关于重量法测定硫酸盐化速率的不确定度分析

碳酸钾溶液浸渍过的玻璃纤维滤膜曝露于空气中,与空气中的二氧化硫、硫酸雾、硫化氢等发生反应,生成硫酸盐。测定生成的硫酸盐含量,计算硫酸盐化速率。不确定度的来源主要包括:样品重复性分析、电子天平称量等。评定结果为硫酸盐化速率的不确定度(2.90±0.06)(SO3),mg/(100cm2碱片·d),k=2。     

Black layers on historical architecture

Background, aim and scope

The external surface of any building in urban polluted environment is unavoidably destined to be covered with layers that assume a grey to black colour and are generally called ‘black crusts’. These, according to standard protocols and glossary, are deteriorated surface layers of stone material; they can have variable thickness, are hard and fragile and can detach spontaneously from the substrate, which, in general, is quite decayed. Plain visual examination may lead to consider ‘black crusts’ all similar, whilst only a careful diagnostic investigation can distinguish ‘black crusts’ and the consequences of their formation on stone substrates. In this paper, various black layers on marble are studied and compared and the morphological and compositional characteristics discussed according to the related mechanisms of formation. Differences between old (hundred years) and recent crusts (30 years) are investigated and pointed out.

Materials and methods

Samples of black crusts collected from the Milan Cathedral façade (Candoglia Marble) have been studied and compared with the careful and synergic employ of traditional techniques: optical (transmission and reflected VIS light) and electron microscopy, X-ray spectrometry and micro-Fourier transform infrared spectroscopy.

Results

Visual examination of loose fragments does not allow to point out outstanding differences amongst the various samples; black layers have similar main mineral components, gypsum and airborne particles, with different spatial distribution. The microscopic studies allowed to point out the porosity differences, the gypsum crystallisation habit, different amount of embedded particles, level and progress of marble decay.

Discussion

The observations lead to define three main types of black crusts: black crust deriving from marble sulphation, compact deposit and encrustation due to exogenic materials deposition. Black crusts show evidence of sulphation in progress, without a clear continuity solution between crust and marble; the lack of separation is particularly evident in ‘recent’ crust, where the sulphation process is more active. Black compact deposits show a higher porosity than black crusts because gypsum is not coming from the chemical corrosion of the substrate but from outside; actually, in the former case, the substrate is sound. Encrustations show a highly regular crystal organisation of gypsum (close packed tabular crystals) that cannot be traced back to casual atmospheric deposit or to corrosion of the substrate but rather to the crystallisation of a solution coming from an external source. Also in this case, the marble is sound; evidence of the effect of some protection treatment is pointed out.

Conclusions

In spite of the apparent similarity of the examined samples, analytical results have evidenced three main types of black crusts: black crust with decayed substrate, compact deposit and black encrustation showing a sound substrate underneath. Experimental evidence of calcite grains sulphation in progress, taking place according to a model recently proposed, has been observed. Sulphation process is prevented where particular conservation treatments had been applied in the past.

Recommendations and perspectives

New experimental studies can be focussed to understand the specific conditions (measurements of micro-climatic and thermodynamic parameters) and mechanisms for black crusts formation in situ. The problem of the kinetic of the sulphation process of marble, the assessment of black layers formation in the case of different carbonate stone materials and the study of acid attack in presence of surface protecting layers deserve further investigation.     

硫酸盐化速率与居民呼吸道疾病的关系

本文通过收集资料和现场调查的形式，以成都市污染程度不同的两区为对比，分析了硫酸盐化速率与呼吸道疾病的动态变化关系。结果发现硫酸盐化速率与呼吸道疾病的发生呈明显正相关，尤以高浓度污染区更明显；且硫氧化物对慢性呼吸道疾病的影响比对急性呼吸道疾病的影响更加敏感、更密切相关。