( ( A ( lower pH ) ) ( ( means ( ( ( ( ( a ( higher acidity ) ) , ) and ) thus ) ( ( a ( higher concentration ) ) ( of ( ( positive ( hydrogen ions ) ) ( in ( the solution ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (JJR lower) (NN pH)) (VP (VBZ means) (NP (NP (DT a) (JJR higher) (NN acidity)) (, ,) (CC and) (ADVP (RB thus)) (NP (NP (DT a) (JJR higher) (NN concentration)) (PP (IN of) (NP (NP (JJ positive) (NN hydrogen) (NNS ions)) (PP (IN in) (NP (DT the) (NN solution)))))))) (. .)))
A lower pH means a higher acidity, and thus a higher concentration of positive hydrogen ions in the solution.
(ROOT (S (NP (NP (DT The) (NN concentration)) (PP (IN of) (NP (NP (NN hydrogen) (NNS ions)) (PP (IN in) (NP (DT a) (NN solution)))))) (VP (VBZ is) (VP (VBN called) (ADVP (RB acidity)))) (. .)))