How does the skin detect solutions with a strong pH? Some burns are not apparent until a day later, so is there a difference between the physical discomfort and physical harm from chemicals?
The skin does not actually detect anything—detection and perception occur in the brain. However, there are receptors in the skin whose job is to respond to chemicals, along with a variety of other receptors designed to respond to other stimuli. The receptors most sensitive to pH are termed nociceptors (or pain receptors). They are in a family of receptors that includes those sensitive to capsaicin, the burning ingredient in hot pepper. When a strongly acidic [or alkaline?]solution touches the skin, these receptors are activated and send electrical signals (action potentials) to the brain via specific nociceptive or pain pathways. These signals then lead to the detection or perception of pain. The pathways conveying these signals from the receptors to the brain do not conduct information very rapidly—perhaps at only a meter per second or so—thus there is a significant delay between the time the solution hits the skin and the time the brain perceives pain.
“Some burns are not apparent until a day later, so is there a difference between the physical discomfort and physical harm from chemicals?"
This is another interesting but somewhat complicated question. The simple answer is yes, there is a difference between the full spectrum of physical harm and the physical discomfort. If a chemical is damaging enough to cause pain, then that pain is usually perceived in a matter of seconds. If a chemical is not damaging enough to cause an immediate burn, but a burn appears later, then the initial exposure has activated an inflammation cascade that requires time to be noticed. Inflammatory cascades involve a variety of mechanisms that depend on specific signals and chemicals the damaged tissues release as a result of the injury or other trigger. Inflammation makes the receptors more sensitive and can cause the area to become reddened and warm due to the dilation of blood vessels and increased blood circulation. Also, fluid can leave the blood vessels due to an increase in vessel permeability, causing swelling or even a blister. These responses are quite distinct from the initial painful experience. All toxic and potentially harmful chemicals should be handled with great care and your skin should be carefully protected while handling them. In the laboratory, we use many personal safety equipment items including lab coats, gloves, face shields, safety glasses, and fume hoods with exhaust circulation to protect people who handle damaging chemicals.