# Methods for Detecting Picric Acid--A Review of Recent Progress
## 1 Introduction  
2,4,6-trinitrophenol (commonly referred to as picric acid) is a nitroaromatic compound bearing a strongly acidic proton in its hydroxyl group [1]. Picric acid (PA) is an explosive that has been widely used for the manufacture of munitions and explosive charges, particularly during World War I [2; 3; 4]. PA was also used in the dye and pharmaceutical industries, in chemical laboratories [5; 6], for the manufacture of fungicides [7; 8] and as one of the components used to monitor the automated synthesis of peptides [9]. In the interwar period (1919-1939), picric acid was rapidly replaced in its military applications by 2,4,6-trinitrottoluene [10], due to the latter being less sensitive to mechanical stimuli (Table 1) and due to the significant toxicity of PA [11].  
Nitroaromatic compounds (NACs), including PA, are known for their harmful hepatotoxic and haematotoxic effects. They cause mutagenesis and carcinogenesis within living cells [13; 14]. The toxicity of picric acid is described as higher than that of nitroderivatives of toluene, xylene or naphthalene [11]. Chronic exposure to PA can damage blood, liver and kidney cells and cause cancer, anaemia, cyanosis and male sterility [15; 16]. In animals, the metabolism of picric acid takes place via the reduction of its nitro groups to amine groups, followed by acetylation of the amino group, whereby picric acid is excreted as 2-amino-4,6-dinitrophenol (picramic acid) derivatives [15; 17; 18; 19].  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Compound**} & **Density** & **Detonation** & **Lead Block** & **Impact** & **Friction** \\  & **(g/cm\({}^{3}\))** & **Velocity** & **Test [cm\({}^{3}\)** & **Sensitivity** & **Sensitivity** \\  & **(g/cm\({}^{3}\))** & **(m/s)** & **Pb/10g** & **[Nm]** & **[N]** \\ \hline PA & 1.77 & 7350 & 315 & 7.4 & 353 \\ Lead Azide & 4.8 & 4500 & 110 & 2.5-4 & 0.1–1 \\ RDX & 1.82 & 8700 & 480 & 7.5 & 120 \\ HMX & 1.87 & 9100 & 480 & 7.4 & 120 \\ TNT & 1.47 & 6900 & 480 & 15 & 353 \\ PETN & 1.76 & 8400 & 523 & 3 & 60 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Summary of selected parameters of commonly used energetic materials [12].  
The toxicity of PA is exacerbated by its high solubility in water [20], which facilitates contamination of various media, posing a potential risk to human health and to the environment. Antiquated munitions are the primary source of PA contamination, as it can be readily leached out from such devices and objects. The presence, toxicity and environmental fate of PA have long been an issue of concern [21], particularly near sites in which post-World War munitions have been stored. To combat the issue of PA contamination and identify any sources of its release into the environment, sensitive and efficient methods for its detection and speciation from among other NACs, often at trace concentration levels, are necessary [22]. Recent years have brought about significant developments in this subject, with a wide variety of materials and approaches being reported, particularly regarding the use of fluorescence quenching, as briefly summarised in this work.