ion in the solution. The problem is easily remedied by homogenizing the contents of the flask immediately before colour measurement, but it is possible that this previously unrecognized problem could be responsible for some of the lack of reproducibility encountered by previous users of the indophenol method.

samples, and then taking them through the complete digestion, distillation and colorimetric procedure to check for any loss of ammonium ion or ammonia. The same procedure was followed with digestion blanks, starting with digestion tubes containing only HF and no added ammonium. The results in Table 1 show the recovery of N I ~ in these experiments. Allowing for a reagent blank of 1.26 mg NH~-, the net recovery in the triplicate standard solutions varied between 95 and 102% of the ammonium present, averaging 98%.

EVALUATION OF THE METHOD

Sensitivity Recovery of ammonium The distillation procedure described here is simpler than that used by other authors in that no steam generator is used, and the acid solution resulting from sample digestion is not make alkaline in the distillation flask but in the digestion vessel. The effectiveness of the modified technique has been tested by measuring out 10 pg aliquots of NH~--bearing standard solution into polythene digestion tubes, adding HF as in the treatment of rock

A solution containing 10 pg NH~- in 50 ml of solution gives an absorbance (net of reagent blank) of 0.434 in a 2-cm cell, with a coefficient of variation of 1.1%. This concentration corresponds to 40 ppm NH~- in the rock if the sample weight and dilution are as given in the recommended procedure. A detection limit of 0.25 pg NH~ should be easily attainable, corresponding to 1 ppm in the rock, but this is relatively small compared with the digestion blank + (typically - 1/~g NH 4 , depending on the purity of the

Determination of ammonium in silicate rocks and minerals

105

Table 2. Reproducibility of the indophenol method applied to rock samples NH~" (ppm) Sample No.

Type of material

Duplicate results

Mean

97/26-3 AGV-1 R43 R46 MV-7 $3-883 P112A PO-310 $2-790 B-R20

Granite Andesite Granite Granite Granite Altered granite Granite Granite Altered granite Biotite

1.4 5.6 8.9 11.3 21.5 40.0 56.2 63.5 106.7 176.8

1.6 6.4 10.2 14.6 21.9 41.1 59.0 67.2 109.6 178.1

K O H reagent), so accurate control of the digestion blanks is essential for measurements at the lowest level. In practice, the detection limit is governed by the reproducibility of the measurements rather than the sensitivity of the coiorimetric reaction. The alternative colorimetric method for the determination of a m m o n i u m utilizes Nessler's reagent (an alkaline solution of potassium mercuric iodide), which forms a yellow complex with the a m m o n i u m ion. With that reagent, 10/~g of NI-I~ in 50 ml of solution gives a net absorbance of 0.071 at 400 nm in a 2-cm cell, so the new m e t h o d represents a 6-fold improvement in sensitivity. Precision

which in turn requires only a small quantity of H F for digestion, and a small quantity of K O H for neutralization. By rendering more feasible a cold digestion in a closed tube, these details permit unsupervised digestion of large sample batches without the use of Pt crucibles, a hotplate or even a fume cupboard (although this is still necessary for measuring out the HF). Digestion in a closed tube reduces errors due to contamination, and cold digestion reduces errors due to ammonia loss (compared with a hot H F - H 2 S O 4 attack). The use of prepared pumice as an antibumping agent also enables a much simpler distillation apparatus to be used than has hitherto been necessary. Editorial handling: Brian Hitchon.

The reproducibility of the m e t h o d in routine use is illustrated by the duplicate determinations listed in Table 2, done over a 2-a period. In each case the duplicated results were obtained on different days, sometimes using different reagent batches. The precision, quoted as the standard error of estimate, is _+3.0 ppm. All of the determinations in Table 2 were done using sample weights between 0.2 and 0.3 g, but for expected a m m o n i u m contents >100 ppm a smaller weight would normally be taken, and the errors would then be proportionately greater. CONCLUSIONS The method described in this paper has adequate sensitivity and reproducibility for the measurement of total a m m o n i u m in geochemical samples. It may also be adapted for the measurement of exchangeable a m m o n i u m , or fixed a m m o n i u m by difference. The use of an H F sample digestion means that any type of geological material can be analysed, not just those decomposable by H2SO 4 or N a O H . The main benefit claimed for the method is the simplified digestion and distillation procedure. The great sensitivity of the indophenol technique means that a relatively small sample weight is required,

AG 8:1-H

1.8 7. I I 1.5 17.8 22.3 42.2 61.7 71.0 112.4 179.4

REFERENCES

BOLLETERW. T., BUSHMANC. J. and TIDWELLP. W. (1961) Spectrophotometric determination of ammonium as indophenol. Anal. Chem. 33, 592-594. HALL A., BENCINIA. and PoLl G. (1991) Magmatic and hydrothermal ammonium in granites of the Tuscan magmatic province, Italy. Geochim. cosmochim. Acta 55, 3657-3664. KYDD R. A. and LEVINSONA. A. (1986) Ammonium halos in lithogeochemical exploration for gold at the Horse Canyon carbonate-hosted deposit, Nevada, U.S.A.: use and limitations. Appl. Geochem. l, 407-417. NGO T. T., PHANA. P. H., YAM C. F. and LENHOFFH. M. (1982) Interference in determination of ammonia with the hypochlorite-alkaline phenol method of Berthelot. Analyt. Chem. 54, 46-49. PATRONC. J. and CROUCHS. R. (1977) Spectrophotometric and kinetics investigation of the Berthelot reaction for the determination of ammonia. Analyt. Chem. 49, 464-469. RIDGWAYJ., APPLETONJ. D. and LEVINSONA. A. (1990) Ammonium geochemistry in mineral exploration--a comparison of results from the American cordilleras and the southwest Pacific. Appl. Geochem. 5,475-489. RILEY J. P. (1975) Analytical chemistry of sea water. In Chemical Oceanography (ed. J. P. RILEY and G. SKInROW), Chap. 19, 2rid edn. Academic Press. URANO H. (1971) Geochemical and petrological study on the origins of metamorphic rocks and granitic rocks by determination of fixed ammoniacal nitrogen. J. Earth Sci. Nagoya Univ. 19, 1-24.