DETERMINATION OF PHOSPHINE FUMIGANT BY SILVER NITRATE

Determinação do fumigante fosfina pelo nitrato de prata

SUMMARY

Two methods for phosphine determination in pellets and tablets are described. They are based on the stoichiometric reaction of phosphine with silver nitrate; this reaction is by the first time established. In the first method the apparatus is very simple, mounted only with erlenmeyer and Soxhlet thimble. In the second method the apparatus is not simple, mounted with separatory funnel, distilling flask and trap flasks.

KEY-WORDS:
Phosphine; argentimetric determination; potentiography.

RESUMO

Os autores estudaram dois métodos para a determinagäo da fosfina com nitrato de prata, em pastilhas e comprimidos de fosfeto de alumfnio. A reagäo estequiométrica entre fosfina e solugäo de nitrato de prata foi, pela primeira vez, estabelecida. No método I, a aparelhagem é simples, consistindo praticamente de um erlenmeyer com um cartucho de Soxhlet. No método Il, a aparelhagem é complexa, envolvendo o acoplamento de funil de separaqäo, baläo destilador e trés frascos lavadores.

PALAVRAS-CHAVE:
Fosfina; argentometria; potenciografia.

INTRODUCTION

For more than ten years phosphine has been used for fumigation of grains with excellent results. In this case, phosphine has been obtained through the use of aluminum phosphide tablets or pellets, which evolve phosphine by the reaction with humidity of the air according to the equation:

$$\text{PAI}+3{\text{H}}_{2}0\to \text{Al}(\text{OH}{)}_3+{\text{PH}}_3$$

Many methods for phosphine determination have been studied and publishied in American periodicals.

Two methods generally used in America, Europe and Asia are the following: one (²), the White and Bushey method based on the combination of phosphine with mercuric chloride and other (¹), in the oxidation of phosphine by potassium permanganate.

It is not known in chemistry literature a method for phosphine determination through its combination with silver nitrate and this is the reason of the present research.

MATERIAL AND METHODS

In this research the following reagents were used: silver nitrate 0.1N, anhydrous sodium acetate 5%, sodium chloride 0.1N, potassium sulphocyanate 0.1N and sulphuric acid 10% (m/v).

As standard samples were used the following fumigants:

a) aluminum phosphide with 53.83% purity in pellets;

b) aluminum phosphide with 50.57% purity in pellets;

c) aluminum phosphide with 61.40% purity in pellets.

ANAL YTICAL PROCEDURE

Two methods were studied for & termination of aluminum phosphide.

Method I - One 500ml erlenmeyer flask was prepared as shown in figure 1, with an extraction thimble suspended by means of a wire hook fixed to the ground stopper with Araldite.

25ml of silver nitrate 0.1N was added to the erlenmeyer flask and also 20ml of a solution of anhydrous sodium acetate 5%, after which a stream of nitrogen was passed through in order to expel the air. The thimble was then removed and the stopper returned to the erlenmeyer. 0.04g of pulverized tablets or pellets was weighed and introduced into the thimble and this was returned to the erlenmeyer which was tightly closed and kept in the dark

FIGURE 1
Apparatus for method I.

After 24 hours, the precipitate was filtrated and the excess of silver nitrate in the filtrate was determined with potassium sulphocyanate 0.1N with ferric alum as indicator or an eletrocmetric titration was made without filtering by adding sodium chloride 0.1N till the equivalent point.

In the former titration, the calculation is:

$$\frac{(25-\mathrm{A})\times 0.1054}{\mathrm{B}}=\mathrm{\%}\mathrm{P}\mathrm{A}\mathrm{l}$$

If the result in phosphine is desirpd the result in PAI above is multiplied by 0.586.

A = ml KSCN 0.1N

B = Weight of the sample

Iml AgN0₃ 0.1N = 0.001054g PAI

Method 11 - The apparatus used is. shown in figure 2.

In which:

A = 250ml separatory funnel;

B = 250ml distilling flask;

C, D and E = gas washing bottles of 250ml each.

FIGURE 2
Apparatus for method ll.

100ml of sulphuric acid 10% (m/v) was put in A and transfered to B by opening the stopcock; 100ml AgN0₃ 0.1N and 80ml of a solution of anhydrous sodium acetate were put in C; 5ml AgN0₃ 0.1 N + 90ml of water in D and 5ml AgN0₃ 0.1 N + 90ml of water in E.

A stream of nitrogen was passed through the apparatus for ten minutes in order to expel the air. 0.15g of the homogeneous sample of the pulverized tablets or pellets was weighed and wrapped carefully in a small filter paper and put in B, the bottle closed and nitrogen stream continued through the apparatus.

Flask B was heated in a water-bath at about 70°C. After 1 hour the three flasks C, D and E were disconnected and the nitrogen stream stopped.

The three solutions from C, D and E were filtered and quantitatively put together in a 500ml beaker, and the excess of silver nitrate titrated with potassium sulphocyanate O. IN as shown in method I.

The calculation is:

$$\frac{(110-\mathrm{A})\times 0.1054}{\mathrm{B}}=\mathrm{\%}\mathrm{P}\mathrm{A}\mathrm{l}$$

The excess of silver nitrate may also determined electrometrically with the addition of NaC1 0.1 N till the equivalent point. For this no filtration of the precipitate is necessary.

RESULTS

A stoichiometric equation was settled meaning the reaction between phosphine and silver nitrate:

$$\begin{array}{rl}2{\mathrm{P}\mathrm{H}}_3& +11{\mathrm{A}\mathrm{g}\mathrm{N}\mathrm{O}}_3+4{\mathrm{H}}_2\mathrm{O}\underleftarrow{\leftarrow }{\mathrm{P}\mathrm{A}\mathrm{g}}_3+\\ & +8\mathrm{A}\mathrm{g}+11{\mathrm{H}\mathrm{N}\mathrm{O}}_3+{\mathrm{H}}_3{\mathrm{P}\mathrm{O}}_4\end{array}$$

Due to the addition of the sodium acetate to the silver nitrate, the reaction from left to right is favoured:

$$\begin{array}{c}2{\mathrm{P}\mathrm{H}}_3+11{\mathrm{A}\mathrm{g}\mathrm{N}\mathrm{O}}_3+11{\mathrm{C}\mathrm{H}}_3{\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{N}\mathrm{a}}^{+}\\ +4{\mathrm{H}}_2\mathrm{O}\to {\mathrm{P}\mathrm{A}\mathrm{g}}_3+8\mathrm{A}\mathrm{g}+{\mathrm{H}}_3{\mathrm{P}\mathrm{O}}_4+\\ +11{\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}+11{\mathrm{N}\mathrm{a}\mathrm{N}\mathrm{O}}_3\end{array}$$

Three samples of aluminum phosphide were analysed three times each by the method of White and Bushey. The medium result (in %) for each sample is shown in table 1.

TABLE 1
Analytical results in percentage of aluminum phosphide from methods I, II and umite and Bushey.

Method White and Bushey	Method I	Method II
	53.80	53.81
53.83	53.90	53.79
	53.85	53.84
	50.50	50.60
50.57	50.58	50.57
	50.62	50.61
	61.40	61.50
61.40	61.50	61.48
	61.51	61.51

The methods I and II were tested three times in each sample. The results (in %) of PAI are shown too in the table 1.

DISCUSSION

The method 1 is very simple as it can be used in laboratory with poor glass material, nevertheless it requires more time.

The method II is more rapid but it needs a more complex apparatus.

According to the obtained results, the two methods are exact and precise. They are equivalent to the well known method of White and Bushey (²), but the method I is simpler.

Large differences among the three methods were not observed.

ACKNOWLEDGEMENT

The authors are gratefull to Mrs. Edith Mackrodt for the revision of the English text.

REFERENCES

• ¹ INDIAN STANDARD INSTITUTION (India). Aluminum phosphide analysis: IS6438, 1972. CM/1-5067.
• ² WHITE, W.E. & BUSHEY, A.H. Aluminum phosphide-preparation and composition. J. Ana Chen soc, 66: 1666-72, 1944.

Publication Dates

• Publication in this collection
  14 Feb 2025
• Date of issue
  Jan-Dec 1985

History

• Received
  30 May 1984