Question

В состав белой краски входят свинцовые белила – основной карбонат свинца, и картины, нарисованные с ее использованием, приобретают при долгом хранении в помещении темный оттенок. Для реставрации потемневшие участки обрабатывают пероксидом водорода. Почему белила темнеют, а при обработке становятся белыми снова? Почему при такой обработке может исчезать желтый цвет? Напишите уравнения реакций.

Answer 1

Свинцовые белила: 
Свинцовые белила— основной карбонат свинца Рb 3 (ОН) 2 (СО 3 ) 2 , растворим в кислотах и щелочах, устойчив к воздействию света и влаги. Применяют для окраски судов и др. От действия сероводорода свинцовые белила темнеют вследствие образования черного сульфида свинца (причина потемнения старинных картин, писанных масляными красками). Реставрировать такую картину можно при помощи разбавленной перекиси водорода: сульфид свинца переходит в белый сульфат свинца. Из-за ядовитости применение свинцовых белил ограниченно.

PbCO3 +H2S = PbS + CO2

PbS + 4H2O2 = PbSO4 + 4H2O

Answer 2

Composition of White Paint and Darkening of Lead Pigments

White paint typically contains lead pigments, specifically basic lead carbonate, which is responsible for its white color. However, over time, paintings created using white lead pigments can develop a darkened appearance when stored in indoor environments. This darkening is primarily due to the reaction of lead carbonate with atmospheric gases, such as sulfur dioxide, which forms lead sulfide.

Restoration with Hydrogen Peroxide

To restore the darkened areas of a painting, hydrogen peroxide is often used. When hydrogen peroxide is applied to the darkened lead pigments, it undergoes a reaction that converts the lead sulfide back into white lead carbonate. This restoration process helps to return the affected areas to their original white color.

Disappearance of Yellow Color

In addition to restoring the white color, the treatment with hydrogen peroxide can also cause the disappearance of any yellow color present in the painting. This is because lead sulfide, which is responsible for the darkening, can also contribute to the formation of yellow lead compounds. When the lead sulfide is converted back into white lead carbonate, any yellow lead compounds that were present are also transformed, resulting in the disappearance of the yellow color.

Chemical Reactions

The reactions involved in the darkening and restoration of lead pigments in white paint can be summarized as follows:

1. Darkening of Lead Pigments: - Reaction: Lead carbonate reacts with atmospheric gases, such as sulfur dioxide, to form lead sulfide. - Equation: PbCO3 + SO2 -> PbS + CO2

2. Restoration with Hydrogen Peroxide: - Reaction: Hydrogen peroxide reacts with lead sulfide to form white lead carbonate. - Equation: PbS + H2O2 -> PbCO3 + H2O

It is important to note that the restoration process should be carried out by professionals with expertise in art restoration to ensure the proper treatment of the artwork.

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