纸的酸碱性或称纸表面的pH值。就纸张本身的纤维而言，无所谓有酸碱性的区别。但是，因为在制浆、漂白过程中带来的残留物以及在调料中添加的物料等，所以才使纸张具有酸性、碱性或中性。一般采取H计来测量:pH值等于7为中性; pH=1~6为酸性;pH=8~14为碱性。pH值是表示水溶液的特性之一，水溶液的酸碱性取决于其中氢离子和氢氧根离子的多少而定。pH值并不代表水溶液中含有的氢离子浓度的绝对值,而是氢离子浓度的指数。pH值两个字母代表溶液的“潜在氢(Potential Hydrogen)”或者“氢离子的活性”( hydrogen power)。这个标度是由丹麦生物化学家塞伦森（Sorenren)于1909年提出的。pH值的定义是:用每升摩尔数表示的氢离子浓度的常用对数的负值，即pH=-lgc(H)。pH值为1~14，并以pH=7为纯水的中性点，区分酸碱性的界限。pH值改变1，离子浓度相差10倍。比如 pH=5的酸度比'pH=6的酸度高10倍。同理，pH=9的碱度是pH =8的碱度的10倍。所以，只要数字相差一点，酸碱性都有不小的改变，这是一般人常常容易忽略的。
纸的酸碱性之测定方法分为热法和冷法。热法是把纸样置于沸水中加热1h,然后将水溶液冷却至20℃，利用玻璃电极测量其pH值。冷法是把纸样泡在室温下的冷水中1h时，再依同样方法测量其pH值。到底哪-个方法更能反映纸的酸碱性?目前尚未有定论，所以一般在测定数字后边注明是热法抑或冷法。
上述这两种方法存在着一个重大缺点，就是以破坏纸张为前提，并且测出的是平均pH值，使得属于珍贵文物的纸质趟案、书刊、画册等，无法检测其酸碱性。为了解决这个难题，又有了无损检测pH值的新方法。它是把纸样固定在一块橡胶板上，用一滴中性的蒸馏水滴在纸面，再将电极伸人水滴并与纸面接触，停留10min后测定其数字。可以在纸样上选择5个不同的“点”，求得平均值。
The acidity and alkalinity of the paper has the greatest influence on the durability of the paper. The lower the pH value of the paper , that is, the stronger the acidity, the more significant the fading and strength decline (self-destruct phenomenon) of the paper, and the shorter its life. This is because the chemical components (cellulose, hemicellulose) in the paper degrade faster under acidic conditions. Of course, if the pH value of the paper is too high, that is, the stronger the alkalinity, the chemical composition in the paper will induce alkaline degradation, which will also greatly reduce the life of the paper.
The acidity and alkalinity of the paper also has a great influence on the printing quality. When the pH value of the paper is lower, its acidic substances and polyvalent metal ions in the printing ink will form oil-insoluble products, which hinders the process of oxidative conjunctiva. As a result, the drying time of the printed matter is prolonged; when the pH value of the paper is high, the hydroxide ions can accelerate the oxidation of the ink, thereby shortening the drying time of the printed matter. Because fountain solution is needed in offset printing, and fountain solution is weakly acidic. Generally, offset paper is weakly alkaline; the pH value of coated paper is suitable between 6 and 7. If the acidity of the paper is too high, the pH value of the dampening plate will be changed during printing, and the acidity of the dampening solution will be increased, so that the ink will not dry well, and the back side adhesion phenomenon is likely to occur. If the strong acidity of the fountain solution is not done well, it will also cause the printing plate to suffer from extensive erosion, damage to the dots, reduce the use time of the printing plate, and cause serious consequences. If the alkalinity of the paper is too high, it will easily react with the acidic dampening solution, thereby increasing its pH value, affecting the normal transfer of ink, resulting in paste, dark ink, and reduced quality of printed matter.
