library(ggplot2)
Volume_mL <- 0:20

pH_sub1 <- c(6.62, 6.66, 6.65, 6.65, 6.62, 6.60, 6.53, 6.48, 6.43, 6.38, 6.31,
             6.25, 6.09, 6.03, 5.95, 5.87, 5.74, 5.69, 5.62, 5.54, 5.45)

pH_sub2 <- c(6.65, 6.76, 6.97, 7.12, 7.54, 8.02, 8.32, 8.53, 8.67, 8.78, 8.87,
             8.93, 9.00, 9.03, 9.09, 9.15, 9.20, 9.23, 9.27, 9.31, 9.34)

DataF <- data.frame(Volume_mL, pH_sub1, pH_sub2)

ggplot(data = DataF) +
  geom_point(aes(x = Volume_mL, y = pH_sub1, color = "Product 1")) +
  geom_line(aes(x = Volume_mL, y = pH_sub1, color = "Product 1")) +
  geom_point(aes(x = Volume_mL, y = pH_sub2, color = "Product 2")) +
  geom_line(aes(x = Volume_mL, y = pH_sub2, color = "Product 2"))